Role of Intravascular Imaging in Primary PCI
Primary percutaneous coronary intervention (PPCI) is always targeted on the angiographically identified culprit lesion. However, the actual culprit lesion may not compromise the lumen and can be located proximally or distally to the angiographic target lesion. As a result, the risk of incomplete lesion coverage can be high when the PPCI is guided solely by angiography. Furthermore, stent implantation must be optimized, as incomplete apposition and/or edge dissection may result in in-stent restenosis or acute thrombotic events. Thus, invasive coronary imaging using intravascular ultrasound or optical coherence tomography can be useful to guide the PPCI procedure by locating the true culprit lesion and may lead to better stent coverage of the lesion. Besides, invasive imaging also helps to resolve diagnostic uncertainty and to identify the mechanisms underlying acute events.
KeywordsPrimary percutaneous coronary intervention Intravascular ultrasound Optical coherence tomography Spontaneous coronary artery dissection
Acute ST-segment elevation myocardial infarction (STEMI) usually results from acute thrombotic occlusion of a major epicardial coronary artery. Primary percutaneous coronary intervention (PPCI) is the reperfusion strategy of choice if it can be done in a timely manner, its aim being to rapidly achieve complete myocardial reperfusion. Overwhelming evidence has shown that prompt reperfusion reduces infarct size, preserves left ventricular function, and improves survival [1, 2].
Studies have demonstrated the benefits of drug-eluting stents (DESs) in patients with acute myocardial infarction (MI) undergoing primary stent implantation. The HORIZONS-AMI (Harmonizing Outcomes with Revascularization and Stents in Acute Myocardial Infarction) trial demonstrated that the use of DESs in patients with STEMI is safe and effective compared with bare metal stents (BMSs) at 1-year follow-up. The use of DESs definitively reduces the need for repeat revascularization without additional risk of death or MI . Besides, a meta-analysis of 13 randomized trials, which compared the outcomes of DES and BMS in 7352 randomized patients, suggested that DES use significantly decreases restenosis compared with BMS use in patients with STEMI. DES use yields a relative and absolute reduction in target vessel revascularization (TVR) of 56% and −7%, respectively (p < 0.001). The meta-analysis also suggested that this benefit does not come at the expense of stent thrombosis, reinfarction, or increased death within 2 years of the PCI .
14.2 Role of Invasive Imaging in the Management of STEMI and Non-STEMI
14.2.1 Intravascular Ultrasound
IVUS uses reflected high-frequency (10–60 MHz) sound waves to visualize the arterial wall in a two-dimensional tomographic format. IVUS permits not only a greater understanding of the characteristics of the coronary plaque and its response to interventional coronary procedures but also allows more precise quantification of the coronary luminal dimensions and atherosclerotic plaque burden [15, 16, 17]. In the setting of elective PCI, randomized trials and meta-analyses indicate that IVUS-guided PCI is associated with a lower incidence of TVR and fewer major adverse cardiac events (MACEs), MI, and stent thromboses than angiography-guided PCI [18, 19]. IVUS provides useful information about the morphology of the coronary lesion, as well as helping stent size selection, optimization of stent expansion, and detection of incomplete apposition and/or edge dissection, resulting in reduced restenosis or stent thrombosis. IVUS- measured minimal stent cross-sectional area (CSA) is the best IVUS predictor of DES failure. The results from the IVUS sub-study of the HORIZONS-AMI trial showed that mechanical problems such as a smaller final stent CSA (<5 mm2) and inflow/outflow disease (residual stenosis or dissection) but not acute mal-apposition were associated with early stent thrombosis after intervention for acute MI. The minimal stent CSA measured by IVUS and the degree of stent expansion were significantly smaller in patients with early stent thrombosis than in the control group. The finding that the minimal stent CSA was significantly smaller in acute MI patients with early stent thrombosis could be either because of tissue protrusion (plaque and/or thrombus) or stent underexpansion or both, as the culprit lesions in acute MI patients are presumed to be thrombus-containing, and thus tissue protrusion into the lumen through stent struts is common . Besides, another IVUS sub-study of the HORIZONS-AMI trial also suggested that the final post-procedure minimal stent CSA in patients with STEMI after primary stent implantation was the only independent IVUS predictor of angiographic binary restenosis, similar to patients with stable coronary disease as previously reported. Angiographic restenosis rates were 26.7% in lesions with a CSA <4 mm2, 22.2% in lesions with a CSA <5 mm2, and 20.5% in lesions with a CSA <6 mm2 . Therefore, a well-expanded stent with a final minimal stent CSA ≥5 mm2 by IVUS is needed in patients undergoing primary PCI to prevent stent thrombosis as well as restenosis at follow-up.
