l AD-A271 039

I SUBMITTED TO: Naval Medical Research and Development CommandNaval Medical Command, National Capital Region

i Bethesda, MD 20814-5044

SUBMITTED BY: Center for Research in Special EnvironmentsSchool of MedicineState University of NY at Buffalo124 Sherman HallBuffalo, NY 14214

PRINCIPAL INVESTIGATORt Claes E.G. Lundgren, M.D., Ph.D.Professor of PhysiologyDirector, Center for Research in

Special Environments

TITLE: BIOMEDICAL CRITERIA FOR OPTIMAL ELASTIC RESISTANCE IN3 DIVERS' UNDERWATER BREATHING APPARATUS

NAVY CONTRACT: PROJECT NO. N0001489J1702

CONTRACT PERIOD: January 1, 1989 - June 30, 1993

1 DTIC, >)19]3 " Buffalo, NY October 5, 1993iii:j

Claes E.G. Lundgren, J.D., Ph.D.

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ITABLE OF CONTENTS

3 page

INTRODUCTION ...................................... 1

3 MATERIALS AND METHODS..............................2

RESULTS. ........................................... 5

I Ergometer Study .................................... 6...... 6End-tidal carbon dioxide .......................... 6Dyspnea ....................................... 6...... 6Breathing difficulty ............................... 6Ventilation ................................... 6...... 6Lung volumes ................................. 7Ventilatory duty cycles ............................ 7Gas exchange .................................. 7...... 7Power of breathing and pressures ............. 7

* Heart rate .................................... 8...... 8

Fin-Study .......................................... 8...... 8End-tidal carbon dioxide ............................. 8Dyspnea....................................... 8Breathing difficulty ............................... 8Ventilation ......................................... 8Lung volumes .................................. 9...... 9Ventilatory duty cycles ............................ 9Gas exchange .................................. 9...... 9Power of breathing and pressures ............. 9Heart rate .................................... 9...... 9

RES-Study ................................................ 10End-tidal carbon dioxide ..................... 10Dyspnea ............................................. 10Breathing difficulty ............................... 10Ventilation ......................................... 11Lung volumes ........................................ 11Ventilatory duty cycles ........................... 11Gas exchange ........................................ 11Power of breathing and pressures ............. 11Heart rate ........................................ .11 r

DISCUSSION ................................................. 12

CONCLUSIONS .............................................. 15

REFERENCES .............................................. 16

TABLES I - XII

ILLUSTRATIONS, Figs 1 - 30

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3 INTRODUCTION

This project is part of a series of investigations aimed atproviding the US Navy with design criteria for acceptablerespiratory impediment in divers' breathing gear so as to enhanceperformance and safety of military divers. Specifically, thepresent project was conceived to study the effects on divers'respiratory performance by elastic resistance, combinations ofelastic resistance and static loading and of flow resistance andstatic loading. The effects of static load and flow resistanceeach acting alone have earlier been subjects of extensive studiesI in this laboratory (Thalmann et al., 1978; Thalmann et al., 1979;Hickey et al., 1987; Norfleet et al., 1987; Warkander et al.,1989; Warkander et al., 1990; Warkander et al., 1992). Elasticresistance may be encountered when breathing on a stiffrebreathing bag (closed and semiclosed gear) or when wearing atight-fitting, unyielding rubber suit or chest harness; staticloading (also known as positive and negative pressure breathing)is the result of differences between breathing gas pressure andmean water pressure on the chest (may be due to depth differencebetween breathinq baq and chest) ; flow resistance is created inbreathing tubes, mouthpieces, C02 absorbers etc.

The subjects, who were breathing air, exercised in the proneposition. Exercise was performed either on an underwaterergometer or by tethered leg-kick swimming with fins.

The ergometer work was chosen for ease of defining workload andto allow comparison with earlier studies, fin swimming wasincluded as a more realistic type of exercise and to determinehow directly the large material we have gathered so far onergometer work might be applicable to real in-the-sea conditions.In the course of the study, differences between the findings withthe two exercise modes were observed which made us suggest moreextensive studies of this aspect while reducing the efforts onassymetrical elastic loading. The latter condition was deemedless important since it would be an unlikely scenario underactual diving conditions. Approval by the Submarine and DivingMedicine Program Manager for this modest modification of theprotocol was obtained. A few other parameters could not berecorded as originally planned. Thus, carbon dioxide productionhas not been measured and derived parameters such as respiratoryexchange ratio and alveolar ventilation not calculated. Thereason for this is technical difficulties with the original, opencircuit breathing apparatus. A novel concept for a closedcircuit apparatus had to be used as detailed in our ProgressReport of August 30, 1990. This apparatus has performed verywell, but did not, because of its CO absorber, allow measurementof C0 2 production. The lack of ?he latter parameter is ofmarginal importance and does not render the project less useful.In the course of the study, it was found that diaphragmatic EMGrecordings did not offer a reliable correlate to C0 2 levels or

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II yspnea. Hence, we did not continue such recordings throughoutthe series.

