Acute Management of Acute ST Elevation Myocardial Infarction

The first step in the management of the patient with an acute ST elevation myocardial infarction (STEMI) is prompt recognition, since the beneficial effects of therapy with reperfusion are greatest when performed soon after presentation. For patients presenting to the emergency department with chest pain suspicious for an acute coronary syndrome (ACS), the diagnosis of STEMI can be confirmed by the ECG. Biomarkers may be normal early.

Once the diagnosis of an acute STEMI is made, the early management of the patient involves the simultaneous achievement of several goals, as outlined by an ACC/AHA task force:

• Relief of ischemic pain. 
• Assessment of the hemodynamic state and correction of abnormalities that are present. 
• Initiation of reperfusion therapy with primary percutaneous coronary intervention (PCI) or fibrinolysis. 
• Antithrombotic therapy to prevent rethrombosis or acute stent thrombosis. 
• Beta blocker therapy to prevent recurrent ischemia and life-threatening ventricular arrhythmias.

This is then followed by the in-hospital initiation of different drugs that may improve the long-term prognosis:

• Antiplatelet therapy to reduce the risk of recurrent coronary artery thrombosis or, with PCI, coronary artery stent thrombosis.
• Angiotensin converting enzyme (ACE) inhibitor therapy to prevent remodeling of the left ventricle.
• Statin therapy.
• Anticoagulation in the presence of left ventricular thrombus or chronic atrial fibrillation to prevent embolization.

GENERAL PRINCIPLES 

The 2004 American College of Cardiology/American Heart Association (ACC/AHA) guidelines on STEMI recommended that all hospitals establish multidisciplinary teams to develop guideline-based, institution-specific written protocols for triaging and managing patients who present with symptoms suggestive of myocardial ischemia. In addition the 2009 focused update recommended that each community develop a STEMI system of care that encourages:

• Ongoing multidisciplinary team (including emergency medical services, non-PCI capable hospitals/STEMI referral centers, and PCI capable hospitals/STEMI receiving centers) meetings to evaluate outcomes and measures of performance.
• A process for prehospital identification and activation.
• Transfer protocols for patients who arrive at STEMI referral centers who are candidates for primary PCI.

An increasing number of centers use structured algorithms, checklists, or critical pathways to screen patients with a suspected ACS. These strategies combine diagnostic evaluation such as electrocardiography and serum biomarkers with therapeutic interventions such as aspirin, beta blockers, antithrombotic therapy, and primary PCI or fibrinolytic therapy.

INITIAL ASSESSMENT  

Clinical assessment of the patient with a possible acute coronary syndrome (ACS) begins as soon as the patient arrives in the emergency department and continues in the coronary care unit. Initial assessment consists of acute triage and early risk stratification. An ECG should be obtained within 10 minutes of arrival, if it has not been obtained already by EMS providers in the prehospital arena.

Acute triage — A focused evaluation on presentation should address, in order of importance, those findings that permit rapid triage and initial diagnosis and management:

• Responsiveness, airway, breathing and circulation — In patients who present with respiratory or cardiorespiratory arrest, the appropriate resuscitation algorithms should be followed. 
• Evidence of systemic hypoperfusion (hypotension; tachycardia; impaired cognition; cool, clammy, pale, ashen skin) — Cardiogenic shock complicating acute MI requires aggressive evaluation and management. This issue is discussed in detail separately.
• Left heart failure with hypoxia — Patients who present with dyspnea, hypoxia, pulmonary edema, and/or impending respiratory compromise require aggressive oxygenation, airway stabilization, diuretic therapy, and afterload reduction in addition to the standard treatments.
• Ventricular arrhythmias — Sustained ventricular tachyarrhythmias in the periinfarction period must be treated immediately because of their deleterious effect on cardiac output, possible exacerbation of myocardial ischemia, and the risk of deterioration into VF.

Early risk stratification — Analyses from several large clinical trials and registries have established a number of clinical predictors of adverse outcomes among patients with STEMI. There are many clinical prognostic factors that are immediately available to the physician based upon the initial history, physical examination, electrocardiogram (ECG), and chest X-ray. Given the speed with which reperfusion therapy is administered in patients with STEMI, their clinical utility in early medical decision making in the ED is often limited. They do provide good prognostic information that has utility in the post-reperfusion period, however, and may provide guidance regarding the optimum method of reperfusion.

