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 post ure 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 recumb ent positions, namely the supine, left lateral decubitus and right lat eral decubitus positions, in healthy subjects by using spectral heart rate variability analysis. Both time- and frequency-domain measures we re 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-frequ ency power is highest in the right lateral decubitus position, followe d in decreasing order by left lateral decubitus and supine positions. The low-frequency power/high-frequency power ratio has the reversed tr end as compared with that of the normalised high-frequency power. Thes e results suggest that cardiac vagal activity is greatest when the rig ht lateral decubitus position is adopted.