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Navy Experimental Diving Unit 

321 Bullfinch Rd 

Panama City, FL 32407-7015 


TA 14-18 
NEDU TR 16-01 
January 2016 



NAVAL SE/lrSYSTEMS COMMAND 


Navy Experimental Diving Unit 


Influence of 

Very High Breathing Resistance 
on Exercise Tolerance, 

Part 1 - Dry Exercise 


Authors: 


Distribution Statement A: 
Approved for Public Release 
Distribution is Unlimited 


Dan Warkander, Ph.D. 
Barbara Shykoff, Ph.D. 




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4. TITLE AND SUBTITLE 

Influence of Very High Breathing Resistance on Exercise Tolerance, 
Part 1 - Dry Exercise 


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Shykoff, B„ Ph.D. 


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TA 14-18 


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Navy Experimental Diving Unit 
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Panama City, FL 32407 


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14. ABSTRACT A breathing apparatus with a partial failure may have higher breathing resistance (R) than expected 
or a breathing apparatus may be needed for harder work than it was designed for. The effects of very high R on 
physical endurance and on breathing was not known. Fifteen subjects took part in this IRB approved study to 
determine such effects during moderate exercise (60% of peak O 2 consumption) on a cycle ergometer on dry land 
at sea level. R was such that the work of breathing per volume (volume-averaged pressure) ranged from nominal 3 
to 9 kPa (J/L), i.e. up to 3 times higher than NEDU’s limits for diving. Individuals’ exercise endurance varied 
greatly. With the least high R, it ranged from 4.5 min to the protocol’s maximum 60 min, with the highest R two 
subjects continued for 60 min, while one other exercised for less than 2 min. The endurance time for the 90 th 
percentile was 12 min at the lowest R and 3 min for the highest. In general, the minute ventilation decreased 
(reduced breathing frequency, unchanged tidal volume and duty cycle) with increasing R and the end-tidal CO 2 
values increased, some subjects reaching levels close to 8% of the dry gas (57 mm Hg). No subject reached the 
abort limit of 65 mm Hg. Some subjects who maintained high CO 2 levels reported no or low dyspnea. Rating of 
perceived exertion did not correlate with R. Reactions to very high R are not predictable. Low scores for dyspnea 
or perceived exertion do not indicate acceptable R. NEDU’s limits for Rina diver's breathing apparatus cannot be 
used at sea level. Values for R found in simulated or real failures of breathing apparatus can be used with the 
endurance times found here to judge likely endurance times. 


15. SUBJECT TERMS 

control of breathing, ventilation, CO 2 , carbon dioxide, hypercapnia, CO 2 retention, dyspnea, exercise, performance, 


16. SECURITY CLASSIFICATION OF: 

17. LIMITATION 
OF ABSTRACT 

18. 

NUMBER 

19a. NAME OF RESPONSIBLE 
PERSON Nancy Hicks 

a. REPORT 

A 

b. ABSTRACT 

Unclassified 

c. THIS PAGE 

Unclassified 

SAR 

OF 

PAGES 

36 

19b. TELEPHONE NUMBER 

(include area code) 850-230-3170 


Standard Form 298 (Rev. 8-98) 
Prescribed by ANSI Std. Z39.18 


































TABLE OF CONTENT 


TABLE OF CONTENT.ii 

INTRODUCTION.1 

METHODS.1 

Determinations of peak oxygen update.2 

Endurance tests.2 

Design of resistance elements.3 

Selection of resistance elements.3 

Calibrations.4 

Data analysis.4 

Abort criteria.4 

Statistical analysis.4 

RESULTS.5 

Endurance times.5 

Minute ventilation.10 

Tidal volume.11 

Breathing frequency.12 

Duty cycle.13 

Peak mask pressures.14 

Heart rate.16 

Inspiratory work of breathing.17 

RPE and dyspnea.18 

Examples of responses to very high breathing resistance.19 

DISCUSSION.24 

Endurance times.24 

End-tidal CO 2 levels.25 

Ventilatory patterns.25 

Inspiratory work of breathing.25 

Dyspnea, RPE scores and symptoms.26 

Comparison to published limits on breathing resistance.27 

SUMMARY.28 

CONCLUSIONS.29 

RECOMMENDATIONS.29 

REFERENCES.29 

APPENDIX A.31 





































INTRODUCTION 


Breathing resistance in a breathing apparatus is unavoidable. Acceptable levels of 
breathing resistance to allow for long term use of the breathing apparatus have been 
found empirically [1] [2] [3] [4] and have been implemented [5] [6] in standards for 
testing of breathing apparatus. However, the effect on a wearer’s exercise endurance 
would be unclear if the breathing resistance were to become far higher than expected, 
due to a partial failure or usage at work rates higher than those for which the apparatus 
had been approved. 

Advance knowledge of how long a wearer will be able to tolerate breathing through a 
particular breathing apparatus can be essential when judging if a certain task is likely to 
be possible in either a long term or short term (emergency) situation. Thus, the main 
purpose of this study was to determine the effects of different levels of very high 
breathing resistance on endurance exercise at a moderate work rate (approximately 
60% VO 2 max)- Primarily, endurance times were determined and the nature of the 
changes in various ventilatory parameters were described. 


METHODS 

The Institutional Review Board at NEDU approved protocol number 14-50/40069, 
’’Influence of very high breathing resistance on exercise tolerance, part 1 - dry 
exercise”. A total of fifteen military personnel from NEDU gave written informed consent 
before beginning the study. Each subject participated in six tests; one to determine peak 
oxygen uptake and five to measure exercise endurance while the subject breathed 
against different elevated breathing resistances. 

During testing, a subject wore an oronasal mask with one-way valves (model 2700, 

Hans Rudolph, Shawnee, KS). The pressure drop was less than 0.8 cm H 2 0 at a flow of 
100 L/min. The valve dead space for dry measurements was 77 mL, and mask dead 
space was approximately 50 mL for a medium mask and 65 mL for a large mask. 
Breathing gas was room air. Experiments were conducted at sea level. 

Breathing resistance was varied in five steps. The least high level was selected to 
impose a total work of breathing per volume (WOB to t/V T ) of 3 kPa, matching NEDU’s 
limit at 1 atm. [6]. WOB t ot/V T for the highest level was three times higher. The resistance 
levels were increased by a factor 3 1/4 , i.e. WOB to t/V T of 100%, 132%, 173%, 228% and 
300% of NEDU’s limit. 

Ventilatory measurements were made using commercial exercise testing equipment 
(Cosmed k4b2, Cosmed USA; Chicago, IL) placed at the common port of the one-way 
valves. The Cosmed also recorded the heart rate measured by a Polar heart rate 
monitor (Polar Electro Inc, Lake Success, NY). A mass spectrometer (MGA 1100) 
analyzed the C0 2 from a sample of gas (60 mL/min) drawn at the mouth. Inspiratory 
flow was measured by a screen pneumotachometer (Microtach II, nSpire Medical, 


1 



Longmount CO) placed at the inlet of the inspiratory valve with a transducer (683- 
5INCHD4V, AllSensors, Morgan Hill, CA) to measure the change in pressure across the 
screen. To measure the mask pressures, a differential pressure transducer (683- 
20INCHD4V, AllSensors, Morgan Hill, CA) was connected to the space in front of the 
subject’s mouth. The non-Cosmed signals were recorded at 100 Hz (BioPac Systems, 
Goleta CA). 

