The PR, QRS, and QT Intervals Case Studies

Home » MEDCLIN530 » EKG » 12-Lead EKG Confidence » II. Methodology: Measurements and their clinical significance » 5. The PR, QRS, and QT Intervals » The PR, QRS, and QT Intervals Case Studies
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Chris Anderson
MD · Clinical Education Director, Pediatrics
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Kevin Hodges
Vice Chair, Emergency Medicine
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Table of Contents

Case 1: Short PR interval—WPW syndrome

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Although the PR interval is measured only in the limb leads, examination of all the leads can help in the diagnosis of WPW syndrome. Leads I and aVL demonstrate a delta wave (the slurred upstroke) at the beginning of the QRS associated with the short PR interval. Leads V1 through V6 do as well.

Case 2: Short PR interval—WPW and supraventicular tachycardia

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The short circuit in WPW bypasses the normal AV node and its safety delay. If the patient develops atrial fibrillation or atrial flutter, the ventricles may be bombarded with impulses at a rate of over 300 bpm, as shown in Figure 5.7e.

Case 3: How to measure the QRS interval

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The QRS interval is measured from the beginning of the QRS to the end of the QRS in the limb leads only. In this example, the QRS interval is 2.0 boxes. Each little box represents 0.04 seconds. Thus, 2 boxes × 0.04 seconds for each box equals 0.08 seconds. Therefore, this QRS interval (2 × 0.04) is 0.08 seconds. Normally, the QRS interval is 0.08 to 0.09 seconds. It is normal if it is less than 0.10 seconds or less. This QRS interval is normal.

Case 4: Long QTc intervals

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The example below demonstrates long QTc interval. The measurement is made only in leads I, II, III, VR, AVL, and AVF. The QT measure 0.50 seconds. The normal QT interval ends before the halfway point to the next R wave.

Drugs and electrolyte effects not only cause prolongation of the QTc, but can also cause ST and T wave changes as shown on next slide.

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