High-risk features include advanced age, low blood pressure, tachycardia, heart failure (HF), and an anterior MI. Specific scoring systems, such as the TIMI risk score, permit a fairly precise determination of the risk of in-hospital mortality.

Patients at high risk require an aggressive management strategy in addition to standard medical management. Direct prehospital transport or, less optimally, prompt interhospital transfer to a facility with revascularization capabilities is recommended for such patients.

INITIAL THERAPY 

The patient with STEMI should have continuous cardiac monitoring, oxygen, and intravenous access. Therapy should be started to relieve ischemic pain, stabilize hemodynamic status, and reduce ischemia while the patient is being assessed as a candidate for fibrinolysis or primary PCI. Other routine hospital measures include anxiolytics, serial ECGs, and blood pressure monitoring. The 2004 ACC/AHA guidelines recommended that all initial therapy be carried out in the emergency department based upon a predetermined, institution-specific, written protocol.

1) Oxygen 

The role of supplemental oxygen in patients without hypoxia has not been well studied. A 2010 Cochrane review evaluated three trials of 387 patients with presumed myocardial infarction (MI) who were randomly assigned to supplemental oxygen or room air. Enrolled patients were either hypoxic and normoxic. The study found no significant difference in mortality (pooled relative risk 2.88, [95% CI 0.88-9.39] in an intention-to-treat analysis and 3.03, [95% CI 0.93-9.83] among those with confirmed MI). No subgroup analysis was performed on those with normoxia.

The suggestion of harm with supplemental oxygen found in this Cochrane review is of concern, particularly in patients with normoxia, as a pathophysiologic basis for such harm has been articulated. Hyperoxia, which might occur with the administration of oxygen to normoxic individuals, has been shown to have a direct vasoconstrictor effect on the coronary arteries.

2) Reperfusion 

Prompt restoration of myocardial blood flow is essential to optimize myocardial salvage and to reduce mortality. A decision must be made as soon as possible as to whether reperfusion will be achieved with fibrinolytic agents or primary (direct) percutaneous coronary intervention (PCI).

• Percutaneous coronary intervention 
• If high-quality PCI is available, multiple randomized trials have shown enhanced survival compared to fibrinolysis with a lower rate of intracranial hemorrhage and recurrent MI. As a result, 2007 focused update of the ACC/AHA 2004 Guidelines for the Management of Patients With STEMI recommended the use of primary PCI for any patient with an acute STEMI who can undergo the procedure within 90 minutes of first medical contact by persons skilled in the procedure. This was not changed in the 2009 update. 
• Patients with typical and persistent symptoms in the presence of a new or presumably new left bundle branch block are also considered eligible.
• For patients presenting 12 to 24 hours after symptom onset, the performance of primary PCI is reasonable if the patient has severe HF, hemodynamic or electrical instability, or persistent ischemic symptoms. Randomized trials of routine late PCI have shown an improvement in left ventricular function but not in hard clinical end points. This approach is not recommended.
• If primary PCI is not available on site, rapid transfer to a PCI center can produce better outcomes than fibrinolysis, as long as the door-to-balloon time, including interhospital transport time, is less than 90 minutes. This door-to-balloon time is difficult to obtain unless rapid transport protocols and relatively short transport distances are in place.
• All patients who undergo primary PCI should be pretreated at diagnosis with anticoagulant and antiplatelet therapy.
• Fibrinolysis 
• The 2007 ACC/AHA focused update (not changed in the 2009 focused update) recommended the use of fibrinolytic therapy in the following patients: 
• Any patient with STEMI who presents within 12 hours of symptom onset has no contraindications for fibrinolysis, and presents to a facility without the capability for expert, prompt intervention with primary PCI within 90 minutes of first medical contact. 
• Patients who present to a facility in which the relative delay necessary to perform primary PCI (the expected door-to-balloon time minus the expected door-to-needle time) is greater than one hour.
• The time interval from first patient contact to initiation of fibrinolytic drug infusion should be less than 30 minutes.
• Fibrinolytic therapy has generally not improved outcomes in patients presenting at 12 hours or later and is therefore not indicated in those who are stable and asymptomatic. However, fibrinolysis can be considered up to 24 hours after symptom onset if the patient has ongoing or stuttering chest pain and PCI is not available.
• A number of different fibrinolytic regimens have been evaluated, and each agent has its own preferred dosing regimen.
• Patients receiving fibrinolytic therapy benefit from pretreatment with clopidogrel but not a GP IIb/IIIa inhibitor.
• Angiography after fibrinolysis 
• Fibrinolysis immediately before primary PCI, previously called facilitated PCI, is not recommended.
•  The use of "rescue PCI" for patients with recurrent ischemia or infarction is better established.
• Bypass surgery 
• Coronary artery bypass graft surgery (CABG) is infrequently performed in patients with STEMI. The main indications are for emergent or urgent CABG related to failure of fibrinolysis or PCI, cardiogenic shock, or life threatening ventricular arrhythmias associated with left main or three-vessel disease.
• The benefit of revascularization must be weighed against the increase in mortality associated with CABG in the first three to seven days after STEMI. Thus, if the patient has stabilized, surgery should be delayed to allow myocardial recovery. Patients with critical anatomy should undergo CABG during the initial hospitalization.

