Myocardial perfusion in patients with transposition of the great arteries after arterial switch operation.

Transposition of the Great Arteries After Arterial Switch Operation To the Editor: Oskarsson et al1 recently reported normal coronary flow reserve (CFR) in children with transposition of the great arteries after anatomic repair assessed by intracoronary Doppler FloWire (Cardiometrics, Inc). CFR can be measured by several techniques, each one with distinct advantages and limitations. An important distinction is between techniques that measure coronary blood flow (eg, positron emission tomography) and those that measure blood flow velocity (eg, Doppler catheters), from which coronary velocity reserve is calculated. Under ideal conditions, there is a linear relation between flow velocity and flow, and therefore the peak-to-rest flow velocity ratio can be substituted for the flow ratio as a measure of CFR. However, coronary velocity reserve is affected by a number of factors, including rheological parameters, the entrance angle of the coronary arteries (which is important in patients after coronary reimplantation), and the shape of the flow profile. While basal coronary blood flow depends on the determinants of myocardial oxygen demand (heart rate, contractility, ventricular load), myocardial blood flow after maximal vasodilatation (adenosine, papaverine) is no longer autoregulated and seems to be unaffected by the parameters mentioned above. Under the condition of general anesthesia, which is necessary to perform invasive Doppler-flow measurements, the myocardial oxygen demand is on a lower level, resulting in low basal coronary blood flow and consecutively increased CFR, which can be expressed as the ratio of hyperemic coronary flow to basal flow. Positron emission tomography imaging is a noninvasive method that can be performed under normal physiological conditions; in comparison to the measurements under general anesthesia, myocardial oxygen demand and thus basal myocardial blood flow are enhanced (effect of the coronary autoregulation), resulting in an attenuated CFR.2 This observation might explain the different and unexpected findings of Oskarsson and colleagues. Positron emission tomography, as the noninvasive method of reference for assessing CFR in humans, measures blood flow in as little as 10 g of myocardium; regional variations in myocardial flow on the basis of small vessel disease and endothelial dysfunction can be detected more reliably than by Doppler-flow measurements; a high percentage of patients with transposition of the great arteries after arterial switch operation show reversible perfusion defects, as mentioned by several authors.2,3 In our opinion, the function of the coronary arteries remains a matter of concern in patients after arterial switch operation; the prognostic implications must be discussed, but close clinical follow-up is mandatory.