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«RATE OF CHANGE OF ALVEOLAR CARBON DIOXIDE AND THE CONTROL OF VENTILATION DURING EXERCISE BY CHRISTOPHER J. ALLEN AND NORMAN L. JONES From the Ambrose ...»

-- [ Page 1 ] --

J. Phy8iol. (1984), 355, pp. 1-9

With 5 text-figures

Printed in Great Britain

RATE OF CHANGE OF ALVEOLAR CARBON DIOXIDE AND THE

CONTROL OF VENTILATION DURING EXERCISE

BY CHRISTOPHER J. ALLEN AND NORMAN L. JONES

From the Ambrose Cardio-Respiratory Unit, McMaster University Medical Centre,

1200 Main Street West, Hamilton, Ontario, Canada

(Received 22 July 1983) SUMMARY

1. The relationship between rate of change of alveolar Pco2 (APA, Co2/Ate)I CO2 output (Pco2) and ventilation (VE) has been determined following a rapid increase in exercise intensity, to test the hypothesis that VE is related to Pco, by a feed-forward control system responding to APA, co2/Ate PC02

2. There was a close relationship between APA Co2/Ate and (APA, C02/Ate = 3-2 C02 + 0-85), but APA, CO2/Ate increased more rapidly than VCO2.

3. Increases in mean inspiratory flow, an index of inspiratory drive, were more closely related to changes in APA, Co2/Ate than to changes in TCo2Increases in PE during transient and steady-state conditions may be described by the equation: VE = 6-76 APA, CO2/Ate - 350, a relationship which is consistent with a feed-forward control system.

INTRODUCTION

Since the early studies of Krogh & Lindhard (1913), physiologists have searched for the mechanisms which mediate the increase in ventilation during muscular exercise. Measurements both in steady state and following a step change in exercise (fco,) show that increases in CO2 output are precisely matched by increases in ventilation (rE), there being little change in arterial PCO2 (Dejours, 1964). This relationship applies equally to increases in tco2 accompanying exercise or venous CO2 loading but is dependent on the presence of intact carotid bodies (Phillipson, Bowes, Townsend, Duffin & Cooper, 1981 a, b).

Swanson (1978) has proposed that both a feed-forward and a feed-back control mechanism are needed to account for the relationship between VCO2 and VE during exercise. In the feed-back system, the relationship between PE and VCo, is indicated by mean arterial 1Co2. A change in mean arterial PCO2 relative to a base-line or reference level generates an error signal; the feed-back control system then adjusts ventilation to minimize the error signal. In the feed-forward system, there is no error signal and ventilation responds directly to changes in VCO2. Yamamoto (1960) suggested that oscillations in both alveolar and arterial PCO2 in phase with respiration might be the link between l'Co2 and VE, and demonstrated that intravenous CO2 loading increased the amplitude of the alveolar PCO2 oscillation without changing 1 PiY 355 2 C. J. ALLEN AND N. L. JONES Pco, (Yamamoto & Edwards, 1960). Band, Cameron & Semple (1970) mean arterial showed that carotid chemoreceptors did indeed respond to oscillations in arterial Pco2.

suggesting that the amplitude of the oscillations could be the feed-forward stimulus to ventilation during exercise. However, the amplitude of this CO2 oscillation was also found to be dependent on the frequency of breathing, which Lamb, Anthonisen & Tenney (1965) showed to have no influence on ventilation during exercise.

Recent studies (Cochrane, Newstead, Nowell, Openshaw & Wolff, 1982) have demonstrated that the slope ofexpired alveolar PCO2 with respect to time (APA Co2/Ate) is correlated with VC02 during steady-state exercise. At the onset of exercise, there is an increase in the alveolar slope, in the rate of change of arterial PCo2, and in the amplitude of the CO2 oscillation (Band, Wolff, Ward, Cochrane & Prior, 1980).

Saunders (1980) suggested that APA C0o2/Ate rather than the amplitude of the oscillation might be the feed-forward stimulus to ventilation during exercise.

The present study was designed to determine the relationship between APA Co2/Ate, CO2~and rE following a rapid increase in exercise intensity, in order to test the hypothesis that fE is related to rco2 during exercise through a feed-forward control system responding to APA, C0o2/Ate.

METHODS

Subjects and Methods Six healthy male subjects exercised for 4 min at 600 kp rn/min (90 W) on a cycle ergometer (Elema AM360) to reach steady state. Measurements were then made for 90 s; the load was then rapidly increased to 1050 kp m/min (160 W) at the onset of an expiration, without warning. In all subjects 1050 kp m/min was less than 75 % of their maximum power output.

