More Than A Trend

Maximizing the potential of the prehospital 12-lead ECG

Tim Phalen
2009 Mar 1

Most EMS providers are aware of the increased attention hospitals are giving the prehospital 12-lead ECG. Although they’ve been around for years, recent research, recommendations and reimbursement structures have led to the increased use and valuation of 12-lead ECGs within the hospital. With this renewed interest, it’s appropriate to examine strategies to maximize the benefits of prehospital 12-lead ECGs.

In terms of improving ST-elevation myocardial infarction (STEMI) care, two goals are primary: First, increase the number of identified STEMI patients and, second, reduce the time to treatment. The answers to the following questions will shed light on strategies to improve EMS contribution to STEMI care. Although the questions may appear overly simplistic, they may yield some surprising answers.

WHO GETS A 12-LEAD?
EMS agencies have chest pain protocols. However, according to one study of more than 434,877 patients with a discharge diagnosis of acute myocardial infarction (AMI), 33% had no chest pain.(1) By implication, if EMS uses chest pain exclusively to "suspect AMI" and run a 12-lead, then one-third of AMIs could be missed.

To maximize the likelihood of catching STEMI on the 12-lead, it’s necessary to go beyond chest pain as the only patient complaint that would prompt a 12-lead ECG. So what other complaints should raise suspicion of possible AMI? Table 1 lists some "pain equivalents," or anginal equivalents, associated with AMI and STEMI.

Table 1: Anginal Equivalents Associated with AMI & STEMI

Respiratory distress
Sense that something is "wrong"
"Weakness"
"Fatigue"
"Dizziness"
"Malaise"
"Syncope or near syncope"
Alterations in blood sugar
Alterations in level of consciousness (particularly in the elderly)

Beyond the anginal equivalents, many AMI patients have pain that may not immediately seem cardiac in nature. Chest pain that’s intermittent, sharp, low intensity or not sub-sternal may be attributed to a variety of other conditions. However, although costracondritis, pleurisy and other conditions may indeed produce these types of complaints, they don’t preclude AMI. Aside from "non-cardiac sounding" chest pain, many AMI patients complain of pain to the abdomen, jaw, shoulder, teeth and elbow. All of these can be categorized as "atypical pain" presentations.

Certainly, not all atypical pain is from AMI; in fact, only a minority of these complaints are due to myocardial infarction. When these complaints are encountered, however, we must seriously consider the possibility of AMI.

With the extensive list of anginal equivalents (pain-free but not complaint-free) and atypical pain presentations (some pain present but not "classic" cardiac pain), it may seem that everyone should be getting a 12-lead ECG. Obviously, not everyone needs a 12-lead, but we can’t wait for classic chest pain to obtain an ECG. So the original question remains, "Who should get a 12-lead?"

One approach to consider is shown in Figure 1. This approach is offered not as a protocol recommendation, but rather as a starting point for discussion and critical thinking. In reviewing Figure 1, it’s obvious that all cardiac chest pain patients should have a 12-lead. However, not all anginal equivalents and atypical pain presentations necessarily require a 12-lead.

When faced with an anginal equivalent or atypical pain presentation, it’s worthwhile to recall the three groups of patients most likely to present in a non-classic manner: the elderly, females and diabetics. Therefore, you should seriously consider obtaining a 12-lead when you encounter a non-classic presentation in an elderly female or diabetic patient.

As a final double-check before deciding against a 12-lead, use your own clinical instinct. If you have a gut feeling this patient might be experiencing AMI, run a 12-lead. No harm will result from obtaining it. Similarly, it may be fruitful to ask about obvious cardiac risk factors.

When using this or a similar approach, the number of 12-leads obtained will certainly increase, but hopefully so will the number of identified STEMIs. Remember, finding STEMI is like panning for gold: You don’t expect to find a gold nugget in every pan, but when you do find one it’s worth all the effort. Likewise, it can be expected that most 12-leads will not identify STEMI; however, when STEMI is identified, and time to treatment is shortened, mortality and morbidity will decrease.

WHEN DO YOU OBTAIN THE 12-LEAD?
Getting early and (preferably) sequential ECGs can help improve the rate of STEMI recognition. Examine the 12-lead ECGs in Figure 2. Both tracings were obtained at the scene of a suspected AMI. ST elevation is obvious in the first ECG, but it disappeared only 12 minutes later when the second ECG was taken. Obviously, if efforts had been limited to a single ECG in the field, STEMI recognition may have been delayed.

HOW MANY LEADS?
Although the 12-lead ECG is currently the best, most available, most economical, most informative screening tool for STEMI, it’s not perfect. The 12-lead ECG has two "blind spots." The 12-lead ECG doesn’t directly "see" the right ventricle or the posterior wall of the left ventricle. Additional leads can be used to allow health-care providers to screen for STEMI in those areas.

