Chapter 13 Joseph P. Ornato, Michael R. Sayre, and James I. Syrett In the United States, someone experiences a myocardial infarction every 26 seconds, and alarmingly the disease claims one life each minute [1]. Acute myocardial infarction (AMI) accounts for almost five times as many deaths in the United States as are attributed to unintentional injuries, which has major implications for EMS systems [2]. About half of those who suffer acute myocardial infarctions are transported to the hospital by EMS, and many more patients call EMS for help because they are experiencing chest pain [3]. The prehospital management of chest pain has improved with better clinical examination, earlier administration of effective medications, and the broad use of 12-lead ECGs to detect acute coronary syndromes (ACS) and myocardial infarction more accurately before arrival in the emergency department (ED) [4]. Because more rapid reperfusion during acute myocardial infarction improves heart function and patient survival, EMS and health care systems have focused on developing strategies to identify chest pain patients with myocardial infarction quickly and to provide effective treatment while transporting them directly to definitive care [5–7]. The goals of management for patients with chest pain include rapid identification of patients with ACS, relief of their symptoms, and transport to an appropriate hospital. This chapter will cover the assessment and treatment of patients with a chief complaint of chest pain and will focus on the scientific basis for prehospital medical care of those patients. It will also review common conditions that can cause chest pain. When evaluating a patient with a complaint of chest pain, EMS professionals should begin by assessing the patient’s stability and then obtain a basic clinical history and examination. Early in the assessment, an EMS provider should apply a cardiac monitor to rapidly identify dysrhythmias, perform a diagnostic 12-lead ECG, and administer specific treatment depending on the results of the initial evaluation. Because only a small minority of the patients with chest pain actually have ACS, maintaining vigilance in this assessment and diagnostic routine can be difficult [8]. Complete accuracy in the diagnosis of chest pain is not always possible in any setting, not even in the hospital [9]. The prehospital provider should not expect to diagnose a patient with a complaint of chest pain definitively. A careful history can lead the provider to a correct “category” of diagnosis much of the time. As a general approach, the patient should be treated as if he or she has the most likely serious illness consistent with the signs and symptoms. Discomfort due to cardiac ischemia is usually, but not always, substernal and may radiate to the shoulder, either arm, both arms, upper abdomen, back, or jaw [9,10]. Other symptoms such as nausea and diaphoresis are commonly present but do not predict the presence or absence of ACS accurately. Cardiac disease is most often seen beginning in middle-aged men and older women. However, even younger adults under the age of 40 with no cardiac risk factors and a normal ECG have a 1–2% risk of ACS [11]. Taking a focused history using the “PQRST method” can be helpful (Box 13.1). There are many causes of chest pain and their incidence changes depending on the characteristics of the population being studied. Patients calling on EMS are more likely to have acute myocardial infarction or other serious causes of chest pain than are patients in the general emergency department (ED) population [3]. Although the majority of this chapter focuses on the management of an ACS, other causes of chest pain are present more commonly. Prehospital care of the patient with a complaint of chest pain begins at emergency medical dispatch. Identification of patients suspected to have ACS allows an EMS system to send advanced-level providers to the patient. Many EMS systems with both basic and advanced-level ambulances use a trained emergency medical call taker who asks the caller a series of questions to determine the nature of the emergency and the likelihood that advanced-level care will be needed (see Volume 2, Chapter 10). A retrospective cohort study from England took a rigorous approach to determining the accuracy of one set of dispatcher questions in identifying patients with ACS [12]. About 8% of calls at the “9-9-9” center were classified as “chest pain.” Subsequent chart review at the hospital identified all patients with the ultimate diagnosis of ACS and found that this represented only 0.6% of all 9-9-9 patients. About 80% of the ACS patients were classified correctly as chest pain at the dispatch level. Another 7% were classified in a variety of other categories that still received a paramedic level response (e.g. severe respiratory distress). Sensitivity of the dispatch system for detecting ACS was 71% and specificity was 93%. However, a great deal of overtriage occurred, and the positive predictive value of the dispatch system for detecting ACS was only 6%. Additional refinement of the dispatch question sequence to reduce overtriage seems possible. The emergency dispatch question sequence for stroke performs much better, with a positive predictive value of 42% and a similar sensitivity to ACS at 83% [13]. The American Heart Association (AHA) and American College of Cardiology (ACC) recommend that emergency medical dispatchers prompt patients with non-traumatic chest pain to take aspirin if they have no contraindications while awaiting EMS arrival [14,15]. This recommendation is based on extrapolation from data showing that patients who take aspirin before hospital arrival are less likely to die and that the practice is likely quite safe [16]. The 12-lead ECG remains the quickest method of detecting myocardial ischemia or infarction. Although ECGs have been used to diagnose ACS since 1932, the technology has now advanced to the point that a prehospital ECG can be done quickly and accurately and can be sent wirelessly to the receiving hospital at a relatively low cost. Additional benefit can be gained by having the prehospital ECG become the first of a series of ECGs, increasing the sensitivity of diagnosis of coronary syndromes [17]. Performing a prehospital ECG on a patient exhibiting signs and symptoms of ACS is a Class I AHA/ACC recommendation [14,15]. This recommendation is based on evidence