Non-invasive prospective targeting of arterial PCO2 in subjects at rest

Accurate measurements of arterial P-CO2 (P-a,P-CO2) currently require blood sampling because the end-tidal P-CO2 (P-ET,P-CO2) of the expired gas often does not accurately reflect the mean alveolar P-CO2 and P-a,P-CO2. Differences between P-ET,P-CO2 and P-a,P-CO2 result from regional inhomogeneities in perfusion and gas exchange. We hypothesized that breathing via a sequential gas delivery circuit would reduce these inhomogeneities sufficiently to allow accurate prediction of P-a,P-CO2 from P-ET,P-CO2. We tested this hypothesis in five healthy middle-aged men by comparing their P-ET,P-CO2 values with P-a,P-CO2 values at various combinations of P-ET,P-CO2 (between 35 and 50 mmHg), P-O2 (between 70 and 300 mmHg), and breathing frequencies (f; between 6 and 24 breaths min(-1)). Once each individual was in a steady state, P-a,P-CO2 was collected in duplicate by consecutive blood samples to assess its repeatability. The difference between P-ET,P-CO2 and average P-a,P-CO2 was 0.5 +/- 1.7 mmHg (P = 0.53; 95% CI -2.8, 3.8 mmHg) whereas the mean difference between the two measurements of P-a,P-CO2 was -0.1 +/- 1.6 mmHg (95% CI -3.7, 2.6 mmHg). Repeated measures ANOVAs revealed no significant differences between P-ET,P-CO2 and P-a,P-CO2 over the ranges of P-O2, f and target P-ET,P-CO2. We conclude that when breathing via a sequential gas delivery circuit, P-ET,P-CO2 provides as accurate a measurement of P-a,P-CO2 as the actual analysis of arterial blood.