The following 26 parameters were obtained in compliance with theoriginal protocol: vital capacity, forced vital capacity, forcedexpiratory volume in 1.0 s and peak flow, forced expiratoryvolume fraction, maximal voluntary ventilation, minute volume,breathing frequency, expiratory reserve volume, tidal volume, endtidal P0 2 and PC02, 02 uptake, maximal inspiratory and expiratoryflows, inspiratory and expiratory times, peak and average mouthpressures, maximal voluntary inspiratory and expiratorypressures, power of breathing, pressure-time product, heart rate,three tiered dyspnea score and Borg-scale rating of perceivedexertion.

The following activity levels characterize this study:

Experiments under pressure, man hours 1789

Dive experiments, 1o 243Man-dives in experiments, 3o 731Testing, Training, Treatments*, man hours 56Testing, Training, Treatment* dives, No 35Testing, Training, Treatment*, man dives 55

MATERIALS AND METHODS

The subjects in each study were five or six non-smoking males,between 19 and 27 years old, and they were all certified scubadivers. The protocol had been approved by the InstitutionalReview Board on Human Subjects and Experimentation of theUniversity at Buffalo and the subjects had given informed consentto participate.

The experimental dives were performed in the wet compartment of ahyperbaric chamber system which has been described earlier(Thalmann, Sponholtz, Lundgren, 1978). The technical crew insidethe chamber consisted of a safety tender who was positioned inthe water within arm's reach of the subject and a tender whocontrolled the breathing apparatus and the respiratory recordingequipment. An intercom system allowed communication between theinside and outside crew. An underwater loudspeaker allowed theoutside crew to talk to the subject.

Except for one case of skin bends which responded well totreatment, no adverse effects of the experiments were noted.

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Exercise was performed submersed in the prone posture at aworkload corresponding to 60% of each subject's maximum oxygenconsumption (as determined when sitting upright on a cycleergometer in air at 1 ATA.). Exercise was either performed on anunderwater cycle ergometer (Collins Inc., Braintree, MA, modifiedfor underwater use or by tethered fin swimming.

The subjects were breathing air. The breathing gear consisted ofa full face mask (modified AGA Divator, Interspiro Inc.,Branford, CT) with an oronasal cup. The dead space in theoronasal cup was about 75 ml (Norfleet, Hickey, Lundgren, 1987).Pressure swings in the mask were measured by a pressuretransducer (Validyne DPl5, Northridge, CA). Static breathing gas

pressure was equal to the water pressure at a plane 7 cm dorsalto the sternal notch (no static load).

The subjects were breathing on a closed circuit breathingapparatus (Fig 1). This functioned as follows: The subject waswearing a full face mask (1). One-way valves (7) and (8) directedthe expired air to the CO2 absorber (4) and then to the bellows(5) or (6) from which the next inspiration was taken. The tensionon the springs (9) and (10) imposed the elastic loads.Spirometric recordings of breathing volumes were obtained bysignals generated by potentiometers (18) and (19) attached to themovable plates of the bellows (5) and (6). The breathing gas wassampled at (13) and analyzed by a mass spectrometer from which asignal proportional to the 02 fraction was fed to the controller(14) which adjusted the flow of make-up 02 through the mass flowI regulator (15) to the inlet (16). Mixing of the make-up 02 andthe air was enhanced by letting the 02 in through the mixinghoses (17). The selector valves (20) and (21) allowed rapidswitching between breathing against an elastic load and freebreathing of chamber air.

The bellows (5) and (6) were placed on the dry side of theLanphier-Morin barrier in the chamber; the CO absorber wasplaced partly immersed (for temperature controls and the facemask (1) and its connecting hoses (2) and (3) were submersed.

The tension of the springs (9) and (10) on the bellows wascalibrated in separate experiments by the use of an electronicpressure transducer and a large calibrated syringe which allowedinjection and withdrawal of breath-sized gas volumes. Due todifferences in the compliance of air at the two depths, twodifferent sets of springs were made and adjusted for each of thefour loads (0, 7, 14 and 21 cm H2 0/L).

The CO2 absorber was designed to impose very low flow resistance.Since this absorber was not going to be carried by a diver we didnot have any weight or size restrictions. The flow resistance forthe absorber for air at the greater depth was 0.6 cm H2 0 per L/sat a flow rate of 8 L/s.

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The bellows (6) provided the final challenge load. The subjectcould be connected to this lead by turning the selector valves(11) and (12). The spring tension of this bellows was set to givean elastic load of 30 cm H2 0/L.

The carbon dioxide in the inspired gas was monitored during thedives and was less than the concentration (0.04%) found inatmospheric air.

