Chapter 22 How to Interpret an ECG
This review chapter summarizes a systematic approach to ECG analysis. Accurate interpretation of ECGs requires thoroughness and care. You should cultivate a systematic method of reading ECGs that is applied in every case.
Many of the most common mistakes are errors of omission, specifically the failure to note certain subtle but critical findings. For example, overlooking a short PR interval may cause you to miss the important Wolff-Parkinson-White (WPW) pattern. Marked prolongation of the QT interval, a potential precursor of torsades de pointes (see Chapter 16) and sudden cardiac arrest (see Chapter 19), sometimes goes unnoticed. These and other major, and avoidable, pitfalls in ECG diagnosis are reviewed in Chapter 23.
Most experienced readers approach an ECG in several “takes.” First, they do an overall gestalt or “big picture” scan to get the “lay of the land.” Then they go over each of 14 features looking at single leads and various combinations. This process may be repeated and rechecked several times before a final interpretation emerges.
You will quickly refine your skills if you get in the habit of “testing hypotheses” in the context of a differential diagnosis. As an example, if you see sinus rhythm with apparent “group beating” patterns, you should ask yourself if they are a manifestation of blocked atrial premature beats (APBs) or of second-degree atrioventricular (AV) block. If the latter, is the block nodal or infranodal, or is it indeterminate in location based on the ECG?
As described here, you should do your reading with the computer analysis, if available, covered up, so as not to be biased. Computer interpretations are frequently incomplete and sometimes completely wrong. Once you have committed to your reading, take a look at the computer interpretation. Sometimes it will point out something you missed. Often, it will miss something you found.
In special cases the ECG may be intentionally recorded at one-half standardization (1 mV = 5 mm) or two times normal standardization (1 mV = 20 mm). However, overlooking this change in gain may lead to the mistaken diagnosis of low or high voltage.
The basic heart rhythm(s) and rate(s) are usually summarized together (e.g., sinus bradycardia at a rate of 40 beats/min). The cardiac rhythm can almost always be described in one of the following four categories:
• Sinus rhythm or some ectopic rhythm (e.g., atrial tachycardia) with second- or third-degree heart block or other AV dissociation mechanism. When complete heart block is present, both the atrial rate and the escape mechanism should be described, along with whether it is narrow or wide complex in duration.
Recall that diagnosis of sinus rhythm requires the presence of discrete P waves that are always positive (upright) in lead II (and negative in aVR). Further, the rate should be physiologically appropriate. (Rarely, a right atrial tachycardia, originating near the sinus node, will mimic sinus rhythm—but the rate is usually faster and more constant than that of sinus tachycardia.)
Be particularly careful not to overlook hidden P waves. For example, these waves may be present in some cases of second- or third-degree AV block, atrial tachycardia (AT) with block, or blocked APBs. Also, whenever the ventricular rate is about 150 beats/min, always consider the possibility of 2:1 atrial flutter—in this instance, classic flutter waves are often hard to discern, or they may be mistaken for the P waves of AT or sinus tachycardia.
If you are unsure of the atrial or ventricular mechanism, give a differential diagnosis if possible. For example, you might say or write: “the rhythm appears to be atrial fibrillation with a noisy baseline, but MAT is not excluded.” If artifact precludes determining the rhythm with certainty, you should also state that and suggest a repeat ECG if indicated.
Calculate the heart rate (see Chapter 2). Normally, the ventricular (QRS) rate and atrial (P) rates are the same (1:1 AV conduction), as implied in the term “normal sinus rhythm.” If the rate is faster than 100 beats/min, a tachycardia is present. A rate slower than 60 beats/min means that a bradycardia is present.