Estimates of the CO2 pressures in systemic arterial blood during rebreathing on exercise

Respiration

ESTIMATES

(1971) 11, 186-l 96 ; North-Holland

Physiology

OF THE

CO2 PRESSURES

DURING

IN SYSTEMIC

REBREATHING

D. DENISON, R. H. T. EDWARDS, Department

of Medicine,

Royal Postgraduate

Publishing

Company,

Amsterdam

ARTERIAL

BLOOD

ON EXERCISE1

G. JONES AND H. POPE Medical

School, London

W12, U.K.

Abstract. Blood was sampled from the brachial arteries of 3 subjects on 120 occasions before or during the prolonged rebreathing of CO-2 in 0~ mixtures. Studies were made at rest and during 3 grades of exercise (3W900 kpm/min). The COZ tension in rebreathed gas was consistently higher than that measured in arterial blood and the difference increased with the level of exertion but at steady loads it did not change with time. The CO:! contents of 45 samples were measured on the same occasions. Their tension-content coordinates fell on the in vitro dissociation curves of normal blood. Previous studies on the same subjects had shown a similar discrepancy between rebreathed gas and pulmonary arterial blood. In spite of these findings the indirect Fick method, based on rebreathing estimates of the mixed venous Pco, and end-tidal estimates of the arterial Pco, yields values for cardiac output which closely agree with those obtained in direct Fick and dye dilution studies published by others. The interpretation of oxygen based estimates of cardiac output is also discussed. Alveolar arterial Pco, difference Cardiac output Indirect Fick method

Mixed venous blood Muscular exercise Rebreathing

This paper reports some further observations on the alveolar-arterial CO, gradient that exists during rebreathing and draws attention to a contradiction that at present we cannot explain. In a previous study of rebreathing, cardiac outputs were estimated by an indirect Fick procedure for a variety of circumstances (DENISON, 1968). The outputs were obtained by measuring metabolic gas exchange and end-tidal gas tensions during open-circuit respiration and mixed venous gas tensions during the rebreathing of a CO2 in N, mixture. Arterial gas tensions were predicted from the end-tidal values using the end-tidal to arterial gradients established by JONES et al. (1966) and arterial and venous blood gas contents were derived using the 0, and CO, dissociation tables of Gomez and Shepherd (GOMEZ, 1961). These are almost identical to the more easily obtained tables of KELMAN and NUNN (1968). Acceptedfor

publication

17 August 1970.

l This work was supported

by a grant from the Medical Research Council of Great Britain. 186

co,

EQUILIBRIUM IN THE LUNG

co2 [ l/min

187

1

Fig. 1. A plot of cardiac output against blood flow. 60 paired estimates from 6 healthy adult males. Output estimated by a rebreathing version of the indirect Fick technique from simultaneous measures of 02 (0) and COZ (0) transfer.

Sixty paired estimates for men in steady-state exercise under normal circumstances showed good agreement between the O,-derived and CO,-based values although the latter were on average slightly higher (fig. 1). These values which also agreed well with direct Fick and dye dilution studies published by others, provided strong indirect confirmation that the rebreathing procedure measured the true gas tensions of blood entering the pulmonary capillaries. However, JONES et al. (1967 and 1969) have shown in man that during rebreathing blood samples from the brachial artery consistently show a CO, tension considerably lower than the gasinthelungs. GURTNER, SONG and FARHI (1967, 1969) have demonstrated in dogs that a similar Pco2 gradient exists between rebreathed gas and blood sampled from the pulmonary artery; this also is true in man (DENISON er al., 1969). So, during rebreathing in man, gas at the lips exhibits a CO, tension that accords well with indirect Fick data but is higher than that measured in the blood entering or leaving the lungs. Recent work on the dissociation curves of blood in vivo would make the contrast between the Indirect and Direct measurements more marked. The contradiction could be explained by a change equivalent to a steepening of the CO2 dissociation curve, in the pulmonary capillaries. The present paper reports some measurements

in man which indicate

that this does not occur.