The subjects breathed through a Hans Rudolph valve (dead space less than 50 ml). Gas was sampled at the lips by a mass-spectrometer (Perkin Elmer MGA 1100), expired flow was measured by a Fleish no. 3 pneumotachograph and differential pressure transducer (Hewlett Packard 270), and tidal volume was derived by integration. Gas concentration and flow were sampled every 50 ms by a PDP-11 computer (Digital Equipment Corporation) and stored on floppy disk without correction for instrumental delay.

After each subject had completed 2 min at 1050 kp mi/min (160 W), expired gas was collected for 1 min both breath-by-breath by the computer, and simultaneously in a meteorological balloon distal to the pneumotachograph. The values for T'E, TCO, O'02, mixed expired PCO2 and PO, derived breath-by-breath were compared to those calculated from the balloon expired gas collection, and the delay between the flow and concentration signals was varied until the computed breath-by-breath measurements agreed with the bag collection.





The pneumotachograph was repeatedly calibrated, using flows of 0-240 1/min, and the mass spectrometer was calibrated using five standard gases with ranges of Fco, from 0-0003 to 0 075 and F0, from 0-13 to 0-21; calibration gases were analysed with the Lloyd-Haldane apparatus. There were no changes in calibration of the instruments throughout the studies.

Data analysis When expired PCO2 and PO, were plotted against expired time or expired volume, there was an early horizontal phase followed by a rapidly changing phase, both of which reflect the wash-out of the airway dead space. This was followed by a third phase ofconstant slope, which reflects wash-out of alveolar gas. VcO, and VO, were derived by integration of Fco, and F0, versus volume, to yield volumes of 02 and CO2 per breath expressed in 1/min s.t.p.d. Least-squares linear regression routines were applied to the points on the third phase of Pco, and PO, versus time, and the slope of the regression equation was calculated for each breath. The beginning of the third phase was arbitrarily chosen as five sampling intervals (250 ms) from the last point of greatest slope during the second phase. Fowler's method (Fowler, 1948) was applied to the PCO2 and Po, versus time plots by the

AL VEOLAR PCO2 AND VENTILATION DURING EXERCISE 3

computer, and each breath was partitioned into a dead-space and alveolar wash-out time. The PSc, and PO, at the mid-point of the alveolar phase of expiration were taken to represent mean alveolar PO, and Pco,. Time constants were derived from the slope of log change in Vo,, Pco or IPE against time (Whipp & Mahler, 1980).

Statistical methods The mean of all breaths collected over 90 s in steady state at 600 kp m/min was calculated for each subject, and used as a base line. Changes from the base-line values for each subject were

–  –  –

calculated for the first breath at 1050 kp m/min, and for breaths corresponding to 15, 30, 45, 60 and 120 s after the increase in load. The mean change for the six subjects at each point was calculated, and 95 % confidence limits were placed on this change using a paired Student's t statistic.

The change from base line (600 kp m/min) was considered significant (P 0-05) if the 95% confidence limits did not overlap the base-line axis.

To allow comparison with the least-square regression equations derived from our data, 95 % confidence limits using the t statistic are given for the slope and intercept. 95 % confidence limits are also given where appropriate for values of Y corresponding to an individual value for X.

–  –  –

11co, and the slope of alveolar Pco, There was a close linear correlation between (APA, co2/Ate) at 600 kp m/min, and at 15, 30, 45, 60 and 120 s after the increase in the exercise load (Fig. 2). The relationship between T'Co and APA, co,/Ate was expressed by the equation APA, co,/Ate = 3-2 Vco, +0 85 (r= 0988, P 0{001). The load was increased within 1 s of the onset of expiration (Fig. 3); 2-3 s later within that same expiration, there was a significant increase in the APA, co./Ate (Figs. 2 and 4), but no change in T'co' TE, or tidal volume until 15 s had elapsed. Thus during the first 15 s at 1050 kp m/min, APA C02/Ate changed more rapidly than co,,.

Mean inspiratory flow rate (VT/TI) was closely correlated to Ico, measured in the same breath (Fig. 5) except during the first two inspirations following the increase to 1050 kp m/min, when there were increases in VT/TI, but no change in 1co, or GE.