Subjects were asked every 3 minutes to give Relative Perceived Exertion (RPE) scores 
(Table 1) and dyspnea scores, where dyspnea scores ranged from 0 to 2 [1], where 0 
indicated no difficulty in breathing, 1 meant that the effort of breathing was noticeable 
but could be sustained for at least 5 minutes, 2 indicated that the subject did not think 
that he could continue for more than five minutes, 3 was assigned if the subject quit 
because of difficulty in breathing. 

Determinations of peak oxygen update 

The first exercise test for each subject was the determination of peak rate of oxygen 
uptake ( VO 2 peak) on a cycle ergometer (Monark, Vansbro, Sweden). Ergometer load 
was increased every three minutes in steps of 50 W initially, then 25 W when the 
subject appeared to be near his exercise capacity, until the subject could no longer 
continue. Subjects were asked to give scores of Relative Perceived Exertion (RPE), 
before each increase in workload. Sub-maximal values were used to estimate K0 2m ax 
using the Astrand nomogram [7], 

Table 1 . Scale for Rating of Perceived Exertion [8]. 


Exertion 

RPE 

no exertion at all 

6 

extremely light 

7 


8 

very light 

9 


10 

light 

11 


12 

somewhat hard 

13 


14 

hard (heavy) 

15 


16 

very hard 

17 


18 

extremely hard 

19 

maximal exertion 

20 


Endurance tests 

All other exercise tests in this study measured endurance on the cycle ergometer set at 
60% of V0 2 max while the subject breathed against a breathing resistance. The order of 
the breathing resistance exposures was randomly assigned. After a three-minute warm- 


2 




up at 50 W, subjects cycled at the workloads selected to produce 60% of their individual 
V O 2 max until they chose to stop. They also would have been told to stop if there had 
been excessive accumulation of C0 2 . 

Design of resistance elements 

A number of resistance elements (Figure 1) were fabricated in-house to fit into the 
inspiratory and expiratory ports of the Hans Rudolph valve assembly. To determine the 
resulting WOB to t/Vi, the mask and valve assembly were placed on a headform and a 
breathing simulator was used to breathe at minute ventilations ranging from 15 to 135 
L/min. Holes for air were tested with diameters (labeled A in Figure 1) varied in 15 even 
steps from 3.0 to 10.2 mm (0.12 to 0.40 inches). The results are illustrated in Figure 2 
and tabulated in Table A1 in the Appendix. The pressure drop was a function of the flow 
and the square of the flow. 

Selection of resistance elements 

An estimate of each subject’s expected minute ventilation at the endurance workload 
was obtained by interpolating the recordings made during the VO 2 peak measurements. 
From this estimate of minute ventilation the data in Figure 2 was used to select the 
resistance element that would most closely match each desired total WOB/Vj. Thus, 
different subjects had differently sized resistance elements for the same desired 
resistance level. 

For simplicity, the letter R will refer to the resistance level (hole size). The lowest R 
(largest hole) will be referred to as R1 and the highest (smallest hole), R5. The same 
size resistance element was applied to the inspiratory side as to the expiratory side, 
thus making the imposed R as symmetrical as practically possible. 



Figure 1 . Sketch of resistance elements (NEDU Design Note number 15-03), 
both viewed from one end and as a cross-section. Dimension A varied, B was 
28 mm (1.1 in), C was 34.9 mm (1.375 in), D was 6.2 mm (0.25 in) and E was 
31.8 mm (1.25 in). 


3 




























Diameter 

of 

opening 


0 


0 25 50 75 100 125 

Minute ventilation (L/min) 



-•- 0.12 


Figure 2. Total work of breathing per tidal volume (WOB/Vj) values measured 
at different minute ventilations for each size of resistance element. The 
diameter is measured in inches. 


Calibrations 

The mass spectrometer was calibrated initially according to the standard procedure to 
correct for gas interactions, then adjusted daily with air and a span gas (5% C0 2 and 
16% 0 2 in nitrogen). The pressure transducer outputs were compared to a water 
manometer. 

Data analysis 

Breath by breath measurements of minute ventilation {V E ), tidal volume (V T ), breathing 
frequency (fR), heart rate and respiratory duty cycle (Ti/T to t), were determined by the 
Cosmed device. End-tidal C0 2 (etCC> 2 ), peak inspiratory and expiratory pressures, and 
inspired flow (t7 in ) were determined from the 100 Hz data. Inspiratory work of breathing 
per volume (WOBi n /V T ) was calculated from V m , inspiratory mask pressure and the 
integrated V m for each breath. 

Values were averaged over the one minute that ended 30 seconds before the 
termination of exercise (to avoid transients at failure or timing errors). 

Abort criteria 

Each subject was free to stop an experiment at any time. An experiment would have 
been stopped if the etC0 2 exceeded a partial pressure of 65 mm Hg. 

Statistical analysis 

The influence of resistance level on each measured parameter was calculated by linear 
regression of the pooled subject data against resistance. The statistical significance of 
the slope was determined, with a = 0.05 used as the limit of significance. 


4 













RESULTS 


The subjects varied widely in height, weight, and apparent aerobic fitness (Table 2). The 
group consisted of both divers and non-divers. 

Table 2. Subject characteristics. Median values, with minimum to maximum 
in parentheses. 



15 men, 1 woman 

Age (years) 

33 (24 - 53) 

Height (cm) 

175 (163- 193) 

Body mass (kg) 

91 (76-107) 

V0 2 peak (L-min -1 ) + 

3.1 (1.9-5.8) 

V0 2 max (L-min -1 ) * 

3.3 (2.4-5.8 ) 

(mL-min -1 -kg -1 ) * 

36 (25 - 57) 

HR at peak (beats/min) 

189 (167- 196) 

Shaft power at 60% (W) 

110 (85- 150) 


^measured values, determined from a nomogram [7] 


During the R exposure, some subjects could not exercise for at least three minutes at 
the exercise load before they stopped work. Presenting their results would be correct for 
illustrating what happened, but also potentially misleading since a cardio-respiratory 
steady state was probably not reached. Therefore, both sets of data are graphed and 
are summarized in Tables A2 and A3 in Appendix A. In addition, the subjects who were 
stopped after 60 minutes had not reached exhaustion and may have had different 
responses to R if they had. Summary data excluding those subjects are shown in Table 
A4 and will be discussed separately. The coefficients of variation (CVar, ratio of SD and 
mean) of the tabulated variables showed no particular influence from the level of R 
(calculated from the data in Tables A2 - A4). 


The graphs that follow here have resistance levels labelled from 1 to 3 (relative to 
NEDU’s limit). The corresponding nominal WOB/V T varied from 3 to 9 kPa. Slopes of 
parameters vs. R are expressed using the nominal WOB/V T , not the relative R or the 
measured WOB/V T . As will be discussed, if minute ventilation was reduced from the 
unloaded condition used to choose resistance elements, actual WOB/V T was less than 
the nominal value. 


The comments given by the subjects are compiled in Table A5. 

Endurance times 

Table 3a summarizes the endurance time results for all subjects. Figure 3 shows the 
endurance times for each of the subjects at each of the resistance levels. The 
endurance times ranged from 4.5 to 60 minutes with R1. Five subjects lasted the full 
hour allowed in the protocol. Even with R5 there were two subjects who lasted an hour, 
but one subject stopped after 1.7 minutes (i.e. still during the warm up period). The 


5 



CVar increased monotonically from 63% with R1 to 131% with R5. The average 
endurance with R5 was 41% of the endurance with R1. 

Table 3b summarizes the endurance times for the subjects who were not stopped by 
the 60-minute limit. The CVar ranged from a low of 50% (R1) to a high of 83% (R3) 
without any particular pattern. The average endurance with R5 was 35% of the 
endurance with R1. 

Table 3a. Summary of endurance times (in minutes) for all subjects for each 
resistance level. 



