Page 63 - Coronary hemodynamics in acute myocardial infarction - Matthijs Bax

The Doppler flow wire in acute myocardial infarction
Introduction
Contemporary mechanical reperfusion therapy in acute coronary syndromes
is aimed at early and complete restoration of myocardial perfusion. However,
successful restoration of epicardial blood flow does not guarantee restoration
of flow at the myocardial tissue level. In about 15–30% of patients, the capillary
structure becomes disorganised owing to endothelial swelling, compression
by tissue, myocyte oedema and neutrophil infiltration, leading to microvascular obstruction.1,2 This inadequate microvascular perfusion is clinically relevant,
as it is associated with larger myocardial infarct size, reduced left ventricular 4 function and a worse clinical outcome than in patients with adequate myocardial reperfusion.3–5
Several diagnostic modalities are currently applied to detect microvascular obstruction. A thrombolysis in myocardial infarction (TIMI) flow grade ≤2 in the absence of macrovascular obstruction is often used as a definition of microvascular obstruction.6 However, even in patients with TIMI flow grade 3, microvascular perfusion can be impaired. The TIMI perfusion grade and the myocardial blush grade are also frequently used to assess myocardial reperfusion using coronary angiography.7,8 Another readily available and widely used marker of tissue-level reperfusion is resolution of ST-segment elevation.9 More accurate, non-invasive imaging modalities such as myocardial contrast echocardiography (MCE) and delayed contrast enhancement using cardiac magnetic resonance imaging (CMR) can also be used to detect microvascular obstruction.4,5; 10,11
Moreover, coronary blood flow can be measured invasively by an intracoronary Doppler-tipped guidewire. Since the 1970s, when catheter-based Doppler systems were first introduced by Benchimol and later Hartley and Cole, many improvements have been made to its design.12–14 A typical contemporary Doppler guidewire transmits and receives pulsed-wave ultrasound signals generated by a piezoelectric ultrasound transmitter (figure 1). Characteristic coronary blood flow patterns in patients with coronary microvascular obstruction are systolic flow reversal, rapid deceleration of diastolic flow and a reduced coronary flow velocity reserve (CFVR).11,15
61