Prehospital electrocardiogram interpretation
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 (see Figure 9.1).
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 clinicians would theoretically not require education in interpretation, which would allow EMS systems to use advanced and basic‐level personnel 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 clinicians. Many prehospital 12‐lead ECG systems use computerized interpretation systems that have high specificity, but the computer interpretation alone can miss up to 20% of true STEMI events [20].
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 [21]. EMS clinician interpretation is another option. Additional extensive education is required, and interpretation accuracy can be affected by both experience and interest in the subject matter [22]. Although several studies have shown that trained paramedics can accurately interpret the presence of STEMI, experience also plays an important role [23–25]. When a paramedic identifies and reports “tombstones” on the 12‐lead ECG, experienced physicians are powerfully motivated to take action.
Figure 9.1 A prehospital 12‐lead ECG showing atrial fibrillation with a rapid ventricular rate and widespread ST‐segment elevation diagnostic for acute myocardial infarction. The ability of EMS clinicians to activate the hospital cardiac catheterization laboratory directly from the scene upon making such a diagnosis, and transport such a patient directly to the laboratory, has been demonstrated to decrease time to definitive care by PCI.
Source: Courtesy of Dare County [North Carolina] Emergency Medical Services.
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 criterion 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 facilitates reliable ECG transmission.
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 [25]. In some cases, systems have been developed that enable simultaneous transmission of the 12‐lead ECG to the receiving ED and to an interventional cardiologist on call [26]. 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 [15]. 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 management is recommended.
No diagnostic test is perfect, and the 12‐lead ECG is no exception. A number of conditions other than acute myocardial infarction can cause ST‐segment elevation, such as left bundle branch block and hyperkalemia (Box 9.2) [27]. Some of the differences between STEMI and the mimics 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