Effort sensation, chemoresponsiveness, and breathing pattern during inspiratory resistive loading J. E. CLAGUE, J. CARTER, M. G. PEARSON, AND P. M. A. CALVERLEY Regional Thoracic Unit, Fazakerley Hospital, and Department of Medicine, University of Liverpool, Liverpool L9 7AL, United Kingdom CLAGUE,J.E.,J. CARTER, M.G. PEARSON,AND P.M.A. CALVERIBY. Effort sensation, chemoresponsiveness, and breathingpattern during inspiratory resistive loading. J. Appl. Physiol. 73( 2): 440-445, 1992.-Although inspiratory resistive loading (IRL) reduces the ventilatory response to CO, (ir~IPco& and increases the sensation of inspiratory effort (IES), there are few data about the converse situation: whether COZ responsiveness influences sustained load compensation and whether awareness of respiratory effort modifies this behavior. We studied 12 normal men during CO2 rebreathing while free breathing and with a 100cmH,O 1-l. s IRL and compared these data with 5 min of resting breathing with and without the IRL. Breathing pattern, end-tidal Pco,, IES, and mouth occlusion pressure (P,,) were recorded. Free-breathing ~E/Pco~ was inversely related to an index of effort perception (IES/~TE; r = -0.63, P < 0.05), and the reduction in ~EIP~o~ produced by IRL was related to the initial free-breathing VE/PCO, (r = 0.87, P < 0.01). IRL produced variable increases in inspiratory duration (TI), IES, and PO,l.at rest, and the change in tidal volume correlated with both VE/PCO, (r = 0.63, P < 0.05) and IES/~E (r = -0.69, P < 0.05), this latter index also predicting the changes in TI with loading (r = -0.83, P < 0.01). These data suggest that in normal subjects perception of inspiratory effort can modify free-breathing COZ responsiveness and is as important as COz sensitivity in determining the response to shortterm resistive loading. Individuals with good perception choose a small-tidal volume and short-T1 breathing pattern during loading, possibly to minimize the discomfort of breathing. l

ventilatory response to carbon dioxide; sensation of inspiratory effort DISEASES disturb the mechanical properties of the lungs and produce complex mixtures of sustained resistive and elastic loads during inspiration and expiration (9,10,21). The ability to maintain ventilation in the face of increased impedance has been extensively studied with inspiratory resistive loading (IRL) as a simple model in normal subjects during wakefulness and sleep.@, 6,31,32). At rest, IRL reduces minute ventilation (VE) and a variety of load compensatory mechanisms, including operational-length compensation; increases in inspiratory time (TI) and changes in respiratory center output have been described (14, 21). During CO2 rebreathing IRL reduces the ventilatory response to CO2 in normal subjects (3,11,26), but there are few data relating individual CO2 chemosensitivity to the response to sustained loads at rest. Despite the obvious discomfort of these maneuvers, it is not clear whether respiratory sensation influences MANY PULMONARY

440

load-compensatory behavior or if the ability to sense respiratory effort modifies chemically stimulated breathing. Most studies of respiratory sensation have examined the sensory response to repeated presentations of single loaded breaths (13,16,19) or studied the sensations experienced during hypercapnia aiming to clarify the mechanisms, producing the sensation, However, they have not considered the influence that sensation might have on breathing pattern and load compensation behavior. We hypothesized that CO, sensitivity would influence breathing pattern during IRL at rest before the CO, tension rose and also that the ability to perceive respiratory sensation would modify the breathing pattern adopted. To test these ideas we measured ventilatory occlusion pressure and effort sensation responses to progressive hypercapnia (both free breathing and loaded) and compared these data with the changes in resting breathing pattern seen with the same load. We anticipated that subjects with the steepest ventilatory response to CO, would show the greatest increase in respiratory effort but this was not the case, and we have had to revise our views about the influence of respiratory sensation on breathing pattern, at least in these normal subjects. METHODS

