Fig. 3

Pulmonary gas exchange panels

Panel 4: The relationships of minute ventilation (\({\dot{{\mathbf{V}}}\mathbf{E}}\)) vs. O₂ uptake (\({\dot{{\mathbf{V}}}\mathbf{O}}_{2}\)) and vs. CO₂ (\({\dot{{\mathbf{V}}}\mathbf{CO}}_{2}\)) output (ventilatory equivalents) as a function of time. The ventilatory equivalents EqO₂ ≈ \({\dot{\text{V}}\text{E}}/{\dot{\text{V}}\text{O}}_{2}\) and EqCO₂ ≈ \({\dot{\text{V}}\text{E}}/{\dot{\text{V}}\text{CO}}_{2}\) indicate how many litres must be ventilated to take up 1 L of O₂ or exhale 1 L of CO₂ (gas exchange efficiency). The same information is found in panel 6 in a linear presentation. The lower the equivalent values, the more effective the gas exchange or work of breathing, and vice versa. Excess \({\dot{\text{V}}\text{E}}\) vs. \({\dot{\text{V}}\text{O}}_{2}\) and \({\dot{\text{V}}\text{CO}}_{2}\) occurs due to augmented ventilatory drive (nonspecific hyperventilation), metabolic acidosis (compensatory hyperventilation) and/or V/Q mismatch (true ventilatory inefficiency). Additional possibility of determining the AT. AT corresponds to the lowest point (nadir) of EqO₂ directly before EqO₂ continuously increases (provided EqCO₂ does not increase simultaneously).

Analysis (target values and response kinetics): Physiological decrease in EqO₂ and EqCO₂ from rest to AT? Significantly elevated EqO₂ and EqCO₂ values at rest or during exercise? Significantly decreased EqO₂ and EqCO₂ values (indicates alveolar hypoventilation)? AT within predicted values or reduced? Cross-validate with panels 3, 7 (3-panel view).

Panel 6: The relationship of ventilation (\({\dot{{\mathbf{V}}}\mathbf{E}}\)) and CO₂ production (\({\dot{{\mathbf{V}}}\mathbf{CO}}_{2}\)): \({\dot{\mathbf{V}}\mathbf{E}}/{\dot{\mathbf{V}}\mathbf{CO}}_{2}\) slope. The \({\dot{\text{V}}\text{E}}/{\dot{\text{V}}\text{CO}}_{2}\) slope is a measure of ventilatory (gas exchange) efficiency at submaximal exercise. The same information can be found in panel 4 (EqCO₂ ≈ \({\dot{\text{V}}\text{E}}/{\dot{\text{V}}\text{CO}}_{2}\)) in a nonlinear presentation, but values are not identical [21]. The \({\dot{\text{V}}\text{E}}/{\dot{\text{V}}\text{CO}}_{2}\) slope is a prognostic indicator in CHF.

Analysis (target values and response kinetics): \({\dot{\text{V}}\text{E}}/{\dot{\text{V}}\text{CO}}_{2}\) slope within normal range (preserved V/Q matching)? Steep increase in the \({\dot{\text{V}}\text{E}}/{\dot{\text{V}}\text{CO}}_{2}\) slope indicative of significant V/Q mismatching (\({\dot{\text{V}}\text{E}}/{\dot{\text{V}}\text{CO}}_{2}\) slope ≥ 39 [27]) and/or nonspecific/compensatory hyperventilation (which is usually paralleled by ↑PETO₂ and ↓PETCO₂)? Initial sharp increase in the \({\dot{\text{V}}\text{E}}/{\dot{\text{V}}\text{CO}}_{2}\) slope that levels off with increasing work rate (suggestive of psychogenic hyperventilation)? Decrease in the \({\dot{\text{V}}\text{E}}/{\dot{\text{V}}\text{CO}}_{2}\) slope indicates alveolar hypoventilation.

Panel 7: End-tidal partial pressures of O₂ (P_ETO₂) and CO₂ (P_ETCO₂) vs. time. Indirect measure of pulmonary gas exchange and V/Q mismatch. The more pronounced the ventilation, the lower the P_ETCO₂ and the higher the P_ETO₂, and vice versa in normal lungs. (Note: P_ETCO₂ ≠ PaCO₂. P_ETCO₂ > PaCO₂ during exercise (approx. 4 mmHg); at rest: P_ETCO₂ < PaCO₂ (approx. 2 mm Hg). Additional possibility of determining the AT. AT corresponds to the lowest point (nadir) of P_ETO₂ directly before P_ETO₂ continuously increases (provided P_ETCO₂ remains constant).

Analysis (target values and response kinetics): Physiological course of P_ETO₂ and P_ETCO₂ at rest and during exercise? Cross-check with BGA. AT within predicted values or reduced? Cross-check with panels 3, 4 (3-panel view). Decrease in P_ETO₂ (indicates exercise-induced hypoxaemia) or abrupt increase at start of exercise (may indicate R-L-shunt or nonspecific hyperventilation)? Significant drop in P_ETCO₂ during exercise (suggests V/Q mismatch and/or hyperventilation)? Significant increase in P_ETCO₂ during exercise (indicates alveolar hypoventilation, e.g., severe COPD, obesity hypoventilation syndrome, neuromuscular disease)?

Note: the determination of P(A-a)O₂ or P(a-ET)CO₂ more sensitively and reliably identifies and quantifies low or high V/Q regions (and/or a R-L-shunt) than end-tidal partial pressures