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Chapter 5. Preoperative EKG

Bessie Kachulis, MD; Ann E. Bingham, MD

Figure 5-1. EKG Waves and Segments
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Reproduced from Fuster V, Walsh RA, Harrington RA. Hurst’s The Heart. 13th ed. Figure 15-1. Available at: www.accessmedicine.com. Copyright © The McGraw-Hill Companies, Inc. All rights reserved.

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Waves, Intervals, and Common Pathologies
Time and voltage scaleWhen the ECG machine rate is 25 mm/s, each small box represents 0.04 s horizontally and 0.1 mV vertically
P wave
  • Represents: atrial depolarization
  • Duration: 80–120 ms
  • Amplitude: 2.5 mV in leads II and III
  • Morphology (see Figure 5-2):
    • Biphasic in the right precordial leads (V1 and V2)
    • Upright in the lateral leads (I, aVL, V5, V6), reflects right to left spread of the activation front
  • Axis: 0–90°
  • Pathology: atrial enlargement—P wave height greater than 2.5 mV
PR (or PQ) interval
  • Represents: impulse propagation from the atria throughout the AV node, bundle of His, bundle branches, and Purkinje fibers until the ventricular myocardium begins depolarization
  • PR duration: 120–200 ms
  • Pathology:
    • Long PR interval—slow AV nodal conduction (see section “First-Degree Heart Block”)
    • Short PR with a delta wave—Wolff–Parkinson–White syndrome or preexcitation
  • Note:
    • PR interval: beginning of P to the beginning of the QRS
    • PR segment: end of P to the beginning of the QRS
Q wave
  • Represents the negative deflection of the QRS before the R wave present in one or more inferior leads in more than 50% of normal adults, but is present in leads I and aVL in less than 50% of normal adults
  • Duration: less than 30 ms in limb leads, less than 50 ms in lead III, less than 30–40 ms in leads I, aVL, V5, and V6
  • Amplitude:
    • Less than 0.4 mV in all limb leads except the amplitude may be up to 0.5 mV in lead III
    • Depth of the Q wave should be less than 25% of the R wave height except in lead III
QRS
  • Represents: ventricular activation
  • Duration: 70–110 ms (measured in the lead with the widest QRS complex)
  • Amplitude: increases through V5 (commonly referred to as “R wave progression”); see LVH
  • Morphology: usually upwardly deflected in leads I and II, may be negative in aVL, usually downwardly deflected in aVR and V1; chest leads V2–V3 are transition leads and are usually isoelectric in appearance, variable in lead III
  • Axis: −30 to +90:
    • More negative than −30°: left-axis deviation
    • Greater than 90°: right-axis deviation
  • Pathology: wide QRS—increased duration of ventricular depolarization: bundle branch block, LVH, ventricle depolarized by an ectopic focus
QT interval
  • Represents: duration of ventricular systole
  • Duration: less than 460 ms for men and less than 470 ms for women; decreases with increasing heart rate, necessitating use of the QT corrected for heart rate, QTc
    • Bazett formula for the QTc: QTc = QT/
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