The increase in inspiratory flow rate resulted from a decrease in inspiration time (T1), total breath duration (Tt t.) was unchanged, and the ratio TI/Tt~t was reduced. The VT/TI during the first inspiration following the increase in work load to 1050 kp m/min differed significantly from the VT/TI predicted from the otherwise close relationship to 'co2 for the same breath (Fig. 5). The VT/TI predicted from the

AL VEOLAR Pco2 AND VENTILATION DURING EXERCISE 5

–  –  –

Fig. 4. Expired PO, and PCO, for the breath before the increase from 600 to 1050 kp m/min superimposed on the breath during which the load was increased in a single representative subject. These two breaths are the middle and left-hand breaths in Fig. 3. The workload was increased at the onset of expiration; 2-3 s later a rise in the alveolar slopes for both CO2 and 0° was evident.

observed Tco. was 1-12 l/s +0 14 (95 % confidence interval), compared to an observed VT/TI of 1P27 1/s (P 0 05). VT/TI was closely correlated to APA CO2/Ate (Fig. 5), and this close relationship was maintained during the first inspiration at 1050 kp m/min (VT/TI predicted from APA CO./Ate was 1-25 1/s; observed VT/TI was 1-26 1/s. P not significant).

6 C. J. ALLEN AND N. L. JONES 2-0

–  –  –

The present study demonstrated a close relationship between the rate of change of alveolar PC02 (APA, CO./Ate) and C02 output (C0o2) during exercise; however, when the exercise load was rapidly increased, APA, CO./Ate rose faster than 1co2 Mean inspiratory flow rate, an index of inspiratory neurogenic drive (Milic-Emili, 1982), was related to APA, Co2/Ate rather than "YCO2 while the exercise load was changing.

During the transition from rest to exercise, tidal volume and end-inspiratory volume increase, and functional residual capacity may fall slightly (Grimby, 1969;

Linnarsson, 1974). The volume changes are greatest during the first few breaths, and may affect measurements of alveolar gas exchange (Beaver, Lamarra & Wasserman, 1981; Wessel, Stout, Bastanier & Paul, 1979). We minimized volume changes by studying the transition from one level of exercise to another, and by introducing the higher exercise load rapidly and early in expiration.

Measurements of alveolar gas exchange and respiratory dead space are critically dependent on the time delay between the pneumotachograph and mass-spectrometer signals. The mass-spectrometer response time is influenced by the transport time from the sampling site to the instrument, the response time of the instrument and the sampling frequency (Noguchi, Ogushi, Yoshiya, Itakura & Yamabayashi, 1982). If the transport time is measured (Beaver, Wasserman & Whipp, 1973; Pearce, Milhorn, Holloman & Reynolds, 1977) or assumed (Cochrane et al. 1982; Linnarsson, 1974, Davies, Hahn, Spiro & Edwards, 1974) but the response time is ignored, there are systematic errors in 202 and yco2 measurements when these are compared to values obtained by simultaneous expired gas collection (Noguchi et al. 1982). Several

ALVEOLAR PC02 AND VENTILATION DURING EXERCISE 7

complex methods to correct for the instrumental response time have been developed (Noguchi et al. 1982; Mitchell, 1979; Arieli & van Liew, 1981; Beaver et al. 1981) that are all standardized against steady-state expired gas collections. In the present study, we standardized directly in each experiment by collecting mixed expired gas in a balloon simultaneously with breath-by-breath analysis. This method allowed us to match precisely the delay during the experiments and obtain the best agreement between breath-by-breath analysis and standard gas collection methods.

Although our protocol differs somewhat from the methodology of other workers, our estimates of the time constants for the increase in ventilation (65 s), VcO2 (60 s), Po, (45 s) are similar to those of Casaburi, Whipp, Wasserman, Beaver & Koyal and (1977) and Miyamoto, Hiura, Tamura, Nakamura, Higuchi & Makani (1982).

PcO, Because the measurements of APA, co2/Ate and both represent rates of CO2 evolution into the lungs, a correlation between them is to be expected, but the measurements themselves are independent. The output of CO2 (i.e. the volume of CO2 per breath) is derived by integration of expired FCO. against expired volume, while APA, co,/Ate is measured from the slope of alveolar PCO, against time. In steady-state experiments at rest, at 600 and at 1200 kp m/min, we have found the relationship between APAco /Ate and Vco. to be identical to that observed in the present experiments (APA,co/Ate = 3-2 ICO2+ 1-01, r = 0-93), (Allen, Jones & Killian, 1984) and similar to that reported by Cochrane et al. (1982), indicating that the relationship is unaffected by mode of exercise. This relationship is not maintained, however, during the first 15 s following a rapid increase in exercise load, during which time APco2/Ate changes independently of "C0o2' VE and 02° The discrepancy between rCo2 APA, Co2/Ate and during the first breaths at the increased exercise load can be attributed to the speed with which APA, Co2/Ate increases (2-3 s, Fig. 4), compared to the slower increase in iCo2 which occurs two breaths (or 15 s) later. This increase in APA, co./Ate slope is consistent with an increase in cardiac output without change in CO2 output (Saunders, 1980).



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