Subjects. Twelve healthy male subjects (aged 25-38 yr) were studied. All were familiar with physiological measurement equipment, but none were trained respiratory physiologists. Measurements were made with the subject seated, wearing a noseclip, and breathing through a mouthpiece from a low-resistance two-way valve (Hans Rudolph 2600). Airflow was measured with the use of a heated pneumotachograph (Fleisch no. 2), and the integrated volume signal was recorded on a strip-chart recorder (Gould 2000 series). Tidal volume (VT), respiratory frequency, their product minute ventilation (VE), inspiratory time (TI), and total breathing cycle duration (TT) were determined. Mean inspiratory flow (VT/TI) and respiratory duty cycle were calculated. Mouth pressure was measured at the mouthpiece with a separate pressure transducer (Gould P23XL), and from this signal, mouth occlusion pressure (P,,) was measured after airway occlusion by a manually operated helium balloon valve (Hans Rudolph 9300) occluded in expiration and recorded onto the chart recorder at a paper speed of 100 mm/s. Occlusion pressure at 100 ms after occlusion was determined from the trace. One to two breaths before

0161-7567/92 $2.00 Copyright 0 1992 the American Physiological

Society

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EFFORT

SENSATION

AND

LOAD

COMPENSATION

441

each occlusion, the subject was asked “How difficult is it during the CO, rebreathing runs. Because the Borg scalto breathe?” and in response pointed to a score on a ing data were ordinal, data were analyzed with the use of modified Borg category scale (8). Occlusion pressure meaSpearman’s rank correlation. Statistical analysis of the surements and effort sensation scores were recorded at meaned slopes during rebreathing were made with Wil30-s intervals during resting breathing. End-tidal PCO, coxon’s signed rank test. Because several of the variables (PET& was measured with a fast-response infrared derived from CO, rebreathing and resting-breathing data CO, analyzer (Gambro Engstrom). were potentially interrelated, stepwise multiple regresVentilatory responses to progressive hypercapnia were sion analyses were performed to determine the residual measured with the use of a modified Read rebreathing variance when more than one variable was found to be technique with initial gas concentrations of 6% CO,-94% correlated. 0,. The circuit consisted of a 6-liter reservoir bag in a sealed box with inspiratory and expiratory connections RESULTS to the low-resistance two-way valve used during resting breathing. Air displacement from the box was measured Ventilatory and sensory responses to CO, and IRL. Data with a turbine spirometer (P. K. Morgan) to determine summarizing the effect of IRL on the variables derived VE, and its accuracy was confirmed by the pneumotachoduring CO, rebreathing are shown in Table 1. All indexes graph in the circuit. CO, levels were measured by a fastshowed a high degree of correlation (r > 0.9) whether response infrared CO, analyzer (P. K. Morgan). Mouth linear regression or ran k correlation was used, and linear occlusion pressure and inspiratory effort sensation were regression data are presented here. measured at 30-s intervals (the same as during resting The unloaded ventilatory response to CO, varied fourbreathing) throughout each rebreathing run, which fold between these subjects. The unloaded-&se in effort lasted 3.5 min. was equally variable (0.06Protocol. Resting breathing data were recorded on each sensation with ventilation 0.22 units 1-l min-l) and was inversely correlated with occasion for 5 min. Recordings were made when the breathing pattern and PET,,, had stabilized. Data were vE/PCO, (r = -0.63, P < 0.05) (Fig. 1). Those subjects obtained for each subject while free breathing (FB; cir- with the greatest increases in sensation for a given showed the flattest ventilatory recuit resistance 1.3 cmH,O 1-l s) and with an IRL (resis- change in ventilation sponses to CO,. When the slopes derived during CO, retance 10 cmH,O l s) in the circuit. The resistor conbreathing were entered in a multiple regression analysisted of 22-mm-ID tubing containing small-bore plastic P, JPco,, IES/Pco,, IES/VE, P, JOE, tubing and was linear up to 3 l/s. Resting breathing was sis (vE/PC02, recorded on 3 separate days. Triplicate recordings of and IES/P, J, IES/~E slope during FB explained 46% of Of vE/PC02. Similarly, IES/P, 1 was resting breathing were made with and without the resis- the residualvariance inversely related to P, ,/Pco, (r = -0.7, P < 0.05), but the tor, in random order for each individual. between’ ~E/Pco, and IES/P,, did not Similarly triplicate ventilatory responses to CO, were relationship reach significance (r = -0.53). No other slopes were sigrecorded for each subject on 3 separate days with and nificantly correlated. PETITE at the end of the rebreathes without IRL, again in random order. (62 t 4.4 (SD) Torr) was unrelated to IES/h (r = 0.24). All subjects had a training session to acclimatize them IRL depressed ~E/Pco, by 30% and increased P,,/ to the equipment and to familiarize them with the use of PCO,by 51% and IES/Pco, by 47% (Table 1). FB IE$/ the modified Borg category scale for effort sensation scoring. Subjects wore headphones playing music to VE still represented 54% of the variance of ~E/Pco, when loaded. Those individuals with the largest resting mask external sounds during the recordings. VE/PCO, showed the greatest fall with loading (Fig. 2). Data analysis. VT measurements and respiratory cycle times were determined from the five breaths preceding Resting ventilation and IRL. Data summarizing the reeach airway occlusion, during resting breathing. Mean sults of the study of resting ventilation, CO, tension, ocresults for the three occasions with and without IRL were clusion pressure, and IES are shown in Table 2. IRL procalculated for each individual and, comparisons between duced a 33% decline in VE despite doubling P, 1 and inmeaned data were made WithStudent’s paired t test. creasing TI. Neither FB ~E/Pco, nor IES/VE predicted The relationships between VE, CO,, sensation of inspithe fall in VE, which resulted from a 56% fall in VT/TI ratory effort (IES), and P,, during CO, rebreathing were rather than from consistent changes in respiratory frecalculated by linear regression with the use of the least quency. Group mean VT was maintained, and PET,,, was squares method. The ventilatory response (v~/Pco,), unaltered compared with FB values. 1 mouth occlusion pressure response (P,,/Pco,), and inVT. Group mean VT did not vary by more than 200 ml spiratory effort sensation response to CO, (IES/Pco,) between FB and breathing against IRL (Table 3). Howwere determined. Each of these values represents the ever there was a marked divergence among subjects, with rate of change of one variable with the other. In addition, five subjects showing an increase in VT of >200 ml and the rate of change of occlusion pressure with ventilation seven subjects a fall of >200 ml (Fig. 3). There were no (PO ,/vE) was determined as a measure of changing respidifferences in the unloaded breathing patterns between ratory system impedance. The rate of change of effort those subjects who defended VT and those who did not. sensation with ventilation and occlusion pressure (IES/ However, those subjects with the steepest ‘\;TE/Pco,reTjE and IES/P, .1, respectively) were also determined as sponses during unloaded CO, rebreathing showed the measures of perceptual sensitivity. The results are ex- greatest increase in VT when breathing against IRL at pressed as the mean value of the three slopes obtained rest (Fig. 4A). There was no difference in the resting l