R1 

R2 

R3 

R4 

R5 

Maximum 

60 

60 

60 

60 

60 

Mean 

33.6 

28.4 

26.0 

19.3 

14.7 

SD 

21.0 

21.1 

22.2 

21.9 

19.3 

Normalized to R1 

100% 

91% 

82% 

55% 

41% 

Median 

30.9 

21.7 

20.5 

9.4 

8.2 

CVar 

0.63 

0.74 

0.85 

1.13 

1.31 

25th percentile 

16.3 

15.4 

7.1 

5.0 

4.1 

10th percentile 

12.3 

7.0 

5.6 

4.3 

2.6 

Minimum 

4.5 

5.4 

5.0 

2.6 

1.7 


Table 3b. Summary of endurance times (in minutes) for subjects who exercised 
for less than 60 min for each resistance level. 



R1 

R2 

R3 

R4 

R5 

Maximum 

34.6 

35.2 

56.0 

27.5 

26.7 

Mean 

20.4 

16.9 

17.5 

9.2 

7.7 

SD 

10.1 

8.7 

15.3 

6.8 

6.5 

Normalized to R1 

100% 

91% 

79% 

45% 

35% 

Median 

17.9 

16.1 

10.9 

7.3 

5.8 

CVar 

0.50 

0.52 

0.87 

0.74 

0.84 

25th percentile 

14.2 

11.3 

5.8 

4.6 

3.9 

10th percentile 

10.5 

6.9 

5.5 

4.2 

2.3 

Minimum 

4.5 

5.4 

5.0 

2.6 

1.7 


For easier viewing, Figure 4A shows just the median (shown also in Figure 3), the upper 
and lower quartiles, and the time that at least 90% of the subjects endured. Figure 4B is 
a “survival” graph that shows the number of subjects remaining at a given time. 

The average slope for the endurance times of all subjects was -3.1 (SE= 0.74) min per 
kPa of WOB/Vj (p<0.001). For the subjects who exercised for at least 3 minutes at load 
(n=8) the average slope was -2.6 (SE=0.86) min per kPa of WOB/Vj (p<0.05). For the 
subjects who exercised for at least 3 minutes at load but less than 60 minutes (n=5) the 
average slope was -3.1 (SE=1.0) min per kPa of WOB/Vj (p<0.05). 


6 




Figure 3. Endurance times for all subjects at each of the resistance loads. 
Some lines overlap at the cut-off time of 60 minutes for all resistance levels. 
Each solid line-symbol pair indicates a subject, while the blue dashed line and 
filled circle show the median times. 



Figure 4A. Endurance times at each of the resistance loads, shown as median, 
upper and lower quartiles, and the time that at least 90% of the subjects 
endured (10 th percentile). 


7 


























Figure 4B. Survival plot of the number of subjects remaining after each 3- 
minute period. 


8 

















End-tidal CO? values 

Figure 5A shows the etC0 2 values for all subjects at the end of exercise. The average 
slope (mean ±SE) for all subjects was 0.09 ±0.03 kPa C0 2 per kPa of nominal WOB/Vt 
( p<0.05). Figure 5B shows the etC0 2 values for the subjects who exercised for at least 
3 minutes at load. For those subjects the slope was 0.23 ±0.07 kPa C0 2 per kPa of 
nominal WOB/V T (p<0.01). For the subjects who exercised for at least 3 minutes at load 
but less than 60 minutes overall (n=11), the slope was 0.31 ±0.09 kPa C0 2 per kPa of 
nominal WOB/V T (p<0.05). No subject was stopped for etC0 2 exceeding the abort 
criterion. See Discussion for interpretation of some of the slope values. 



3 _|—.—. — . — . — | — . — ,—. — . — | — .—.—.—.—|—.—.—.—. — | " 

1.0 1.5 2.0 2.5 3.0 “ •-median 

Resistance level (relative to NEDU's limit) 



Resistance level (relative to NEDU's limit) 




1 

2 

3 

4 

5 

6 

7 

8 

9 

10 
11 


-*-12 


13 

14 

15 


•median 


Figure 5. One-minute average end-tidal C0 2 values at the end of exercise. 
Panel A shows values for all subjects. Panel B shows values for subjects who 
continued for at least 3 minutes at load. 


9 























Minute ventilation 

Figure 6A shows one-minute average V E at end of exercise for all subjects, The 
average slope (mean ±SE) for all subjects was -3.4 ±0.4 L/min per kPa of nominal 
WOB/Vj (p<0.0001). Figure 6B shows V E only for the subjects who completed at least 3 
minutes of exercise at load. For them, the slope was -4.7 ±1.0 L/min per kPa of nominal 
WOB/Vj (p<0.001). For the subjects who exercised for at least 3 minutes at load but for 
less than 60 minutes overall, the slope was -4.3 ±1.4 kPa CO 2 per kPa of nominal 
WOB/Vj (p<0.05). 




Resistance level (relative to NEDU's limit) 



-median 



+ 



1 

2 

3 

4 

5 

6 

7 

8 

9 

10 
11 


-*-12 
—e— 13 
—*— 14 
—e— 15 
—•—median 


Figure 6. One minute average V E at the end of exercise. Panel A shows values 
for all subjects. Panel B shows values for subjects who continued for at least 3 
minutes at load. 


10 
































Tidal volume 

Figure 7A shows the one-minute average V T at the end of exercise. Figure 7B shows 
the V T for the subjects who exercised for at least 3 minutes at load. The slopes were not 
significantly different from zero. 



i 


0.5 -I 

0 -F— 1 — 1 — 1 — 1 —l— 1 — 1 — 1 — 1 —l— 1 — 1 — 1 — 1 —l— 1 — 1 — 1 — 1 —l 

1.0 1.5 2.0 2.5 3.0 

Resistance level (relative to NEDU's limit) 



Resistance level (relative to NEDU's limit) 



1 

2 

3 

4 

5 

6 

7 

8 

9 

10 
11 


—*—12 


13 

14 

15 


•median 


1 

-»— 2 

3 

-*— 4 

-*— 5 
-•—6 

- 1—7 

-*—8 
-B— 9 

-i-10 

-*-11 

-*—12 

-©—13 

14 

-©— 15 
-•—median 


Figure 7. One minute average V T at the end of exercise. Panel A shows values 
for all subjects. Panel B shows values for subjects who continued for at least 3 
minutes at load. 


11 
































Breathing frequency 

Figure 8A shows the one-minute average breathing frequency (f R ) at end of exercise. 
The average slope of f R vs. R-load (mean ±SE) for all subjects was -1.4 ±0.4 
breaths/min per kPa of nominal WOB/V T (p<0.0001). Figure 8B shows f R for the 
subjects who exercised for at least 3 minutes at load. For them, the change in f R with R 
was -1.8 ±0.6 breaths/min per kPa of nominal WOB/V T (p<0.001). For the subjects who 
exercised for more than three minutes at load but for less than 60 minutes overall, the 
slope of f R with R was -2.6 ±0.9 kPa C0 2 per kPa of nominal WOB/V T (p<0.05). 




Resistance level (relative to NEDU's limit) 



1 

2 

3 

4 

5 

6 

7 

8 

9 

10 
11 


—*— 12 


13 

14 

15 


•median 


1 

-•— 2 

-±— 3 
4 

-*— 5 
-•—6 

-1—7 

-a—8 
-B— 9 

-Bs-10 

-*-11 

BK-12 

-e— 13 
—*— 14 
© 15 

-•—median 


Figure 8. One minute average breathing frequency at the end of exercise. 
Panel A shows values for all subjects. Panel B shows values for subjects who 
continued for at least 3 minutes at load. 


