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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.