ST-Segment Elevation Myocardial Infarction
Vikas Soma
Kishore Nallu
Eyal Herzog
Mun K. Hong
ST-segment elevation myocardial infarction (STEMI; often referred to as Q-wave myocardial infarction [MI]) accounts for approximately 30% new cases of MI every year in the United States.1 STEMI represents the most urgent group of patients for immediate therapy and thus, represents the priority group of the Priority risk, Advanced risk, Intermediate risk, and Negative/low risk (PAIN) pathway.2 Prompt recognition is the key step in the management of these patients because reperfusion therapy is most beneficial when performed early in presentation. Early reperfusion is the key to maximizing the preservation of ischemic myocardium at risk, and minimizing the acute and late morbidity and mortality associated with STEMI.3,4
PATHOPHYSIOLOGY
The likely mechanism of STEMI is the plaque rupture of a previously non-flow-limiting lesion or less likely, endothelial erosion.5 These events result in the sudden development of a flow-limiting stenosis with eventual thrombus formation and cessation of blood flow. The inciting factors are unknown, although hypersympathetic state, including early morning hours and associated with strenuous physical or mental stress, has been hypothesized to increase the likelihood of plaque rupture.
INITIAL ASSESSMENT
The American College of Cardiology/American Heart Association (ACC/AHA) guidelines on STEMI6 suggest that all patients presenting to the emergency department (ED) with a history of chest pain or symptoms suggestive of STEMI should undergo the following:
1. Triaged and managed through a predetermined, guidelinebased, institution-specific chest pain protocol such as the PAIN pathway.2 The protocol should address, in order of importance, those findings that permit rapid triage and initial diagnosis and management, and should include the following top six differential diagnostic possibilities:
a. Acute coronary syndrome (ACS)
b. Acute aortic dissection
c. Acute pulmonary embolism
d. Tension pneumothorax
e. Acute perforation of peptic ulcer or esophageal tear or rupture
f. Acute pericarditis.
In addition, the 2009 ACC/AHA STEMI guidelines focused update7 recommended that each community develop a STEMI system of care that includes the following:
a. Ongoing multidisciplinary team collaboration (including emergency medical services, nonpercutaneous coronary intervention [non-PCI] capable hospitals/STEMI referral centers, and PCI capable hospitals/STEMI receiving centers) to evaluate outcomes and measures of performance.
b. A process for prehospital identification and activation
c. Transfer protocols for patients who arrive at STEMI referral centers and are candidates for primary PCI.
d. Destination protocols for STEMI receiving centers.
2. The 12-lead ECG is the most critical data for establishing the diagnosis of STEMI, as initial biomarkers can be negative. A 12-lead electrocardiogram (ECG) should be performed and interpreted by an experienced physician within 10 minutes of ED arrival.
a. If the initial ECG is nondiagnostic, but there is a high clinical suspicion of STEMI, then an ECG should be repeated every 5 to 10 minutes to assess for evolving ST-elevation.
b. In patients with suspected inferior STEMI, a right-sided ECG should be obtained to assess for right ventricular (RV) infarction, which manifests as ST-elevation in leads V3R and V4R.
c. In patients with suspected posterior wall or high lateral wall infarction, leads V7, V8, and V9 should be obtained by extending the lateral chest leads to the back.
3. If STEMI is present, the decision to establish reperfusion either by primary PCI, the preferred revascularization option,8 or fibrinolytic therapy should be made within 20 minutes of ED arrival. The goal is to achieve a door-to-needle time for fibrinolytics within 30 minutes or perform primary PCI within 90 minutes of first medical contact by persons skilled in the procedure.6 These goals should not be understood as the ideal times for reperfusion but as the longest times that should be considered acceptable for effective reperfusion therapy.