12 


































Duty cycle 

Figure 9A shows the one-minute average respiratory duty cycle (T/Ttot) at end of 
exercise. Figure 9B shows the duty cycle for the subjects who exercised for at least six 
minutes overall. The slope of Ti/T to tas a function of R was not different from zero. Some 
subjects varied their duty cycle very much, but others kept theirs essentially constant. 




Figure 9. One minute average duty cycle at the end of exercise. Panel A shows 
values for all subjects. Panel B shows values for subjects who continued for at 
least 3 minutes at load. 


13 
























Peak mask pressures 

Figures 10A and 10B show the one minute average peak inspiratory and 
expiratory mask pressures at end of exercise. The magnitude of the pressures 
increased with R. 




Figure 10. One minute average peak mask pressures at the end of exercise. 
Panel A shows values for all subjects. Panel B shows values for subjects who 
continued for at least 3 minutes at load. 


For the inspiratory pressures for all subjects the magnitude of change was (mean 
±SE) 0.37 ±0.06 kPa per kPa of nominal WOB/Vj (p<0.0001). For the subjects 
who exercised for at least three minutes at load, it was 0.41 ±0.08 kPa per 


14 














































nominal kPa of WOB/Vj (p<0.001). For the subjects who exercised for at least 
three minutes at load but less than 60 minutes overall, the slope of inspiratory 
pressure with R was 0.49 ±0.15 kPa CO 2 per kPa of nominal WOB/Vj (p<0.01). 

For the expiratory pressures for all subjects the magnitude of change was (mean 
±SE) 0.20 ±0.04 kPa per kPa of nominal WOB/V T (p<0.0005). For the subjects 
who continued for at least three minutes at load it was 0.25 ±0.05 kPa per kPa of 
nominal WOB/Vj (p<0.001). For the subjects who exercised for between three 
minutes at load and 60 minutes overall, it was 0.34 ±0.09 kPa per kPa of nominal 
WOB/Vt (p<0.005). 


15 



Heart rate 


Figures 11A and 11B show the one-minute average HR at end of exercise. The average 
slope of HR vs. R-load (mean ±SE) for all subjects was -3.7 ±1.0 beats/min per kPa of 
nominal WOB/V T (p<0.005). For the subjects who continued for at least three minutes at 
load, it was -5.9 ±1.8 beats/min per kPa of nominal WOB/Vj (p<0.01). For the subjects 
who exercised for between three minutes at load and 60 minutes overall the slope was - 
8.7 ±2.1 beats/min per kPa of nominal WOB/Vj (p<0.05). 




Resistance level (relative to NEDU's limit) 


1 

■*—2 

-*—3 

■*—4 
-*— 5 
■*—6 

-I— 7 

-a—8 
e-9 
^-10 
*-11 
12 
e— 13 
*— 14 
e—15 
•—median 


-1 

-2 

3 

4 

5 

■6 

7 

-8 

-9 

10 

-11 

12 

13 

14 

15 

median 


Figure 11. One-minute average heart rates at the end of exercise. Panel A 
shows values for all subjects. Panel B shows values for subjects who continued 
for at least 3 minutes at load. 


16 









































Inspiratory work of breathing 

Figures 12A and 12B show the average WOB in /V T during the last minute of exercise. 
The average slope of WOB in /V T vs. R-load for all subjects was (mean ±SE) 0.26 ±0.05 
kPa per kPa of nominal WOB/V T (p<0.0001). For the subjects who continued for at least 
three minutes at load it was 0.30 ±0.06 kPa per kPa of nominal WOB/Vj (p<0.001). For 
those who exercised between three and 57 min. at load, it was 0.27 ±0.08 kPa per kPa 
of nominal WOB/Vj (p<0.01). The ratio of median WOBi n /V T to the nominal inspiratory 
value (half the total) was 83% for the subjects who exercised for at least 3 min. at load 
and 85% for those who exercised between 3 and 57 min. at load. 



-r 

T»- 


■e 

-A- 



1 

2 

3 

4 

5 

6 

7 

8 

9 

10 
11 
12 

13 

14 

15 


■median 



Figure 12. One minute average WOBi n /V T at the end of exercise. Panel A 
shows values for all subjects. Panel B shows values for subjects who continued 
for at least 3 minutes at load. The dotted lines labelled “design line” shows 
WOBin/Vj = 0.5 ■ WOBtot/V T . 


17 
































RPE and dyspnea 

Figures 13A and 13B show the RPE at end of exercise, and Figure 14 shows dyspnea 
scores. The average slope of RPE vs. R-load (mean ±SE) for all subjects was -0.74 
±0.21 per kPa of WOB/V T (p<0.01). For the subjects who continued for at least three 
minutes at load it was -0.58 ±0.20 per kPa of WOB/Vt (p<0.05). For the subjects who 
exercised between three minutes at load and 60 minutes overall, it was -1.1 ±0.40 kPa 
per kPa of WOB/V T (p<0.05). 

i 

■»— 2 
-±— 3 
-*—4 
-*—5 
■•—6 
-1—7 
-a—8 
e-9 
-A-10 
*-11 
12 

13 

14 

15 

median 




Figure 13. RPE at the end of exercise for all subjects at the end of exercise. 
Panel A shows values for all subjects. Panel B shows values for subjects who 
continued for at least 3 minutes at load. 


18 






























Figure 14. Dyspnea scores reported by each subject at the end of exercise. 
A score of 3 was assigned unless they reported that something other than 
breathing stopped them. For clarity, lines are separated vertically (the 
possibilities were 0, 1,2 or 3). 


Examples of responses to very high breathing resistance 

In terms of etC0 2 values, and RPE and dyspnea scores, the subjects showed very 
different reactions to the breathing resistances. Figure 15 illustrates changes in a 
subject who managed 60 minutes with all levels of R. In early exercise (warmup ended 
at minute 3) the etC0 2 gradually climbed, leveled off and remained fairly constant after 
about 5 minutes. The etC0 2 plateau levels clearly increased from about 6.2% with R1 to 
about 7.2% with R5. Figure 15 also shows that the RPE scores increased gradually with 
time, but never became higher than 14 (“somewhat hard” to “hard”). The influence of R 
on RPE was less distinct than on etC0 2 . The subject reported a dyspnea score of 1 for 
all resistances except R5, where it was 0. 

Figure 16 shows a second person’s reaction to the resistance loads. This subject 
managed the full 60 minutes with R1 and R2, but the endurance time decreased slightly 
with R3, and very considerably with R4 and R5. The etC0 2 levels tended to increase 
with the magnitude of R (the influence of R was already noticeable during warmup), but 
etC0 2 decreased gradually after approximately 15 minutes. The RPE score for R3 
reached a peak (16, “hard) after about 15 minutes, but then actually dropped two steps 
before the subject stopped exercise. The dyspnea scores stayed at 1 until the subject 
stopped with an assigned dyspnea score of 3 (for R3 and R4), except for R5 where the 
need to stop became urgent and the score jumped from 0 to an assigned value of 3. 

