Reproducibility of Data on Steady-State Gas Exchange and Indices of Maldistribution of Ventilation and Blood Flow

Abstract

The reproducibility of data on steady-state gas exchange and arterial blood gas levels was examined in 45 patients with similar pathophysiologic abnormalities (idiopathic scoliosis) by comparing the data obtained during two sequential periods breathing air. Comparison in all patients of 12 variables by paired t-test demonstrated statistically significant differences in the following three variables: the ratio of physiologic dead space to tidal volume (Vd/TV), the alveolar-arterial oxygen pressure difference (P[A-a]O2), and the arterial oxygen tension. By excluding four patients whose respiratory quotient was greater than two standard deviations from the mean value and one patient with a marked change in the pattern of ventilation, only the Vd/TV was significantly different (P less than 0.05). The range of the coefficients of variation was wide. The highest, which was for P(A-a)O2, was 18.8 percent; the majority were in range of 6 to 13 percent (minute ventilation, TV, frequency, oxygen consumption and carbon dioxide production, respiratory quotient, Vd/TV, dead space, and alveolar ventilation), and the coefficients of variation of partial pressure of oxygen and carbon dioxide in the arterial blood were, respectively, 3.6 and 3.4 percent.