Methods Measurements were made on the three adult males who had been subjects of the previous study (DENISON et al., 1969). On the morning of an experiment a catheter was introduced into the brachial artery of the subject’s left arm and attached to a manifold of seven syringes. The subject sat on an Elema cycle ergometer and either rested or pedalled against loads of 300, 600 or 900 kpm/min. Before and during each experiment the subject breathed oxygen to keep his expired nitrogen tension below 7 mm Hg. After at least 10 minute open-circuit respiration during rest or exercise at

188

D. DENISON,

R. H. T. EDWARDS,

G. JONES

AND

H. POPE

a constant load, a sample of arterial blood was drawn, then the subject turned a tap to rebreath 3-5 liters of a CO, in 0, mixture for as long as he could manage (1-2 min). During this time a further four to six blood samples were drawn from the brachial artery while respired gas tensions were recorded continuously using an AEI mass spectrometer. The subject then rested for at least 20 min. Experiments were duplicated on each individual at rest and each work load. Each blood sample was drawn into an ungreased heparinized glass syringe containing a stainless steel washer for mixing. The dead-space of the catheter and syringe was flushed with blood prior to sampling and then 2-4 ml of blood was collected, mixed and analyzed. To ease rapid sampling six syringes with stopcocks were arranged in parallel as “piglets” to draw from the catheter which terminated in a larger syringe for flushing the catheter. Samples were drawn over periods that varied from 6 to 14 set and immediately inspected for bubbles. Small bubbles appeared in 3 of the 120 samples and were ejected immediately. None were seen in the other samples. Blood CO, tensions were measured on all samples within 2 min of withdrawal, using a Severinghaus electrode. The CO, contents of the samples drawn during open-circuit respiration and the second sample taken in each rebreathing were measured with a Natelson micro-gasometer. Further details of sampling and calibration procedures together with anthropometric data are given in the previous paper. Results Figure 2 shows an example of the pattern of results seen on both occasions in all three subjects They have been plotted assuming the measured tension represented that existing in blood at the midpoint of sampling and that the lung-arm circulation time was 5 set (JONES et al., 1967). During rebreathing the PcoZ in respired gas rises continuously and consistently exceeds the CO, tension in blood drawn from the brachial artery. The rate of rise and the (a-A) Pco, difference increase with work load but the difference remains constant with time. In fig. 3 the observed (a-A) PcoZ differences are plotted against work load and respired Pco2. In both plots continuous lines show the regression of the CO, gradient on the other variable. The broken lines indicate the regressions of the gradient between respired gas and pulmonary arterial blood seen in these subjects under the same circumstances in the previous study. The present gradients are greater at each work load but smaller at each level of respired Pco2 drawn upstream of the lung. There were no significant The gradients seen in blood drawn downstream of the following equations which agree well with the data of under almost identical circumstances: (A-a) Pco2 = 0.21 (rebreathing =0.013

P&-992.47

(work load kpm/min)+2-&2.0

than those seen in the blood differences between subjects. lung can be expressed by the JONES et al. (1969)’ obtained

mm Hg mm Hg

l The regression quoted in their paper is a misprint which should read: Pcoz = 0.23 (rebreathing PcoL) - 10 & 1.1 mm Hg (JONES, 1970, personal communication)

co,

I

I

0

I

I

I

189

EQUILIBRIUM IN THE LUNG

I

1

I

I 2

1

0

2 min

min

900

kpm lmin

80

80

r 0

1

2 min

0

,

I

I

1

1 min

Fig. 2. Simultaneous estimates of alveolar c,Af and arterial (a) COZ tensions during rebreathing in one subject at rest and working against loads of 300,600 and 900 kpm/min.

Figure 4 shows conventional CO2 dissociation curves for normal oxygenated whole blood in vitro, calculated from the equations of KELMAN and NUNN (1968). The lower right-hand graph indicates how little these are affected by small variations in temperature and haemoglobin content. The other three graphs show the CO, tensions and contents observed in the blood samples drawn from each subject. They have been superjmposed on curves calculated for various degrees of mild metabolic acidosis, allowing for measured deviations in haemoglobin content. The open circles represent values for arterial blood drawn during open-circuit respiration: the closed circles indicate values seen during rebreathing (presumed oxygenated mixed venous values). In all subjects at each level of exertion the arterial and “venous” values fell on a curve normal for blood in vitro at degrees of acidosis consistent with the relative work loads. In summary, arterial blood sampled downstream of the lung from three adult males during rebreathing at rest or on exertion at 300, 600 and 900 kpm/min showed

190

D. DENISON, R. H. T. EDWARDS, G. JONES AND H. POPE

A%02 -

A%02 -

16 -

v

/‘.

50

16 -

100 PAW,

R

600

400

power load &pm

(mm Hg 1

min:

Fig. 3. (a-A) Pco2 gradients observed during rebreathing plotted against work load and respired CO2 tension (98 observations from 3 subjects). Continuous lines show regressions for the present data. Broken lines show regressions obtained from a previous study of pulmonary arterial blood.

