Chapter 22 How to Interpret an ECG
ECG Interpretation: Big Picture and General Approach
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.
Fourteen Specific ECG Features to Analyze
On every ECG, 14 features (parameters) should be analyzed. These features are listed in Box 22-1 and discussed in the following sections.
Standardization and Technical Features
Make sure that the electrocardiograph has been properly calibrated so that the standardization mark is 10 mm tall (1 mV = 10 mm) (see Chapter 2).
Also check for limb lead reversal (see Chapter 23) and ECG artifacts (discussed later in this chapter).
Rhythm
• Sinus rhythm (or a sinus variant such as sinus bradycardia or sinus tachycardia)
• Sinus rhythm with extra (ectopic) beats such as APBs or ventricular premature beats (VPBs)
• An entirely ectopic (nonsinus) mechanism such as paroxysmal supraventricular tachycardia (PSVT), atrial fibrillation or flutter, ventricular tachycardia, or an AV junctional escape rhythm
• 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.
Heart Rate(s)
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.