Signals from the bellows were displayed on a Grass Polygraph anddivided such that one channel represented respiratory volumesdirectly while the other channel was electronicallydifferentiated to display respiratory gas flow. End-tidal P0 2and end-tidal PCO2 were determined from samples takencontinuously by a catheter located in the oronasal cup andconnected to a mass spectrometer (Perkin-Elmer Model 1100,Pomona, CA). The time of inspiration and expiration werecalculated from the spirometer trace. Expiratory reserve volume(ERV) was determined from the spirometer trace by requesting thesubject to exhale to residual volume at intervals during theexperimental run and comparing this volume on the spirometertrace with his preceding end-expiratory volume.

The subjects provided dyspnea scores by hand signals every 5 minthroughout a run: a closed fist indicating no shortness ofbreath, one outstretched finger indicating a feeling of dyspneanot strong enough to make the subject doubt his ability tocontinue for another 5 min, two fingers indicating dyspneapronounced enough to make the subject doubt his ability tocontinue for another 5 min and three fingers for severe dyspneanecessitating immediate termination of the experiment. In thelatter case dyspnea scores that were subsequently missed wereassigned scores of 3 for the purpose of averaging. It should berecognized that the number scale for dyspnea scores from 0 to 3should not be thought of as being linear and that an averageddyspnea score may be misleading from the practical point of view.For instance, a mean score of I based on three subjects reportingNo 1 dyspnea stands for a different reality than a mean score of1 based on two No 0 and one No 3 score. The averaging of scoresprovided a way of crudely illustrating tendencies.

After the chamber was pressurized the subject entered the waterand rested for five minutes. Determinations of VC, FEV, and MVVwere performed before the subject started exercise. Havingexercised for 25 minutes the subject was connected to the finalchallenge load. He remained connected until he was forced to quitbut not for longer than 90 s. Challenge loads were only used inthe ergometer study and the fin-swimming study. The reason fornot using the challenge loads in the RES study was that in thetwo preceding studies the challenge load did not appear to make adifference in the outcome of the experiments with the lighter and

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heavier elastic loads. The reason for this might have been thatthe subjects, for safety reasons, only were exposed to thechallenge load for the last 90 s of an experimental run. Eachexperiment was divided into 5-minute periods in which parametersrelated to ventilation were sampled for at least one minute. Atthe same intervals one dyspnea score was obtained (except duringrest) and one vital capacity and one maximum pressure maneuverwere performed.

The criteria for premature termination of an experiment were thatthe end-tidal PCO 2 exceeded 65 mm Hg (8.7 kPa) or that thesubject did not cooperate adequately. In addition, the subjectswere free to terminate the experiment at any time.

Work of breathing against the imposed elastic loads wascalculated separately for inspiration and expiration from themouth pressure and volume signals (ZP dV) . As outlined byMorrison and Reimers (1982) only positive work was included inthe calculations (i.e. when the product of pressure and volumewas greater than zero) . Work of breathing per liter of 7t(volume-weighted mean pressure, WOBtot/Vt, where WOBtot is thesum of inspiratory and expiratory work of breathing) was alsocalculated.

All recordings were stored on tape by an FM-recorder (Honeywell101, Honeywell Inc., Denver, CO). Reduction of data was largelyperformed on a computer by programs written in-house. Signalswere sampled at 100 Hz.

Statistical analysis was by means of analysis of variance fordeviations from control values with Newman-Keuls test. Eachsubject served as his own control. Significance was noted atp<0.05.

RESULTS

The results will, for the most part, be presented in threesections referring to the ergometer-study which describes studiesof elastic loading with cycle ergometer exercise, the fin-studywhich deals with elastic loading with fin swimming and the RES-study which combined marginally acceptable static lung loadingwith either elastic loading or with flow resistance, whileperforming ergometer exercise.

Because of the modest number of subjects in these studies theretention of subjects from one study to the next was a concern.Therefore, one of the criteria in the subject selection was thelikely continuation into the next study. In the fin-study threeout of five subjects were retained from the ergometer-study andin the RES-study four out of five subjects were retained from thefin-study.

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Parts of the results described below have been published andpresented at scientific meetings as the results became available(Warkander & Lundgren, 1992A; Warkander & Lundgren, 1993A &1993B).

Elastic Loading with Cycle Ergometer Exercise (Ergometer Study)

Six subjects participated in this study. Their mean age was 22years. Numerical results &Ad levels of statistical significanceare presented in Tables I-IV and graphic presentations in Figs 2-9.

End-tidal carbon dioxide

The two highest elastic loads caused slight increases in thegroup means of the end-tidal CO2 levels during exercise at thegreater depth (Fig 2). in no other condition did the elastic loadaffect the CO., levels.

It is noteworthy that during exercise the increase in depth perse caused increases in PetC0 2 of between 5 and 10 torr.

