How is the efficacy of the initial treatment assessed? There is no simple answer to this question. The integration of several types of information is required as usual in medicine. Simple, non-invasive, widely available, and easily repeatable tools are preferable. They should be applied timely, when it is necessary to make a medical decision. First, chronologically, it is important to appropriately select patients in whom rescue PCI is indicated after failed thrombolysis. Persistence of pain and absent or insufficient ST segment resolution can be useful. The precise time and cut-off points of ST segment resolution remain, however, unclear. MCE can assess the absence or presence of microvascular perfusion, a prerequisite of tissue viability. This can be done rapidly, at the bedside, by intravenous bolus injection and/or continuous infusion of a contrast agent consisting of small microbubbles passing first through the pulmonary capillaries, and then opacifying the left ventricular cavity and enhancing the myocardium. MCE detects contrast microbubbles at the capillary level and allows assessment of myocardial blood volume and red cell velocity. Several modalities are available, mainly high power triggered imaging or low power continuous imaging after a flash echo at high mechanical index in order to destroy all microbubbles and then observe microbubble replenishment, indicating refilling of myocardial capillaries. For example, fig 1 illustrates the usefulness of early MCE at the end of thrombolytic infusion. The left panel shows an end systolic triggered image in the four chamber view. The left ventricle, septum, and apex are opacified. The lateral wall is thin, as a result of akinesis, and does not show contrast enhancement. Coronary angiography was immediately performed after MCE in this patient: the circumflex artery was occluded. Rescue PCI was successful. The right panel shows a similar image before hospital discharge. The lateral wall is well opacified and its end systolic thickness is similar to that of the remote regions, indicating both microvascular and contractile recovery.
Figure 1 End systolic triggered images in the apical four chamber view after intravenous contrast administration in a patient with acute lateral myocardial infarction. Left: No reflow at the end of thrombolytic infusion. Rescue percutaneous coronary intervention (more ...)
Similarly, MCE can be performed immediately after primary PCI to assess the absence or presence and extent of microvascular salvage. Early identification of patients with extensive no reflow would indeed be particularly useful if novel pharmacological adjunctive agents may enhance more optimal reperfusion. A potential pitfall of immediate MCE study is the frequent occurrence of post reflow hyperaemia of the risk area, which may significantly underestimate the degree of necrosis. However, even if perfusion remains preserved in necrotic regions early after restoration of patency, microvascular reserve is impaired. Vasodilatation by adenosine administration can unmask abnormalities in flow reserve. MCE in conjunction with a coronary vasodilator predicts infarct size more accurately, early after recanalisation in experimental studies.8
Other methods can be used to determine infarct size, but are less widely available or cannot be performed at the bedside. Magnetic resonance imaging (MRI) has been shown, in both experimental and clinical studies, to precisely measure infarct size with necrosis-avid MRI contrast agents.9