of acute ST‐segment elevation are subtle and easily missed.
Box 9.2 Causes of ST‐segment elevation on 12‐lead electrocardiogram
Acute myocardial infarction
Normal ST‐segment elevation and normal variants
Left bundle branch block
Acute pericarditis and myocarditis
Hyperkalemia
Brugada syndrome and arrhythmogenic right ventricular cardiomyopathy
Pulmonary embolism
Transthoracic cardioversion
Prinzmetal angina
Source: Based on ref. [27].
Medications
Several medications are important for EMS management of the patient with chest pain. Providing the chest pain patient with medication for relief of pain whenever safe and feasible and regardless of the etiology of the pain is fundamental. Treatment of pain reduces anxiety in addition to relieving the patient’s discomfort. For ACS patients, treatment of pain can reduce catecholamine levels and thus improve the balance between oxygen demand and supply for ischemic cardiac muscle.
Oxygen
Despite its historical use, the evidence review leading up to the AHA Guidelines does not recommend the routine use of oxygen therapy in patients with uncomplicated AMI or ACS who have no signs of hypoxemia or heart failure [15]. Among patients with STEMI and without hypoxia, a randomized trial comparing supplemental oxygen versus no supplemental oxygen, initially in the EMS setting and continued into the hospital, showed no evidence of benefit and larger infarct size among those given oxygen [28]. The guidelines do, however, recommend oxygen administration if the patient is dyspneic or has an arterial oxyhemoglobin saturation <94%, signs of heart failure, or shock.
Aspirin
Aspirin is inexpensive, readily available, and has been shown to benefit patients having myocardial infarction or other ACS. The ISIS‐2 study established that the absolute benefit of aspirin administration for myocardial infarction patients results in 26 fewer deaths per 1,000 patients treated, with the maximal benefit occurring in the first 4 hours [29]. Prehospital administration of aspirin is safe, may improve outcome, and should be given as soon as possible to patients with suspected ACS unless contraindicated [15, 16, 30–32].
A sex difference has been documented in aspirin administration in the prehospital management of patients presenting with chest pain. Analysis of data in the National EMS Information System for about 2.4 million prehospital patients evaluated for chest pain showed that, for every 100 EMS chest pain calls, 2.8 fewer women received prehospital aspirin than did men [33].
Despite strong evidence of the benefit of aspirin in the treatment of chest pain, in a similar study of 198,232 patients eligible to receive aspirin by protocol, only 45.5% actually did. This highlights the importance of EMS systems to focus training on reinforcing aspirin administration to eligible patients and monitoring aspirin administration through established quality assurance programs [34].
Varying doses of aspirin have been proposed, but for ACS the most widely used dose is four 81‐mg baby aspirin tablets. These tablets are well tolerated, easy to swallow, and more rapidly absorbed than other preparations. Rectal preparations (300 mg) should be considered in patients unable to swallow. Acceptable contraindications to aspirin administration include definitive aspirin allergy or a history of active gastrointestinal bleeding.
Nitroglycerin
Nitroglycerin is a time‐honored treatment to relieve chest pain due to angina by decreasing myocardial oxygen demand and increasing collateral blood flow to ischemic areas of the heart. Somewhat surprisingly, nitroglycerin is not effective at reducing STEMI patient mortality [35]. Also, the response, or lack thereof, to nitroglycerin administration is not an accurate diagnostic test to determine whether cardiac ischemia is the underlying cause of a patient’s chest pain [36]. For example, because it relaxes smooth muscle, nitroglycerin may also relieve pain in patients with esophageal spasm.
Nitroglycerin can be administered as sublingual tablets or an oral spray. The usual dose for either method of delivery is 0.4 mg. Although up to three doses can be given, at intervals of 5 minutes between doses, current AHA/ACC recommendations for self‐administered patient use of nitroglycerin is for people to call EMS if chest pain is not improved 5 minutes after single dose of nitroglycerin. In so doing, STEMI patients avoid a potential 15‐ to 20‐minute delay before activating the EMS system [15, 16].
Although there is no difference in the incidence of nitroglycerin‐induced hypotension between groups of patients with inferior versus noninferior wall STEMI, nitroglycerin should be avoided in several groups of patients with chest pain [37]. Patients who have used phosphodiesterase inhibitors and then take nitrates can have profound, refractory hypotension. Nitrates generally should be avoided for 24 hours following sildenafil or vardenafil use and for 48 hours following tadalafil use.
Patients with a right ventricular infarction are dependent on right ventricular filling pressure to maintain cardiac output and a normal systolic blood pressure. If the patient has a systolic blood pressure below 100 mmHg or a heart rate below 60 beats per minute, nitroglycerin should be avoided until a 12‐lead ECG, including right‐sided leads, documents the absence of a right ventricular infarction. Nitroglycerin should also be avoided or used with caution in patients who already have systolic blood pressures <90 mmHg or heart rates <50 or >100 beats per minute [38].
Opiate analgesics
A large retrospective case series of hospitalized patients with non‐ST segment elevation ACS found that patients who received morphine had a higher mortality than those who did not [39]. It is unclear whether this was a causal effect or simply indicated that those who required morphine may have had more severe disease. A possible mechanism of harm from opiate therapy may be an interaction between opiate administration and platelet reactivity [40]. The AHA/ACC treatment guidelines for patients with unstable angina or non‐ST‐elevation MI (NSTEMI) reduce the strength of recommendation for morphine from Class I to Class IIa for patients with non‐STEMI [41]. The 2013 AHA/ACC STEMI Guidelines give morphine a Class I recommendation in STEMI patients because those patients are going to have reperfusion therapy [16]. The recommended dose of morphine for the patient with chest pain is 2–4 mg intravenously with increments of 2–8 mg intravenously repeated at 5‐ to 15‐minute intervals when pain is not adequately controlled with nitroglycerin.
Morphine sulfate has traditionally been the treatment of choice for prehospital patients suspected of ACS/STEMI. A common prehospital alternative is fentanyl. A prospective, randomized, prehospital clinical trial found the latter to be a safe and effective alternative to morphine sulfate, resulting in similar analgesia with no significant difference in hypotension [42].
Beta‐Blockers
Older guidelines recommended IV beta‐blocker (typically metoprolol) administration early in the course of acute myocardial infarction because of data suggesting reduced rates of reinfarction and recurrent ischemia when
