3.3 Waves, intervals & a systematic 5-step rhythm analysis

The Waveforms of One Cardiac Cycle

Each normal beat produces a repeating set of deflections. Knowing what each one represents lets you connect the strip to the underlying conduction.

• P wave — atrial depolarization. Normally small, rounded, and upright in Lead II, < 0.12 s wide and < 2.5 mm tall. One upright, uniform P before every QRS suggests a sinus origin.
• QRS complex — ventricular depolarization. The Q is the first negative deflection, R the first positive, and S the negative after R. Normal duration is < 0.12 s (under three small boxes). A wide QRS points to a ventricular origin or a bundle-branch block.
• T wave — ventricular repolarization; slightly asymmetric and normally upright in Lead II. Tall, peaked T waves suggest hyperkalemia, while inverted T waves may indicate ischemia.
• U wave — a small deflection after the T wave, best seen in V2-V3 and at slow rates; prominent U waves are associated with hypokalemia.

Intervals and segments

• PR interval — from the start of the P wave to the start of the QRS. Normal is 0.12-0.20 s (3-5 small boxes) and represents conduction from the atria through the AV node. Prolonged (> 0.20 s) means first-degree AV block; short (< 0.12 s) may indicate a pre-excitation pathway.
• QRS duration — start to end of the QRS; normal < 0.12 s.
• QT interval — from the start of the QRS to the end of the T wave; it varies inversely with heart rate, so a corrected QT (QTc) is used. As a rule of thumb the QT should be less than half the R-R interval at normal rates.
• ST segment — from the end of the QRS (the J point) to the start of the T wave; normally flat on the isoelectric baseline. Elevation or depression greater than 1 mm signals injury or ischemia.
• Isoelectric baseline (TP segment) — the flat line between the T wave and the next P wave; it is the reference against which ST deviation is measured.

Reference table of normal values

Measuring accurately

Measure every interval from the same reference points each time: the PR interval from the beginning of the P wave to the beginning of the QRS, and the QT from the start of the QRS to the end of the T wave where it returns to baseline. Count small boxes and multiply by 0.04 second — a PR spanning four small boxes equals 4 x 0.04 = 0.16 second, comfortably normal. When atrial and ventricular activity are dissociated, count the P-P interval for the atrial rate and the R-R interval for the ventricular rate separately; a faster atrial rate marching independently of a slower ventricular rate is the hallmark of complete (third-degree) AV block.

A Repeatable 5-Step Method

The CRAT exam rewards a systematic approach. Apply the same five steps to every strip so you never miss a finding.

Step 1 — Regularity. March out the R-R intervals with calipers. Is the ventricular rhythm regular, regularly irregular, or irregularly irregular? Check the P-P intervals for atrial regularity too.

Step 2 — Rate. Calculate the ventricular rate (300, 1500, or 6-second method) and, if the P waves differ, the atrial rate separately. Classify as bradycardia (< 60), normal (60-100), or tachycardia (> 100).

Step 3 — P waves. Is there one P wave before every QRS? Are they uniform and upright (sinus)? Look for absent P waves (junctional), sawtooth flutter waves, chaotic fibrillatory waves, or more P waves than QRS complexes.

Step 4 — PR interval. Measure it. Is it within 0.12-0.20 s? Is it constant, progressively lengthening (Wenckebach), or variable? A constant PR with a fixed relationship confirms normal AV conduction.

Step 5 — QRS width. Measure the QRS duration. Narrow (< 0.12 s) means a supraventricular origin conducted normally; wide (>= 0.12 s) suggests a ventricular focus, a bundle-branch block, or aberrant conduction.

Answer these five questions in order and the rhythm's identity nearly always falls out. For example: regular rhythm, rate 75, one upright P before each QRS, PR 0.16 s, QRS 0.08 s indicates normal sinus rhythm. Change only the P waves to "absent, with an irregularly irregular baseline" and the same framework points to atrial fibrillation. Discipline with the sequence — not memorizing every arrhythmia by shape — is what carries you through the exam.

Consider a second walk-through. The R-R intervals are irregular, the ventricular rate averages 40, there are more P waves than QRS complexes with a constant P-P interval, the PR interval on conducted beats is fixed at 0.18 s, and the QRS is 0.09 s. Steps 1 and 2 flag a slow, irregular ventricular response; step 3 reveals extra, non-conducted P waves; and steps 4 and 5 show normal conduction on the beats that do get through — a picture consistent with a second-degree AV block. Once again the framework, not a memorized snapshot, delivered the answer, and that is exactly the habit the CRAT exam is designed to reward.