Figure 17 shows a third person’s reaction, a subject who exercised for 60 minutes with 
the lowest resistance load, but stopped exercise much earlier with the more elevated 


19 





















resistances. With R1, etCC >2 increased at the start of exercise to about 6.9%, but 
decreased after about 7 minutes to about 6.3% after 60 minutes of exercise. RPE 
scores were moderate mid-exercise and were at 17 at the end of the hour. Dyspnea 
scores were 0 throughout. With R2, the etC0 2 increased considerably to about 7.2% at 
the start of exercise, but dropped quickly and was about 6% at the end of exercise. RPE 
scores climbed until exercise ended with a very high RPE score of 19, and dyspnea 
scores were 2 for about 9 minutes until exercise ended with an assigned dyspnea score 
of 3. With R3, etC0 2 increased similarly to that with R2 but remained slightly higher, 
with RPE scores distinctly lower than with R2, while dyspnea scores were similar to 
those with R2. With R4, etC0 2 followed a pattern similar to that of R2, RPE scores 
remained only slightly higher than with R3, and the subject reported a dyspnea score of 
2 at 15 minutes, but then reduced it to a 1 while also lowering the etC0 2 values. With 
R5, etC0 2 increased similarly to R3 and R4, but stayed slightly higher for longer and 
then abruptly dropped before the subject quit with a RPE score of 13. 


20 




Figure 15. Time plot of subject A’s end-tidal CO 2 (a nine-breath moving average), RPE and dyspnea scores for 
each of the resistance loads. For clarity, the lines for RPE and dyspnea have a slight vertical separation (only 
whole numbers were reported). 


21 


Dyspnea 



























Figure 16. Time plot of subject B’s end-tidal CO 2 (a nine-breath moving average), RPE and dyspnea scores for 
each of the resistance loads. For clarity, the lines for dyspnea have a slight vertical separation (only whole 
numbers were reported). 


LU 

CL 

CE 


22 


Dyspnea 




















8 




x 


19 



LU 

CL 

CC 


25 30 35 

Time (min) 


Figure 17 . Time plot of subject C’s end-tidal CO2 (a nine-breath moving average), RPE and dyspnea scores for 
each of the resistance loads. For clarity, the lines for RPE and dyspnea have a slight vertical separation (only 
whole numbers were reported). 


23 


Dyspnea 






















DISCUSSION 


The average response is often a good descriptor for a response. However, in some 
circumstances, like this study where tolerance is measured, the average does not tell 
the whole story. Similarly, it is not sufficient when limits are set. Hence, results will be 
discussed mostly as averages, but some values, such as endurance times, will also be 
expressed as a value that the vast majority of the subjects could tolerate. 

Endurance times 

The average change in exercise endurance was -3.1 min/kPa of nominal WOB/V T , 
which would mean an average decrease in endurance time of about 28 minutes if a 
change of WOB/V T from 3 to 9 kPa could be tolerated. This calculation does not reflect 
the subjects’ actual endurance time (many did not even make 28 minutes with R1). In 
fact, the subjects’ responses to these very high breathing resistances varied drastically; 
some subjects managed to endure all the resistances for the full hour, while others 
endured them only for minutes. With R1, the endurance time met or exceeded by 75% 
of the subjects (calculated as the 75 th percentile) was 16 minutes and for the 9CT 
percentile it was just over 12 minutes. With R5, the 75 th percentile of the endurance it 
was four minutes and for the 90 th percentile it was less than three minutes. Overall, the 
endurance at a higher R was shorter than at a lower R (Figure 4), but this was not 
always the case. A subject’s endurance at one R level is not necessarily a good 
predictor for the endurance at a different breathing load. 

Most subjects showed a gradual reduction in endurance with increasing R. Two 
subjects were able to put in enough respiratory effort to last the entire 60-minute period 
with all levels of R. Their minute ventilations decreased with increased R, resulting in an 
increase in etC02. Their RPE scores increased with R and show that these two subjects 
sensed the necessary increase in respiratory effort as a component of total body 
exertion. The subject with the shortest endurance showed a minimal change in 
endurance when the R varied. 

Based on a sample of 15 subjects of varying stature and fitness, 90% of the population 
can be expected to continue moderate exercise for at least 3 minutes at the warmup 
load of 50W with a nominal WOB to t/V T of 9 kPa. Put differently, 10% of the population 
will have endurance times of less than 3 minutes, and those at only the warm-up load. 
With a nominal WOB to t/V T of 6.8 kPa, 90% of the subjects could exercise for 4 minutes 
or more, a nominal WOB to t/Vi of 5.2 kPa for 5 minutes or more, a nominal WOB t ot/V T of 
3.9 kPa for about 7 minutes or more and a nominal WOB to t/V T of 3 kPa for about 12 
minutes or more. A summary of these values is shown in Table 4. 

Simulated failure modes (e.g. kinked hoses, low supply pressure) can be imposed on a 
breathing apparatus and the resulting WOB/V T measured. These values and values 
from existing breathing apparatus can be compared to the endurance times in Table 4 
to judge how long wearers may last. 


24 



Table 4. The minimum time that at least an estimated 90% of the subjects could 
endure. The times stated include the 3 minute warmup period before the 
workload was increased to 60% of each subject’s capacity. 


Nominal WOB/V T 
(kPa) 

3.0 

3.9 

5.2 

6.8 

9.0 

time 

(minutes) 

12 

7 

5 to 6 

4 

3 


End-tidal CO? levels 

At no time did any subject show an etC0 2 close to the abort criterion of 65 Torr. This is 
in contrast to previous studies with lower resistances but with inspired C0 2 and heavier 
exercise either underwater [9] or dry [10]. 

The slope of 0.31 kPa C0 2 /kPa WOB/Vj (90% confidence interval: 0.09 to 0.54) for the 
group that exercised at least 3 minutes at load would indicate that, when the nominal 
WOB/Vj increased by 6 kPa (from R1 to R5), the etC0 2 would increase by about 1.9 
kPa. Thus, the etC0 2 would go from 6.2 to 8.1 kPa, a value not seen. The reason for 
this apparent impossibility is that 4 out 11 subjects had only two data points each for the 
calculation of the slope. For two of these four, the slope was 0.93 and for the third it was 
0.52 and for the fourth it was 0.02 kPa C0 2 /kPa. When the four were excluded, the 
slope became 0.14 kPa C0 2 /kPa (p<0.001). 

Subjects for whom the etC0 2 increases steeply with increasing R cannot tolerate the 
higher R; the climb in etC0 2 represents respiratory failure in face of the load. 

Ventilatory patterns 

On the average, V E decreased by 10-15% when the intended WOB/Vj increased three¬ 
fold (R1 to R5). The decrease was caused by reduced breathing frequencies, as V T did 
not change. The lack of influence of R level on the duty cycle may be because the 
imposed inspiratory and expiratory resistances were symmetrical. The large variations 
in duty cycle within a subject may reflect attempts to somehow ease the breathing 
difficulty, but as one subject put it, he “hunted for a good breathing pattern, but didn't 
find one”. 

Inspiratory work of breathing 

The calculated WOB in /V T was lower than the intended value. The calculated value was 
about 83% to 85% of the intended one, Figure 12. This is an expected phenomenon 
since subjects typically reduce their V E when challenged by increased R (Figure 6). 
Individuals who lower their WOB/Vj from the designed value do so at the expense of 
increased etC0 2 . 

The values for WOBi n /V T (Table A1) are the ones that must be used to compare 
breathing resistances across breathing apparatus, because that is how those are 
characterized during unmanned testing. 


25 




Inspiratory and expiratory peak pressures 

On the average, peak inspiratory pressures increased more with increasing R than did 
the expiratory pressures. 

Population values for maximum expiratory pressures for single breaths have been 
reported by one source as 14.9 (SD=3.5) kPa in women, 24.2 (SD=4.6) kPa in 29 year- 
old men and 15.6 (SD=6.4) kPa in 59 year-old men [11]; and by another source, as 13 ± 
2.6 kPa in men and 10 ±1.8 kPa in women [12]. Maximum inspiratory pressures have 
similarly been reported at -10.2 (SD=1.9) kPa in women, -13.6 (SD=4.0) kPa in 29 year- 
old men and -11.1 (SD=3.1) kPa in 59 year-old men [11]; and in gender-mixed subjects 
as -15 kPa [13]. 

