Study to determine the repeatability of supra-sternal Doppler (ultrasound cardiac output monitor) during general anaesthesia: effects of scan quality, flow volume, and increasing age.

Background: The ultrasound cardiac output monitor (USCOM) is a continuous wave Doppler system designed to measure cardiac output (CO) non-invasively and intermittently either from the pulmonary or from the aortic valve. USCOM scan quality is critical to obtaining reliable data and during anaesthesia it is said to deteriorate with increasing age. The aim of this study was to investigate the effect of age on supra-sternal USCOM scan repeatability during anaesthesia.
Methods: We performed a series of 6 USCOM scans in 180 patients of all ages after induction for routine surgery. A 12-point Cattermole (CS) score and 10-point insonation (IS) score were used to evaluate scan quality and ease of insonation. The coefficients of variation (CVs) of USCOM variables [CO, peak velocity, stroke volume index (SVI) and the corrected flow time] were derived from the series of six readings.
Results: In >95% of young patients (age <50 yr), it was easy to obtain a good-quality USCOM scan (CS>8). In these patients, repeatability of serial readings was good with CVs<5% and precision of less than ±10%. In older patients (>50 yr), scan quality and ease of insonation declined, with >25% of patients >60 yr having unreliable USCOM scans (CS<5). In these patients, the CV was >5-10%. In several elderly patients (>65 yr), we failed to locate the USCOM signal. Average scan time increased with age (30 to >60 s). SVI was also strongly correlated with scan quality (R(2)=0.77).
Conclusions: Increasing age has a significant effect on USCOM scan quality and data reliability.