Comparison between uncalibrated cardiac output using the femoral and radial arterial pressure waveform in critically ill patients

Introduction

Cardiac output (CO) monitoring is often required to manage critically ill patients. Nominal values can be determined from analysis of arterial pressure waveforms. It is assumed that arterial waveforms from different arterial sites give similar CO values. The aim of this study was to compare the values of uncalibrated CO derived from simultaneous radial (COr) and femoral (COf) blood pressures.

Methods

We enrolled 17 medical and surgical ICU patients, requiring haemodynamic monitoring and vasoactive drugs. Simultaneous recordings of the arterial radial and femoral waveforms were made from arterial pressure monitors via an A–D converter and analysed to obtain CO values using the PulseCO^® algorithm of the LiDCOplus (LiDCO, London, UK). Paired CO values were selected at several points on each recording. Calibrations were not done at each time point so the comparison required examination of the ratio of uncalibrated COr and COf.

Results

The median value of the CO ratio was 0.95 (IQR 0.88–1.02), with a high variability across the patients, ranging from 0.3 to 1.41, whereas intrapatient variability was low, with a median CV of 3.26% (IQR 1.1–5.3%). Although the ratio in COs between the two sites varied greatly, the difference between the median (range) arterial pressures was 2 mmHg (-3 to 8 mmHg). However, the pulse pressure difference between the two sites was generally large with a median (range) of 2 mmHg (-26 to 44 mmHg).

Conclusion

CO derived from blood pressure records at radial and femoral sites can appear similar when a patient population as a whole is considered. However, in individual patients, the difference between the two sites is large enough to be clinically unacceptable without a site-specific recalibration.