4.2 Core Workflows and Decision Points

Sinus Rhythms

When the sinoatrial (SA) node sets the pace, the strip shows an upright, uniform P wave before every QRS in lead II, a constant PR interval, and a narrow QRS. The four sinus patterns differ mainly by rate and regularity:

Sinus arrhythmia is the key trap: it is irregular yet benign, because every beat still originates at the SA node with a normal P wave. Do not mistake its respiratory variation for a pathologic irregular rhythm.

Atrial Rhythms

When an irritable focus above the ventricles fires, the P-wave morphology changes. Key atrial patterns:

• Premature atrial contraction (PAC): an early beat with an abnormal or differently shaped P wave that interrupts the underlying rhythm; the QRS is usually narrow.
• Atrial flutter: a re-entry circuit produces classic sawtooth flutter (F) waves at an atrial rate near 250-350 bpm. The AV node blocks many impulses, so a 2:1, 3:1, or 4:1 conduction ratio gives a slower, often regular ventricular rate.
• Atrial fibrillation: multiple chaotic atrial foci produce no discrete P waves, only a wavy fibrillatory baseline, and the ventricular response is classically irregularly irregular. Use the 6-second method to estimate rate.
• Supraventricular tachycardia (SVT): a regular narrow-complex tachycardia, often 150-250 bpm, so fast that P waves are hidden in the preceding T wave.
• Multifocal atrial tachycardia (MAT): at least three differently shaped P waves with varying PR intervals, irregular and over 100 bpm.
• Wandering atrial pacemaker (WAP): the same shifting P-wave morphology as MAT but at a normal rate (under 100 bpm).

Distinguishing the Fast and the Irregular Atrials

Two pairs of atrial rhythms are routinely confused, and the exam exploits that. Use rate and P-wave variability to separate them:

The clinical stakes differ too: uncontrolled atrial fibrillation and flutter raise stroke risk because blood pools in the fibrillating atria, which is why a new irregularly irregular rhythm is always worth reporting. SVT may be terminated by vagal maneuvers ordered by a clinician, but the technician's role is to capture a clean 12-lead during the episode so the provider can classify it.

Junctional Rhythms and Decision Logic

If the SA node fails, the AV junction can take over as an escape pacemaker, intrinsically firing at 40-60 bpm. Because the impulse spreads backward into the atria, the P wave is inverted, absent, or buried in the QRS, while the QRS itself stays narrow. Rate variants include a junctional escape rhythm (40-60), accelerated junctional rhythm (60-100), and junctional tachycardia (over 100).

Use this decision sequence when a tracing puzzles you:

1. Are P waves present and upright with one per QRS? If yes, suspect a sinus rhythm and confirm by rate.
2. Are P waves abnormal, multiple, sawtoothed, or absent? Suspect an atrial rhythm; check whether the baseline is a flutter sawtooth or fibrillatory wave.
3. Are P waves inverted, missing, or buried with a narrow QRS at 40-60 bpm? Suspect a junctional rhythm.
4. Is the ventricular response irregularly irregular with no P waves? That pattern is atrial fibrillation until proven otherwise.

This ordered logic prevents the common error of naming a rhythm from the rate alone before checking the P waves.

A useful cross-check is to confirm that the atrial rate and ventricular rate agree. In normal sinus rhythm and most sinus and junctional rhythms, every P conducts, so the two rates match. When they diverge, the cause is informative: in atrial flutter the atrial rate (P or F waves, 250-350) far exceeds the ventricular rate because the AV node blocks most impulses; in complete heart block the atrial and ventricular rates are simply unrelated. Counting both rates separately, rather than assuming one rate for the whole strip, catches these high-yield patterns.

Finally, remember that the default escape pacemakers form a hierarchy: the SA node fires at 60-100, the AV junction at 40-60, and the ventricles at 20-40. A slow rhythm with the wrong P-wave pattern for its rate usually means a lower pacemaker has taken over.