Dyspnea

No dyspnea was reported during resting conditions. Duringexercise the dyspnea scores increased with increasing elasticload at both depths, reaching statistical significance with allthree loads at the shallow depth and with the two highest loadsat the greater depth (Fig 3) . There were large differencesbetween the subjects. For instance, extreme dyspnea forcedsubject A to abort his experiments prematurely when exposed thehighest load at both depths. Under the same conditions subjectsC and D did not report any dyspnea.

Breathing difficulty

The Borg-scale ratings of breathing difficulty were increasedwith each load at both depths (Fig 4).

Ventilation

The ventilation (VE), (Fig 5) was not influenced very much by theelastic loads. The only change was a slight increase by thehighest load during exercise at the shallow depth. However,changes were seen in both tidal volume (Fig 6) and breathingfrequency (Fig 7). During exercise the elastic loads caused thetidal volume to decrease (by up to 25%) and the breathingfrequency to increase (by up to 40%). At rest, the same patternwas seen with all loads at the greater depth and with the highestload at the shallow depth.

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Maximal voluntary ventilation, which was only measured duringrest, decreased with the lowest and the highest loads at theshallow depth but not at all at the greater depth. The effect ofdepth was to lower the MVV at the greater depth by, on theaverage, almost 50% compared to the shallow depth. The FEV % wasnot changed by the added elastic loads.

Lung volumes

The vital capacities (VC) were reducea by all loads duringexercise. At rest the same pattern was seen at the shallow depthand at the greater depth the VC was reduced by the two highestloads. On the average, the highest load reduced the vitalcapacity by about 25%.

A similar pattern was seen for the expiratory reserve volume(ERV). It was reduced by all loads during exercise and by thehighest load during rest. On the average, the highest loadreduced the ERV by about 25%.

Ventilatory duty cycles

The duration of inspiration relative to the duration of a breath(Ti/Ttot) did not change with any of the elastic loads duringrest or during exercise at the shallow depth. During exercise atthe great depth it showed a small decrease.

Gas exchange

The oxygen uptake (V02) was not influenced by the elastic loadsduring rest nor at the great depth. During exercise at theshallow depth it was increased with all loads, probably becauseof lower control values for subject F.

Power of breathing and pressures

The external work of breathing per volume (WOB/V) was increasedwith all loads under all conditions.

The expiratory power of breathing (Fig 8) increased withgradually increasing loads in all conditions except at rest withthe lowest load at the shallow depth. The two highest loadsincreased the inspiratory power of breathing (Fig 9) duringexercise at the shallow depth and the highest load did the sameduring exercise at the great depth.

Mouth pressures during spontaneous ventilation increased with allelastic loads during exercise. At rest, increases were alsocaused by all loads in the expiratory mouth pressures whileincreases in inspiratory mouth pressures were caused by the twohighest loads.

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Heart rate

The heart rate was influenced by the two highest loads duringexercise but not at rest. Depth caused a slight reduction duringexercise by an average nr 77 beats per minute (p<O.01).

Elastic Loading with Fin Swimming (Fin-Study)

The mean age of the five subjects was 22 years. Numericalresults and levels of statistical significance are shown inTables V-VIII and graphic presentations in Figs 10-17.

End-tidal carbon dioxide

The results of the end-tidal CO2 measurements are shown in Fig10. At 15 fsw the resting levels were consistently around 40torr regardless of the elastic load; at work there was asignificant increase in the DetCO2 with the two highest elasticloads. At 190 fsw no remarkable effects were induced by theelastic load during rest while during exercise the group as awhole showed a small but significant increase from a mean in thecontrol situation of 48.4 torr to 51.5 torr with the highestelastic load.

There wer-e large differences between the levels of CO2 maintainedby the subjects. Subject B had higher levels of CO2 (reachingalmost 60 mm Hg) than any of the other subjects, while subject Chad near-normal levels even during the most severe conditions.

DVspnea

Dyspnea was only reported during exercise, (Fig 11). The scoresdid not change with the elastic loads.

Breathing difficulty

The Borg-scale ratings of breathing difficulty were increased bythe highest load at the shallow depth and by the two highestloads at the greater depth (Fig 12).

Ventilation

During rest a slightly higher VE (Fig 13) was evident at thegreater depth (16.8 + 0.5 SE L/min) than at the shallow depth(14.0 + 0.3 SE L/min) (p < 0.05). However, during exercise theV at the greater depth (44.2 + 1.7 SE, L/min) was the same as

the shallow depth (47.3 + 2.0 SE, L/min) (p > 0.05). Nosystematic effects of the elastic loads were seen. The breathingfrequency (Fig 14) was increased in response to the two highestelastic loads both during rest and exercise at both depths (also

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with the lowest load during exercise at the shallow depth). Thetidal volume decreased with all loads during exercise (Fig 15).The same pattern was seen at rest at the shallow depth, while itdecreased with the highest load during rest at the great depth.

The MVV was not affected by any elastic load. It was, however,significantly lower (by almost 50%) at the greater depth. Forcedexpiratory volume after 1 s (FEV1%) showed slight increases withincreasin- elastic loads. The FEVl% was depressed at the greaterdepth.