14.2.2 Optical Coherence Tomography
The high resolution of OCT clearly enables a better understanding of the atherosclerotic plaque and helps to identify the mechanism underlying the acute event. Studies aiming to investigate an association between OCT and improvement in clinical outcomes may further strengthen the evidence in favor of using OCT to guide PCI in ACS patients. The DOCTORS (Does Optical Coherence Tomography Optimize Results of Stenting) trial is the first randomized, prospective, multicenter trial to investigate the use of OCT in optimizing the results of PCI for non-ST-segment elevation ACS. The results showed that OCT findings led to a change in procedural strategy in 50% of the patients in the OCT-guided group, mainly driven by the optimization of stent expansion, and were associated with higher fractional flow reserve values at the end of the procedure than angiography-guided PCI alone. However, this benefit was obtained at the cost of longer fluoroscopy and procedural times, as well as a greater volume of contrast medium and a higher dose of radiation, but without an increase in periprocedural complications, MI, or kidney dysfunction .
14.3 Role of Invasive Imaging in the Management of Spontaneous Coronary Artery Dissection
14.3.1 Pathogenesis of Spontaneous Coronary Artery Dissection
The usual pathogenesis of acute MI involves unstable plaque rupture or plaque erosion that is distinct from spontaneous coronary artery dissection (SCAD). SCAD is a rare cause of acute MI, and is due to hemorrhage within the arterial wall, resulting in separation of the intimal-medial layers rather than atherosclerotic plaque rupture or erosion. The presence of true arterial lumen compression by the hematoma in the intimal-medial layers can subsequently result in myocardial ischemia or acute MI. Early recognition and precise diagnosis of SCAD is very important in order to implement the appropriate medical treatment; otherwise it will lead to disastrous consequences. Two potential mechanisms for the cause of SCAD have been proposed. The first is that an intimal tear creates an entry point that promotes intramural bleeding inside the false lumen, leading to separation of the intimal-medial layers. The second is that rupture of the vasa vasorum leads to an intramural hematoma, which increases the pressure and potentially causes an intimal rupture into the true lumen. Thus SCAD caused by the latter mechanism can occur with or without a distinct intimal rupture . SCAD is not uncommon in young females presenting with acute MI in the absence of traditional cardiovascular risk factors  and is the most frequent cause of acute MI among pregnant women .
14.3.2 Intravascular Ultrasound
14.3.3 Optical Coherence Tomography
Even though angiography is the gold standard to guide procedural decision-making during primary PCI, it has various well-known limitations. Angiography is only a luminology without providing any information on the vessel wall and atherosclerotic plaque characteristics. Besides, it is suboptimal in detecting stent underexpansion, stent edge dissection, plaque protrusion, or thrombi. Invasive imaging techniques like IVUS and OCT provide complementary details to help primary PCI guidance, such as to ensure stent coverage of the culprit lesion and optimization of stent implantation. OCT is superior in the visualization of superficial structures, such as stent edge dissection, intimal tear, tissue protrusion, and intraluminal thrombi, whereas IVUS is better for deep vessel imaging, vessel sizing, and identifying positive remodeling. However, it remains controversial as to whether routine invasive imaging guidance improves outcomes in patients receiving primary PCI.
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