The relationship between the area of peripherally-derived pressure volume loops and systemic vascular resistance

Arterial and photoplethysmographic (PPG) waveforms have been utilized to non-invasively estimate stroke volume from the pulse contour. The ability of these pulse contour devices to accurately predict stroke volume is degraded when afterload changes significantly. There is a need for a non-invasive device capable of identifying when vascular tone has changed. Shelley et al. previously described a qualitative relationship between peripheral pressure volume (PV) loops (in which pressure waveforms from an intra-arterial catheter are combined with volume waveforms from the PPG waveform) and changes in vascular tone. The purpose of this study was to quantitatively compare changes in the area of peripheral PV loops with changes in systemic vascular resistance (SVR) in a patient population undergoing major surgery. Physiologic data from ten patients undergoing liver transplantation was extracted from a hemodynamic database. A peak detection algorithm was applied to both the arterial and PPG waveforms, which were manually aligned so that the troughs occurred at identical time points. PV loop area (PVA) for each heartbeat was calculated and median PVA was recorded for each minute. PVA for each patient was indexed to the average value for the first 5 min (because PPG amplitude has no standard and is not comparable between patients) and compared to indexed SVR at all points for which SVR was available. SVR and PVA were plotted as a function of time and outliers (3.1 %) removed. The Pearson correlation coefficient describing the relationship between PVAi and SVRi was 0.67 (1,728 min of data, p = 0.0020, sign test over 10 patients) and between MAP and SVR was 0.71. There was no meaningful correlation between ΔSVR and either ΔPVA or ΔMAP (based on minute-to-minute changes). Indexed values of PVA are correlated with indexed values of SVR and may serve as a useful monitor for changes in afterload but in their present form do not offer added value above the measurement of MAP. Incorporation of different (e.g. finger, forehead) and redundant (e.g. bilateral) sites may significantly improve the accuracy of this technique.