Chapter 19. Basic ECG Reading

The formal procedure for obtaining an ECG is given in Chapter 13, Electrocardiogram. Every ECG should be approached in a systematic, stepwise way. Many automated ECG machines can give a preliminary interpretation of a tracing; however, all automated interpretations require analysis and sign-off by a physician. Determine each of the following:

• Standardization. With the ECG machine set on 1 mV, a 10-mm standardization mark (0.1 mV/mm) is evident (Figure 19–1).
• Axis. If the QRS is upright (more positive than negative) in leads I and aVF, the axis is normal. The normal axis range is –30 degrees to +105 degrees.
• Intervals. Determine the PR, QRS, and QT intervals (Figure 19–2). Intervals are measured in the limb leads. The PR should be 0.12–0.20 s, and the QRS, < 0.12 s. The QT interval increases with decreasing heart rate, usually < 0.44 s. The QT interval usually does not exceed one half of the RR interval (the distance between two R waves).
• Rate. Count the number of QRS cycles on a 6-s strip and multiply that number by 10 to roughly estimate the rate. If the rhythm is regular, you can be more exact in determining the rate by dividing 300 by the number of 0.20-s intervals (usually depicted by darker shading) and then extrapolating for any fraction of a 0.20-s segment.
• Rhythm. Determine whether each QRS is preceded by a P wave, look for variation in the PR interval and RR interval (the duration between two QRS cycles), and look for ectopic beats.
• Hypertrophy. One way to detect LVH is to calculate the sum of the S wave in V1 or V2 plus the R wave in V5 or V6. A sum > 35 indicates LVH. Some other criteria for LVH are R > 11 mm in aVL or R in I + S in III > 25 mm.
• Infarction or Ischemia. Check for ST-segment elevation or depression, Q waves, inverted T waves, and poor R-wave progression in the precordial leads (see Myocardial Infarction).

Equipment

Bipolar Leads

• Lead I: Left arm to right arm
• Lead II: Left leg to right arm
• Lead III: Left leg to left arm

Precordial Leads:

V1 to V6 across the chest (see Figure 13–9).

ECG Paper:

With the ECG machine set at 25 mm/s, each small box represents 0.04 s, and each large box 0.2 s (see ...