Estimates of mean alveolar PCO2 during steady-state exercise in man: a theoretical study

The Role of CO2 on Respiration and Metabolism During Hypercapnic and Normocapnic Recovery From Exercise

High intensity exercise can lead to depletion of CO₂ from the body (hypocapnia). This disturbance becomes more noticeable during recovery or between seasons of intermittent exercise, putting the subject in a neural fatigue state. Objectives: A possible hypothesis to address this condition would be to provide high CO₂ mixtures (hypercapnic) during the recovery period from exercise in order to relieve hypocapnia. Methods: Eight men (23.8 ± 1.2 yrs, VO_2max = 45 ± 1.9 ml▪kg-1▪min-1) performed cycling exercise at 80%VO_2max for 6-7 min. During recovery (23 min) they inhaled hypercapnic air (EXP-21%O2, 3%CO₂, and 76%N₂) or normal air (CON-21%O₂, 0.003%CO₂, and 79%N₂). Respiratory parameters were collected with open spirometry and heart rate was measured. Results: Exercise caused mild hypocapnia {9.9 mmHg drop of CO₂ end-expiratory partial pressure (P_ETCO₂)} in CON condition after exercise (p < .005). P_ETCO₂ elevated close to the rest values during the three hypercapnic phases in EXP condition (main effect of condition p < .001 between EXP and CON), but after hypercapnic breathing it returned to hypocapnia similarly with CON. The ventilatory response (V_E▪P_ETCO₂^-1) and the exhaled volume of CO₂ (VCO₂) progressively increased during and also after ventilatory manipulations in EXP compared to CON condition (V_E▪P_ETCO₂^-1: post hoc p < .001, VCO₂: pVCO₂: p < .05-.001), and VO₂ became lower after the end of second hypercapnic manipulation (p < .05 between EXP and CON). Conclusion: It seems that hypercapnic breathing after exercise is not a good strategy to reverse exercise hypocapnia, because of great hyperventilation caused by CO₂ and exercise mechanisms during the recovery period leading to increased CO₂ removal from body. This intervention may also decrease O₂ supply and muscles blood flow.

Evaluation of cardiac output from a tidally ventilated homogeneous lung model

We used the direct Fick measurements to validate a method for estimating cardiac output by iteratively fitting VCO(2) at the mouth to lung model values. This model was run using a series of 50, 30 and 10 breaths to test sensitivity to number of breaths used for fitting. The lung was treated as a catenary two-compartment lung model consisting of a dead space compartment connected with a single alveolar space compartment, perfused with constant pulmonary blood flow. The implemented mathematical modeling described variations in O(2) and CO(2) compartmental fractions and alveolar volume. This model also included pulmonary capillary gas exchange. Experimental data were collected from measurements performed on six healthy subjects at rest and during 20, 40, 60 and 85-90% of peak V(O)(2). The correlation between the two methods was highest and the average agreement between the methods was best using 50 breaths R = 095; P < 0.0001; Q(model) = 1.1Q(Fick) - 2.3). The mean difference and lower to upper limits of agreement between measured and estimated data were 0.7 l/min (-2.7 to 4.1 l/min) for cardiac output; -0.9 ml/100 ml (-1.3 to -0.5 ml/100 ml) for arterial O(2) content; -0.8 ml/100 ml (-3.8 to 2.2 ml/100 ml) for mixed venous O(2) content and -0.1 ml/100 ml (-2.9 to 2.7 ml/100 ml) for arteriovenous difference O(2) content. The cardiac output estimated by the lung model was in good agreement with the direct Fick measurements in young healthy subjects.

Assessment of arterial gas pressures and cardiac output using a breathing lung model

PURPOSE

The aim of this investigation was to evaluate a breathing lung model to estimate arterial gas partial pressures and cardiac output at rest and during exercise.

METHODS

A mathematical model was used to describe variations in gas fractions, alveolar volume, and gas exchange in the pulmonary capillaries during the breathing cycle. Experimental data were obtained from 17 healthy subjects at rest and during exercise at 40, 50, 65, and 75% VO(2max) on a cycle ergometer. VO(2), VCO(2), and P(ET,CO2) were monitored continuously with a MedGraphics CPX/D gas exchange system. Arterial gases were measured in brachial artery blood samples drawn simultaneously with gas exchange. Cardiac output was measured using the CO(2) rebreathing method corrected by the arterial blood sample data. The model parameters including cardiac output, end-expiratory alveolar volume, and mixed-venous gas contents were estimated by fitting modelVCO(2) to experimental values over 50 breaths.

