Look at these photos of volcanic eruptions. Each time the shutter clicked, that’s exactly how the volcano looked. But from one moment to the next, something changed. Everything changes, and changes are often dramatic. This is especially true with 12-lead ECGs.
Think of it this way: Acute myocardial infarctions (AMIs) aren’t like broken bones and, therefore, ECGs aren’t static like X-rays. If an„EMS crew were treating a hip fracture and could somehow perform the X-ray on scene, what would it show? A broken hip, of course. And if the X-ray wasn’t performed until the patient arrived at the emergency department (ED), would the broken hip still be visible? Absolutely.
When dealing with a fracture, whether the X-ray is obtained immediately, in 10 minutes or in 10 hours, the interpretation and diagnosis usually won’t change. But what’s true for X-rays isn’t necessarily true for ECGs. In fact, an ECG can significantly change in a very sort period of time — as can the corresponding interpretation.
Examine the 12-lead ECGs presented in Figure 1 (p. 68). Both tracings were obtained on scene at a suspected AMI.„Obvious ST elevation is present in the first ECG, but is gone only eight minutes later when the second ECG was taken.
Why it matters„
Transient changes on the ECG are more than just an interesting little quirk of electrocardiography. In some cases, they can completely alter the diagnosis and resulting treatment a patient receives. What follows are several specific ways that the presence of transient changes can make a difference.
Better recognition of AMI: Acute myocardial infarction can be very difficult to recognize, whether in the field or in the ED. Missed AMI remains the number one dollar loss for malpractice claims among ED physicians.1 One study found that among more than 400,000 patients with a discharge diagnosis of AMI, approximately 33% didn’t have chest pain, back pain, abdominal pain, etc.2 Therefore, although chest pain is the most common presentation in AMI, it’s important to understand that ˙atypicalÓ presentations are by no means infrequent.
Sometimes the only way to recognize AMI is with a 12-lead ECG, but if the 12-lead happened to have been obtained at a moment when the changes weren’t present, the wrong conclusion could be drawn. Serial (consecutive) ECGs can increase the likelihood of catching those changes. Researchers have determined that when compared with the initial 12-lead ECG at 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.3,4
Improved identification of reperfusion candidates: Not all infarct patients improve with immediate reperfusion. ST-segment elevation myocardial infarction (STEMI) is the primary indication that a patient would benefit from either fibrinolytics or percutaneous intervention (PCI), such as angioplasty and stenting. Serial ECGs not only help better identify 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.Ó5
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 preexisting 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 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 is 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.
Causes of ischemic changes
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.7 In addition, nitroglycerin can dilate the target coronary artery and also reduce or eliminate ST elevation.5
Vasospastic angina: Prinzmetal’s angina is due to 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.
EMS has the logistical advantage when it comes to performing serial ECGs. In the ED, patients outnumber the staff, whereas 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. 72).
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 earlier. As mentioned above, this process is worthwhile but should be done without delaying treatment.
Given its potential, a strong case can be made for the value of early and repeat 12-lead ECGs.„EMS is in a privileged position to take on this standard. Think about it: Who else can obtain an early 12-lead ECG while the patient has complaints, before they receive medications and acquire serial ECGs? 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.
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?Ó
The author wishes to thank the Marin County (Calif.) Fire Department for providing the ECGs used in Figure 1.
ST-Segment Trending Capabilities
Because of the dynamic nature of acute coronary syndromes, when STEMI is suspected but the initial ECG is non-diagnostic, EDs are to either manually obtain a 12-lead ECG every five to 10 minutes or use an ST-segment monitor to trend the ST segment (a Class I recommendation).
At least one prehospital monitor offers a feature known as ˙ST segment trending,Ó which emulates the function of an ST-segment monitor. In units with ST trending, once the ˙12-leadÓ button is pressed, the device not only samples and prints a 12-lead ECG but will then automatically re-sample a 12-lead every 30 seconds thereafter. This re-sampling is done internally, and the 12-lead is not printed out or displayed on the screen.
The monitor analyzes the re-sampled ECGs and identifies changes of at least 1 mm in the ST segment, whether upward or downward. Such a change in one sample may be due to an ischemic event or may simply be the result of patient movement or artifact. Therefore, the ST segment trending algorithm requires the change to persist for five samples, or about two and a half minutes, before meeting the threshold for an alert.
If the ST segment change meets that threshold, a new 12-lead is printed, alerting the care provider and documenting the event. The ECGs in figure 1 captured the STEMI by use of this feature.
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