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).

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.

Also check for limb lead reversal (see Chapter 23) and ECG artifacts (discussed later in this chapter).

Rhythm

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:

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.)

Sometimes, the ECG will catch an abrupt change between two of the preceding mechanisms, for example, paroxysmal atrial fibrillation spontaneously converting to sinus rhythm.

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.

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.

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, there will be more P (or with atrial flutter or fibrillation, F or f) waves than QRS complexes, as with sinus rhythm with complete heart block.

PR Interval

The normal PR interval (measured from the beginning of the P wave to the beginning of the QRS complex) is 0.12 to 0.2 sec. A uniformly prolonged PR interval is often referred to as first-degree AV block (see Chapter 17). A short PR interval with sinus rhythm and with a wide QRS complex and a delta wave is seen in the Wolff-Parkinson-White (WPW) pattern. By contrast, a short PR interval with retrograde P waves (negative in lead II) generally indicates an ectopic (atrial or AV junctional) pacemaker.

P Wave Size (Amplitude and Width)

Normally the P wave does not exceed 2.5 mm in amplitude and is less than 3 mm (120 ms) wide in all leads. Tall, peaked P waves may be a sign of right atrial overload (P pulmonale). Wide (and sometimes notched P) waves are seen with left atrial abnormality.

QRS Width (Interval or Duration)

Normally the QRS width is 0.1 sec (100 ms) or less, measured by eye, in all leads (or 110 ms if measured by computer). The differential diagnosis of a wide QRS complex is described in Chapter 10. The differential diagnosis of wide complex tachycardias is described in Chapter 20.

QT/QTc Interval

A prolonged QT/QTc interval may be a clue to electrolyte disturbances (hypocalcemia or hypokalemia), drug effects (quinidine, procainamide, amiodarone, or sotalol), or myocardial ischemia (usually with prominent T wave inversions). Shortened QT intervals are seen with hypercalcemia and digitalis effect. The finding of a prolonged QT/QTc interval as determined by computer interpretation should always be rechecked manually. Conversely, computers sometimes miss prolonged QT intervals that are present. Formulas for computing the calculation of a rate-corrected QT interval (QTc) are discussed in Chapter 2.

QRS Voltage

Look for signs of right or left ventricular hypertrophy (see Chapter 6). Remember that thin people, athletes, and young adults frequently show tall voltage without left ventricular hypertrophy (LVH).

Low voltage may result from pericardial effusion or pleural effusion, hypothyroidism, emphysema, anasarca obesity, myocardial disease, or other factors (see Chapter 24).

Mean QRS Electrical Axis

Estimate the mean QRS axis in the frontal plane. Decide by inspection whether the axis is normal (between −30° and +100°) or whether left or right axis deviation is present (see Fig. 5-13).

R Wave Progression in Chest Leads

Inspect leads V₁ to V₆ to see if the normal increase in the R/S ratio occurs as you move across the chest (see Chapter 4). The term “poor” or, preferably, “slow” R wave progression (small or absent R waves in leads V₁ to V₃) refers to a finding that may be a sign of anterior myocardial infarction (MI), but may also be seen in many other settings, including: altered lead placement, LVH, chronic lung disease, left bundle branch block, and many other conditions in the absence of infarction.

The term reversed R wave progression is used to describe abnormally tall R waves in lead V₁ that progressively decrease in amplitude. This pattern may occur with a number of conditions, including right ventricular hypertrophy, posterior (or posterolateral) infarction, and (in concert with a limb lead reversal pattern) dextrocardia.

Abnormal Q Waves

Prominent Q waves in leads II, III, and aVF may indicate inferior wall infarction. Prominent Q waves in the anterior leads (I, aVL, and V₁ to V₆) may indicate anterior wall infarction (see Chapter 8).

ST Segments

Look for abnormal ST segment elevations or depressions.

T Waves

Inspect the T waves. Normally they are positive in leads with a positive QRS complex. They are also normally positive in leads V₃ to V₆ in adults, negative in lead aVR, and positive in lead II. The polarity of the T waves in the other extremity leads depends on the QRS electrical axis. (T waves may be normally negative in lead III even in the presence of a vertical QRS axis.) Remember that one of the P waves (with blocked ABPS or AT with block) can “hide” in the T waves, giving it a slightly different appearance from the other T waves.

U Waves

Look for prominent U waves. These waves, usually most apparent in chest leads V₂-V₄, may be a sign of hypokalemia or drug effect or toxicity (e.g., amiodarone, dofetilide, quinidine, or sotalol).