Relevance of the waterfall phenomenon in continuous finger cuff blood pressure measurements.

1. Finger cuff blood pressure monitors that provide continuous presentation of arterial pressure at the finger are available. 2. The operation of such instruments is based on 'clamping' arterial volume under a pressurized encircling cuff applied to a digit. Blood volume is detected by measuring infra-red transmission across the finger. 3. The compressed veins adopt cuff pressure and combine with the resistance of the peripheral vascular bed at the finger tip to control the blood flow. This results in waterfall behaviour in the veins under the cuff. 4. With the cuff set just 5 mmHg below the undisturbed arterial pressure, adequate flow to the distal finger is maintained. 5. Two potential sources of error affect the finger cuff blood pressure method. These are related to the increase in systolic pressure associated with moving peripherally and the variable value of occluding transmural pressure, both of which depend on muscular tone.