4. During this time, a focused history and physical examination should be performed with the following objectives:
a. Risk stratify patients using scoring systems such as the thrombolysis in myocardial infarction (TIMI) risk score9 or Grace score.10 As opposed to patients with unstable angina or NSTEMI, in whom risk stratification identifies a high-risk subset of patients who derive the greatest potential benefit from an early invasive approach, all patients with STEMI (presenting < 12 hours of symptoms onset) benefit from early reperfusion. Risk stratification in STEMI patients helps to identify those with higher morbidity and mortality. These high-risk features include patients with advanced age, cardiogenic shock or heart failure, large anterior wall MI, and those who develop electrical (e.g., ventricular arrhythmias) or mechanical complications from the STEMI.11,12 Patients who do not either receive any reperfusion therapy or achieve successful reperfusion after primary PCI or fibrinolytic therapy also have a higher risk of in-hospital mortality.
b. Perform a quick but thorough review of exclusion criteria for fibrinolytic treatment whether STEMI is confirmed and whether the patient is in a nonprimary PCI center with long (> 60 minute) transfer time for primary PCI.
MANAGEMENT STRATEGIES
INITIAL APPROACH
Immediately after the diagnosis of STEMI, the patients should receive oxygen and adjunct pharmacology based on predetermined, guideline-based, institution-specific protocol, including but not limited to aspirin, clopidogrel/prasugrel, statin, and possibly glycoprotein IIb/IIIa (GP IIb/IIIa) inhibitor. All patients should be placed on a cardiac monitor with a defibrillator on standby.
MODE OF REPERFUSION
The current ACC/AHA guidelines for the treatment of STEMI recommend that a reperfusion strategy, based on either pharmacological management with a fibrinolytic agent or mechanical management with primary PCI, should be implemented as soon as possible after arrival in the ED (class I-A recommendation).7 Factors affecting the choice of reperfusion therapy include time between onset of chest pain and presentation, patient characteristics at presentation (high vs. low risk), and access to a skilled facility capable of primary PCI.
Time from symptom onset. The most important determining factor is the time from symptom onset, which directly correlates with the clinical outcome in patients with STEMI regardless of the reperfusion method. If the symptom duration is < 3 hours, and the door-to-balloon minus door-to-needle time is < 1 hour, primary PCI is preferred. However, if the estimated time from first medical contact to balloon inflation is > 90 minutes (or door-to-balloon minus door-to-needle time is > 1 hour), fibrinolytic therapy is preferred.7 PCI is the favored strategy when there is a delay between symptom onset and presentation (> 3 hours) and when PCI can be performed in a timely manner with a door-to-balloon time goal of 90 minutes.
High-risk patients. Primary PCI has been shown to have better clinical outcomes in high-risk patients, including those elderly who develop shock within 36 hours of MI, patients with severe heart failure and/or pulmonary edema (Killip class III or IV), and symptom onset within 12 hours. If the clinical scenario describes a STEMI with high-risk features, transfer for primary PCI is favored even if the time from first medical contact to balloon inflation modestly exceeds 90 minutes.
Access to skilled facility capable of primary PCI. The biggest obstacle to the use of primary PCI for all STEMI patients is the lack of a skilled facility capable of primary PCI. For those patients presenting to facilities without this capability, rapid transfer to a PCI center can produce better outcomes than fibrinolysis, as long as the door-to-balloon time, including inter hospital transport time, is <90 minutes. If, however, rapid transfer is not possible, fibrinolytic therapy should be administered. If patient presents very early after symptom onset (<1 hour), and there are no contraindications to fibrinolytic therapy, the therapy should be administered as it may abort the infarction.13 In patients with high-risk features, rapid transfer to a PCI center is preferred as long as the door-to-balloon time is <90 minutes. Patients who do receive fibrinolytic therapy, especially those with high-risk features (e.g., elderly, anterior MI location), should be electively transferred to a PCI center in the event that rescue PCI is needed.14 Table 2.1 summarizes the criteria for selecting a reperfusion therapy.
PRIMARY PCI
The current STEMI guidelines6 indicate that the benefit of PCI over fibrinolytic reperfusion pertains to primary PCI performed in a skilled facility. A skilled facility is defined as a laboratory that performs approximately 400 PCI procedures annually, and at least 36 of them are primary PCI. The PCI should be performed by experienced interventionalists, defined as operators who perform >75 PCI procedures annually, and at least 11 of them are primary PCI.
