Comparison of Transit-Time and Doppler Ultrasound Methods for Measurement of Flow in Aortocoronary Bypass Grafts during Cardiac Surgery

Summary

To evaluate the accuracy of flow measurements in aortocoronary bypass grafts with the ultrasound transit-time method, an in vitro and in vivo 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 two flow probes were placed on the graft 4-5 cm apart to avoid acoustic interference, and blood was pumped through the System at different flow rates. The comparison between the methods showed excellent agreement 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 flow, both methods overestimated flow slightly with mean error 4.4 and 7.3 ml/min for the transit-time and Doppler, respectively. Both methods showed excellent correlation with true flow (correlation coefficient 0.998 for the transit-time and 0.997 for the Doppler mothod). The in vivo accuracy was evaluated by comparison of the two methods in 9 patients. 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 × Doppler flow + 1.3. In terms of flow, tho mean error was 1.5 ml with limits of agreement ± 17.2 ml.

Conclusions: tho transit-time flow-meter showed excellent agreement with the Doppler method for both in vitro and in vivo studies. Tho diameter-independence of the transit-time flowmeter was an advantage in the clinical Situation. A disadvantage was the probe design which made acoustic contact more difficult to obtain than for the Doppler method.

Zusammenfassung

Um die Genauigkeit der Flußmessungen im aortokoronaren Bypass zu bestimmen, wurden In-vitro- und In-vivo-Untersuchungen vorgenommen. An einer montierten Vena saphena wurden die beiden Meßköpfe im Abstand von 4-5 cm angebracht und dann wurde das Blut in verschiedenen Flußgeschwindigkeiten gepumpt. Der Vergleich zeigte eine gute Übereinstimmung der Methoden mit einem linearen Korrelations-koeffizienten von 0,996. Gegenüber dem wahren Fluß überschätzten beide Methoden jedoch den Fluß leicht mit einem mittleren Irrtum von 4,4 ml/min für die Transit-time und 7,3 ml/min für den Doppler. An 9 Patienten wurden In-vivo-Messungen vorgenommen und zwar 34 Messungen an 17 Venenbrücken in einer Ausgangslage und nach Papaverinanwendung. Auch hierbei fand sich eine gute Übereinstimmung zwischen den beiden Meßmethoden.

Key words

Transit-time - Doppler shift - Ultrasound - Aortocoronary grafts - Blood flow