The origin of inverted waveforms in the reflection plethysmogram.

In reflection plethysmography at the finger inverted pulsewaves are sometimes observed, especially when, during anaesthesia, arterial pressure is measured in the same arm with an inflatable cuff. The origin of this inversion is investigated to two series experiments with volunteers. In the first series of experiments the influence of the pressure in the upper arm cuff was investigated and in the second series the influence of the application pressure of the transducer on the finger. It is concluded that inversion of the pulse waves of the plethysmogram is local phenomenon restricted to the reflection method. It is caused by a relative increase in the optical density of the surrounding tissue in relation to the arterial vessels. In the finger it is brought about by venous engorgement and it is dependent on the applied pressure.