60

60

-

cc% 50

-

20

-

D.D.

H

.-2 0-5

mEq

1 &z+

5.

20 -

60

60 R.E. bodes

cC% /):

20

G.J

CC@2

cc02

5oI

20

I

,

I

I

,

I

I 100

50 %

I

I

I

I

I

I

1 100

50 %

Fig. 4. COZ content and tension estimates on arterial blood samples drawn in open circuit respiration (0) and during rebreathing (0) in 3 subjects. Values are superimposed on dissociation curves calculated from the subjects’ haematocrit and the equations of KELMAN and NUNN (1968).

191

CO, EQUILIBRIUMIN THE LUNG CO,

tensions

gas and blood,

O-21 mm Hg lower than in respired which was similar

but not identical

gas. The CO, gradients to those observed

between

between

pul-

monary arterial blood and rebreathed gas, increased with work load but not with time during rebreathing (up to 2 min). The observations confirm and extend those made under very similar conditions by JONES et al. (1969). In addition, simultaneous CO, content measurements, on samples presumed to be representative of normal arterial and oxygenated mixed venous blood, provided tension-content coordinates that fell on normal dissociation curves for whole blood in vitro. Discussion Hesser, Mithoefer, Lanphier and others have drawn attention to the rise in alveolar Pcoz that must accompany the shrinkage of the lungs caused by oxygen uptake during a breathhold. Normally in a lung moderately inflated with air or oxygen at a pressure of one atmosphere, alveolar Pco2 rises faster than the CO, tension in mixed venous blood causing a reversal of CO, flow after 50 set or so. (MITHOEFER, 1965; HESSER, 1965; HESSER, KATSAROS, MATELL, 1968). However in rebreathing the effective lung volume is increased by 3+ to 5 liters and the rate of rise of Pcoz is correspondingly lower. Campbell and his colleague have estimated the immediate storage capacity of the body in man as judged by the rate of rise of PV,oz and found it to be about 0.5 ml/kg/mm Hg at rest and 1.0 ml/kg/mm Hg in exercise. They have shown this rise balances the concentration of alveolar CO, during rebreathing at rest causing the transfer of less than 20 ml of CO, between blood and lungs over a 3 min period. (FOWLE and CAMPBELL, 1964a; FOWLE, MATTHEWS and CAMPBELL, 1964; CLODE, CLARK and CAMPBELL, 1967). In the present experiments the predicted rises in alveolar Pco2 due to concentration were calculated from known (steady-state) oxygen consumption and lung and bag volumes and the rises in PV,,, due to body stores estimated from known CO, excretions and Campbell’s values for body CO, storage capacity. At all levels of exercise the predicted rises of Pcoz on each side of the alveolar membrane were within 3 mm Hg of each other and the observed values throughout the period of measurement. At rest the calculated rise in venous blood equaled that observed in respiration but exceeded that predicted for concentration of alveolar gas alone. If reversal of CO, flow in the lungs occurred at any time, it is unlikely that it exceeded 50 ml/min. The present and previous studies on the same subjects confirm that, in man, during rebreathing, respired gas exhibits a CO, pressure a few to several mm Hg higher than the COz tensions seen in samples drawn from the blood entering or leaving the lung. The carefully argued and beautifully substantiated experiments of GURTNER el al. (1969) provide strong evidence that this is due not to a rise of Pco2 in the pulmonary capillaries but to a pH gradient maintained across the capillary wall by polarised lipoprotein molecules in the vessel walls. If this is true, the CO, tension of pulmonary capillary blood should be the same as that seen in pulmonary arterial blood, a few to several mm Hg lower than that seen in rebreathed gas. Nevertheless, other studies using identical methods of rebreathing and respired gas analysis suggest that the

192

D. DENISON, R. H. T.EDWARDS,G.

rebreathing

values provide

a more appropriate

JONESAND H. POPE estimate

blood (DENISON, 1968, 1970 and fig. I). As the reliability can be questioned, its nature is discussed below.

