The pulse oximeter as a non-invasive measure of cardiovascular response to lower body negative pressure

Ewe, C., Parker, K., White, P. and Henry, J.A. (2001) The pulse oximeter as a non-invasive measure of cardiovascular response to lower body negative pressure. Scientific Meeting of the Faculty of Accident and Emergency Medicine. 01 Jan 2001. A8 .

Abstract

Objectives: Current assessment of early haemorrhagic shock relies on non-specific and relatively unreliable parameters such as change in blood pressure and heart rate. The aim of this study was to evaluate the use of the pulse oximeter waveform (POW) as a non-invasive measure of early changes in the cardiovascular system. This was achieved by the simulation of graded hypovolaemia using lower body negative pressure (LBNP).
Method: LBNP of up to –40 mmHg was used to simulate a reduction in circulating blood volume of up to 15% in 12 healthy male volunteers in the supine position. A pulse oximeter probe was attached to the left index finger. Cardiovascular parameters were continuously monitored using a Propaq^TM monitor and defined safety end point criteria were applied. The POW was recorded using MicroMedical^TM pulse oximeter software and the traces analysed spectrally and non-spectrally on a customized Matlab^TM programme.
Results: A rise in heart rate of 20 beats/min and a rise in diastolic blood pressure occurred at –40 mmHg. The amplitude of the POW did not change significantly but the ratio of peak to notch height changed significantly (p<0.05) at LBNP levels of –10 mmHg and –20mmHg. Frequency domain analysis showed a mean increase in frequency (p<0.02) at –30 mmHg and –40 mmHg LBNP. Change in spectral power in the low frequency range analysed (0.01-0.1Hz) was not significant.
Conclusions: The pulse oximeter has been shown to be a sensitive indicator of POW response to early hypovolaemic stress. LBNP produced haemodynamic changes and sympathetic activation simulating a mild hypovolaemic state. However, its use as a model for studying changes in POW due to wave reflection may be complicated by the introduction of complex vascular responses originating from the lower body.

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Contributors

Author: P. White

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