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

Estimation of mixed venous PCO2 for determination of cardiac output in children

STUDY OBJECTIVES

Cardiac output (Q) can be estimated noninvasively during exercise by employing CO2-rebreathing techniques (equilibrium and exponential) to estimate the oxygenated mixed venous PCO2 (PvCO2). It has been found in adults and children that the equilibrium method underestimates Q as a result of overestimation of PvCO2, unless PvCO2 is "downstream corrected." In adults, it has been found that the exponential method does not require downstream correction and yields values similar to those obtained by the equilibrium method with downstream correction. The objectives of this study were as follows: to test whether the exponential method gives similar results to the equilibrium method with downstream correction in children; to verify that downstream correction is required in children; and to test whether a single equation could be used in adults and children to predict Q from oxygen consumption (VO2).

DESIGN

Descriptive.

SETTING

Exercise laboratory of a university hospital.

PARTICIPANTS

23 children (16 boys, 7 girls) with a mean age of 11.0 +/- 1.9 years (7.1 to 13.9 years), and 12 adults (7 men, 5 women) with a mean age of 33.6 +/- 7.2 years (24 to 48 years).

INTERVENTIONS

While performing steady-state exercise on an ergometer, PvCO2 was determined in 14 children using both the equilibrium and exponential methods, and in all other subjects using the equilibrium method alone.

MEASUREMENTS AND RESULTS

For the 14 children who underwent testing by both the equilibrium and exponential methods, the uncorrected equilibrium PvCO2 was significantly different from both the corrected PvCO2 and the exponential PvCO2. We found a strong relationship between Q (L/min), calculated using the downstream corrected values of PvCO2, and VO2 (L/min) (r2 = 0.95), and this relationship was similar to that obtained by dye dilution in other studies. When weight was included, it was determined that one equation could be used for children and adults: Q (L/min) = 1.42 + 5.80.VO2 (L/min) + 0.06.wt (kg), r2 = 0.97, SEY = 0.67.

CONCLUSIONS

CO2-rebreathing techniques can be used to determine Q in children; the exponential method gives values that are similar to the equilibrium method with the downstream correction; and one prediction can be used for Q in adults and children.

Dynamic control of breathing during exercise and hypercapnia

The dynamic influences of end-tidal CO2 and exercise on ventilation are compared when CO2 and exercise are imposed separately and when they are imposed simultaneously. Five human subjects are studied. The subjects performed three trials: random work rate forcing, random CO2 inhalation and their simultaneous loading. The work rate was varied between 20 and 80 W as a pseudorandom binary sequence. The concentration of inspired CO2 was varied randomly between 0 and 7 per cent, adjusted so that it produced approximately the same amount of ventilatory fluctuations as the random work load. The relative contribution of each variable was analysed using multivariate autoregressive analysis at frequencies ranging from 0.1 to 1 cycle min-1. The results show that the dynamics of the response to CO2 inhalation, exercise and their combination are nonlinear and that the combination of CO2 inhalation and exercise magnifies the nonlinear behaviour. Ventilation is largely unaffected by either work rate or end-tidal CO2 at 1 cycle min-1. During simultaneous CO2 and work rate forcing, ventilation tends to follow the change in the end-tidal CO2.

Rebreathing method for the simultaneous measurement of oxygen consumption and effective pulmonary blood flow during exercise

This paper describes a rebreathing method for the simultaneous measurement of oxygen consumption (VO2) and effective pulmonary blood flow (QP. eff) at rest and during exercise. Subjects rebreathed a test gas consisting of 35% oxygen, 3.5% chlorodifluoromethane (freon-22), and 10% argon in nitrogen for 30 seconds or until the respired oxygen tension fell to below 13.3 kPa. Sixty normal subjects were studied on a motorized treadmill, the Bruce protocol being used. The rebreathing manoeuvre was performed at three minute intervals, and was initially practised sitting down. Measurements were then made with the subjects standing at rest, and subsequently during the last minute of each stage of the Bruce exercise protocol until the subjects were exhausted. Heart rate was recorded from the electrocardiogram. Oxygen uptake plotted against calculated power (watts) showed a discontinuity between resting and exercise values, probably because power output during treadmill exercise is underestimated. The arbitrary addition of 30 watts to the exercise power output abolished this discontinuity. There was good agreement between rebreathing estimates of oxygen consumption and values measured during a second exercise test by the conventional open circuit argon dilution method. Coefficients of variation of oxygen consumption and effective pulmonary blood flow measured by rebreathing were usually less than 10% even during maximal exertion. At rest mean (SD) effective pulmonary blood flow corrected for body surface area was 2.2 (0.46) l/min/m2. Effective pulmonary blood flow rose linearly with oxygen consumption. At rest the arteriovenous oxygen content difference for pulmonary blood (VO2/QP eff) was 9.1 (1.6) ml/dl, rising to a maximum of 16.4 (1.8) ml/dl. The stroke volume index was 27.5 (6.8) ml/m2, rising to a maximum of 46.5 (7.1) ml/m2 during exertion.

Measurement of mixed venous carbon dioxide pressure by rebreathing during exercise

This study compared rebreathing methods currently used for the measurement of mixed venous PCO2 (PVCO2) during exercise. Four mathematical procedures were used to derive the asymptote of the exponential rise in end-tidal PCO2 during 15 sec rebreathing of low (2-6%) CO2 mixtures ('exponential' methods); the derived PVCO2 was compared to that obtained with an 'equilibrium' method, in which high CO2 mixtures were rebreathed to obtain an equilibrium of CO2 between the lungs and rebreathing bag. Precision was established by analysing duplicate rebreathings at each power output. The most precise of the four exponential procedures (coefficient of determination 0.98) used iterative statistical analysis to obtain an equation for the best fit curve of end-tidal PCO2 with time (t), solved for t = 20 sec. The PVCO2 derived by this procedure was similar to that obtained by the equilibrium method (r = 0.96), and both yielded estimates of cardiac output that were within the previously published expected range. Variations in the initial rebreathing bag volume and CO2 concentration, and in the breathing frequency during rebreathing had little effect on the derived PVCO2.

