A rhythm strip shows an early beat preceded by a P wave that is shaped differently from the other P waves, followed by a narrow QRS complex. Which abnormality is this?

Premature atrial contraction (PAC). An early beat with an abnormally shaped P wave and a narrow QRS is a premature atrial contraction: the impulse starts from an ectopic atrial site but still travels down the normal conduction pathway. A PVC has no preceding P wave and a wide, bizarre QRS, and escape beats come late, not early.

On a rhythm strip, the PR interval grows longer with each beat until a P wave is not followed by a QRS, and then the pattern repeats. What rhythm is this?

Second-degree AV block type I (Wenckebach). Progressive PR lengthening followed by a dropped QRS, repeating in groups, is second-degree AV block type I (Mobitz I or Wenckebach). First-degree block has a long but constant PR with no dropped beats, second-degree type II has a constant PR with sudden drops, and third-degree block has no relationship between P waves and QRS complexes.

A patient's strip shows regular P waves at 80 per minute and regular wide QRS complexes at 36 per minute, with no consistent relationship between them. The patient is dizzy. What should the PCT recognize and do?

Third-degree (complete) heart block; stay with the patient and report it immediately. When P waves and QRS complexes march independently, with the atrial rate faster than the ventricular rate, the rhythm is third-degree (complete) heart block. A slow, wide ventricular escape rhythm with dizziness can quickly worsen, so the PCT stays with the patient and reports immediately; the patient may need emergency pacing.

A patient suddenly develops a regular, narrow-complex heart rate of 190 per minute and says their heart is pounding. No P waves are clearly visible. What is the most likely rhythm and the PCT's best action?

Supraventricular tachycardia (SVT); check the patient and vital signs, print the strip, and notify the nurse right away. A sudden, regular, narrow-complex tachycardia around 150 to 250 per minute with hidden P waves is typical of SVT. The PCT assesses the patient, obtains vital signs, saves the strip, and notifies the nurse promptly; treatment is given by the licensed team. The patient is awake with a pulse, so this is not ventricular fibrillation, and a regular rhythm rules out atrial fibrillation.

Premature Beats, SVT & AV Heart Blocks — Free Study Guide 2026

Premature Beats (PACs, PJCs, PVCs), SVT & AV Heart Blocks

NCCT's rhythm recognition tasks ask PCTs to recognize atrial and ventricular abnormalities (for example, PVC, PAC) and recognize cardiac rhythm classifications and rates. Section 12.2 covers sinus rhythms, atrial fibrillation and flutter, junctional rhythms, and the lethal ventricular rhythms. This section covers the early (premature) beats, supraventricular tachycardia, and the atrioventricular (AV) blocks. On the exam, you are recognizing and reporting patterns, not diagnosing.

Premature (Ectopic) Beats

A premature beat comes earlier than the next expected beat of the underlying rhythm. It is named for where it starts: the atria, the AV junction, or the ventricles. Finding the early beat is step one; step two is looking at the P wave and the QRS width.

Premature Atrial Contraction (PAC)

Timing: early.
P wave: present, but shaped differently from the sinus P waves because it starts from a different spot in the atria. It may hide in the preceding T wave, making that T wave look peaked or notched.
PR interval: may differ from the sinus beats.
QRS: usually narrow (under 0.12 seconds), because the impulse still travels down the normal conduction pathway.
Pause: usually noncompensatory; the early beat resets the SA node.
Causes: caffeine, nicotine, stress, fatigue, alcohol, low oxygen, electrolyte problems, and heart disease. Frequent PACs can precede atrial fibrillation.
Variations: a nonconducted (blocked) PAC shows an early P wave with no QRS after it. An aberrantly conducted PAC has a wide QRS that can mimic a PVC; look for the early P wave in front of it.

Premature Junctional Contraction (PJC)

Early, with a narrow QRS.
The P wave is inverted in lead II and may appear just before the QRS (with a PR under 0.12 seconds), be hidden inside it, or appear just after it.

Premature Ventricular Contraction (PVC)

Early, with no P wave before it.
Wide, bizarre QRS of 0.12 seconds or more, with the T wave pointing the opposite direction from the QRS.
Usually followed by a full compensatory pause.
Section 12.2 covers PVC patterns (couplets, bigeminy, multifocal PVCs, R-on-T) and why they can signal danger.

Side-by-Side Comparison

Feature	PAC	PJC	PVC
Timing	Early	Early	Early
P wave	Present, different shape	Inverted in lead II, or hidden	None before the QRS
QRS width	Narrow (under 0.12 s)	Narrow	Wide (0.12 s or more), bizarre
T wave	Normal	Normal	Opposite the QRS
Pause	Usually noncompensatory	Usually noncompensatory	Usually fully compensatory
Report priority	Report frequent or new PACs	Report	Report new, frequent, multifocal, paired, or R-on-T PVCs promptly

Premature beats can form patterns: bigeminy (every other beat is early), trigeminy (every third beat), and couplets (two in a row) apply to PACs as well as PVCs, for example "atrial bigeminy."

Supraventricular Tachycardia (SVT)

Regular, narrow-complex tachycardia, usually at about 150 to 250 beats per minute.
P waves are often hidden in the preceding T wave because the rate is so fast.
It often starts and stops suddenly (paroxysmal SVT).
Patients may feel palpitations, dizziness, shortness of breath, or chest discomfort.
PCT action: check the patient and vital signs, print the strip, and notify the nurse right away. Treatment such as vagal maneuvers or IV adenosine is given by the licensed team.

Compare this with sinus tachycardia (usually 101 to 150 per minute, visible P waves, gradual onset from a cause such as fever or pain) and atrial flutter with 2:1 conduction (often exactly 150 per minute, with sawtooth waves).

Atrioventricular (AV) Heart Blocks

AV blocks slow or stop impulses between the atria and ventricles. The key is the relationship between each P wave and the QRS that should follow it.

Rhythm	PR Interval	Dropped Beats	Key Pattern
First-degree AV block	Longer than 0.20 s, constant	None; every P conducts	"Long but steady"
Second-degree type I (Mobitz I, Wenckebach)	Gets progressively longer with each beat	One QRS dropped, then the cycle repeats	Grouped beating; regularly irregular
Second-degree type II (Mobitz II)	Constant (normal or long)	QRS suddenly dropped without warning	QRS often wide; can progress to complete block
Third-degree (complete) heart block	No consistent PR; P waves and QRS are unrelated	Atria and ventricles beat independently	Regular P waves "march through"; slow escape rhythm

A classic memory aid:

"Longer, longer, longer, drop, then you have a Wenckebach."
"If some P's don't get through, then you have a Mobitz II."
"If P's and Q's don't agree, then you have a third degree."

In third-degree block, the atrial rate is faster than the ventricular rate. The ventricles are driven by an escape pacemaker: a junctional escape (narrow QRS, about 40 to 60 per minute) or a ventricular escape (wide QRS, about 20 to 40 per minute). Patients are often dizzy, weak, short of breath, or faint, and they may need emergency pacing.

What the PCT Does

Look at the patient first, not just the monitor. Check responsiveness, breathing, color, and vital signs.
Print or save the strip and label it with the patient's name, date, time, and lead.
Report new or changing rhythms to the nurse. Second-degree type II and third-degree block, SVT with symptoms, and new frequent or multifocal PVCs call for immediate reporting.
Stay with a symptomatic patient and be ready to call a rapid response or code (Section 2.3).

NCCT NCPCT National Certified Patient Care Technician

12.3 Premature Beats (PACs, PJCs, PVCs), SVT & AV Heart Blocks

Key Takeaways