l

1-l

l

l

l

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442

EFFORT

1. Ventilatory

TABLE

AND

LOAD

COMPENSATION

and effort sensation responses during CO, rebreathing

VEIPCO,, 1 min-’ Torr-’ l

FB IRL

SENSATION

PO.1/PC% I cmH,O/Torr

l

2.57tl.O 1.7kO.5 *

0.37kO.16 3.73kO.2 *

IESlPco,, units/Torr

cmH,O

0.3tO. 12 3.15kO.15 *

PO.&, 1-l min-’ l

units

l

0.14+0.1 0.29kO. 1 *

Slopes (means + SD) of ventilatory variables during CO, rebreathing. VE, minute ventilation; inspiratory effort; FB, free breathing; IRL, inspiratory resistive loading. * P < 0.01.

IESI~E, 1-l min-’ l

l

0.13kO.l 0.2620.1 *

Posl, mouth occlusion pressure; IES, sensation of

5

r=-0.63 pgo.05

r=0.87 pco.01

.-=1 Q IL

a

0.0 1

2

hPCO2

3

4

5

(FB) L/min/mmHg

Relationship between unloaded ventilatory responses to CO2 (%/Pco,) and unloaded effort sensation rise with ventilation during rebreathing (IES/%). FIG.

1.

0 ;

kPC0:

I

0 I

I

1

(FB; (LlmithnmHg;

FIG. 2. Relationship between fall in ventilatory response to COP with resistor (IRL) and free-breathing ventilatory response (FB).

