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  • Basic Principles

    • Equipment

    • Normal ECG Complex

  • Axis Deviation

    • Clinical Correlations

  • Heart Rate

  • Rhythm

    • Sinus Rhythms

    • Atrial Arrhythmias

    • Nodal Rhythm

    • Ventricular Arrhythmias

    • Atrioventricular Block

  • Cardiac Hypertrophy

    • Atrial Hypertrophy

    • Ventricular Hypertrophy

  • Myocardial Infarction

  • Electrolyte and Drug Effects

    • Electrolyte Effects

    • Drug Effects

  • Miscellaneous ECG Changes

    • Pericarditis

    • Hypothermia

    • WPW (Wolff-Parkinson-White)

Chapter update by Allison L. Bailey, MD, FACC, FAACVPR and Steven A Haist, MD, MS


The formal procedure for obtaining an electrocardiogram (ECG) is given in Chapter 19, Electrocardiogram. Every ECG should be approached in a systematic, stepwise manner. Many automated ECG machines can give a preliminary interpretation of a tracing; however, all automated interpretations require analysis and sign-off by a physician. When reading an ECG 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 27-1).

  • Axis. If the QRS is upright (more positive than negative) in leads I and aVF, the axis is normal. The normal axis is –30 degrees to +105 degrees.

  • Intervals. Determine the PR, QRS, and QT intervals (Figure 27-2). Intervals are measured in the limb leads. The PR should be 0.12–0.20 s, and the QRS, <0.10 s (0.10–<0.12) incomplete bundle branch block (BBB), ≥0.12 s complete BBB. 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) between two QRS complexes 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 mm indicates LVH. Additional 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).

Figure 27-1

Examples of 10-mm standardization mark and time marks and standard ECG paper running at 25 mm/s.

Figure 27-2

Diagram of the ECG complexes, intervals, and segments. The U wave is normally not well seen. VAT = ventricular ...

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