RESULTS

The fit of model parameters gave arterial gas partial pressures not significantly different from measured data. Measured P(a,C02) and P(a,O2) were significantly correlated with model outputs (R(2) = 0.991 for P(a,CO2) and R(2) = 0.999 for P(a, CO2); P < 0.0001). The cardiac output values estimated using the breathing lung model were significantly correlated with the values obtained with the corrected CO rebreathing method (R(2) = 0.71; P < 0.0001). There was, however, a significant 2.3 L x min(-1) difference between these two methods.

CONCLUSION

Results obtained with the proposed method were in good agreement with measured arterial gas partial pressures. Despite a certain degree of bias, the promising results for cardiac output demonstrate the reliability of this method that should be now evaluated using a gold standard method.

Modeling of end-tidal and arterial PCO2 gradient: comparison with experimental data

PURPOSE

The aim of this study was to test whether a tidally ventilated homogeneous lung model can correctly describe arterial and end-tidal gas partial pressures and thus the difference in end-tidal and arterial gas partial pressures at rest and during exercise.

METHODS

The implemented mathematical modeling described variations during the breathing cycle in CO2 and O2 fractions, alveolar volume, and pulmonary capillary gas exchange. Experimental data were obtained from measurements performed by 17 healthy subjects at rest and during 40, 50, 65, and 75% exercise .VO(2max) on a cycle ergometer. VO2, VCO2, and PET,CO2 were continuously measured using the MedGraphics CPX/D gas exchange system. Arterial gases were measured in brachial artery blood samples drawn simultaneously with gas exchange. Cardiac output was measured using the CO2 rebreathing method corrected by the blood sample data. The model was driven using experimental data for ventilation, VO2, VCO2, and cardiac output.

RESULTS

The mean difference and the upper and lower limits of agreement between measured and simulated data were -0.004, +0.84, and -0.84 Torr for Pa,CO2; -0.06, +0.64, and -0.76 Torr for Pa,O2; -1.96, +2.84, and -6.76 Torr for PET,CO2; and +7.20, +25.80, and -11.40 Torr for PET,O2. Actual PET,CO2-Pa,CO2 difference increased significantly with workload (P < 0.0001) from 0.3 +/- 3 Torr at rest to 4.7 +/- 2.5 Torr at 75% .VO(2max). Model-simulated PET,CO2-Pa,CO2 difference also increased significantly with exercise (P < 0.0001) from 0.7 +/- 1.7 Torr at rest to 9.1 +/- 3.4 Torr at 75% .VO(2max).

CONCLUSION

The lung model described actual arterial CO2 partial pressures better than variations in end-tidal CO2 partial pressures and thus better than the gradient in end-tidal arterial CO2 partial pressures.

Effect of external chest wall oscillation on gas exchange in healthy subjects

Effective gas exchange can be maintained in animals without the need for endotracheal intubation using external chest wall oscillation (ECWO). The clinical application of this technique has been limited by equipment which was either impractical or uncomfortable. We evaluated a prototype of a new oscillator in which an oscillatory profile of negative and positive pressure was imposed on a negative baseline pressure within a cuirass. In seven healthy subjects, we identified an oscillatory cuirass pressure that could effectively ventilate but would not result in severe hypocapnia over 5 min. We then measured the influence of changing the frequency of oscillation (fo) on PaCO2 and spontaneous ventilation. Lastly, we evaluated the capability of this prototype to achieve targeted changes in chamber pressure. Subjects were ventilated with an inspiratory chamber pressure of -20 +/- 4 cm H2O, an expiratory chamber pressure of 5 cm H2O and an inspiratory-expiratory ratio of 1:1 at 9 oscillatory frequencies (fo: 1 to 5 Hz at 0.5-Hz increments). Each subject was ventilated for 5 min with consecutive periods of ECWO being separated from each other by 10 min of unassisted breathing. Oscillatory tidal volume (Vo) was sampled and PaCO2 was determined from the expired carbon dioxide concentration (FECO2) measured at the mouth. The change in PaCO2 (delta PaCO2) was the difference in PaCO2 immediately before and after ECWO. We found that delta PaCO2 and Vo were inversely related to fo. At 1 Hz the delta PaCO2 was -13 +/- 1 mm Hg and Vo was 344 +/- 34 mL in the absence of spontaneous breathing (fb = 0). At 3 Hz and above, at the chamber pressures used, the delta PaCO2 was small (-1 to -2 mm Hg) and the Vo was less than the predicted dead space. Subjects breathed spontaneously but at a frequency below that of their resting fb. With this prototype, chamber pressure changes up to 30 cm H2O could be accurately achieved at 1, 2.5, and 4 Hz. In conclusion, ECWO can provide effective ventilation among healthy adults in the presence or absence of spontaneous breathing, and further studies are warranted to explore its effectiveness in a variety of clinical circumstances.