Lung volumes

Vital capacities were depressed both at rest and exercise atboth depths. The highest elastic load decreased the VC by about25%. During exercise at the shallow depth the ERV was decreasedby the two highest loads while at the greater depth it wasaffected by all elastic loads.

Ventilatory duty cycles

The Ti/T-Ot showed statisticaily significant but functionallyinsignificantly decreases (0.49 to 0.47 or 0.48) during exerciseat the great depth.

Gas exchange

The V0 2 was not influenced by the elastic load except by thehighest load at the great depth which caused it to be 8% higheron the average, than the unloaded control value.

Power of breathing and pressures

Work of breathing per L of V increased with elastic loads in allconditions. During exercise the inspiratory and expiratory powerof breathing was increased by all elastic loads. During rest thePOBex (Fig 16) was increased by all elastic loads but the POBin(Fig 17) was only influenced by the highest load. The pressureswings in the mask were influenced by the elastic loads in allconditions.

Heart rate

Added elastic loads had no effects on the HR except duringexercise at the shallow depth when the highest elastic loadincreased the HR. Hyperbaric bradycardia was evident duringexercise by changing from 124.9 b/min at the shallow depth to113.8 b/min at the great depth (p < 0.05).

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Marginally Acceptable Static Lung Loading Combined with EitherMarginally Acceptable Elastic Loading or Flow Resistance duringErgometer Exercise (RES-study)

Five subjects participated. Their mean age was 23 years.Numerical results and levels of statistical significance areshown in Tables IX-XII and graphic presentations in Figs 18-25.

In compliance with the original research proposal's plan,marginally acceptable static lung loads were tried together witheither a marginally acceptable elastic load (ES) or a marginallyacceptable resistive load:

a) A -10 cm H2 0 static loading with a 7 cm H2 0/L elastic load(ES).

b) A -10 cm H 0 static load with a flow resistance that causedan externaf work of breathing of 1.5 to 2.0 J/L (RS).

End-tidal carbon dioxide

At rest at the shallow depth PS caused a slight hypocapnia (Fig18). No changes were seen with either PS or ES at rest at thegreater depth. During exercise ES caused increases in the groupmean at both depths while RS caused an increase at the greaterdepth. It must be noted that with the ES combination at thegreater depth subject G reached levels of CO 2 that were above theabort limit (65 mm Hg) . He had to be told to stop and when heegressed f- .m the water he was unaware of the severe C0 2 -loadthat he had been exposed to.

The experiments at rest revealed a slight hypocapnia 36.C + 0.8SE, mmHg at 190 fsw compared to 15 fsw 41.7 + 0.7 SE, mmHg (p <0.05).

Dyspnea

The dyspnea scores (Fig 19) were not affected by either of theload combinations at the shallow depth. However, at the greatdepth both combinations caused increases in the group means, theRS producing an average slightly above the acceptable (0.5)limit.

Breathing difficulty

The Borg-scale ratings of breathing difficulty were increased byboth load combinations at both depths (Fig 20).

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i3 Ventilation

The VE (Fig 21) was not changed during rest, or during exerciseat the shallow depth. During exercise at the greater depth itwas slightly suppressed by both load conminations.

During exercise at the shallow depth the tidal volume (Fig 22)was increased by RS and decreased by ES. At the greater depthduring exercise it was reduced by ES. The ES combination causedincreases in the breathing frequency (Fig 23) at rest but notduring exercise. The RS combination induced a slight reduction inf during exercise at the shallow depth.

The MVV was reduced by RS at the shallow depth. The MVV wasreduced by about 33% at the great depth and was not furtherreduced by either load.

Lung volumes

The VC and the ERV were reduced by ES in all conditions. The RScombination also reduced the ERV and the VC was minimally reducedduring exercise at the great depth.

Ventilatory duty cycles

During exercise at both depths the T./Ttot was reduced by ES and

increased by RS. In both instances t&e changes were small.

I Oxygen consumption

The V0 2 was not affected during rest at either depth nor duringexercise at the shallow depth. It was slightly reduced by RS atthe great depth.

I Power of breathing and pressures

The POBe increased with both loads at both depths duringexercise 'Fig 24). The POBin was decreased by ES during rest atthe greater depth and increased by RS during exercise at theshallow depth (Fig 25). The WOB/V was increased by both loads

i during exercise.

Mouth pressures were increased by both loads during exercise.

3 Heart rate

The HR was not changed by either load in any condition.IU

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Ij DISCUSSION

Our criteria for judging if a given external respiratoryimpediment was acceptable or not were the same as those appliedin our studies of respiratory flow resistance (Final ReportContract N00014-86-0106 and Warkander & Lundgren, 1992B).Briefly, they consisted of maximum C0 2 levels and dyspnea levels.The group average of end tidal PC02 should not exceed 55 mmHg(7.3 kPa) and no individual value should exceed 60 mmHg (8kPa).Dyspnea score averages should not exceed 0.5 and individualvalues should not be higher than 1. These limits were chosensince it was felt that higher values would either endanger thediver or adversely affect task performance.

