Analysis and interpretation of turning points and run lengths in breath-by-breath ventilatory variables

Three healthy young male volunteers were studied for 10 min periods of steady-state ventilation under the following conditions: air breathing, 50% inspired oxygen, hyperoxic hypercapnia and hypoxic hypercapnia. Eight variables were recorded breath-by-breath: inspiratory and expiratory tidal volumes, mean flows and durations, total breath duration and expiratory ventilation. Each resulting sequence of breath-by-breath data was analysed in terms of the number of 'turning points' in the sequence and the concomitant mean 'run length' between turning points. The mean run length for all these variables was 1.600, 1.599 and 1.592 in the three subjects. These values are significantly greater than the expected value of 1.5 for random data sequences. There were no consistent significant differences between variables or subjects. Under conditions of hypoxic hypocapnia the mean run length of all variables was found to be 1.666, 1.724 and 1.568 in the three subjects. The two higher values corresponded with periodic breathing of period 10-30 s. These observations are lower than previously published values. A number of ways of interpreting a mean run length of greater than 1.5 are suggested.