-0.83) between IES/~E and the change in TI, and exPETamong those subjects with a rise in VT and those plained 68% of the residual variance. Changes in TI were in whom VT fell. FB occlusion pressure responses to CO, during re- less well correlated with the FB IES/P, .1 slopes (r = 0.58, breathing (P,,/Pco,) were unrelated to the change in P < 0.05). resting VT with loading, nor were changes in VT correlated with IEWP, l (r = 0.49, P > 0.1). However, the rate DISCUSSION Conventional analyses of the involuntary control of of increase of inspiratory effort with ventilation during of stable rebreathing (IEWE) was correlated with the change in breathing have focused on the maintenance VT during loading at rest; individuals with the steepest blood gas tensions, proprioceptive input from the lung and chest wall, together with respiratory center output IES/~~E slopes showed the smallest increases in VT (Fig. 4B). Stepwise regression analysis entering the slopes, de- (30). However, even at rest there is significant variation rived during rebreathing and the change in VT in both VT and total cycle duration (24). Higher mental functions as well as personality (2) can modify breathing with IRL at rest, showed a significant correlation with the IES~E slope during CO, rebreathing (r = 0.69, patterns producing more rapid and shallow respiration P < 0.05). when assessed noninvasively (20). Increased mental activity produces similar changes even when breathing is TI. The changes in TI were also variable when breathing against IRL (percentage of change in TI varied from involuntarily stimulated by CO, (27), and indeed cortical -6.5 to +506%). The increase in TI, whether considered influences operating during CO, rebreathing have been (22). With the as an absolute change in duration or as a percentage in- demonstrated with magnetic stimulation onset of stage 4 sleep or during anesthesia, breathing crease from the FB level, was correlated with VE/PCO, and was inversely related to IES/~E during FB and becomes less variable (31, 32). loaded CO, rebreathing (Fig. 5). Stepwise regression analThese data support the view that cortical influences ysis, entering all the slopes derived from rebreathing and alter breathing patterns, although there is uncertainty as the change in TI with loading, revealed a correlation (r = to whether the sensations associated with voluntary and Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (137.154.019.149) on January 14, 2019.

EFFORT TABLE

443

AND LOAD COMPENSATION

2. Breathing patterns before and after IRL VE, l/min

FB IRL

SENSATION

12.2k2.5

8.0t2.5 *

VT,

liter

0.98t0.21

0.9wo.42

f,

breathdmin

12.9t3.1

9.6k5.2 t

VT/TI, l/s

0.61~0.15

0.26kO.14 *

?‘I, TI/TT

0.35t0.04 0.57kO.11 *

S

1.75t0.49

4.6k2.6 *

P 0.19 cmH,O

1.32kO.37

3.71t1.66 *

IES

0.42kO.65 3.3t1.6 *

%F

40*4.1 41~~3.2

Resting ventilatory variables (means k SD) during FB and IRL. VT, tidal volume; f, respiratory frequency; TI, inspiratory time; TY, total breathing cycle duration; PETIT,, end-tidal Pco~. Statistically significant differences between FB and IRL: * P < 0.01;f P < 0.05.

involuntary respiratory activity are the same (1). Our data suggest that the involuntary perception of inspiratory effort is at least as important as CO, responsiveness in IRL compensation and may influence the response under unloaded conditions. A number of limitations of our study require comment. These studies were conducted with the use of a mouthpiece and noseclip, which is likely to increase resting ventilation compared with uninstrumented subjects (4). All subjects were familiarized with the procedure before data recording, and the levels of ventilation at rest were similar to those in other studies using a mouthpiece or facemask (31). Alterations in resting VT resulting from the added dead space and the respiratory stimulation associated with the system may affect the ability to perceive volume change (17) but are not likely to qualitatively change the relationship between sensation and loading. We used a well-established category scale to assess effort sensation (8, 23), which has the advantage of simplicity in use by different subjects and of being relatively linear (11). Despite falls in VE, we saw no change in PETITElevels during the periods of loaded breathing even though some subjects were hypoventilating. This may reflect the effects of the different days of recording, and possible different starting levels of PETITEon these different days, although we saw no systematic change in our data to support this. Similar findings have been reported previously (31) and may reflect the slower gas equilibration during these quasi-steady-state conditions (12). Differences in final PETITEdid not explain the variation in IES/~E nor did differences in test duration because this was standardized to avoid overscoring of the final points. Our system is limited by potential for interactions between the CO,-stimulated variables. Nonetheless, the consistency of the load compensation responses during rebreathing and at rest suggests to us that the measure of respiratory sensation IES/~E that we used is a valid one. TABLE

