Arterial-pulse oximetry loops: a new method of monitoring vascular tone.

Objective: We report the off-line calculation of the vascular compliance of the finger and suggest the continuous on-line use of this methodology as an aid to monitoring the peripheral vascular resistance. This method consists of the simultaneous analysis of the waveform signals from the pulse oximeter monitors and the arterial pressure as indicators of "volume" and pressure respectively to continuously calculate the vascular "compliance" (volume change per unit pressure change). This should be seen as a "relative compliance" as the pulse plethysmograph signal is not calibrated. This new methodology allows for continuous monitoring of peripheral vascular compliance as a beat-to-beat indicator of peripheral vascular resistance. The vaso-constrictors, phenylephrine and ephedrine, were shown to decrease the compliance as predicted.

Methods: The arterial pressure and pulse oximeter waveforms were obtained during routine anesthetic care. The waveforms were collected with a computer data-acquisition system and then analyzed "off-line" as an indirect indicator of total vascular tone. Demographic and clinical information including drug administration were recorded.

Results: A case report is presented using this new form of analysis. Vascular compliance changes induced by phenylephrine and ephedrine were studied. A dose response curve of peripheral vascular compliance to phenylephrine was generated from these data.

Conclusions: By plotting the pulse oximeter waveforms versus the arterial waveforms, multiple volume versus pressure (relative compliance) loops were obtained. Analysis of these loops may assist in the monitoring of vascular compliance.