1. Measurement of blood pressure and heart rate over a 24 h period was peformed in 10 quadriplegic spinal cord injury patients and 10 immobilized, neurologically intact orthopaedic subjects by using the Spacelabs 90207 automated ambulatory monitoring system.

2. Systolic and diastolic blood pressure fell significantly at night in orthopaedic subjects but not in quadriplegic patients, and night-time blood pressures were similar in both groups.

3. Cumulative summation of differences from a reference value (cusum analysis) confirmed a markedly diminished diurnal blood pressure variation in the quadriplegic patients.

4. These findings could not be accounted for on the basis of blood pressure variations during chronic postural change.

5. Heart rate fell significantly at night in both groups.

6. The findings suggest that the increase in blood pressure during waking hours in neurologically intact subjects is a consequence of a diurnal variation in sympathetic activity (absent in quadriplegic patients with sympathetic decentralization) which is independent of changes in physical activity.

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