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Interpreting ECGs: An Integrative Approach
ECG Reading: General Principles
This review chapter details a systematic approach to electrocardiogram (ECG) analysis. Accurate interpretation of ECGs requires thoroughness and care. Trainees should be encouraged to cultivate a comprehensive, disciplined method of reading ECGs that can be applied in every case.
Many of the most common mistakes are errors of omission, specifically the failure to note subtle but critical findings. For example, overlooking a short PR interval may cause you to miss the Wolff–Parkinson–White (WPW) pattern. Marked prolongation of the QT interval and/or prominent U waves, a potential precursor of sudden cardiac arrest induced by torsades de pointes (see 16, 21 ), may go unnoticed until a code blue is called. Atrial fibrillation (AF) is mistaken for other supraventricular tachycardias (e.g., flutter, paroxysmal supraventricular tachycardia [PSVT] or even sinus tachycardia with atrial ectopy) with surprising frequency. Conversely, sometimes the diagnosis of AF is missed, for example, with underlying ventricular pacing because of the regularized ventricular response. These and other major, and avoidable, pitfalls in ECG diagnosis are reviewed in Chapter 24 .
The most experienced readers approach an ECG in several “takes,” analogous, to the way your expert colleagues examine medical imaging studies. First, they get an overall gestalt , a “big picture” scan to survey the “lay of the land.” Next, they home in on each of the 14 features in the next sections, looking at single leads, usually beginning with the rhythm strip, and then at various sets of leads. This process should be repeated in an iterative way several times before you formulate an integrative interpretation. The final step is writing out a concise summary.
Trainees can more quickly refine their ECG skills if they get into the mode of testing hypotheses in the context of working through a differential diagnosis. Take for instance the general finding of sinus rhythm with group beating patterns that are formed by clusters of QRS complexes. The general differential diagnosis of group beating includes two pathophysiologic mechanisms: prematurity and/or block. Thus you can address the key question of whether the group beating pattern represents (1) sinus rhythm with premature atrial complexes, which could be blocked or conducted, or (2) sinus rhythm with intermittent block in the atrioventricular (AV) (more rarely the sinoatrial [SA]) node. If the nonconducted P waves “march out” with “missing” QRS complexes suggesting second-degree AV block, you should then ask whether the second-degree AV block is nodal (Mobitz I) or infranodal (Mobitz II) or whether it is indeterminate in location from the surface ECG (see Chapter 17 ).
Also get in the habit of doing your reading with the computer (electronic) analysis, if one is available, covered up. This way you will not be biased or misled. Computer interpretations are frequently incomplete and sometimes partly or fully wrong. Once you have committed to your own reading, take a careful look at the computer interpretation. It may point out something you missed. On the other hand, the electronic reading may miss something you found. Be aware that even computer-measured intervals, which should be the most reliable part of electronic assessments, may need to be amended. This important caveat is discussed in the following sections.
The Importance of Being Systematic: 14 Points
On every ECG, 14 features (parameters) should be analyzed. These “must-check” items are listed in Box 23.1 and discussed in the next section. Note that items 2 to 4 are best considered as a group because they are interrelated. An example is given in Fig. 23.1 .
Box 23.1
Be Systematic: 14 Features to Analyze on Every ECG
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1
Standardization (Calibration and Technical Quality)
As a “reading reflex,” make sure that the electrocardiograph has been properly calibrated so that the standardization mark is 10 mm tall (1 mV = 10 mm) (see Chapter 3 ). 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). Overlooking this change in gain may lead to the mistaken diagnosis of low or high voltage. The paper speed of 25 mm/sec may also be changed in some situations. Finally, check for limb lead reversal (see Chapter 24 ) and ECG artifacts (discussed later in this chapter).
2
Heart Rate(s): Atrial and Ventricular
Calculate the heart rate(s)—atrial (if feasible) and ventricular (see Chapter 3 ). Normally, the atrial (P) and ventricular (QRS) rates are the same (sinus rhythm with 1:1 AV conduction), as implied by the term “normal sinus rhythm.” If the P or QRS rate is faster than 100 beats/min, a tachycardia is present. A rate slower than 50 to 60 beats/min means that a bradycardia is present.
Remember: You can see a combination of both fast and slow rates with certain arrhythmias and conduction disturbances (e.g., atrial flutter with a slow ventricular rate). If second- or third-degree (complete) heart block is present with underlying sinus rhythm, there will be more P than QRS complexes. In contrast, ventricular tachycardia with underlying sinus rhythm and AV dissociation produces a situation where the QRS rate exceeds the P wave rate (see Fig. 19.15 ).
3
Rhythm/AV Conduction
The basic heart rhythm(s) and rate(s) are usually summarized together (e.g., sinus bradycardia at a rate of 40 beats/min; AF with a mean ventricular response of 80 beats/min at rest). The cardiac rhythm can almost always be described in one of the following five categories:
- 1.
Sinus rhythm (including sinus bradycardia and sinus tachycardia)
- 2.
Sinus rhythm with extra (ectopic) beats, usually premature atrial complexes (PACs), premature ventricular complexes (PVCs), or, more rarely, escape beats
- 3.
Ectopic (non-sinus) mechanism, such as paroxysmal supraventricular tachycardia (PSVT a group of arrhythmias), AF or atrial flutter, monomorphic or polymorphic ventricular tachycardia, accelerated idioventricular rhythm (AIVR), or a slow ventricular escape rhythm.
- 4.
Sinus rhythm or an ectopic rhythm (e.g., atrial tachycardia) with second- or third-degree AV block. When complete AV heart block is present, you need to specify both the atrial and ventricular components (e.g., sinus rhythm at 70 beats/min with complete AV block and a narrow complex ventricular escape rhythm at 30 beats/min).
- 5.
Paced rhythm (single- or dual-chamber). Note: the rhythm may be fully or partially paced as described in Chapter 22 .
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 AF with a noisy baseline, but multifocal atrial tachycardia (MAT) is not excluded.” If artifact precludes determining the rhythm with certainty, you should also state that and suggest a repeat ECG or rhythm strip if indicated.
4
PR (AV) Interval
The normal PR interval (measured from the beginning of the P wave to the beginning of the QRS complex) is 100-200 msec. A prolonged PR interval is referred to as first-degree AV block or, preferably, as PR prolongation (see Chapter 17 ). A short PR interval with sinus rhythm and with a wide QRS complex and a delta wave is seen in WPW patterns. By contrast, a short PR interval with retrograde P waves (negative in lead II) generally indicates an ectopic (atrial or AV junctional) pacemaker. With AF, there is no PR interval. With atrial flutter the “FR” interval is usually not reported as such. With other rhythms, the PR interval is variable, as in second- or third-degree AV blocks or with MAT or wandering atrial pacemaker (WAP).
5
QRS Interval (Width or Duration)
Normally the QRS interval is 100 msec or less, measured by eye, in all leads (or 110 msec if measured electronically by computer algorithm). The general differential diagnosis of a wide QRS complex is described in Chapter 11 . The specific differential diagnosis of wide complex tachycardias is described in Chapter 19 .
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