The effect of the lateral decubitus position on vagal tone

The average person spends about one-third of their time in a recumbent position. However, little is known about the effect of recumbent posture on autonomic nervous activity. Manoeuvres which can increase vagal tone have been sought both in the normal subject and in patients with heart disease. We have studied the autonomic effect of various recumbent positions, namely the supine, left lateral decubitus and right lateral decubitus positions, in healthy subjects by using spectral heart rate variability analysis. Both time- and frequency-domain measures were calculated and compared between the three recumbent positions. The normalised high-frequency power was used as the index of cardiac vagal activity, the normalised low-frequency power as the index of cardiac sympathetic activity and the low-frequency power/high-frequency power ratio as the index of sympathovagal balance. The normalised high-frequency power is highest in the right lateral decubitus position, followed in decreasing order by left lateral decubitus and supine positions. The low-frequency power/high-frequency power ratio has the reversed trend as compared with that of the normalised high-frequency power. These results suggest that cardiac vagal activity is greatest when the right lateral decubitus position is adopted.