Change in posture during sleep causes errors in non-invasive automatic blood pressure recordings.

When measured with automatic non-invasive monitors blood pressure is seen to fluctuate during sleep. The recorded blood pressure is influenced by the vertical distance between the heart level (zero reference) and the level of the brachial artery at the point of compression by the inflated cuff. In 20 randomly selected men, blood pressure was measured every 20 min during the night. The difference between consecutive recordings was 7.6 mmHg +/- 5.7/5.0 (s.d.), range 0-23/0-20 mmHg. The maximum blood pressure difference was 19 +/- 6.8/15 +/- 6.0, range 7-32/6-26 mmHg. These results were compared with the difference in blood pressure recorded on the left arm in 20 volunteers changing posture between four standardized recumbent postures. The posture change caused an average blood pressure difference of 9 +/- 6.1/9 +/- 6.5 mmHg, range 0-28/0-30 mmHg. The maximum blood pressure difference was 15 +/- 5.1/12 +/- 5.5 mmHg, range 5-27/(-)2-23 mmHg recorded between lying supine and lying on the right side. The similarity between differences and the variation in blood pressure during the recumbent posture indicates that changes in posture cause most of the night-time blood pressure variation recorded with non-invasive devices.