Noninvasive rodent cardiac output: comparison of a compact clinical monitor with echocardiography and proposal of a simple correction factor

Introduction

Rodent models are often studied in critical care research. Noninvasive cardiac output (CO) measurement is desirable but often impractical. The present study aimed to compare a commercially available human CO monitor, USCOM^® (USCOM Ltd, Sydney, Australia), with specialised rodent echocardiography for noninvasive measurement of rat CO.

Methods

With institutional ethics committee approval (UQ AEC protocol 675/05), 21 anaesthetised, mechanically ventilated male Sprague–Dawley rats (573 ± 96 g) were studied during refinement and study of an endotoxic shock model. Pulsed–wave Doppler echocardiography (15 MHz rodent probe) was used to measure the left ventricular outflow velocity and to calculate the stroke volume and CO. USCOM (v1.7, human neonatal algorithm; 2.2 MHz) measurements followed each echocardiographic examination. USCOM CO was measured by combining continuous wave Doppler with the predicted outflow tract diameter (OTD-U).

Results

Twenty-one paired measurements were analysed. The mean echocardiographic CO was 113 ml/min (range 46–236). The mean USCOM CO was 245 ml/min (range 75–553). Paired echocardiographic and USCOM measurements demonstrated significant correlations for heart rate (r = 0.92, P ≤ 0.0001) and CO (r = 0.68, P = 0.001). Bland–Altman analysis of CO demonstrated a mean bias of -131 ml/min and precision of 52 ml/min. Linear regression analysis yielded a simple correction factor for USCOM OTD estimation. Following application of this correction factor (0.68 × OTD-U), the mean bias improved to -0.1 ml/min with precision of 38 ml/min.

Conclusion

Measurement of rat CO using the USCOM human neonatal algorithm (v1.7) is not interchangeable with specialised pulsed-wave Doppler echocardiography. We propose a simple correction factor that should improve performance of this device in the rodent laboratory. Incorporation into a rat-specific algorithm should be evaluated prospectively across a range of potential applications.