Effect of cardiac cycle on ultrasound assessment of endothelial function

To investigate the effects of non-ECG-triggered imaging during ultrasound studies of endothelial function, brachial artery diameters were measured throughout the cardiac cycle at rest, during reactive hyperemia, and after administration of nitroglycerin. R-wave-triggered imaging using a 7.5-MHz ultrasound transducer with acquisition every 41.7-66.7 ms was performed in 24 subjects. Cardiac cycle-related variation was computed as the maximum per cent change from the end-diastolic diameter. The range of possible errors in flow-mediated dilation (FMD) and nitroglycerin-mediated vasodilation that may result from ignoring cyclic variations in diameter was determined for each condition. True FMD, true nitroglycerin-mediated vasodilation, and the maximum and minimum values that could be erroneously calculated for FMD if timing was ignored all differed dramatically (p < 0.05). The range of apparent FMD values that could be measured was nearly three times the true FMD value. Ignoring temporal position within the cardiac cycle artifactually increased calculated FMD into the normal range, despite truly impaired FMD. Peak arterial dilation occurred before end-systole and greater baseline vessel compliance was associated with greater FMD. Brachial arterial diameters vary significantly throughout the cardiac cycle. The magnitude of this variation is similar to the arterial dilation induced by reactive hyperemia and nitroglycerin, making ECG-triggered imaging mandatory for accurate and reproducible clinical and research measurements of artery diameters and FMD. Measurement of diameters at end-diastole may be preferred to other time-points in the cardiac cycle.