Temporal variation in the VT-TI relationship in humans

The variation with time of the relationship between tidal volume (VT) and inspiratory duration (TI) was assessed by analysis of 34 breathing sequences, nominally of 300 breath duration, during eupnea and hypercapnic hyperpnea in 6 human subjects. Two approaches were used: (1) each sequence was divided into consecutive 50-breath blocks and standard regression techniques used to characterize VT as a function of TI; 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 greater than 0.05) using the clustering approach; in 27 experiments, it was indicated that the regression slope estimates of VT on TI should not be considered homogeneous. Changes in the VT-TI slope were not correlated with changes in mean levels of VT, TI, minute ventilation (V), mean inspiratory flow (VT/TI), nor alveolar PCO2 (PACO2). Thus it is apparent that the VT-TI relation cannot be considered temporally invariant, but changes with time over periods ranging from less than 20 to more than 100 breaths.