dispatch. Identification of patients suspected to have ACS allows an EMS system to send advanced level clinicians 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 Chapter 88).
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]. Approximately 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. Approximately 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 over‐triage 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 over‐triage 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].
Although an ACS patient can present with a variety of clinical symptoms, a study in Utah revealed that more than half of patients proven to have AMI complained of chest pain or a breathing complaint at the point of dispatch [14]. The percentage of AMIs significantly increased for patients aged 35 years and older and varied significantly by sex, dispatch level, and chief complaint.
The American Heart Association (AHA) and American College of Cardiology (ACC) recommend that emergency medical dispatchers prompt patients with nontraumatic chest pain to take aspirin if they have no contraindications while awaiting EMS arrival [15, 16]. This recommendation is based on extrapolation from data showing that patients who take aspirin before hospital arrival are less likely to die, and it is likely quite safe [17].
The 12‐lead electrocardiogram
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 [18].
Performing a prehospital ECG on a patient exhibiting signs and symptoms of ACS is a Class I AHA/ACC recommendation [15, 16]. This recommendation is based on evidence demonstrating that, despite minimal 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 [19].
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)
