4.1 EKG Analysis and Interpretation Overview

Reading the ECG Grid

Every measurement in rhythm interpretation depends on the standardized ECG paper. At the default paper speed of 25 mm/sec with standard calibration, the grid is read as follows:

Because one large box equals 0.20 second, five large boxes equal one full second and 300 large boxes equal one minute. These two facts drive nearly every rate calculation and interval measurement on the exam. A normal calibration signal is a 10 mm (two large boxes) tall rectangular deflection; if the standardization mark is taller or shorter, the voltages on the tracing are distorted and the strip should be recalibrated before measuring amplitudes.

Calculating Heart Rate

The CET must estimate rate quickly and correctly. Three methods are tested:

• 300 method (large-box / regular rhythms): count the number of large boxes between two consecutive R waves and divide into 300. R waves three large boxes apart = 300/3 = 100 bpm; five boxes apart = 60 bpm. Memorize the cascade 300-150-100-75-60-50.
• 1500 method (small-box / precise): count small boxes between two R waves and divide into 1500. R waves 20 small boxes apart = 1500/20 = 75 bpm. This is the most accurate method but only valid when the rhythm is regular.
• 6-second method (irregular rhythms): count the QRS complexes within a 6-second strip (30 large boxes) and multiply by 10. This is the preferred method for irregularly irregular rhythms such as atrial fibrillation, where R-R intervals vary beat to beat and the box methods fail.

The 300 and 1500 methods assume regularity; if the rhythm is irregular, always default to the 6-second count.

Normal Intervals and the 5-Step Method

Waveform components map to cardiac electrical events: the P wave is atrial depolarization, the QRS complex is ventricular depolarization, and the T wave is ventricular repolarization. Memorize these normal durations:

Apply the same 5-step analysis to every strip: (1) determine the rate; (2) assess regularity of the R-R and P-P intervals; (3) examine the P waves (present, upright, one per QRS, uniform?); (4) measure the PR interval and check it is constant; (5) measure the QRS width. A tracing that is regular at 60-100 bpm with an upright uniform P before every QRS, a PR of 0.12-0.20 s, and a narrow QRS is normal sinus rhythm.

Waveforms, Segments, and What They Mean

Before measuring anything, name each deflection and tie it to a cardiac event. A clear mental map prevents mislabeling a segment, which is the root of most measurement errors:

• P wave: atrial depolarization. Normally upright in lead II, rounded, and uniform, with one P preceding each QRS.
• PR segment: the brief flat line after the P; reflects the conduction delay through the AV node that lets the atria empty before the ventricles fire.
• QRS complex: ventricular depolarization. The Q is the first downward deflection, R the first upward, and S the downward after the R.
• ST segment: the flat line between the QRS and the T wave; its level relative to the baseline signals injury or ischemia.
• T wave: ventricular repolarization; normally upright and slightly asymmetric.
• U wave: a small deflection sometimes seen after the T, often tied to slow heart rates or low potassium.

Measure intervals from the correct landmarks: the PR interval runs from the start of the P to the start of the QRS, the QRS from the start to the end of the complex, and the QT from the start of the QRS to the end of the T wave. Mislandmarking the QT (for example, stopping at the U wave) is a frequent error that throws off the measurement.

CET Scope and Escalation

The Certified EKG Technician (CET) acquires high-quality tracings and recognizes rhythms; the technician does not diagnose disease or prescribe treatment. That distinction shapes many exam answers. When a strip shows a life-threatening pattern, the correct action is to obtain a clean recording and escalate immediately to a licensed clinician rather than wait, treat independently, or ignore the finding.

The rhythms that demand the fastest escalation are ventricular tachycardia, ventricular fibrillation, asystole, third-degree (complete) heart block, and a new ST-elevation pattern. Knowing the normal values above lets you flag deviations confidently: a PR over 0.20 s suggests an AV conduction delay, a QRS of 0.12 s or wider suggests a ventricular or bundle-branch origin, and a chaotic baseline with no organized complexes suggests fibrillation or artifact. Always confirm the patient is responsive and the leads are attached before declaring an emergency rhythm.

Good interpretation also depends on a clean acquisition: the rhythm strip is most often read from lead II because its axis aligns with the heart's normal depolarization, producing the clearest upright P waves. If P waves are hard to see, switch to lead II before concluding that they are absent, which prevents over-calling junctional or atrial rhythms.