The suggested relationship between PCI-capable centers and improved outcomes is illustrated by findings from a retrospective analysis of 1997 Medicare claims data demonstrating that the need for coronary artery bypass graft (CABG) surgery after PCI occurred more frequently (2.25% vs. 1.55%; P < .001) when the procedure was performed by inexperienced staff (> 60 vs. < 30 cases per year) and that the risk of 30-day mortality was higher (4.29% vs. 3.15%; P < .001) for patients treated at lowvolume PCI centers (< 80 vs. > 160 Medicare cases per year).15
BENEFITS OF PRIMARY PCI OVER FIBRINOLYTICS
RESTORATION OF TIMI 3 FLOW
Several studies have shown that primary PCI results in substantially better rates of normal flow in the culprit artery compared with treatment with fibrinolytic agents (TIMI grade 3: 74% to 93%16,17 vs. 60% to 63%18,19 and 20). Fibrinolysis fails to restore blood flow completely in 30% to 40% of patients with STEMI, and analyses of randomized clinical trials comparing primary PCI and fibrinolysis have shown improved clinical outcomes with primary PCI.8,21
TABLE 2.1 Criteria for selecting reperfusion therapy | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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ADVERSE EVENTS
Meta-analysis of 23 randomized trials has shown that primary PCI was superior to fibrinolytic therapy in reducing mortality (7% vs. 9%, P = .0002), nonfatal reinfarction (3% vs. 7%, P < .0001), stroke (1.0% vs. 2.0%, P = .0004), and the combined endpoint of death, nonfatal reinfarction, and stroke (8% vs. 14%, P <.0001).8
REOCCLUSION
Fibrinolytic therapy, albeit widely available, is limited by early and late reocclusions of the infarct-related artery. The incidence of reocclusion after successful fibrinolysis increases over time as shown in a series of studies.22,23 and 24 It is up to 25% to 30% within months of STEMI.25,26 Reocclusion of a previously patent artery is a cause of increased in-hospital mortality and is detrimental to the long-term recovery of left ventricular function.27 However, coronary stenting virtually eliminates vascular recoil, and multiple studies have shown low reocclusion rates between 0% and 6% on follow-up angiography ranging from 6.2 days to 7.7 months after angioplasty with stent placement for STEMI.28,29,30,31 and 32
ANATOMIC RISK STRATIFICATION
A subset of patients may have severe three-vessel or left main disease or anatomic features unfavorable for PCI and may be candidates for urgent or emergency CABG. These patients may be missed if a fibrinolytic strategy is used. Another subset of patients will have spontaneously reperfused coronary arteries or may have acute pericarditis or other nonthrombotic causes of ST-segment elevation, such as epicardial or microvascular spasm or Tako-tsubo cardiomyopathy (TCM).33 These patients can be treated medically and conservatively, avoiding the risks of fibrinolytic therapy. In addition, identification of high-risk patients by cardiac catheterization, such as those with concomitant valvular disease, may facilitate additional strategies that will improve outcome, whereas low-risk patients may be eligible for early hospital discharge.
HIGH-RISK PATIENTS
Primary PCI in patients with anterior STEMI reduces mortality compared with fibrinolytic therapy although there is no difference in patients with non anterior STEMI.34,35 Patients with cardiogenic shock treated with coronary revascularization experienced an absolute 9% reduction in a 30-day mortality compared with those managed with medical stabilization.36 In Second National Registry of Myocardial Infarctions (NRMI-II)12 acute MI patients with congestive heart failure (CHF) had a 33% relative risk reduction in mortality with primary PCI compared with a 9% relative risk reduction with fibrinolytic therapy.
STRATEGIES TO MINIMIZE THE DOOR-TO-BALLOON TIME
There are many strategies to reduce the door-to-balloon time, and a report by Bradley et al.37 suggests practical ways to achieve this important goal. In their survey of hospitals and review of 28 different strategies, 6 of them were found to significantly reduce the door-to-balloon time. They included the following, and we already have majority of them in place at our institution:
ED physicians activate the catheterization laboratory or the MI team (reducing 8.2 minutes).
A single call to a central page operator activates the catheterization laboratory or the MI team (13.8-minute reduction).
ED physicians activate the catheterization laboratory or the MI team while the patient is being transported by the ambulance to the hospital (saving 15.4 minutes).
The on-call staff is expected to arrive in the catheterization laboratory within 20 minutes after being paged (reducing 19.3 vs. > 30 minutes).
