COMPARISON OF TRANSIT-TIME AND DOPPLER ULTRASOUND METHODS FOR MEASUREMENT OF FLOW IN AORTOCORONARY BYPASS GRAFTS DURING CARDIAC-SURGERY

To evaluate the accuracy of flow measurements in aortocoronary bypass grafts with the ultrasound transit-time method, an in vitro and in viv o comparison was carried out. The in vitro comparison with evaluation against both true flow and the ultrasound Doppler method, was carried out with a fresh saphenous vein mounted in a pulsatile flow rig. The t wo flow probes were placed on the graft 4-5 cm apart to avoid acoustic interference, and blood was pumped through the system at different fl ow rates. The comparison between the methods showed excellent agreemen t with a linear correlation coefficient of 0.996, and a mean error of -2.9 ml/min with limits of agreement +/- 13.1 ml/min (+/- 2 SD = 95 % of measured differences between these limits). However, against true f low, both methods overestimated flow slightly with mean error 4.4 and 7.3 ml/min for the transit-time and Doppler, respectively. Both method s showed excellent correlation with true flow (correlation coefficient 0.998 for the transit-time and 0.997 for the Doppler method). The in vivo accuracy was evaluated by comparison of the two methods in 9 pati ents. The two probes were placed on the same saphenous vein grafts 4 - 5 cm apart, and a total of 34 measurements in 17 grafts were carried out including measurements at baseline and after papaverine injection. The correlation coefficient was 0.990 and linear regression analysis gave the equation: Transit-time flow = 1.00 x Doppler flow + 1.3. In t erms of flow, the mean error was 1.5 ml with limits of agreement +/- 1 7.2 ml. Conclusions: the transit-time flow-meter showed excellent agre ement with the Doppler method for both in vitro and in vivo studies. T he diameter-independence of the transit-time flowmeter was an advantag e in the clinical situation. A disadvantage was the probe design which made acoustic contact more difficult to obtain than for the Doppler m ethod.