PR, QRS, QT: Understanding EKG Intervals for Monitor Techs

Master the three critical EKG intervals every monitor tech must know — PR, QRS, and QT — plus ST segment changes and when to escalate findings.

Why Intervals Matter at the Central Station When you watch a cardiac monitor, you are watching electricity move through a heart. The waveforms — the P wave, QRS complex, T wave — show you what the electricity is doing at each moment. But the spaces between those waveforms, and the width of the waveforms themselves, tell you something equally important: how long the electricity takes to get from one place to another. Those time measurements are intervals. And for monitor techs, three intervals matter more than everything else combined: the PR interval, the QRS duration, and the QT interval. Add the ST segment to that list, and you have the four measurements that will drive the majority of your clinical notifications. You do not need to memorize every possible cause of an abnormal interval. You need to know what normal looks like, recognize when something has changed, and understand what that change might mean — well enough to communicate it clearly to the nurse. The PR Interval: AV Node Conduction Time The PR interval measures the time from the beginning of atrial depolarization (the start of the P wave) to the beginning of ventricular depolarization (the start of the QRS complex). In practical terms, it represents how long it takes the electrical impulse to travel from the SA node through the atria, through the AV node, and down to the ventricles. Normal range: 0.12 to 0.20 seconds (3 to 5 small boxes on EKG paper). Prolonged PR Interval (Greater Than 0.20 Seconds) When the PR interval stretches beyond 0.20 seconds, the impulse is taking too long to get through the AV node. This is first-degree AV block — though "block" is a misleading name because every impulse still gets through. A more accurate description would be "first-degree AV delay." What to watch for: First-degree AV block by itself is usually benign. Many patients live with it for years. But a PR interval that is progressively lengthening over the course of a shift is a different story — it may be a sign of worsening AV conduction that could progress to a higher degree of block. If you notice the PR interval getting longer on serial strips, report it. Practical tip: First-degree block is easy to miss if you are not specifically looking for it. The rhythm looks normal in every other way — regular rate, upright P waves, narrow QRS. The only abnormality is that the space between the P wave and QRS is a little too wide. Train yourself to glance at the PR interval as part of your systematic strip review. Short PR Interval (Less Than 0.12 Seconds) A PR interval shorter than 0.12 seconds means the impulse is arriving at the ventricles faster than it should. This raises the possibility of Wolff-Parkinson-White (WPW), a condition where an accessory pathway bypasses the AV node and conducts the impulse directly to the ventricles. Why it matters: WPW is not just a curiosity on the EKG. Patients with WPW are