Maximum voluntary inspiratory and expiratory pressures during dive experiments [1] 
were about ±10 kPa, independent of depth (15 and 190 fsw, 4.5 and 57 fsw). In this 
study, the subject’s individual, single-breath, maximum pressures were not measured, 
but the highest inspiratory and expiratory mask pressures seen for R5 (±6 to 7 kPa, ca 
±60 to 70 cm H 2 0), Figure 10, were below 70% of the lowest pressures reported in the 
literature. A large expiratory pressure makes the mask try to lift off the face, limiting 
possible expiratory pressures. The straps holding the masks had to be very tight, but 
even so, for one subject in particular the seal was hard to maintain during expiration. 

Dyspnea, RPE scores and symptoms 

One might think that an increasing R would always induce the same or increasing 
dyspnea, akin to the dyspnea scores shown for one subject in Figure 17. Flowever, this 
is contradicted by the scores shown in Figure 15 where the subject had the lowest 
dyspnea scores with the highest R. Thus, a report of low dyspnea does not necessarily 
correspond to low or moderate breathing resistance. What individuals detect as 
dyspnea is not as straight-forward as simply the pressure needed to move air or the 
ability to maintain desired C0 2 levels. In fact, the sensation of dyspnea is possibly 
related to the ability to match respiratory drive with ventilation, and respiratory drive may 
be decreased in the presence of large WOB/V T [9]. 

For some subjects the need to stop exercise came on very quickly. 

Some subjects noticed inspiratory R more than expiratory R, while for others it was the 
opposite, although resistance was always symmetrical. 

Even R1 provided high enough WOB t ot/V T that subjects had a minimal chance to “catch 
up” on V E after a short breathing interruption due to a cough or a sniffle. 

If the limitation to exercise as R increases, becomes the difficulty in breathing, one 
might expect that the RPE scores at the end of exercise would decrease with increasing 
R. This appears to be the case; the median RPE score at the end of exercise with R5 in 
those subject who completed at least 3 minutes at load corresponded to “somewhat 
hard” and only one subject exceeded an RPE of 14. Two people scored the effort “very 


26 



light” after stopping. A factor other than perceived effort is causing subjects to stop 
exercise. 

Comparison to published limits on breathing resistance 

The National Institute for Occupational Safety and Health (NIOSH) certifies respiratory 
protective devices for the U.S. [14], including those for Navy non-diving applications. 
The Navy has some 200,000 non-diving units for escape from ships. Many of the tests 
use a constant flow to judge breathing resistance, but for the closed-circuit escape 
respirators like those owned by the U.S. Navy, testing is done with sinusoidal breathing 
generated by a breathing machine (subpart O of 42 CFR 84). Subpart O includes two 
limits: “the peak expiratory-to-inspiratory pressure swing shall not exceed 200 mm H 2 0” 
(ca. 2 kPa) and “excursions (lasting less than 1 minute) shall not exceed -300 to +200 
mm H 2 0” (ca. -3 to +2 kPa). For the subjects in this study the first limit was exceeded 
with R1. However, the much larger excursion range was not exceeded below R4. Thus, 
according to NIOSH, R3 and lower would be acceptable for use for less than 1 minute. 
For R3 the minimum endurance time was 5 minutes and 90% of the present group of 
subjects managed 6 minutes. 

The physiologically acceptable values [3] adopted by the International Standards 
Organization [5] allow a maximum WOB to t/V T of 1.8 kPa from breathing apparatus 
designed for extended use. That limit was, by design, exceeded in this study. The short 
endurance times achieved confirm that the R levels used in this study are indeed 
excessive for long term exposure. However, for short term exposures the present 
results can provide guidance for minute ventilations in the range tested here. This range 
was 36 to 63 L/min (10 to 90% of measured values) for subjects who exercised for at 
least three minutes at load. These minute ventilations can be expected from a 70 kg 
man who works “full work shifts including breaks” to “continuous work for up to 2 h 
without breaks” [14]. 

Thus, if an endurance time of 10 minutes is needed for moderate work, then a 
WOBtot/Vj of 3.2 kPa is likely to be manageable by at least 90% of wearers (using linear 
interpolation between adjacent values). Similarly, a 5-minute endurance time can be 
expected to be achieved with a WOB t ot/V T of up to 5.5 kPa. Linear extrapolation to an R 
lower than R1 and R2 indicates that a WOB to t/V T of 2.6 kPa is likely to be manageable 
by 90% of the population for 15 minutes. A summary of these values is shown in Table 
5. 


Table 5. The maximum WOB/Vj that is likely to allow a desired endurance time 
(estimated from the 50, 75 and 90 th percentiles). The times stated include the 3 
minute warmup period before the workload was increased to 60% of each 
subject’s capacity._ 


maximum desired time 
(minutes) 

5 

10 

15 

maximum, nominal 

50 th 

- 

6.8 

5.8 

WOB/Vj 

^5^ 

6.7 

4.7 

3.4 

(kPa) 

90 th 

6.0 

4.0 

2.6* 


*see text 


27 




NEDU’s limits for work of breathing, the current Navy diving standards, allow higher 
WOBtot/Vj than do the NIOSH and ISO standards. The acceptable WOB/V T under the 
NEDU limits varies with depth, but R1 for this study was designed to match the 1 atm 
(surface) limit, 3.0 kPa [6] [15]. NEDU’s limits originate from a set of 96 diving 
experiments [1] conducted at two depths at the University at Buffalo. Those limits were 
set such that all of the subjects could manage 25 minutes of moderate exercise without 
excessive end-tidal CO 2 or excessive dyspnea. In this non-diving situation, the median 
endurance time with R1 exceeded 30 minutes. Only 9 of 15 subjects managed 25 
minutes of moderate exercise. Calculated as the 90 th percentile the endurance time was 
only 12 minutes or more. Thus, by the criteria used to determine the diving limits, the 
present study finds that a WOB t ot/V T of 3.0 kPa is too high for use in the dry at 1 atm. 
However, any suggested changes for diving standards should await results of the 
second phase of this study which will be carried out under water. 

Demographics 

The subjects represented a large variety in age, size and physical fitness. Due to the 
population of potential subjects that we could draw from, only one woman took part. 


SUMMARY 

The response to high R varied greatly from subject to subject. On the average, the high 
breathing resistance reduced the endurance time. However, the spread in endurance 
times with any R ranged from a few minutes to the maximum permitted time of one 
hour. 

NEDU’s diving limit for WOB t ot/V T of 3 kPa at the surface cannot be extrapolated for use 
in the dry at 1 atm. The NIOSH and ISO limits may fit better. 

The high breathing resistance reduced the Ve through a reduction in breathing 
frequency with unchanged V T . There was no consistent change in respiratory duty 
cycle. 

On the average, the expired C0 2 levels increased with increased R, with a slope of 0.1 
kPa per kPa of nominal WOB/Vj. The inter-individual spread in end-tidal C0 2 was large, 
ranging from 5% to 7.7%. There was no more serious C0 2 retention. 

Even with R being imposed equally on inspiration and expiration, some subjects noticed 
R more on one phase than on the other. Some subjects reported that they couldn’t 
breathe fast enough and stopped exercise, while some reported that leg fatigue made 
them stop. One subject was close to removing the mask, while another felt 
claustrophobic after 2 minutes but recovered and continued to exercise for the 
maximum time of an hour. 


