Page 65 - Coronary hemodynamics in acute myocardial infarction - Matthijs Bax

The Doppler flow wire in acute myocardial infarction
depends on multiple factors, including myocardial resistance, metabolic demands, neurohumoral activation, filling pressures and vascular resistances of epicardial coronary arteries and distal microvascular bed. Ishihara et al observed an abnormal CFVR in the IRA directly after angioplasty, while CFVR gradually improved at 14 days and 6 months. However, even at 6 months, CFVR in the IRAs was still impaired (mean CFVR 2.34±0.38) in comparison with angiographically normal coronary arteries in reference patients (mean CFVR 3.13±0.48).
A similar experiment was conducted in a larger cohort by Bax et al, who measured 4 CFVR in both IRAs and non-IRAs immediately after the primary angioplasty in
73 patients with a first anterior MI, at 1 week and at 6 months.18 Figure 2 shows
CFVR, and baseline and hyperemic average peak flow velocity in IRAs and non-
IRAs. Immediately after primary PCI, CFVR was reduced in both IRAs and non- IRAs, although more pronounced in IRAs. At 1 week, CFVR was still impaired in IRAs, but in non-IRAs CFVR had almost returned to normal. Unlike the findings by Ishihara et al, CFVR was found to be normalized in IRAs at 6 months. This discrepancy can possibly be explained by the fact that all patients in the Japanese cohort were treated with balloon angioplasty alone, rather than coronary stenting. Furthermore, the Japanese study was hampered by small sample size (n=14). The reduced CFVR was mainly due to a decreased hyperemic blood flow velocity. The explanation for decreased hyperemic blood flow velocity during the acute phase of MI is multifactorial. Neurohumoral responses to ischemia lead to microvascular vasoconstriction in both IRAs and non-IRAs, and distal (micro-)embolization in IRAs. Moreover, microvascular damage and endothelium dysfunction as a result of ischemia and reperfusion lead to disturbed autoregulation. The microvascular resistance index was measured as the ratio of transvascular pressure gradient (mean aortic pressure minus right atrial pressure) to hyperemic blood flow velocity. The microvascular index was found to be increased during the acute phase of MI, and almost normalized at 1 week. Therefore, this study suggests that reduced CFVR after MI is partly explained by increased microvascular resistance, but to a greater extent by disturbed autoregulation.
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