For example, a patient may have an isolated posterior wall infarction. In that case, the 12-lead ECG may show some depression in the range of V1 to V3 or even V4 but would not demonstrate ST elevation. If additional leads were obtained from the patient’s back, ST elevation might be found.

Figure 3 shows an example of when an initial 12-lead did not show ST-segment elevation. However, the paramedic on the call suspected AMI clinically and noted the ST depression in the range of V1–V4. This prompted the acquisition of V4r (of the right ventricle) and V8–V9 (of the posterior wall). Because the additional leads were obtained, this STEMI was identified and directed for reperfusion.

HOW OFTEN DO CHANGES OCCUR & WHY?
A clear answer is still emerging, but departments have reported a range of 7–34% of prehospital 12-lead ECGs capturing dynamic changes in STEMI. Although not necessarily representative of all response-area demographics, the findings of one community are summarized on Figure 4. In this case, 34% of the STEMI patients had either ST elevation that was gone in later ECGs or had ST elevation present only in later ECGs.

Dynamic ischemic changes on an ECG can result from many potential causes. It may be impossible to tell exactly which is responsible in any given situation, but here are a few possibilities.

Infarct is dynamic in nature: An ongoing interplay of factors contributes to ECG changes. Among them are variations in myocardial oxygen demand and chemical factors in the clotting process, which can induce coronary artery vasoconstriction. For these and other reasons, ST changes can occur simply as part of the infarct process.

EMS treatment: Oxygen has been shown to reduce or eliminate ST change.(2) In addition, nitroglycerin can dilate the target coronary artery and also reduce or eliminate ST elevation.(3)

Vasospastic angina: Prinzmetal’s angina results from coronary artery vasospasm. During episodes of vasospasm, the ST segment typically elevates. Nitroglycerin often relieves the vasospasm and the ST elevation along with it. Trending can help capture this (see ST-SEGMENT Trending Capabilities).

DO ECG CHANGES MATTER?
Transient changes on the ECG are more than just interesting little quirks of electrocardiography. In some cases, they can completely alter the diagnosis and resulting treatment, and there are several specific ways that the presence of transient changes can make a difference.

Better recognition of STEMI: The ECG changes associated with AMI and STEMI can be dynamic. Serial (consecutive) ECGs can increase the likelihood of catching those changes. Researchers have determined that, when compared with the initial 12-lead ECG at emergency department (ED) presentation, ST monitoring improves the sensitivity and specificity in recognizing acute coronary syndrome (ACS) and AMI. In the case of AMI, one study has shown that diagnostic sensitivity improved from 55.4% in the initial ECG to 68.1% with serial ECGs.(4,5)

Improved identification of reperfusion candidates: Not all infarct patients improve with immediate reperfusion. STEMI is the primary indication that a patient would benefit from either fibrinolytics or percutaneous intervention (PCI), such as angioplasty and stenting. Serial ECGs help better identify not only MI, but also the subset of infarct patients who are candidates for immediate reperfusion.

The 2004 ECC Guidelines make serial ECGs a Class I recommendation in the ED: "If the clinical ECG is not diagnostic of STEMI but the patient remains symptomatic and there is a high clinical suspicion for STEMI, serial ECGs at five- to 10-minute intervals or continuous 12-lead ST-segment monitoring should be performed to detect the potential development of ST elevation."(3)

LBBB & other confounding patterns: Left bundle branch block (LBBB) frequently causes ST elevation when no infarct exists and, in that sense, is an imitator of infarct. However, AMI can also produce a new onset LBBB, and in that setting immediate reperfusion is indicated.

Unfortunately, it can be difficult to determine if the presence of LBBB on the ECG of a suspected AMI patient is pre-existing or is a new onset. If the LBBB is infarct-induced, it has a high mortality rate—up to 60%. Therefore, the patients who may need reperfusion the most are the least likely to receive it. However, dynamic changes on serial ECGs shed light on the situation. A hallmark of infarct is change over time. If a patient has had an LBBB for the past 15 years, it’s not likely to change much during the next 15 minutes. But when changes occur in a short period of time, suspect AMI.

When AMI is suspected clinically, LBBB is present on the 12-lead and changes are observed in serial ECGs, then new onset LBBB is presumed to be infarct-induced. Such patients are potential candidates for immediate reperfusion.

Better identification of high-risk unstable angina: AMI and STEMI are two points on the continuum of ACS. Another point on that continuum is unstable angina. In this condition, the coronary artery is often blocked by a blood clot but shows no evidence of tissue death. Hence, it can’t be called infarction. Treatment of unstable angina varies depending on certain findings. High-risk unstable angina patients, although not eligible for immediate reperfusion, may receive an urgent catheterization. Dynamic ECG changes are one criterion used to identify high-risk unstable angina patients.