3. Resting breathing data in subjects maintaining VIZ, llmin

FB IRL

9.38t2.0

FB IRL

12.2t2.1 6.9222.5

12.3k3.0

VT liters

0.93t0.21

1.4t0.21 l.OlkO.23 0.68t0.21

f, breathdmin

13.4k2.1

7k2.4 12.6k3.8 11.5?6*

CO, rebreathing is a very incomplete model of ventilatory control but is a convenient means of providing a uniform ventilatory stimulus. The maximum ventilation reached lay well within the potential ventilatory capacity of the subjects and was unrelated to the individuals static maximal inspiratory pressure. The CO, response varied widely among subjects (15), and our data suggest that awareness of respiratory effort explains much of this variance whether FB or loaded. Subjects experiencing the most effort for a given increase in ventilation appeared to limit their response to CO,. As noted previously (26), individuals with the largest ventilatory response to CO, showed the greatest fall with loading and were those least able to perceive increases in ventilation when FB. These behavioral factors may explain the previous finding that ventilatory responses in such subjects do increase during elastic loading (26) when the breathing pattern that minimizes discomfort differs from that during resistive loading. Effort perception, even though assessed with CO, stimulation, was also an important determinant of load compensation at rest. Our subjects did not defend VE in any consistent fashion, but ventilatory depression resulted from reductions in VT/TI with each component affected to a variable degree. We found a bimodal distribution of VT response to loading, a finding previously noticed in first breath-response studies and also with inspiratory loading during sleep (5,14). A steep CO, response predicted subjects who increased their VT when loaded before changes in PETITE occurred. Conversely, good perceivers with steep IEWVE slopes had small VT responses to loading. This may reflect the relatively greater effect of FB IES/~E on respiratory timing. Good perceivers defended TI when loaded at rest, presumably to limit the discomfort experienced with each inspiration. These data are in keeping with previous studies suggesting that the breathing pattern adopted is that most likely to minimize discomfort (18). The ability to do

VT and subjects in

VTITI, l/s

TIP TI/TT

0.32t0.18

VT held 0.34t0.05 0.57t0.18

0.61t0.17 0.2ltO.l

VT fell 0.36t0.03 0.57t0.05

0.61t0.14

whom

S

1.53t0.19

5.7k2.9 1.9t0.59

3.83t2.2

VT fell P 0.19 cmH,O

PQ02,

IES

Torr

1.38~0.36 3.93t1.4

0.36kO.57 2.82t1.4

41t4.7 4025.0

1.3t0.4

0.46t0.74 3.71~1.72

4OA3.9 41kl.6

3.5t1.93

Resting ventilatory variables with FB and IRL (means k SD) in group of patients in whom VT was held with IRL (n = 5) and group in whom VT fell with IRL (n = 7). For abbreviations, see Tables 1 and 2. Statistically significant differences, * P < 0.01. Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (137.154.019.149) on January 14, 2019.

444

EFFORT

SENSATION

AND LOAD

COMPENSATION

r=-0.83 6

VTiiB VT-‘IRL 3. Tidal volume (VT) during resting breathing and change produced by IRL. FIG.

for 12 subjects

this successfully depends on the number of degrees of freedom within the system. During short-term unloaded CO, rebreathing, increases in ventilation can be limited to reduce discomfort as is also the case during loaded eucapnic respiration. However, during loaded CO,-stimulated breathing, both sensation and neuromuscular drive are increased producing a more complex response. These acute studies in normal subjects must be viewed cautiously when considering the complex chronic loading occurring in respiratory disease. The IES/~E relationship appears to measure the “gain” of the sensory system but probably summarizes more complex relationships between stimulus and response, including a variable contribution from chest wall proprioception (28). No account is taken of tachyphylaxis as seen in asthmatic subjects whose Weber fraction is elevated (7). The presence of a large background load, especially one that affects resting Bl

0 a \

0.0

0.2

0.1

0.3

IES/irE (FB) (units/L/min) FIG. 5. Relationship between change in inspiratory time (TI) produced by IRL at rest and rate of change of effort sensation with ventilation

(IES/~E)

during

unloaded

rebreathing.

