Augmented photoplethysmographic low frequency waves at the onset of endotoxic shock in rabbits

A rabbit model of endotoxaemia was employed to study the regional changes in photoplethysmogram (PPG) waveform and its low frequency fluctuations, and how they are related to the physiological events during the time course of endotoxic shock. Endotoxin (1 mg kg(-1) lipopolysaccharide) was injected into eight anaesthetized and mechanically ventilated rabbits. The 90 min monitoring period was divided into six stages, with the onset of hypotension separating the first three (EDTX1-3) and last three (HYPO1-3) stages. The most significant finding was a substantial but transient elevation in sympathetic-related toe PPG variability (PPGV) spectral power in EDTX3 and HYPO1 (P < 0.01 versus EDTX1). The group average response showed that the rapid rise started 15 min before and peaked at the onset of hypotension, which indicated a surge in sympathetic vasomotor activity that preceded the decompensatory blood pressure fall. Ear skin vasoconstriction was evident by a marked and sustained fall in ear PPG amplitude along with a decrease in ear skin temperature at the onset of hypotension, during which the sympathetic-related ear PPGV spectral power was elevated (P < 0.01, HYPO1 versus EDTX1). The results demonstrate the value of PPGV in characterizing regional vascular control and provide important insights into the physiological mechanisms of endotoxic shock.