Introduction

Interpretation of the electrocardiogram (ECG) must be done systematically, and the essential components need to be examined and synthesized into a coherent analysis. No single interpretation strategy is correct; in fact, many approaches can be used, considering patient needs, interpreter ability, the clinical setting, and the purpose of ECG. Regardless of the chosen method, efficiency and accuracy of interpretation are vital issues common to all such strategies of ECG review. The clinician is encouraged to develop his/her own style of approach, considering these issues and stressing these goals.

It cannot be overemphasized that a systematic (or “check‐list”) approach is absolutely necessary even for experienced clinicians. A systematic approach needs to be strictly followed with every ECG; however, it is especially important with a 12‐lead ECG, where the goal is not only to determine the rhythm but also to identify the underlying pathology. One recommended strategy is to consider the ECG in a stepwise manner, determining the rate, rhythm and regularity, axis, intervals, and morphology and then summarizing findings in a final interpretation.

In many cases, particularly with critically ill patients, following a systematic approach is understandably challenging because the clinician is simultaneously managing the airway, securing intravenous access, and ordering medications and other studies, in addition to obtaining and interpreting the ECG. In this instance, an initial rhythm interpretation followed by a careful systematic review of the ECG is a reasonable approach. The most important point in this interpretation is that every ECG obtained is carefully studied because a glaring and obvious finding can divert attention from a different but critical and more subtle finding that is easily missed if a systematic approach is not used. The ECG can provide important clinical information, involving not only the patient’s rhythm but also the cardiac and non‐cardiac conditions. Each ECG should be reviewed carefully looking for signs of disease such as pathologic rhythms, dysfunction of the conduction system, acute coronary ischemia, or infarction, and the impact of various toxins, electrolytes, and other disease states. Reviewing the individual ECG in an orderly manner by evaluating the rate, rhythm, axis, intervals, and morphology will help ensure that all abnormalities are recognized rapidly and efficiently.

The use of the ECG in a single‐ or multilead analysis mode is most appropriate for rhythm evaluation. In the hospital setting this is often readily available with bedside monitoring devices. When thinking about the difference between single‐lead monitoring and the multilead ECG (including the 12‐lead ECG), it is important to remember that the real value of the multilead ECG is that each lead provides a simultaneous yet different view of the same heart beat (Figure 3.1). Figure 3.1 demonstrates an analogy comparing two views of an automobile involved in a motor vehicle collision (MVC); from one perspective, the automobile does not appear to be significantly damaged, while a different view reveals significant damage to the car. The 12‐lead ECG in a suspected acute coronary syndrome (ACS) presentation demonstrates similar varying perspectives: one lead demonstrates a non‐worrisome ST segment contour, while another lead reveals ST segment elevation myocardial infarction (STEMI).

In most cases, the 12‐lead ECG offers little additional information about rhythm identification and has little immediate impact on patient management. However, in the setting of potential acute coronary syndrome (ACS), the 12‐lead ECG can provide important information critical for the diagnosis of ACS and also guide therapy, predict risk, and suggest alternative diagnoses. For ACS, the use of the 12‐lead ECG in “diagnostic mode” is the most appropriate electrocardiographic tool. Single‐lead monitoring remains important, however, for the detection of cardiac arrhythmias that frequently complicate ACS. Though the use of single‐lead rhythm monitoring is less valuable for diagnosing ACS because evaluation of the ST segment can be affected by device algorithms designed to reduce artifact.

Most single‐lead monitors are programmed by default to display lead II, as typically all elements essential to rhythm interpretation (P, QRS, and T) are well seen in this lead. Although the majority of rhythms can be determined by looking at only one lead, the experienced provider knows that at times it can be helpful to look at other leads if a particular element of the ECG is not well seen or the ECG does not match the patient condition (Figure 3.2). All monitors with basic limb leads can at least view leads I, II, and III, and it is always reasonable to look at another lead when presented when faced with a challenging rhythm. In the case shown in Figure 3.2, lead I appears to demonstrate aystole or fine ventricular fibrillation, while simultaneous leads II and III reveal a widened QRS complex, consistent with ventricular tachycardia.

Rate

The heart rate is expressed in beats per minute (bpm). In a normally conducted beat, every atrial contraction is followed by a ventricular contraction; thus, the atrial and ventricular rates are equal. If a dysrhythmia is present, these rates may not be identical and each must be calculated separately.

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Wolff–Parkinson–White Syndrome The Electrocardiogram in the Pediatric Patient Electrocardiographic Differential Diagnosis of Bradyarrhythmia Ischemic Heart Disease Clinical Applications of the Electrocardiogram Atrioventricular Conduction Block

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