Applying the acceptability criteria described above to theelastic loads, loads higher than 14 cm H2 0/L were foundunacceptable during ergometer work in terms of C0 2 balance (Fig2) and above 7 cm H2 0/L in terms of dyspnea scores (Fig 3). Bycontrast, during fin swimming, all loads were acceptable. Thus,at the same V0 2 , ergometer work rendered the subjects moresensitive to elastic loading than fin swimming. The rezson forthis is not clear. A hypothetical explanation would be that theV02max would be greater in fin swimming than in submergedergometer work. This would have placed the subjects at a morefavorable (less demanding) V02/VVO2max relationship during

swimming than during ergometer work since the V0 2 was the same inboth conditions. From the practical point of view, the greatertolerance to respiratory impediment during fin swimming thanduring ergometer work is important since determinations oftolerance performed during ergometer work may be consideredrelatively conservative and leave a margin of safety for finswimming which is a common activity under real diving conditions.It is noteworthy that all earlier Navy supported studies in thislaboratory on respiratory performance of divers have been3 conducted with ergometer work.

The data from the ergometer study are presented in Fig 26 to showthe distribution of acceptable and unacceptable outcomes relativeto work of breathing and ventilation. A boundary zone between0.8 and 1.0 J/L distinguishing acceptable and unacceptableoutcomes of exposure to elastic loads suggests itself. Itappears that elastic loading became unacceptable at a lower work-of-breathing level than did flow resistive work in an earlierstudy (Warkander & Lundgren, 1992B) where the boundary zone wasbetween 1.5 and 2.0 J/L (Fig 27). (Note though, that only onesubject was common to the two studies). Possibly the differencejust described is due to the fact that the elastic work ofbreathing per volume increases as inspiration progresses and thework culminates when the recoil of the respiratory organs is thelargest. At the same time the respiratory muscles are at lessadvantageous points on their length/tension curves at theextremes of inspiration and expiration. By contrast, resistive

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Iwork per volume peaks at roughly 50% of the tidal volume. Theserelationships are illustrated in Figs 28. Similar considerationsapply during expiration on the condition that it is deep enoughto proceed past the relaxation volume of the external elastic-load device.

The plots of power of breathing show some remarkable features.Thus, there are large differences between individuals. Comparefor instance, subjects D and F who expended about 0.8 W and 2.8W, in expiratory power (Fig 8, Panel C, 21 cm H2 0/L) and 1.0 and1.7 W in inspiratory power (Fig 9, Panel C, 21 cm H2 0/L)respectively, while their ventilations were very similar, namelyabout 75 L/min (Fig 5). This reflects differences in breathingpattern, the tidal volumes of D being about 65% of F's thusrequiring less work against the elastic resistance. In thiscase, not surprisingly, F reported more dyspnea than D (Fig 3).A similar difference is shown by their Borg-scale scores. Adifferent picture is offered by comparing subjects A and D whohad about the same inspiratory and expiratory power yields (Figs8 and 9, panels C, 21 cm H2 0/L) but who reported greatlydifferent dyspnea levels (Fig 3) while their Borg-scale scoreswere about the same (Fig 4). The possibility exists that thedyspnea scale and the Borg-scale measure different things. Thisappears to be born out by the lack of correlation between the twoscales (Fig 29). Based on the differences in dyspnea scoresbetween the two subjects just mentioned, one might ask if A hadweaker respiratory muscles which consequently would be morestressed by the same power output as that of D. However, this isnot born out by the recordings of maximal inspiratory andexpiratory pressures in the two subjects (Table III).Alternatively, one may look at the ventilatory effect of acertain power expenditure. Subject D, for instance had among thehighest ventilatory yields relative to power of breathing andalso the lowest dyspnea scores. On the other hand, subject A forinstance, who had an average ventilation/power quotient reportedamong the highest dyspnea scores. Thus, these comparisons do notoffer a consistent picture but it is possible that it wouldimprove if alveolar ventilation (not available - seeIntroduction) could be substituted for overall ventilation whencalculating ventilation/power.

3 By design, the loads in the experiments which combined elasticloading with static loading and resistive with static loadingwere chosen to each be marginally acceptable. When thecombinations were tried they were all found to be unacceptable.That is to say: in the load combinations, the loads acted in aninterdependent manner. Put differently: in their action toincrease CO2 or induce dyspnea the loads acted at least partlythrough the same mechanisms. However, generally the loadcombinations did not cause large excursions above the

acceptability limits for C02 readings and dyspnea scores (Figs 18and 19). An exception was one end-tidal PCO 2 reading in one

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Isubject exceeding 65 mmHg. The question arises as to what levelsof combined loads are acceptable. This can only be answered withprecision if a series of lower loads are tested. However, as atentative, relatively conservative estimate, we propose that loadlevels 50% below the ones tested might be acceptable for thecombinations tried. Thus, one would accept an elastic load of 2-4 cm H2 0/L or a flow resistance causing a work of breathing of0.75 - 1.0 J/L in combination with a static lung load of -5 cmH2 0. It furthermore appears safe to predict that one type ofload may be increased above 50% if a corresponding reduction is5 made in the other.

