Special Case of Pericarditis
Pericarditis is an inflammation of the pericardial sac. As the pericardium surrounds the entire heart and is closely adherent to the heart’s surface, inflammation of the pericardium will cause diffuse and widespread ST-segment elevation in practically all leads of a 12-lead ECG.43
When the ST-segment elevations appear unrelated to the pattern of ischemia seen with any specific coronary arteries, there should be a suspicion of pericarditis. Further assessment, including auscultation for a pericardial rub and an assessment of the chest pain while lying and seated, are in order.
Table 34-7 Etiologies of Normal ST-Segment Elevation
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• Therapeutic digoxin |
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• Pre-excitation syndromes |
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o WPW |
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o LGL |
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• Early repolarization syndromes (Congenital) |
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o Brugada syndrome |
Figure 34-18 Digitalis effect on the ST segment.
Figure 34-19 Hypokalemia. Nonspecific ST Changes
In some instances, the ST segment changes do not fit a pattern of ischemia, nor are they contributory toward another diagnosis. In some cases, the ST changes are transitory but not evolutionary. In those cases, the Paramedic merely notes that the 12-lead ECG has nonspecific ST changes.
There are a number of causes of nonspecific ST changes including improper lead contacts, electrolyte abnormalities, drug-induced changes, and hyperventilation. Even a drink of cold water can cause nonspecific ST changes.
Vasospastic Angina
One of the causes of transient patterns of myocardial is chemia is vasospasm. This vasospasm may be the result of many etiologies, including hyperventilation. These patients may present with symptoms consistent with acute coronary syndrome. 12-lead ECGs taken during this time will demonstrate ST-segment elevations that seemingly disappear spontaneously. This condition is called variant or Prinzmetal’s angina.
Approach to 12-Lead ECG Interpretation
There are several published approaches to the analysis and interpretation of the 12-lead ECG and each of these methods has one common characteristic. Success in accurate 12-lead interpretations requires a disciplined approach to the analysis as well as the avoidance of any presumptions. Most Paramedics read a 12-lead ECG from left to right, starting at the left corner. The left corner provides information about calibration, speed, and diagnostic quality.
Calibration
The 12-lead ECG machine is a scientific instrument. As such, it needs to be calibrated to ensure its accuracy. Unlike the past, when Paramedics had to physically calibrate the ECG machine, current machines are self-calibrating. To demonstrate this internal calibration, the ECG machine marks the calibration as a squared off calibration mark at the beginning of the recording (Figure 34-20). Standard gain is 1 mV to 10 mm (10 small boxes) of amplitude.
Speed
The paper speed (Figure 34-21) is critical for the analysis of the 12-lead ECG. The correct paper speed should be 25 mm/ second. In some instances, the Paramedic may have slowed the paper speed to better analyze slope characteristics. (The delta wave of WPW is sometimes difficult to discern when the paper speed is 25 mm/second.) However, a slower, or faster, paper speed will impact on the measurement of intervals (i.e., PRI, QRS, and QT), leading to errors in interpretation.
Diagnostic Quality
The last information in the left lower corner of the 12-lead ECG is related to frequency response (Figure 34-22). When the ECG is used as a monitor for dysrhythmia, the machine reduces the frequency response (i.e., the sample from the signal) to 0.5 Hz and 20 to 50 Hz. This helps eliminate some of the artifact but also diminishes the quality of the ECG.
The 12-lead ECG must have a frequency response of 0.05 (not 0.5) Hz to 150 Hz. By "opening" the range, minor changes in the ECG are observable. In this way, the 12-lead ECG can be diagnostic. Unfortunately, the artifact and noise eliminated by the narrow sample supports the importance of proper skin preparation and proper electrode placement.
12-Lead ECG Analysis
Having confirmed that the 12-lead is accurate (calibrated) and that the 12-lead ECG is diagnostic, the Paramedic can then proceed to analysis. There are several systems of analysis. The P, Q, R, S, T method helps to ensure that no change or abnormality is left undetected. Regardless of the methodology of analysis, the Paramedic should always maintain a detailed approach to analysis and never rush to judgment over what appears to be obvious signs of ECG changes. The decision to label a 12-lead ECG as indicative of acute coronary syndrome must be coupled with the patient’s clinical picture and the whole picture taken into consideration.
The start of every 12-lead ECG analysis is to confirm that the patient is not experiencing a dysrhythmia. The first mission of a Paramedic and emergency physician remains the treatment of dysrhythmia. This point is emphasized by the placement of a rhythm strip on the bottom of some 12-lead ECGs. Once the Paramedic has confirmed that the patient rhythm is normal (i.e., there is a P wave associated with a QRS) and sinus in origin, then the Paramedic can proceed to the rest of the analysis.
Figure 34-20 Calibration marking on the 12-lead ECG.
Figure 34-21 The paper speed is indicated on the 12-lead ECG.
Figure 34-22 The frequency response is indicated on the 12-lead ECG.
Some Paramedics analyze the 12-lead ECG by proceeding in a left-to-right and top-to-bottom fashion. Other Paramedics, with a trained eye, look for Q waves in the leads that are associated with specific coronary arteries (e.g., Leads II, III, and aVF) overlying the right coronary artery located in the inferior wall. Paramedics look for pathologic Q waves that are indicative of either an old myocardial infarction or a new myocardial infarction that is late in its evolution. The latter finding, Q waves in a late evolving MI, have some implications for further complications.
Following a search for Q waves, the Paramedic should take a moment and look at the R wave progression. A reverse R wave progression (RRWP) is suggestive of anterior ischemia. A RRWP can be likened to an early warning system, alerting the Paramedic to the possibility of sudden cardiac death before other ECG changes, such as ST-segment elevation, occur. Anterior wall MI (AWMI) can rapidly progress to either heart failure or sudden cardiac death.
Next, the Paramedic should look for ST-segment elevations and ST-segment depressions, which are indicative of reciprocal changes. The importance of delaying to find ST-segment elevations, suggestive of an ST elevation myo-cardial infarction (STEMI), is to reinforce the importance of a disciplined approach and to prevent the Paramedic from leaping to conclusions.
Finally, the Paramedic would turn to analysis of the T waves. While T waves are supportive of an argument for ACS, isolated T wave abnormalities may have no significance at all. Therefore, T wave changes should only be considered in the context of other ECG changes and the patient’s history.
