Rhythm Identification and Interpretation
Rhythm interpretation is the largest single domain on the Certified EKG Technician exam, roughly 35% of the questions, and it is the part of the job where recognition speed genuinely matters to a patient. This chapter builds the skill in the order the exam tests it: first the waveforms and the numbers that define normal, then the three ways to calculate a heart rate, then a rhythm-by-rhythm reference covering sinus, atrial, junctional, ventricular, blocked, and paced rhythms. It closes with the two judgment calls a technician makes every shift, telling artifact from a real rhythm and knowing which rhythms require someone at the bedside immediately.
The Waveforms and the Numbers That Define Normal
Every strip is read with the same five-step method: rate, regularity, P waves, PR interval, QRS duration. The P wave is atrial depolarization, the QRS complex is ventricular depolarization, and the T wave is ventricular repolarization. Atrial repolarization also happens but is hidden inside the QRS. The PR interval is measured from the beginning of the P wave to the beginning of the QRS and represents the time from atrial depolarization through the deliberate delay at the AV node. The QRS is measured from the first deflection off the baseline to the point where it returns. The QT interval runs from the start of the QRS to the end of the T wave and represents the entire ventricular cycle of depolarization and recovery. Because EKG paper runs at 25 mm/sec, one small box is 0.04 second and one large box is 0.20 second, so all of these measurements are made by counting boxes.
Three Ways to Calculate the Heart Rate
The exam expects all three methods and expects you to know when each one is valid. The 300 method divides 300 by the number of large boxes between two R waves and is the fastest estimate for a regular rhythm. The 1500 method divides 1500 by the number of small boxes between two R waves and is the most precise, again only for regular rhythms; the number comes from the 1500 small boxes that pass in one minute. The six-second method counts the QRS complexes in a six-second strip and multiplies by 10, and it is the only correct choice for an irregular rhythm such as atrial fibrillation. EKG paper is marked at three-second intervals across the top, so six seconds spans two marked intervals, or 30 large boxes. When atrial and ventricular rates differ, as in flutter or complete heart block, calculate them separately using P-P for the atrial rate and R-R for the ventricular rate.
Rhythm Criteria Reference
Rhythms are named by where the impulse starts and how fast it goes. Sinus rhythms start in the SA node and have an upright P wave before every QRS. Atrial rhythms start elsewhere in the atria, so the P wave looks different, is absent, or becomes a flutter or fibrillatory wave, while the QRS stays narrow. Junctional rhythms start at the AV junction, so the atria depolarize backward and the P wave is inverted, hidden, or after the QRS with a PR under 0.12 second; the QRS remains narrow. Ventricular rhythms start below the junction, so there is no related P wave and the QRS is wide and bizarre. AV blocks are defined by what happens to the relationship between the P waves and the QRS complexes. Use the table of criteria below the way you would use a checklist, matching rate, regularity, P wave, PR, and QRS one at a time.
Ectopic Beats and the AV Blocks
Premature beats interrupt an underlying rhythm and are named for where they start. A premature atrial contraction is early with an abnormally shaped but upright P wave and a narrow QRS. A premature junctional contraction is early with an inverted or absent P wave and a short PR. A premature ventricular contraction is early, wide, and bizarre with no preceding P wave and a T wave that points opposite the QRS. PVCs are further described by pattern and by origin: bigeminy is every other beat, trigeminy every third, quadrigeminy every fourth, a couplet is two in a row, unifocal PVCs all look alike, and multifocal PVCs vary in shape. The AV blocks are a separate family, defined by whether and how impulses cross from atria to ventricles. Working them out is a matter of measuring every PR interval and checking whether every P wave is followed by a QRS.
Paced Rhythms, Artifact, and the Lethal Rhythms
A pacemaker impulse appears as a thin vertical spike. A spike followed by a P wave indicates atrial pacing, a spike followed by a wide QRS indicates ventricular pacing, and a dual-chamber device may show both. Failure to capture is a spike with no waveform after it; failure to sense is a spike that appears at the wrong time in the cycle. Artifact, on the other hand, is any deflection that is not generated by the heart, and the single most important skill is refusing to treat the monitor instead of the patient. Chaotic-looking noise in a patient who is awake and talking is motion artifact, and often the underlying regular complexes can be seen marching straight through the noise. The lethal rhythms are the reverse situation: they demand that you leave the machine and go to the patient immediately.
Last updated: July 2026