The on-call interventionalist stays in the hospital for these emergencies (14.6-minute reduction).
A collaborative feedback is provided between the ED and the catheterization laboratory (8.6-minute reduction).
Most of these strategies can be implemented without undue stress on the system, except for the attending interventionalist taking the call in the hospital. These reductions in precious minutes are especially important when many patients still present late after the onset of their symptoms.
TRANSFER FOR PRIMARY PCI
Data from a meta-analysis of five trials evaluating safety and feasibility of emergent transfer for PCI showed that despite the inherent delay involved in transfer, primary PCI was associated with a significant reduction in nonfatal reinfarction, total stroke, and the combined endpoint of death, nonfatal reinfarction, and stroke compared with on-site fibrinolysis.8 This was true even in high-risk patients. However, most of these transfer studies were performed in Europe where the distances are shorter, the time required for transfer is shorter, and the use of dedicated emergency vehicles resulted in efficient transfer. Thus, it is unknown whether these results could be replicated elsewhere.
PREHOSPITAL FIBRINOLYTICS AND PRIMARY PCI IN FACILITIES WITHOUT CARDIAC SURGERY
An observational cohort study in patients with STEMI showed lower mortality rates in patients treated with primary PCI compared to both in-hospital fibrinolysis and prehospital fibrinolysis.38 The randomized comparison of primary angioplasty and prehospital fibrinolysis in acute myocardial infarction (CAPTIM) trial (stopped prematurely because of poor recruitment) showed no significant difference in the rate of death, nonfatal MI, and stroke between patients treated with primary PCI and those treated with prehospital fibrinolytic therapy.39
The cardiovascular patient outcomes research team (C-PORT) trial evaluated whether treatment of acute MI with primary PCI is superior to thrombolytic therapy at hospitals without on-site cardiac surgery. At 6-month follow-up, the composite endpoint of death, reinfarction, and stroke was significantly lower in the primary PCI group (12.4% vs. 19.9%, P = .03).40 The ACC/AHA guidelines for the management of patients with STEMI considers the performance of primary PCI in a community hospital without on-site cardiac surgery as a class IIb indication, provided the hospital meets the skilled PCI facility requirements and the procedures performed by an experienced interventionalist. Nonetheless, a protocol for rapid transfer to a hospital with cardiac surgery should be in place in case of procedural complications or other surgical emergencies, such as concomitant mechanical complications.
FIBRINOLYTIC THERAPY
As stated earlier, the time from onset of symptoms to fibrinolytic therapy is an important predictor of MI size and patient outcome.41 The efficacy of fibrinolytic agents in lysing thrombus diminishes with the passage of time.42 Fibrinolytic therapy administered within the first 2 hours (especially the first hour) can occasionally abort MI and dramatically reduce mortality.43,44 The National Heart Attack Alert Working Group recommends that EDs strive to achieve a 30-minute door-to-needle time to minimize treatment delays.45 Prehospital fibrinolysis by skilled emergency medical service (EMS) personnel reduces treatment delays by up to 1 hour and reduces mortality by 17%.46
CONTRAINDICATIONS TO FIBRINOLYTIC THERAPY IN STEMI
ABSOLUTE CONTRAINDICATIONS
Any prior intracranial hemorrhage.
Known structural cerebral vascular lesion (arteriovenous malformations, aneurysms, etc.).
Known malignant intracranial neoplasms (primary or metastatic).
Ischemic stroke within 3 months except acute ischemic stroke within 3 hours.
Suspected aortic dissection.
Active bleeding or bleeding diathesis (excluding menses).
Significant closed-head or facial trauma within 3 months.
RELATIVE CONTRAINDICATIONS
History of chronic, severe, and poorly controlled hypertension.
Severe uncontrolled hypertension on presentation (systolic blood pressure [SBP] >180 mm Hg or diastolic blood pressure [DBP] >110 mm Hg).
History of prior ischemic stroke >3 months, dementia, or known intracranial pathology not covered under absolute contraind ications.
Traumatic or prolonged (>10 minutes) cardiopulmonary resuscitation (CPR).
Major surgery within the last 3 weeks.
Recent internal bleeding (within 2 to 4 weeks).
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