To date, little work has been done to determine the number of patients whose diagnosis or treatment decision could be made from information exclusively present on the prehospital ECG. However, one recent study looked at how often the prehospital 12-lead contained information that would identify high-risk unstable angina patients. It found that 22% of patients with ACS (not necessarily AMI or STEMI) had evidence of ischemia that was not present on arrival at the hospital.(6) This is an important finding.

HOW DO YOU COMMUNICATE YOUR ECG FINDINGS?
Once specific ECG findings suggest STEMI has been identified, minutes matter. It’s imperative to communicate that the patient is a potential candidate for immediate reperfusion. Some possible communication strategies include presenting the 12-lead at arrival or using the radio to alert the receiving facility of your ECG findings and transmitting the ECG.

Several studies have looked at various strategies. One recent example of the importance of early notification demonstrated the results shown in Table 2.

Table 2: Patients with Treatment Time of 90 Minutes or Less

Patients with no 12-lead ECG                                        37.5%
Patients with 12-lead presented at arrival                       51.0%
Patients with STEMI Alert from field                                85.7%

When deciding which communication strategy works best for a particular community, several factors, such as terrain, cellular coverage and budget, must be taken into consideration. Bear in mind, recent technological improvements make 12-lead transmission much more reliable and practical than even a few years ago.

PRACTICAL CONSIDERATIONS
EMS has a logistical advantage when it comes to performing serial ECGs. In the ED, patients outnumber the staff, but in the field an entire team focuses on one cardiac patient. In the ED, patients aren’t typically assigned their own 12-lead machine, but in the field, that’s precisely the case.

In the ED, repeat ECGs are often done at 30-minute intervals; EMS can easily get a repeat ECG with every set of vitals, or if ST trending is available, automatically obtain a 12-lead every 30 seconds (see sidebar, p. 22).

With practice, 12-leads can be obtained on scene with little or no increase in scene time. In many situations, it’s possible to work the 12-lead into the call early on, even before nitroglycerin would be administered. When this is feasible, it provides an opportunity to establish a baseline ECG before medications are administered. As mentioned above, this process is worthwhile but should be done without delaying treatment.

CONCLUSION
When it comes to recognizing STEMI, EMS is in a privileged position. Who better to obtain early ECGs, serial ECGs as often as every 30 seconds and even get additional leads when indicated? No one. Although dynamic changes won’t occur in every patient, or even every shift, routine acquisition of early and serial 12-lead ECGs increases the likelihood of recognizing STEMI, thus shortening the time to lifesaving treatment.

Increasing our level of suspicion as to who should get a 12-lead, striving to obtain the first 12-lead as early as possible and prioritizing the importance of serial ECGs are three important steps to improve STEMI care. Considering that EMS is in the unique position to obtain early and repeat ECGs, the logical question to ask ourselves is, "Are we seizing this valuable opportunity?"

Tim Phalen has presented 12-lead education to more than 35,000 participants. He is the co-author of the textbook The 12-lead ECG in Acute Coronary Syndromes and developer of online 12-lead and STEMI educational programs. He can be reached through his Web site at ECGSolutions.com.

Disclosure: Tim Phalen serves as a consultant to Physio-Control, Inc. He has also provided education sponsored by Physio-Control, Inc.

REFERENCES
1. Canto JG, Shlipak MG, Rogers WJ, et al: "Prevalence, clinical characteristics and mortality without chest pain among patients with myocardial infarction presenting." JAMA. 283(24):3223–3229, 2000.

2. Harvey RA, Fuller FP: "The dynamic nature of ST segment and T-wave changes during acute MI." Prehospital and Disaster Medicine. 12(4):313–317, 1997.

3. Antman EM, Anbe DT, Armstrong PW, et al: "ACC/AHA guidelines for the management of patients with ST-elevation myocardial infarction." Circulation. 110(9):e82–292, 2004.

4. Fesmire FM, Percy RF, Bardoner JB et al: "Usefulness of automated serial 12-lead ECG monitoring during the initial emergency department evaluation of patients with chest pain." Annals of Emergency Medicine. 31(1):3–11, 1998.

5. Jernberg T, Lindhal B, Wallentin L: "ST-segment monitoring with continuous 12-lead ECG improves early risk stratification in patients with chest pain and ECG nondiagnostic of acute myocardial infarction." Journal of the American College of Cardiology. 34(5):1413–1419, 1999.

6. Drew BJ, Dempsey ED, Joo TH, et al: "Pre-hospital synthesized 12-lead ECG ischemia monitoring with trans-telephonic transmission in acute coronary syndromes: Pilot study results of the ST SMART trial." Journal of Electrocardiology. 37(suppl.):214–221, 2004.

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More Than A Trend

Maximizing the potential of the prehospital 12-lead ECG

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