[Influence of the dead space induced by the face mask on the measure of heart rate variability].

The respiratory pattern influences the power spectrum of heart rate variability and therefore control or monitoring of respiration is needed. The effect of the dead space induced by the face mask used in the expiratory gas exchange analysis on the measurement of heart rate variability was evaluated in 22 subjects (15 males and 7 females aged 39-76 years, mean 42 years) using electrocardiography recorded for 5 min during spontaneous respiration under the conditions of supine rest, sitting on the bicycle ergometer with and without a face mask. The power spectrum of heart rate variability was obtained by the coarse-graining spectral analysis (CGSA) method and the total power (TP : 0-0.5 Hz), low frequency component (LF: 0-0.15 Hz), high frequency component (HF: 0.15-0.50 Hz), HF/TP and LF/HF were measured. The volume of the face mask was 320 ml and the tidal volume during sitting with the face mask was 596 +/- 110 ml (444-808 ml). The value of LF/HF increased from supine rest to sitting in accordance to the change of body position (p < 0.05), but the value of LF/HF when sitting with the face mask decreased to the level during supine rest. The value of HF/TP decreased from supine rest to sitting (p < 0.05), but when sitting with the face mask returned to that during supine rest. To evaluate the effect of tidal volume, the subjects were divided into two groups (each of 11 subjects) by the median value of tidal volume (570 ml). The value of LF/HF decreased from supine rest to sitting with the face mask in the smaller tidal volume group (tidal volume < 570 ml) (p < 0.02). Moreover, there was a significant correlation between the change of the value of LF from supine rest to sitting with the face mask and the tidal volume (r = 0.44, p < 0.05). There results suggest that the power spectrum of heart rate variability is strongly influenced by the dead space induced by the face mask used in expiratory gas exchange analysis. In particular, the sympathetic activation from supine rest to sitting in subjects with the smaller tidal volume is unclear. Thus, interpretation of the results of heart rate variability with or without the face mask used in expiratory gas exchange analysis requires care.