Changes in partial pressure of carbon dioxide with time in carotid arterial blood in cats

1. A rapidly responding CO2 sensor and cuvette were placed in the carotid artery of anaesthetized cats and changes of PCO2 in post-pulmonary capillary blood were recorded when flow through the cuvette was suddenly stopped. 2. Under control conditions when the cats breathed spontaneously or were ventilated artificially, stop-flow caused Pa, CO2 to decay by 2--5 mmHg reaching a new equilibrium in 10--15 sec. The amount by which CO2 decayed was reduced by the inhalation of high O2. The decay was enhanced by hypoxia, the inhalation or infusion of CO2. It was reversed by the administration of acetazolamide: now with stop-flow, Pa, CO2 rose by 8--15 mmHg. 3. the mechanism of this decay is uncertain. We propose that it is due to the transfer of CO2 from plasma to red cells in post-capillary blood in response to the reduction of [H+]i as H+ binds to Hb following the oxylabile release of CO2 from Hb.

Blood/gas equilibrium of carbon dioxide in lungs. A critical review

(1) The scope of this review is to examine the experimental evidence for the existence of negative PCO2 differences between pulmonary capillary blood and lung gas, [delta PCO2(b-G)], which have been observed both during rebreathing, when CO2 was at equilibrium, and during steady state gas exchange, particularly in hypercapnia. (2) The mechanism that have been invoked to explain negative delta PCO2(b-G) include (i) slow equilibration of the system CO2/HCO3-/H+ in blood, and (ii) effects of a negatvely charged surface of the pulmonary capillary endothelium. While the first postulated mechanism appears to be quantitatively insufficient to explain the results, the second seems to lead to serious qualitative difficulties. (3) Existence of negative delta PCO2(b-G) in CO2 equilibrium would invalidate the basis of the conventional analysis of alveolar gas exchange. (4) A critical analysis of the experimental evidence for the existence of negative delta PCO2(b-G) is presented. It includes the identification of directional experimental errors leading to spurious negative delta PCO2(b-G), and a critical review of the literature data in this regard. (5) Results of own experiments, conducted in an attempt to consider all possible sources of error, are reported, revealing (i) perfect PCO2 equality between alveolar gas and blood in rebreathing equilibrium of CO2; (ii) absence of negative delta PCO2 (b-G) during steady state gas exchange in hypercapnia. (6)Both experiments and model calculations show that negative delta PCO2 between mixed venous blood and end-expired gas observed in birds at steady state of gas exchange are explained by a particular action of the Haldane effect in avian parabronchial lungs with cross-current arrangement of gas and blood flow. (7) It is concluded that the negative delta PCO2(b-G) reported in the literature are probably artifactual and that there is no adequate evidence to invalidate the traditional view according to which blood/gas CO2 equilibration in lungs leads to equal PCO2 in both media.

CO2 dissociation curves of oxygenated whole blood obtained at rest and in exercise

In vitro CO2 dissociation curves for oxygenated whole blood were determined in 19 healthy male subjects at rest and during submaximal and maximal bicycle work. Hemoglobin concentration and blood lactate increased with increasing work load and accordingly buffer value of the whole blood increased while bicarbonate and Base Excess (BE) decreased, resulting in a downward shift of the CO2 dissociation curve during exercise. Despite the marked increase in buffer values of the blood, the slopes of the CO2 dissociation curves during exercise were found to be about the same as those obtained at rest. It was inferred that the increasing effect of increased buffer value, on the dissociation slope, was essentially compensated by the decreasing effect of diminished bicarbonate content. The advantages of this relatively constant CO2 dissociation slope for the indirect measurement of cardiac output by the Fick principle are discussed.

Comparison of methods to calculate cardiac output using the CO2 rebreathing method

A comparison was made of methods used to calculate cardiac output by the indirect (CO2) Fick procedure (equilibrium method). Alternative methods for calculation of arterial PCO2, mixed venous PCO2, and conversion of gas tension to content were tested. Cardiac output values determined with a "corrected" equilibrium PCO2, to approximate mixed venous PCO2, were observed to be closest to cardiac output values determined on similar populations by the dye dilution method. Arterial PCO2 was best estimated from the Bohr equation using a dead space in exercise from prediction equations in the literature applicable to the populations under study. CO2 dissociation curves used to derive the veno-arterial CO2 content difference, were shown to differ considerably. For the present, the curve by McHardy [25], as modified by Jones (personal communication), and similar to the standard Comroe curve [5], was chosen.

Human cardiorespiratory responses to acute cold exposure

1. Respiratory and circulatory functions of minimally clad human subjects were studied before and during acute exposure to ambient temperatures of 4.5-6.5 degrees C. 2. After 1 h of cold exposure, subjects showed increases of ventilation, O2 UPTAKE AND CARDIAC OUTPUT. Rectal temperatures fell. 3. During exercise in the cold conditions, oxygen uptake and cardiac output were greater than during the same exercise at normal temperature. 4. The increased cardiac output during cold exposure was achieved by an increase of stroke volume rather than heart rate; this finding is in contrast to changes during bicycle exercise and isometric exercise at normal ambient temperatures. 5. We conclude that the cardiorespiratory effects of cold exposure are not superseded by the response to moderate exercise. The difference between heart rate and stroke volume at increased levels of cardiac output during exercise at normal temperatures and during rest and exercise in cold conditions may be explained by changes of arterial baroreceptor input and of blood catecholamine levels.