Temporal variation in the V t-T i relationship in humans

Abstract

The variation with time of the relationship between tidal volume (V t) and inspiratory duration (T i) was assessed by analysis of 34 breathing sequences, nominally of 300 breath duration, during eupnea and hypercapnic hypernea in 6 human subjects. Two approaches were used: (1) each sequence was delivered into consecutive 50-breath and standard regression techniques used to characterize V t as a function of T i; and (2) a piecewise linear regression technique was applied to cluster consecutive breaths into regression regimes. Analysis of covariance was used with the results of both approaches to determine the likelihood that a single regression line was adequate to describe the entire data set. Similar results obtained regardless of the approach used. In only 4 of the 34 experiments would the hypothesis of regression slope homogeneity be accepted ( p > 0.05) using the clustering approach; in 27 experiments, it was indicated that the regression slope estimates of V t and T i should not be considered homogeneous. Changes in the V t-T i slope were not correlated with changes in mean levels of V t, T i, minute ventilation (V̇, mean inspiratory flow (V t/T i), nor alveolar P CO 2 (P a CO 2 ). Thus it is apparent that the V t-T i relation cannot be considered temporally invariant, but changes with time over periods ranging from less than 20 to more than 100 breaths.