of the tensions

in capillary

of this indirect

evidence

In healthy subjects, resting or exercising in normal surroundings, arterial oxygen saturations can be predicted, with little error, from end-tidal 0, and CO, tensions, providing the respired oxygen pressure remains above 100 mm Hg. Rebreathing measures of mixed venous oxygen tension, which can be resolved to the nearest I mm Hg, lie within 1 mm Hg of the 0, tensions measured in simultaneously drawn pulmonary arterial blood. If oxygen consumption, end-tidal and rebreathing estimates are made under steady state conditions and arterial and mixed venous oxygen contents derived from standard in vitro 0, dissociation tables, cardiac output can be calculated. Such estimates are highly reproducible, rise linearly with work load and pulse-rate and agree reasonably with the published results of direct Fick and dye dilution studies for similar exercise in the same posture. Under the same experimental circumstances rebreathing estimates of mixed venous CO, tension can also be resolved to the nearest I mm Hg but arterial CO, tensions can be predicted from end.tidal values with less accuracy. Nevertheless, the studies of JONESef ~1. (I 966) using the mass spectrometer employed in the present experiments, give a regression of arterial on end-tidal CO, tensions, Paco2 =Pet,o,-( -0.004 V co2 -0.13f+0.75 mm Hg), that is reasonably confirmed by other investigators (MATELL, 1963; RAHN and FARHI, 1964). If CO, production and end-tidal and rebreathing CO, pressures are measured under steady state conditions at the same time as the oxygen measurements and arterial and mixed-venous CO, contents are derived from Jones’ regression and standard in vitro CO, dissociation tables for whole blood, cardiac output can be estimated independently. Figure 1, which plots 60 pairs of such estimates shows that CO, based values accord with, but slightly exceed, the more reliable 0, derived estimates. It should be noted that the simultaneous estimation of PVoz and PV,,, by rebreathing arrests the flux of 0, and CO, at the same time, obviating problems of concentration or body storage during the period before recirculation. Figure 5 plots 228 estimates of cardiac output by O,-based direct Fick methods during supine exercise in healthy men. The values, taken from the papers of DONALD et al. (,1955), FREEDMAN et al. (1955), DEXTER et al.(1951), BEVEG~RD, HOLMGREN and JONSSON (196O), HOLMGREN, JONSSON and SJOSTRAND (1960) and REEVES et ul. (1961), can be represented Q=6.0

(\joz)+6f

by the regression: 1.1 L/min.

Figure 6a compares these values with regression lines for the two types of measure from fig. 1. The indirect estimates give lower resting values (probably because of the absence of a cardiac catheter), but rise more steeply with exercise. Recently CRUZ, RAHN and FARHI (1969) have presented evidence that the metabolic acidosis of exercise causes a progressive shift of the O2 dissociation curve which would lead to an overestimate of blood flow in moderate and heavy exertion. The heavy line

co,

193

EQUILIBRlUM IN THE LUNG

Fig. 5. Published estimates of cardiac output, measured by an Oz-based direct Fick technique, plotted against oxygen consumption. (228 observations from >30 subjects.) Also shown are the regression line and 95 ‘A confidence limits. &PIUS

30

JO-

6

6 /’ 20-

,’

“IN

“IN

VITRO”

VIVO”

Fig. 6. Comparisons of the results of indirect Fick studies from fig. I with the direct Fick studies of fig. 6. (a) uncorrected comparisons. (b) indirect estimates corrected for in L+XIdissociation curve shifts and for the observed (V-A) PCO~gradient.

in fig. 6b shows the regression line for the 0, based output the correction suggested by CRUZ et ul. : Q(O,)corr. Cruz,

Farhi

=2.8 +0.7 (Qo,)uncorr.

of fig. 1 recalculated

(upright

using

exercise)

and Rahn derived their correction from studies of upright cycling. BEVEGARD et ul. have shown that cardiac output is on average 2.2 L/min higher at all levels of exercise in the supine posture. This would reduce the metabolic acidosis

194

D. DENISON, R. H. T. EDWARDS,

G. JONES AND H. POPE

seen at each level. Until appropriate studies have been made, we propose studies of supine exercise Cruz’s correction should be halved:

that for

Qo, corr. = 1.4+0.85 (Q(0,) uncorr.) . . . (supine exercise) The lower border of the shaded area in fig. 6b shows the regression for the CO,derived outputs of fig. 1 recalculated applying the (V-A) Pcol gradient reported in our the previous paper [PVCo2=5.85+0.78 (P,,, rebr.) mm Hg]. This demonstrates disparity between such estimates and those simultaneous O,-based measures or direct Fick studies. Recently ICHIYANAGI et al. (1969) have confirmed observations by COHEN, BRACKETTand SCHWARTZ (1964), BRACKETT, COHEN and SCHWARTZ (1965), PRYS-ROBERTS,KELMAN and NUNN (1966), MICHEL, LLOYD and CUNNINGHAM(1966), and others that, i11ho,

