Time domain, geometrical and frequency domain analysis of cardiac vagal outflow: effects of various respiratory patterns

The purpose of this study was to compare the applicability of four differen t measures of heart rate variability (HRV) in the assessment of cardiac vag al outflow, with special reference to the effect of breathing pattern. The anticholinergic effects of an intravenous glycopyrrolate infusion (5 mug kg (-1) h(-1) for 2 h) during spontaneous and controlled (15 min(-1)) breathin g rate were investigated in eight volunteers, and the effects of different fixed breathing rates (6-15-24 min(-1)) and hyperventilation in 12 subjects . Cardiac vagal activity was assessed by ECG recordings in which the follow ing measures of HRV were computed: the high-frequency (HF) spectral compone nt, the instantaneous RR interval (RRI) variability (SD1) analysed from the Poincare plots, the percentage of differences between successive RRIs grea ter than 50 ms (pNN50), and the square root of the mean squared differences of successive RRIs (RMSSD). On average, glycopyrrolate reduced the HF spec tral component by 99.8%, SD1 by 91.3%, pNN50 by 100% and RMSSD by 97.0%. Th e change of breathing pattern from controlled to spontaneous decreased sign ificantly the HF component and pNN50, but did not affect SD1 or RMSSD. Rapi d breathing rate (24 min(-1)) decreased the HF component, but had no effect s on the other measures. A controlled breathing rate is needed for a reliab le assessment of cardiac vagal outflow by the spectral analysis technique. The quantitative geometrical analysis of short-term RRI variability from th e Poincare plots and the time domain measure RMSSD were not significantly a ffected by changes in the breathing rate, suggesting that these indices are more suitable for the measurement of cardiac vagal outflow during the 'fre e-running' ambulatory conditions.