Interval Measurement — Measure PR interval, QRS duration, and QT interval

The ability to measure cardiac intervals on a surface tracing arose from more than a century of innovation in electrophysiology. In the 1890s, Augustus Waller published the first human electrocardiogram using a capillary electrometer, yet the technology lacked the resolution needed to distinguish discrete waveform segments. It was Willem Einthoven who, through the invention of the string galvanometer, provided the fidelity required to identify the P wave, QRS complex, and T wave as separate electrical events—each corresponding to a distinct phase of the cardiac cycle. Einthoven's assignment of alphabetic labels (P, Q, R, S, T) gave clinicians a common language, and within a decade, physicians began correlating the time intervals between these waves with specific pathologies such as heart block, ventricular hypertrophy, and drug toxicity.

Despite advances in computerized interpretation, the fundamental question that motivated early electrocardiographers persists: how long does each phase of electrical conduction take, and does that duration fall within normal limits? Answering this question with precision is the core competency addressed in this lesson.

Before measuring any interval, you must understand the relationship between the EKG grid and real time. Standard EKG paper runs at 25 mm/s. Each small square on the horizontal axis represents 0.04 seconds (40 ms), and each large square (composed of five small squares) equals 0.20 seconds (200 ms). All interval measurements depend on counting these horizontal divisions between the onset and offset of defined waveform components. An interval encompasses both the waveform itself and any isoelectric segment between waveforms, while a segment refers to the flat baseline between two waveforms only.

The diagram above represents a single PQRST complex from a lead II rhythm strip. Notice that every interval measurement begins and ends at a precisely defined waveform boundary: the onset is defined as the first deflection from the isoelectric baseline, and the offset is the point where the waveform returns to baseline. In practice, the widest QRS complex across all 12 leads should be used for the QRS duration measurement, because a narrower lead may not capture a delayed conduction vector that is perpendicular to its axis. Similarly, lead II or V5 is often preferred for QT measurement because the T wave is typically most distinct in these leads.

Interval measurement on the EKG is fundamentally a linear distance-to-time conversion. Because the paper feeds at a constant rate, every millimeter of horizontal displacement corresponds to a fixed duration. At the standard paper speed of 25 mm/s, the relationship is straightforward: 1 mm equals 0.04 s. If your institution uses a non-standard speed (e.g., 50 mm/s for pediatric tracings), the conversion factor halves and each millimeter represents 0.02 s.

Knowing the normal ranges is only half the skill; recognizing and categorizing abnormal intervals is where clinical value lies. A prolonged PR interval (> 0.20 s) indicates delayed atrioventricular conduction—first-degree AV block—while a short PR interval (< 0.12 s) raises suspicion for pre-excitation syndromes such as Wolff-Parkinson-White (WPW), where an accessory pathway bypasses the AV node. A widened QRS (> 0.12 s) points to a bundle branch block or intraventricular conduction delay, and the morphology of the QRS in leads V1 and V6 determines whether it is right or left. A prolonged QTc exceeding 0.50 s is a red flag for torsades de pointes, a potentially fatal polymorphic ventricular tachycardia.

A 58-year-old patient presents for a routine pre-operative EKG. You are asked to verify the machine-generated interval measurements. The rhythm strip (Lead II, 25 mm/s) shows the following: the P wave begins 4 small squares before the QRS onset, the QRS complex spans 2.5 small squares, the R-R interval is 20 small squares, and the QT interval spans 10 small squares. Determine the PR interval, QRS duration, heart rate, and corrected QT interval.

Even experienced technicians make measurement errors when they neglect certain technical and physiological nuances. The table below summarizes the most common pitfalls alongside practical strategies to avoid them. Mastering these distinctions separates competent technicians from those who merely rely on the machine's automated readout.

The surface EKG intervals you learn as a CET candidate reflect electrophysiological events that can be measured far more precisely with intracardiac recordings during an electrophysiology (EP) study. In the EP lab, catheters placed inside the heart's chambers record local activation times, breaking the PR interval into discrete sub-intervals—the PA interval (intra-atrial conduction), the AH interval (AV nodal conduction), and the HV interval (His-Purkinje conduction). Understanding the surface correlates prepares you to communicate effectively with cardiologists and electrophysiologists who depend on these more granular measurements.

As you advance in your career, familiarity with these deeper measurements will enhance your ability to anticipate the cardiologist's interpretation and communicate abnormal findings promptly. For instance, if you recognize a PR prolongation pattern that progresses across serial tracings, you can alert the clinical team to the possibility of advancing AV block—potentially averting an emergent situation.