Page 70 - Coronary hemodynamics in acute myocardial infarction - Matthijs Bax

Chapter 4
dependence of CFVR is mainly because resting coronary flow velocity is very sensitive to changes in myocardial oxygen consumption. Furthermore, as the extent of microvascular obstruction is known to increase within the days after primary PCI, single Doppler flow velocity measurements immediately after primary PCI might underestimate the degree of microvascular obstruction subsequently present. Another limitation is the difficulty of detecting an adequate flow signal. A novel guidewire tipped with both a Doppler flow and a pressure sensor has made signal acquisition more cumbersome. A possible explanation might be the change in display from an analogue signal to a digital signal. Based upon our own experience, reversing the tip of the guidewire to make the sensors face the proximal part of the coronary artery may improve signal quality.
The Doppler flow wire interrogates the resistance of the entire vessel and may not differentiate diffuse epicardial disease or residual epicardial stenosis from microvascular obstruction. Combined pressure–flow velocity measurements are better suited for distinguishing between epicardial and microvascular resistance. Fearon et al reported the index of microcirculatory resistance, measured by a pressure sensor-tipped guidewire in combination with flow using thermodilution to be an independent predictor of recovery of echocardiographic left ventricular wall motion score in 29 patients after acute MI.26 Currently, assessment of microvascular injury by this index has not been compared with the ‘gold standard’ for detection of microvascular obstruction using contrast-enhanced CMR.
The introduction of the dual-sensor (Doppler velocity and pressure) tipped guidewire led to the introduction of physiological indices based upon combined pressure and flow measurements. These novel indices—most notably, hyperaemic microvascular resistance and hyperaemic stenosis resistance, have not yet been tested in the setting of acute MI.27
Another potential modality to assess microvascular injury is wave intensity analysis (WIA). On the basis of measurements of coronary arterial pressure and velocity, WIA allows for a better understanding of aortic, left ventricular and
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