3) Medications 

• Antiplatelet therapy — Antiplatelet therapy including aspirin, thienopyridine, and, in patients undergoing primary PCI, a GP IIb/IIIa inhibitor improve outcomes. Initial thienopyridine dosing varies with the reperfusion strategy.
• Anticoagulant therapy — The evidence to support parenteral anticoagulant therapy in most cases of STEMI is strong. However, the evidence to recommend one agent over another is less robust, in part because it is derived from many studies that were performed before the current era of aggressive antiplatelet therapy or studies which conflict with each other. The choice of agent depends upon the overall treatment strategy designed for each patient: 
• fibrinolytic therapy with either fibrin specific or non-fibrin specific agents, 
• primary percutaneous coronary intervention, or 
• no reperfusion.
• Nitrates — Intravenous nitroglycerin is useful in patients with persistent chest pain after three sublingual nitroglycerin tablets, as well as in patients with hypertension or heart failure. However, nitrates must be used with caution or avoided in settings in which hypotension is likely or could result in serious hemodynamic decompensation, such as right ventricular infarction or severe aortic stenosis. In addition, nitrates are contraindicated in patients who have taken a phosphodiesterase inhibitor for erectile dysfunction (or pulmonary hypertension) within the previous 24 hours.
• Morphine — Intravenous morphine sulfate at an initial dose of 2 to 4 mg, with increments of 2 to 8 mg repeated at 5 to 15 minute intervals, should be given for the relief of chest pain or anxiety.
• Beta blockers — Oral beta blockers are administered universally to all patients without contraindications who experience an acute STEMI. Contraindications include: 
• heart failure, 
• evidence of a low output state, 
• high risk for cardiogenic shock, 
• bradycardia, 
• heart block, or 
• reactive airway disease.

The 2007 focused update of the ACC/AHA 2004 guidelines on the management of STEMI suggest that an intravenous beta blocker may be considered in hypertensive patients who are not at high risk of cardiogenic shock on the basis of clinical risk factors. This was not changed in the 2009 focused update.

4) Other Treatment

• Arrhythmia management 
• Both atrial and ventricular arrhythmias can be seen during and after the acute phase of STEMI. These include atrial fibrillation or flutter, which can cause symptomatic hypoperfusion due to a rapid rate, and life-threatening ventricular tachycardia or ventricular fibrillation.
• Prophylactic intravenous or intramuscular lidocaine to prevent VT/VF in the acute MI patient is NOT recommended. Recommended prophylactic measures include early administration of an intravenous beta blocker and treatment of hypokalemia and hypomagnesemia.
• Sinus bradycardia can occur in patients with STEMI, especially when the inferior wall is involved. If the patient is symptomatic, therapy with atropine is indicated. Persistent sinus bradycardia may require temporary pacing.
• Atrioventricular nodal and intraventricular conduction abnormalities also may be seen in STEMI, particularly of the anterior wall. If the patient is symptomatic, temporary pacing is indicated. Asymptomatic patients with certain types of conduction abnormalities may also require prophylactic temporary pacemaker therapy, and some may require permanent pacemaker implantation.
• Non-steroidal antiinflammatory drugs — Nonsteroidal antiinflammatory drugs (except aspirin) should be discontinued immediately due to an increased risk of cardiovascular events associated with their use.
• Potassium and magnesium — Although there are no clinical trials documenting the benefits of electrolyte replacement in acute MI, the ACC/AHA guidelines recommend maintaining the serum potassium concentration above 4.0 meq/L and a serum magnesium concentration above 2.0 meq/L (2.4 mg/dL or 1 mmol/L).

Reference: UTD