The mechanisms behind dyspnea and respiratory failure are thefocus of much interest by respiratory physiologists. While thepresent experiments were not directly designed to explore thosemechanisms, some related observations were made. Thus, issuggestive that dyspnea might reflect respiratory muscle fatigue.However, the indices of respiratory muscle fatigue, viz. maximalinspiratory and expiratory pressures failed to substantiate thisnotion. Increasing elastic load had no effect on the pressuresand, moreover, the subjects' abilities to generate respiratorypressures did not deteriorate as experiments wore on. This is inkeeping with our earlier observations using flow resistive loads(Final Report Contract N00014-86-0106 and Warkander & Lundgren,1992B) . Furthermore, in the latter study, we found that flowresistive loads met with essentially two types of reactions fromthe subjects (during ergometer work). Either they showed atendency to accumulate C02 without suffering dyspnea or they keptnear-normal C02 levels at the expense of suffering dyspnea (FinalReport Contract N00014-86-0106 and Warkander & Lundgren, 1992B).

Interestingly, elastic loads induced the same types of responsesin the present study using a new set of subjects (8/9) andincluding all experimental conditions except rest (Fig 30). Itfollows that there must be a common final path for these,physically very different, types of respiratory impedimentsto interfere with respiratory performance. Practically importantis that the present observations confirm our earlierrecommendation that tests of the effects of respiratoryimpediments must include both subjective evaluation by thesubject, such as dyspnea scoring, and measurements of C02 levels(Final Report Contract N00014-86-0106 and Warkander & Lundgren,1992B).

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Ig CONCLUSIONS

These conclusions were based on observations in 5 or 6 subjects.The size of the material should be considered when makinggeneralizations.

1. Respiratory elastic loading in excess of 7 cm H2 0 L-1 wasnot acceptable during prone underwater exercise at 60% ofmaximal oxygen uptake. Higher loads were unacceptablebecause they generated excessive end-tidal C0 2 levels and/orexcessive dyspnea.

2. At an equal oxygen uptake, a given respiratory elastic loadduring exercise on an underwater leg ergometer is less welltolerated than during fin swimming. Hence, testingtolerance to respiratory impediment while exercising on theergometer appears to yield more conservative (safer)tolerance limits than when exercise is performed by finswimming.

3. Elastic and static respiratory loads which, when applied oneat a time, were marginally acceptable were not acceptablewhen combined, due to C02 accumulation and/or dyspnea.

4. Resistive and static respiratory loads which, when appliedone at a time, were marginally acceptable were notacceptable when combined, due to C0 2 accumulation and/or3 dyspnea.

5. While tests according to (3) and (4) caused C02 and dyspnealevels to exceed acceptability standards they generally didso only slightly. Hence, it is suggested that designingbreathing gear where either elastic and static loads orresistive and static loads are combined, reducing the loadlimits to 50% of the individually tolerable would providefor acceptable effects of the load combinations.

6. An earlier recommendation from this laboratory that tests ofacceptable external respiratory impedance must consider bothend-tidal C0 2 levels and dyspnea scores was reaffirmed in

i this study.

7. Assigning acceptability limits to the Borg-scale forperceived respiratory effort will have to await further

I study.

1I

I

g REFERENCES

Hickey, D.D., W.T. Norfleet, A.J. PAsche, and C.E.G. Lundgren.Respiratory function in the upright, working diver at 6.8 ATA(190 fsw). Undersea Bicmed Res 14(3):241-262, 1987.

Morrison, J.B., S.D. Reimers. Design principles of underwaterbreathing apparatus. In: Bennett, P. and Elliott, D., eds. ThePhysiology and Medicine ot Diving. San Pedro CA: BestPublishing Co., pp. 55-98, 1982.

Norfleet W.T., D.D. Hickey, C.E.G. Lundgren. A comparison ofrespiratory function in divers breathing with a mouthpiece or afull face mask. Undersea Biomed. Res. 1987; 14:503-527.

Thalmann, E.D., D.K. Sponholtz and C.E.G. Lundgren. Chamber-based system for physiological monitoring of submerged exercising3I subjects. Undersea Biomed. Res. 5(3):293-300, 1978.

Thalmann, E.D., D.K. Sponholtz and C.E.G. Lundgren. The effectsof immersion and static lung loading on submerged exercise atdepth. Undersea Biomed. Res. 6(3):259-290, 1979.