demonstrating that, despite at most slightly increased time spent on scene for patients receiving ECGs, the time to definitive treatment for ST-elevation myocardial infarction (STEMI) with fibrinolysis or percutaneous coronary intervention (PCI) is shortened overall, with a significant reduction in mortality [18]. With the ease of obtaining a prehospital 12-lead ECG comes the need for its accurate interpretation. Precise interpretations can influence decisions to transport patients to more appropriate but more distant facilities, as well as immediate management strategies on hospital arrival. A 12-lead ECG is required to diagnose STEMI and can often provide evidence that ACS is present. Currently three methods of out-of-hospital ECG interpretation exist: computer algorithms integrated into the ECG machine, direct interpretation by paramedics, or wireless transmission of the ECG to a physician for interpretation. One, two, or all three can be used in a given EMS system. All prehospital 12-lead ECG machines contain computer programs that will interpret the ECG, and the machines can be configured to print the interpretation on the ECG. If this technology is sufficiently sensitive and specific for STEMI, the EMS professionals would theoretically not require education in interpretation, which would allow EMS systems to use advanced- and basic-level providers to acquire 12-lead ECGs. Additional benefits of using the computer’s interpretation include avoidance of the technical issues and cost of establishing base stations dedicated to receiving incoming ECGs, as well as the provision of consistent interpretation that does not depend on the variable skills and experience of EMS providers. Many prehospital 12-lead ECG systems use computerized interpretation systems which have high specificity, but the computer interpretation alone can miss up to 20% of true STEMI events [19]. Despite the high specificity, many emergency physicians and cardiologists do not place enough trust in the computer interpretation alone to routinely activate the cardiac catheterization PCI team that can provide rapid reperfusion treatment for a STEMI patient [20]. EMS provider interpretation is another option. More extensive training is required, and interpretation accuracy can be affected by both experience and interest in the subject matter [21]. Although several studies have shown that trained paramedics can accurately interpret the presence of STEMI, experience also plays an important role [22–24]. Having a paramedic identify and report “tombstones” on the 12-lead is a powerful motivator for action by experienced physicians. The third method of interpretation is by transmission of the acquired ECG to a base station for interpretation by a physician. This method has generally been used as the gold standard when comparing other methods of interpretation, and its accuracy has been shown to be slightly better than other methods. It relies both on the availability of the interpreting physician and on an infrastructure that allows reliable transmission of the ECG. In one observational cohort study, positive predictive value of prehospital 12-lead ECGs was improved by transmitting them to emergency physicians compared with interpretation solely by paramedics [24]. In some cases automated systems have been developed that allow simultaneous transmission of the 12-lead ECG to the receiving ED and to an invasive cardiologist on call [25]. These systems have the potential to decrease treatment times further because both the ED staff and the PCI team are activated early. The AHA guidelines state that the ECG may be transmitted for remote interpretation by a physician or screened for STEMI by properly trained paramedics, with or without the assistance of computer interpretation [14]. Advance notification should be provided to the receiving hospital for patients identified as having STEMI. Implementation of 12-lead ECG diagnostic programs with concurrent medically directed quality assurance is recommended. No diagnostic test is perfect, and the 12-lead ECG is no exception. There are a number of conditions other than acute myocardial infarction that can cause ST-segment elevation, such as left bundle branch block and hyperkalemia [26] (Box 13.2). Some of the differences between STEMI and the mimics of acute ST-segment elevation are subtle and missed easily. 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. Despite its historical use, the evidence review leading up to the 2010 AHA guidelines did not find sufficient evidence to recommend the routine use of oxygen therapy in patients with uncomplicated AMI or ACS who have no signs of hypoxemia or heart failure [14]. 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 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 maximum benefit occurring in the first 4 hours [27]. Prehospital administration of aspirin is safe [28] and may improve outcome [29,30], and should be given as soon as possible to patients with suspected ACS unless contraindicated [14,15]. 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 [31]. 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 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 [32], nor is the response, or lack thereof, to nitroglycerin administration an accurate diagnostic test to determine whether cardiac ischemia is the underlying cause of a patient’s chest pain [33]. 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 of either method of delivery is 0.4 mg. Although up to three doses can be given at an interval of 5 minutes between doses, current AHA/ACC recommendations for self-administered patient use of nitroglycerin is for them to call EMS if chest pain is not improved 5 minutes after only a single dose of nitroglycerin to avoid a 15–20-minute delay before activating the EMS system among STEMI patients [14,15]. Nitroglycerin should be avoided in several groups of patients with chest pain. 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 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 in patients who already have systolic blood pressures <90 mmHg or heart rates <50 or >100 beats per minute.
Chest pain and acute coronary syndrome
Introduction
General approach
Role of emergency medical dispatch
The 12-lead electrocardiogram
Prehospital electrocardiogram: interpretation
Medications
Oxygen
Aspirin
Nitroglycerin
Morphine sulfate
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