28 



CONCLUSIONS 


The purpose of this study was to determine the effects of different levels of very high 
breathing resistance on endurance exercise at a moderate work rate. These values that 
can be estimated are listed in Table 5. However, individual reactions to very high 
breathing resistance are not predictable. If breathing resistance increases beyond 
normal limits, exercise endurance at moderate work will be severely restricted for the 
vast majority of people, but there may be some people who can overcome even very 
high breathing resistance. 


RECOMMENDATIONS 

Existing values from unmanned tests of breathing apparatus can be compared to the 
results found in this study to judge likely endurance times of wearers. Similarly, possible 
failure modes can be imposed on existing breathing equipment and the resulting 
WOBtot/Vj values used to judge likely endurance times. 

NEDU’s limits for acceptable breathing resistance should not be relaxed; they may need 
to be tightened. 


REFERENCES 

1. D. E. Warkander, W. T. Norfleet, G. T. Nagasawa and C. G. E. Lundgren, 
"Physiologically and subjectively acceptable breathing resistance in divers' 
breathing gear," Undersea Biomedical Research, vol. 19, no. 6, pp. 427-445, 
1992. 

2. D. E. Warkander, "Comprehensive Performance Limits for Divers' Underwater 
Breathing Gear: Consequences of Adopting Diver-focused Limits," Navy 
Experimental Diving Unit, 2007. 

3. B. Shykoff and D. E. Warkander, "Physiologically acceptable resistance of an air 
purifying respirator," Ergonomics, vol. 54, no. 12, pp. 1186-1196, 2011. 

4. D. Warkander, "NEDU TR 10-14 Work of breathing limits for heliox breathing," 
Navy Experimental Diving Unit, Panama City, 2010. 

5. International Standards Organization, ISO 16976-4 Respiratory protective 
devices — Human factors — Part 4: Work of breathing and breathing resistance: 
Physiologically based limits, Geneva: International Standards Organization, 
2012 . 

6. Navy Experimental Diving Unit, "U.S. Navy Unmanned Test Methods and 
Performance Limits for Underwater Breathing Apparatus, NEDU TM 15-01," 
Panama City, 2015. 

7. P. O. Astrand and K. Rodahl, Textbook on Work Physiology. Physiological 
Bases of Exercise, McGraw-Hill, 1977. 

8. Borg, "Perceived exertion as an indicator of somatic stress.," Scand. J Rehabil. 


29 



Med., vol. 2, no. 2, pp. 92-98, 1970. 

9. B. Shykoff and D. E. Warkander, "Exercise Carbon Dioxide (C02) Retention 
with Inhaled C02 and Breathing Resistance," Undersea and Hyperbaric 
Medicine, vol. 39, no. 4, pp. 815-828, 2012. 

10. B. Shykoff, D. E. Warkander and D. Winters, "Effects of Carbon Dioxide and 
UBA-like Breathing Resistance on Exercise Endurance," Navy Experimental 
Diving Unit, Panama City, FL, 2010. 

11. C. Cook, J. Mead and M. Orzalesi, "Static volume-pressure characteristics of the 
respiratory system during maximal efforts," J. Appl. Physiol, vol. 19, no. 5, 1964. 

12. W. Man, T. A. Kyroussis, A. Fleming, Chetta, F. Flarraf, N. Mustfa, G. F. 

Rafferty, M. I. Polkey and J. Moxham, "Cough Gastric Pressure and Maximum 
Expiratory Mouth Pressure in Humans," Am J Respir Crit Care Med, vol. 168, 
pp. 714-717, 2003. 

13. N. A. S. and T. J. Gal, "Cough Dynamics during Progressive Expiratory Muscle 
Weakness in Healthy Curarized Subjects," J. Appl. Physiol.: Respirat. Environ. 
Exercise Physiol., vol. 51, no. 2, pp. 494-498, 1981. 

14. International Standards Organization, "Respiratory protective devices — Human 
factors — Part 1: Metabolic rates and respiratory flow rates," Geneva, 
Switzerland, 2007. 

15. D. E. Warkander, "Recommended Amendment to NEDU Technical Manual 01 - 
94: U.S. Navy Unmanned Test Methods and Performance Goals for Underwater 
Breathing Apparatus.," Navy Experimental Diving Unit, Panama City, 2008. 

16. National Institute of Occupational Safety and Health, "www.ecrf.gov," 2015. 
[Online]. Available: www.ecfr.gov. [Accessed 18 Nov 2015]. 

17. D. E. Warkander and B. Shykoff, "Exercise carbon dioxide (C02) retention with 
inhaled C02 and breathing resistance," Undersea and Hyperbaric Medicine, vol. 
39, no. 4, pp. 795-808, 2012. 


30 



APPENDIX A 


Table A1. Work of breathing values per tidal volume in kPa (i.e. the volume average pressure, WOB/V T ) for each 
combination of minute ventilation and opening size. 


Minute 

ventilation 

L/min 


f Size of opening in the resistance element (inches and mm) 

breaths/ 0.40 0.38 0.36 0.34 0.32 0.30 0.28 0.26 0.24 0.22 0.20 0.18 0.16 0.14 0.12 
minute 10.2 9.7 9.1 8.6 8.1 7.6 7.1 6.6 6.1 5.6 5.1 4.6 4.1 3.6 3.0 


15 

1.5 

10 

0.20 

0.22 

0.25 

0.29 

0.33 

0.39 

0.50 

0.62 

0.76 

1.00 

1.31 

1.62 

2.12 

2.59 

3.30 

22.5 

1.5 

15 

0.35 

0.40 

0.48 

0.56 

0.64 

0.77 

1.01 

1.25 

1.54 

2.01 

2.62 

3.16 

4.00 

4.74 

5.74 

34 

1.9 

18 

- 

- 

- 

- 

- 

1.61 

2.07 

2.58 

3.13 

3.93 

4.93 

5.77 

6.88 

7.58 

- 

40 

2.0 

20 

0.91 

1.03 

1.27 

1.53 

1.73 

2.14 

2.73 

3.35 

4.01 

4.96 

6.08 

6.95 

7.91 

- 

- 

50 

2.5 

20 

- 

- 

- 

- 

2.57 

3.06 

3.84 

4.64 

5.49 

6.53 

7.64 

8.29 

- 

- 

- 

62.5 

2.5 

25 

2.02 

2.30 

2.81 

3.28 

3.76 

4.47 

5.56 

6.61 

7.63 

- 

- 

- 

- 

- 

- 

75 

3.0 

25 

2.77 

3.13 

3.77 

4.38 

4.99 

5.85 

7.10 

8.24 

- 

- 

- 

- 

- 

- 

- 

85 

2.5 

34 

3.41 

3.87 

4.64 

5.37 

6.07 

6.99 

8.09 

- 

- 

- 

- 

- 

- 

- 

- 

90 

3.0 

30 

3.82 

4.27 

5.14 

5.94 

6.72 

7.78 

- 

- 

- 

- 

- 

- 

- 

- 

- 

105 

3.0 

35 

4.73 

5.26 

6.19 

7.03 

7.68 

- 

- 

- 

- 

- 

- 

- 

- 

- 

- 

135 

3.0 

45 

6.92 

7.62 

8.52 

- 

- 

- 

- 

- 

- 

- 

- 

- 

- 

- 

- 


Note. The purpose of the measurements was to obtain values close to the desired range of 3 to 9 kPa. Hence no data was 
collected at some minute ventilations. The breathing simulator has an automatic system that stops it if the instantaneous 
pressure is high enough (ca 7 kPa) to risk causing damage to the simulator system. This safety feature restricts the WOB 
values to less than approximately 8.5 kPa. Extrapolation was used to get values of 9 kPa. 


31 









Table A2. Mean and standard deviation (SD) of respiratory parameters and heart rate for all subjects. 