Warkander DE, Nagasawa GK, Lundgren CEG. Criteria for mannedtesting of underwater breathing apparatus. In: Lundgren,C.E.G., D.E. Warkander, eds. Physiological and Human EngineeringAspects of Underwater Breathing Apparatus. Fortieth Workshop ofthe Undersea and Hyperbaric Medical Society. Bethesda, MD:Undersea and Hyperbaric Medical Society, Inc., Publ. No. 76(UNDBR) 10/1/89. pp. 77-86, 1989.

Warkander, D.E., W.T. Norfleet, G.K. Nagasawa, C.E.G. Lundgren.CO retention with minimal symptoms but severe dysfunction duringwet simulated dives to 6.8 ATA. Undersea Biomed Res 17(6):515-

1 523, 1992.

Warkander, D.E. .nd C.E.G. Lundgren. Physiologically andsubjectively acceptaole elastic loads in divers' breathing gear.Undersea Biomed Res 19(suppl) :140, 1992A.

Warkander, D.E., W.T. Norfleet, G.K. Nagasawa and C.E.G.Lundgren. Physiologically and subjectively acceptable breathingresistance in divers' breathing gear. Undersea Biomed Res19(6):427-445, 1992B.

I Warkander, D.E. and C.E.G. Lundgren. Cowparison of two modes ofunderwater exercise in tests of tolerance to elastic respiratoryload. Undersea Biomed Res 20(suppl):45, 1993A.

Warkander, D.E. and C.E... T nidgren. Respiratory performance indivers during exposure to combinations of ventilatoryimpediments. Undersea Biomed Res 20(suppl):46, 1993B.

Physiological Design Criteria for the Breathing Resistance inDivers' Gear. Final Report. Contract N00014-86-0106, 1989-92.

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I Dyspnea (RES-study)B Exercise at 190 fsw (57 msw, 6.8 atm abs, 690 kPa)

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: B

21 G

I.1.5- H

I --0 mean

0 ES RSLoad

Fig 19 Dyspnea scores plotted for the different loadcombinations during ergometer exercise. Each barrepresents one subject, the horizontal line shows thegroup mean, dots indicate the highest score reported byeach subject. Panel A: data from rest at the shallowdepth, panel B: rest at the greater depth, panel C:exercise at the shallow depth, panel D: exercise at thegreater depth.

I Breathing difficulty: Borg scale (RES-study)Exercise at 15 fsw (4.5 msw, 1.45 atm abs, 147 kPa)

* 20-

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-. 16-

U G14-

HI| -- -10-

*ES RS 0mean

LoadUBreathing difficulty: Borg scale (RES-study)3 Exercise at 190 fsw (57 msw, 6.8 atm abs, 690 kPa)

20"

* ~18 A

BS16-I G

ýi 14-

_ HZ 12-

ICD 10-

8.

I6 mean,0 ES RS ma

3 Load

Fig 20 Breathing difficulty (Borg-scale) plotted for thedifferent load combinations during ergometer exercise.Each bar represents one subject, the horizontal lineshows the group mean. Panel A: data from rest at theshallow depth, panel B: rest at the greater depth,panel C: exercise at the shallow depth, panel D:exercise at the greater depth.I

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II

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0.5 -- ~ - -~ - ----

.......

0 0 , , ,3 20 30 40 50 60 70 80 90 100Ventilation (L/min BTPS)

I Fig 26 Work of breathing per volume (volume averaged pressure)plotted against ventilation (VE). Data from both depthsduring ergometer exercise. For interpretation, seetext.

II

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Fig28 shew difeensm ft hstw pressureopnts. durin tatsine-wv

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I ~ressrc eered a gant the eags ugvlasta(nce. ofuhechesata

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0.5i i i i

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I Comparison of dyspnea scores and Borg scaleall subjects (ergometer study)

I

I 2,

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aI 1.5CUU - •.a

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08 10 12 14 16 18 20

I Breathing difficulty

IFig 29 Comparison of the scale for dyspnea scores and the

Borg-scale. See text for interpretation.

Bogsae e et o nepeain

II

I

I Ergometer, fin and RES studies

Exercise at 15 fsw (4.5 msw, 1.45 atm abs, 147 kPa)

~0

030

S.l-tida0 p 0 40

CO2 (trm Hg) 30u

Exercise at 190 fsw (57 msw, 6.8 atm abs, 147 kPa)

IM

I Z

0 3O

SI •n,+7.... 50 " - -" I -

End-tidall p, 40

C02 ('MM Hg) 30

Fig 30 A 3D-diagram showing the number of data points forcombinations of dyspnea scores and end-tidal CO 2I values. Data obtained from all experimental conditionsexcept rest. The top graph shows data from the shallow

is graph and the bottom graph shows data from the greater3 depth. Note that high dyspnea scores occurred with

Exrelatively low (570s, valu8 and vice14 vera.