Wagner_Marriot's Practical Electrocardiography, 12e

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F I G U R E 7 . 7 .

A. Slow onset of the QRS complex following a normal PR interval ( arrows ). B. Short

PR interval preceding a normal QRS complex duration ( arrows ).

However, the PR interval is not always abnormally short and the QRS complex is not always abnormally prolonged. Figure 7.7A illustrates an abnormally slow onset of the QRS complex following a normal PR interval (0.16 second). Figure 7.7B illustrates an abnormally short PR interval preceding a QRS complex of normal duration (0.08 second). Conduction through the bundle of Kent may be relatively slow, or the bundle of Kent may directly enter the His bundle. Among almost 600 patients with documented ventricular preexcitation, 25% had PR intervals of 0.12 second and 25% had a QRS complex dura- tion of 0.10 second. 5 When ventricular preexcitation is suspected in a patient with tachyarrhythmia but no ECG evidence of preexcitation, the following diagnostic procedures may be helpful: 1. Pace the atria electronically at increasingly rapid rates to induce conduction via any existing accessory pathway. 2. Produce vagal nerve stimulation to impair normal conduction through the AV

node so as to induce conduction via any existing accessory pathway. 3. Infuse digoxin intravenously for the same purpose as in procedure 2.

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CHAPTER 7: Ventricular Preexcitation