lung volume, may restrict the range of possible scoring by increasing the initial score. There are, however, data in chronic obstructive pulmonary disease patients suggesting that the greater the sensory exponent for a given mouth pressure, the greater the fall in VT during subse-

quent resistive loading (25). Likewise the onset of a rapid shallow breathing pattern in patients starting to wean from assisted ventilation

is more likely to reflect patient

distress than respiratory muscle fatigue (29). In conclusion, our data in normal subjects suggest that variation in the ability to perceive respiratory effort can explain much of the variability seen between subjects of the CO, response and is an important determinant of the type of load-compensating breathing pattern adopted both during CO, stimulated and resting resistive loaded breathing. Address for reprint requests: P, M. A. Calverley, Regional Thoracic Unit, Fazakerley Hospital, Longmoor Lane, Liverpool L9 7AL, UK.

r=-0.69

Received

2 January

1991; accepted

in final

form

25 February

1992.

REFERENCES 1. ADAMS, L., R. LANE, S. A. SHEA, A. COCKCROFT, AND A. Guz. Breathlessness during different forms of ventilatory stimulation: a study of mechanisms in normal subjects and respiratory patients. CZin. Sci. Land.

69: 663-672,

1985.

2. AITKEN, R. C. B., A. K. ZEALLY, AND S. V. ROSENTHAL. Some psychological and physiological -1 : 1

liEIPCO2

r 2

I 3

(FB)

r 4

(L/min/mmHg)

1

5

-1

:

r

0.0

0.1

ES/h

(FB)

I

0.2

1

0.3

(units/L/min)

FIG. 4. Correlation between change in VT produced by IRL and FB ventilatory response to CO, (A) and rate of rise of effort tion with ventilation (B).

at rest sensa-

considerations

of breathlessness.

In: &e&zing. London: Churchill, 1970, p. 253-273. (Hering-Breuer Centenary Symp.) 3. ALTOSE, M. D., S. G. KELSEN, N. N. STANLEY, R. S. LEVINSON, N. S. CHERNIACK, AND A. P. FISHMAN. Effect of hypercapnia and flow-resistive loading on tracheal pressure during airway occlusion. J. Appl. Physiot. 40: 345-351, 1976. 4. ASKENAZI, J., P. A. SILVERBERG, R. J. FOSTER, A. I. HYMAN, J.

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AND LOAD

MILIC-EMILI, AND J. M. KINNEY. Effect of respiratory apparatus on breathing pattern. J. Appl. Physiol. 48: 577-580, 1980. 5. AXEN, K., F. HAAS, D. GAUDINO, AND S. S. HAAS. Effect of mechanical loading on breathing patterns in women. J. Appl. Physiol. 56: 175-181,1984.

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p. 751-786. (Lung Biol. Health Dis. Ser.) 11. CLAGUE, J. E., J. CARTER, M. G. PEARSON, AND P. M. A. CALVERLEY. Relationship between inspiratory drive and perceived effort in normal man. Clin. Sci. Lond. 78: 493-496, 1990. 12. FARHI, L. E., AND H. RAHN. Gas stores of the body and the unsteady state. J. Appl. Physiol. 7: 472-484, 1955. 13. GOTTFRIED, S. B., M. D. ALTOSE, S. G. KELSEN, C. M. FOGARTY, AND N. S. CHERNIACK. The perception of changes in airflow resis-

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K. J., D. B. BUCENS, AND E. J. M. CAMPBELL. Effect of breathing patterns on the perceived magnitudes of added loads to breathing. J. Appl. Physiol. 52: 578-584, 1982. 17. KILLIAN, K. J., S. C. GANDEVIA, E. SUMMERS, AND E. J. M. CAMPBELL. Effect of increased lung volume on perception of breathlessness, effort, and tension. J. Appl. Physiol. 57: 686-691, 1984. 18. KILLIAN, K. J., AND N. L. JONES. Respiratory muscles and dyspnea. Clin. Chest. Med. 9: 237-248, 1988. 19. KILLIAN, K. J., C. K. MAHUTTE, J. B. L. HOWELL, AND E. J. M. CAMPBELL. Effect of timing, flow, lung volume, and threshold pressures on resistive load detection. J. Appl. Physiol. 49: 958-963, 1980.

COMPENSATION

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Effort sensation, chemoresponsiveness, and breathing pattern during inspiratory resistive loading.

Although inspiratory resistive loading (IRL) reduces the ventilatory response to CO2 (VE/PCO2) and increases the sensation of inspiratory effort (IES)...
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