whole blood

has a flatter CO,

content-tension

dissociation

curve than in vitro, due to redistribution

of HCO, ions in body fluids. If this process, minutes to complete itself, does occur in the lungs, it will inbetween CO, and 0, based estimates of output. The upper area in fig. 6b fits CO,-derived outputs that have been recalthe shift in dissociation curve of oxygenated blood in viva et al. Dependent on the extent to which this occurs in the lung, will lie somewhere within the shaded area shown. The present suggest that in the lungs the dissociation curve shift will be

which requires some crease the discrepancy margin of the shaded culated also applying reported by Brackett CO,-derived estimates content measurements small. Obviously if the P ,-o2 gradient correction is applied, CO,-based estimates exceed and disagree with simultaneous O,-derived values and with published data. It is possible that this discrepancy reflects an error in the estimates of arterial rather than mixed venous CO, tensions but particularly in heavy exercise this would demand arterial CO, tensions up to 10 mm Hg lower than those reported in a host of careful exercise studies. We believe that although this evidence is indirect the observed discrepancies in cardiac output estimates are far too large to be accounted for by errors in assumption of the gas tensions in arterial blood or in the measurement of metabolic gas exchange. If the observed Pco, gradients actually exist across the pulmonary exchange membrane, the CO, dissociation curve in uiuo must be very much steeper than current views admit although the present content estimates refute this. If conversely CO, equilibration does occur between alveolar gas and blood in the pulmonary capillary, a mechanism

has to be found to oppose the one predicted by the studies of Gurtner that the lipoprotein lining layer of alveoli could provide this. We would have preferred to present the results of comparison between simultaneous direct and indirect Fick studies in the same subjects, however undesirable complications occurred in all three subjects in the previous experiments. The less satisfactory arguments from indirect comparisons are recounted for this reason. At present we can offer no explanation for the contradiction they describe. Differences between upstream and downstream estimates of the PcoZ g radient (figs. 3a, 3b) are probably related to the level of ~~~~~ and the continued excretion of CO2 in the downstream studies. et al. It is possible

Co, EQUILIBRIUM IN THE LUNG

195

Acknowledgement We

are grateful

for exceptional

JUNIPER and ROSEMARY

technical

help

from

PRISCA CHANG,

ELIZABETH

CHALMERS.

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JONES, N.

L., G. J. R. MCHARDY, A. NAIMARKand E. 5. M. CAMPBELL(1966). Physiological dead space and alveolar-arterial gas pressure differences during exercise. C/in. Sri. 31 : 19-29. JONES,N. L., E. J. M. CAMPBELL,G. J. R. MCHARDY,B. HIGGS and M. CLODE(1967).The estimation of carbon dioxide pressure of mixed venous blood during exercise. C/in. Sri. 32: 311-327. JONES,N. L., E. J. M. CAMPBELL,R. H. T. EDWARDSand W. G. WILKOFF(1969). Alveolar to blood Pco, difference during rebreathing in exercise. J. Appl. Physiol. 27: 356-360. KELMAN,G. R. and J. F. NUNN (1968). Computer produced Physiological Tables. London, Butterworths. MATELL,G. (1963). Time courses of changes in ventilation and arterial gas tensions in man induced by moderate exercise. Acta Physiol. Stand. 58 (Suppl. 206): l-53. MICHEL,C. C., B. B. LLOYDand D. J. C. CUNNIXGHAM(1966). The in oiuo carbon dioxide dissociation curve of true plasma. Respir. Physiol. 1 : 12 I-l 37. MITHOEFER,J. C. (1965). Breathholding. In: Handbook of Physiology. Section 3. Respiration. Vol. II, edited by W. 0. Fenn and H. Rahn. Washington, D.C., American Physiological Society, pp. 1011~1025. PRYS-ROBERTS, C., G. R. KELMANand J. F. NUNN (1966). Determination of the in zv’cocarbon dioxide titration curve of anaesthetized man. Brit. J. Anaesth. 38: 500-509. RAHN, H. and L. E. FARHI (1964). Arterial-alveolar COZ difference. In: Handbook of Physiology. Section 3. Respiration. Vol. I, edited by W. 0. Fenn and H. Rahn. Washington D.C., American Physiological Society, pp. 751-754. REEVES, J. T., R. F. GROVER, G. F. FILLEY and S. G. BOUNT (1961). Circulatory changes in man during mild supine exercise. J. Appl. Physiol. 16: 279-282.