Changing Flow Pattern of the Internal Thoracic Ar tery Undergoing Coronary Bypass Grafting, Continuous-wave Doppler Assessment

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Abstract

Surgeons have limited ability to intra-operatively evaluate the patency of internal thoracic artery graft as a bypass for coronary artery revascularisation. Continuous-wave Doppler ultrasonography was used to study the velocity of the internal thoracic artery before harvesting and after grafting (scanning probe, 8MHz). Systolic and diastolic frequency shift (in kilohertz) and systolic frequency/diastolic frequency index were analysed. Twenty-four internal thoracic artery grafts in 15 patients were studied. Fourteen internal thoracic artery grafts were anastomosed to the left anterior descending artery, one to a diagonal artery and nine to the circumflex artery. The mean systolic frequency before harvesting was 1.19 ├é┬▒ 0.40kHz and no significant differences were found between the right and the left internal thoracic artery (right, 1.17 ├é┬▒ 0.37; left, 1.19 ├é┬▒ 0.42kHz). There was a 40% drop in systolic frequency related to the harvesting. Mean systolic frequency decreased after grafting (1.19 ├é┬▒ 0.40 versus 0.87 ├é┬▒ 0.32kHz; p < 0.01), whereas mean diastolic frequency doubled (0.32 ├é┬▒ 0.12 versus 0.83 ├é┬▒ 0.4 kHz; p < 0.001) and mean diastolic frequency/systolic frequency index increased from 28% ├é┬▒ 11% to 101% ├é┬▒ 39% (p < 0.001), indicating an increased myocardial vascularisation during diastole. No significant difference was found between grafted arteries (left anterior descending versus circumflex). All patients had an uneventful post-operative course and no perioperative myocardial infarction (MI) was reported. Doppler flow quantification of internal thoracic artery bypasses may give the surgeon an opportunity to intraoperatively evaluate the physiologic features and patency of the internal thoracic artery before and after coronary artery bypasses (J. Thorac Cardiovasc. Surg. (1996) 112: pp. 52├óÔé¼ÔÇ£58).

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Internal thoracic artery (ITA) is universally recognised as the optimal conduit for myocardial revascularisation.1,2 However, despite its universal acceptance, intra-operative ITA graft patency assessment remains mostly qualitative. Indirect signs of antegrade flow such as distal vascular bed filling or spontaneous electrical myocardial activity at the unclamping of the ITA pedicle are used as indices of patency. However, these do not give the surgeon a direct flow quantification. Traditionally, cardiac and vascular surgeons have used an electromagnetic flow probe to evaluate saphenous vein bypasses with which magnitude of the blood flow is derived from voltage variation of ions flowing across a fixed magnetic field.3

During the past 15 years, surgeons have become familiar with the use of ultrasonography for blood flow evaluation. Continuous-wave Doppler ultrasonography has been commonly used by vascular surgeons for non-invasive evaluation of peripheral vascular disease and also by cardiac surgeons for assessment of coronary artery bypass.4-8 Continuouswave Doppler ultrasonography applies the principle of the Doppler shift described by the physicist and mathematician Christian Johann Doppler (1803├óÔé¼ÔÇ£1853).9 Instead of producing a magnetic field, the piezoelectric crystal contained inside the Doppler probe transmits an ultrasonic beam that is reflected by the circulating red blood cells. A separate transducer crystal receives the reflected sound waves. The velocity of the blood flow is proportional to the frequency shift between transmitted and reflected ultrasonic beam. Contrary to the magnetic flow probe, the ultrasonic flow probe is effective without vessel denudation and only requires a limited access to the vessel circumference.

However, continuous-wave Doppler ultrasonography measures flow velocity, whereas the electromagnetic ├óÔé¼´åİencircling™ flowmeter as a result of known crosssectional areas can measure volumetric flow, therefore the former is not as accurate as the latter. Nevertheless, the continuous Doppler assessment allows systolic and diastolic flow velocity characterisation of the ITA flow pattern during a complete cardiac cycle. Native ITAs are predominantly perfused during systole, whereas coronary arteries are mainly perfused during diastole. Evaluation of systolic and diastolic velocity of ITA graft flow may provide the surgeon with a quantitative evaluation of the flow pattern changes as the ITA grafts adapts its haemodynamics to the coronary artery circulation, therefore possibly becoming useful in determining graft patency.

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