Parameter 

units 

R1 

mean 

SD 

R2 

mean 

SD 

Resistance level 

R3 

mean SD 

R4 

mean 

SD 

R5 

mean 

SD 

etC0 2 

% 

6.18 

0.48 

6.44 

0.62 

6.53 

0.63 

6.65 

0.53 

6.69 

0.49 

Ve 

L-min' 1 * 

55.4 

10.7 

50.7 

12.9 

43.6 

11.8 

40.0 

14.0 

33.0 

10.5 

VT 

L* 

2.44 

0.51 

2.31 

0.48 

2.32 

0.65 

2.50 

0.75 

2.05 

0.52 

f 

min' 1 

23.9 

5.6 

21.6 

4.4 

19.7 

4.8 

16.5 

4.1 

15.9 

4.3 

duty cycle 


0.48 

0.03 

0.48 

0.03 

0.50 

0.05 

0.46 

0.06 

0.46 

0.06 

P mask,ex 

kPa 

17.5 

7.2 

18.2 

7.4 

20.6 

7.8 

23.7 

7.9 

29.2 

14.1 

P mask,in 

kPa 

-17.4 

6.6 

-20.1 

6.9 

-26.0 

12.9 

-33.4 

11.9 

-38.3 

16.9 

Heart rate 

min' 1 

146 

23 

138 

25 

138 

23 

128 

23 

124 

24 

WOB in /V T 

kPa 

1.24 

0.43 

1.53 

0.53 

2.05 

0.77 

2.32 

0.90 

2.84 

1.27 

RPE 


13.5 

4.3 

14.1 

3.5 

12.2 

3.7 

10.8 

4.2 

9.7 

4.7 


"Volumes are given in BTPS. 


32 









Table A3. Mean and standard deviation (SD) of respiratory parameters and heart rate for all subjects who 
continued exercise for at least 3 minutes at load. 


Parameter 

units 

R1 

mean 

SD 

R2 

mean 

SD 

Resistance level 

R3 

mean SD 

R4 

mean 

SD 

R5 

mean 

SD 

etC0 2 

% 

6.14 

0.46 

6.36 

0.55 

6.33 

0.60 

6.60 

0.60 

6.78 

0.58 

Ke 

L-min' 1 * 

57.0 

9.0 

52.4 

11.9 

47.8 

10.4 

47.9 

9.2 

39.1 

8.0 

V t 

L* 

2.45 

0.53 

2.24 

0.40 

2.36 

0.74 

2.67 

0.84 

2.23 

0.56 

f 

min' 1 

24.5 

5.3 

22.4 

3.4 

21.1 

4.6 

18.5 

3.0 

17.9 

2.5 

duty cycle 


0.48 

0.03 

0.48 

0.03 

0.50 

0.05 

0.44 

0.06 

0.45 

0.06 

P mask,ex 

kPa 

18.3 

6.8 

18.8 

7.4 

22.2 

8.5 

25.9 

6.9 

37.9 

12.2 

P mask,in 

kPa 

-17.8 

6.7 

-20.7 

6.7 

-27.2 

14.9 

-37.0 

13.1 

-47.1 

15.7 

Heart rate 

min' 1 

149 

21 

143 

24 

147 

20 

138 

20 

140 

15 

WOB in /V T 

kPa 

1.27 

0.42 

1.59 

0.52 

2.16 

0.85 

2.67 

0.84 

3.43 

1.27 

RPE 


14.0 

3.9 

14.3 

2.6 

13.5 

2.4 

12.1 

2.8 

11.4 

2.7 


"Volumes are given in BTPS. 


Table A4. Mean and standard deviation (SD) of respiratory parameters and heart rate for all subjects who 
continued exercise for at least 3 minutes at load, but less than 60 minutes. 


Parameter 

units 

R1 

mean 

SD 

R2 

mean 

SD 

Resistance level 

R3 

mean SD 

R4 

mean 

SD 

R5 

mean 

SD 

etC0 2 

% 

6.12 

0.57 

6.35 

0.66 

6.17 

0.63 

6.60 

0.70 

6.70 

0.60 

Ke 

L-min' 1 * 

57.3 

8.1 

47.1 

20.9 

47.2 

12.2 

48.3 

11.2 

39.7 

9.3 

VT 

L* 

2.61 

0.52 

1.98 

0.77 

2.29 

0.85 

2.89 

0.93 

2.35 

0.56 

F 

min' 1 

23.3 

5.6 

22.0 

3.6 

21.6 

5.2 

17.0 

1.9 

17.1 

1.5 

duty cycle 


0.50 

0.03 

0.48 

0.03 

0.52 

0.04 

0.46 

0.05 

0.46 

0.07 

P mask,ex 

kPa 

18.8 

5.1 

20.1 

7.6 

23.1 

9.2 

28.2 

6.4 

40.5 

11.6 

P mask,in 

kPa 

-18.8 

3.7 

-21.4 

6.7 

-26.5 

13.0 

-39.7 

11.7 

-50.7 

13.9 

Heart rate 

min' 1 

150 

21 

140 

27 

152 

21 

140 

23 

143 

12 

WOB in /V T 

kPa 

1.36 

0.23 

1.46 

0.72 

2.24 

0.77 

2.89 

0.75 

3.79 

1.18 

RPE 


15.3 

3.7 

13.6 

5.3 

14.3 

2.4 

11.4 

3.1 

11.2 

3.1 


"Volumes are given in BTPS. 


33 











Table A5. Compiled list of subject comments related to breathing and exercise. The order is not related to the 
subject order, nor consistent across columns. Some subjects had no comments. 


R1 

R2 

Resistance level 

R3 

R4 

R5 

Legs stopped the subject. 

Felt exactly like the MK16 
diving rebreather. 

Got behind on breathing 
and couldn't catch up 
when sniffling 
condensation through 
nose. 

Respiratory muscle ache. 
Could feel the muscle 
workload from the 
previous test (which was 
two days earlier, with R5). 

Suspects that this is the 
easiest R. 

Easier to exhale than to 
inhale. You can always 
push it out. 

Nose was runny, couldn't 
clear nose fast enough. 

Inhalation was hard, got a 
headache behind the right 
eye in phase with 
inhalation. Went away 
when mask was removed. 
Exercise was steady, not 
limiting. 

Legs were 100% of 
reason for stopping. 

Tried different breathing 
patterns and pedal 
speeds. 

After 2 min felt claustro¬ 
phobic, but improved and 
continued for 60 minutes. 

Easier to exhale than to 
inhale, just couldn't get 
enough. 

Could inhale. Couldn't 
exhale enough. Would 
have removed mask after 
another 2 min. 

Inspiration is a pain. 

Couldn't exhale fast 
enough before he needed 
to inhale. 

Couldn't breathe enough. 

Harder to exhale than to 
inhale. 

Was able to inhale. 
Exhalation was much 
harder. 

Couldn't catch up. 

Couldn't catch up after 
yawning. Harder to 
inhale. 

Hunted for a good 
breathing pattern, but 
didn't find one. 

Couldn't exhale enough 
before subject needed to 
inhale. Workload was 
very easy. 

Felt panicky. 

A sudden onset of need 
to stop. 

Inhalation was harder 
than exhalation. Ears 
kept popping. 

Couldn't inhale enough. 
Swallowed and couldn't 
catch up. 

Harder to exhale. 

Couldn't exhale fast 
enough before 1 had to 
breathe in. 

Thought that this was the 
hardest. 

Couldn't exhale enough 
before subject had to 
inhale again. 

"Definitely the hardest so 
far". Slightly harder to 
exhale than to inhale. 
Harder to exhale. 


34