ECG criteria
- The PP pause equals an integer multiple of the baseline sinus cycles (most commonly 2×, with 3× or more possible) — this supports the diagnosis of second-degree SA block, but even exact multiplicity does not prove it definitively: reliably distinguishing a block from sinus arrest on a surface ECG is frequently impossible
- The expected sinus P wave with the corresponding QRS is absent; a concealed premature (blocked) P wave and a replacement complex must be ruled out
- The sinus P waves conducted before and after the pause generally share identical morphology; a potential ectopic atrial escape rhythm following the pause may present a different P wave shape
- The PQ (PR) interval in conducted complexes is evaluated to detect concomitant AV block; a normal PQ interval is not a mandatory criterion for SA block
- In type I (Wenckebach periodicity), the PP intervals progressively shorten prior to the pause
- Third-degree SA block cannot be reliably diagnosed on a standard ECG: the picture may correspond to an absence of sinus P waves accompanied by an atrial, junctional, or ventricular escape rhythm, and is indistinguishable from sinus arrest without special methods
- Must be distinguished from a sinus pause without strict multiples and from AV block, where the P wave is present but fails to be conducted to the ventricles
What it looks like on ECG
Evaluate the rhythm strip, preferably in lead II, where P waves are clearly visible.
| Type | PP intervals before the pause | Pause duration |
|---|---|---|
| Grade I | do not change (not diagnosed on surface ECG) | absent |
| Second-degree, type I (Wenckebach) | progressively shorten | less than twice the PP interval |
| Second-degree, type II | constant | equal to an integer multiple of the baseline PP intervals (most commonly ×2, less frequently ×3 or more) |
| Grade III | cannot be reliably diagnosed on a surface ECG | an atrial, junctional, or ventricular escape rhythm is possible in the absence of sinus P waves |
In second-degree type II, the pause appears as a «drop» — the expected P-QRS-T complex is missing, and the next sinus P wave appears approximately after two (less commonly three) baseline PP intervals during a stable sinus rhythm; a concealed extrasystole or escape complex must be ruled out. The morphology of conducted P waves before and after the pause is usually identical, which differentiates the block from an ectopic atrial escape rhythm (whose P wave may differ in morphology) or pacemaker shifting. The QRS width in conducted cycles is not determined by the SA block itself: it depends on intraventricular conduction (which may be narrow or wide with a concomitant bundle branch block) and, during a pause, on a potential escape complex with its own morphology.
Mechanism
The sinus node generates an impulse, but conduction through the perinodal tissue to the atrial myocardium is blocked — impulse output is impaired (exit block). Analogous to AV block, three degrees are distinguished: I — conduction delay without dropouts (not visible on surface ECG, diagnosed only electrophysiologically), II — intermittent conduction dropout with pauses, III — complete exit block, where no sinus impulse reaches the atria. During a pause, escape rhythms from atrial, junctional, or ventricular pacemakers may take over. Confirming an exit block rather than sinus arrest via surface ECG is impossible because intrinsic sinus activity is not recorded on it; for the same reason, a surface ECG does not allow assessment of whether the automaticity of the node itself is preserved or impaired — SA block is frequently combined with sinus node dysfunction and sick sinus syndrome.
Causes
- Increased vagal tone (vagotonia), including in athletes and during sleep
- Ischemia or inferior wall myocardial infarction (the right coronary artery supplies the sinus node in the majority of individuals)
- Fibro-degenerative changes of the sinus node (sick sinus syndrome)
- Myocarditis, cardiomyopathies, infiltrative heart diseases
- Drug intake: beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, class I and III antiarrhythmics
- Electrolyte disturbances (hyperkalemia)
- Post-cardiac surgery, trauma, or surgery in the sinus node area
Differential diagnosis
- Sinus pause (arrhythmia/arrest) — the pause is not a multiple of the baseline PP interval and can be of any duration; on a surface ECG, it is often indistinguishable from SA block. A multiple pause only supports block rather than sinus arrest, while a non-multiple pause supports sinus arrest rather than block, though this sign lacks definitive evidence
- Second-degree AV block — the P wave is present during the pause but is not conducted to the ventricles (only QRS drops out)
- Blocked atrial extrasystole — the pause begins with a premature, deformed P wave rather than a drop-out against a regular rhythm
- Sinus arrhythmia — gradual PP fluctuations without a sharp, multiple-length elongation
- Pronounced sinus bradycardia — PP intervals are uniformly lengthened, but without the pattern of an entire cycle dropping out
Clinical significance and management
Clinical significance depends on the frequency and duration of the pauses and the presence of symptoms. Asymptomatic SA block, especially vagotonic, in young individuals and athletes often requires no intervention. Pronounced pauses with presyncope, syncope (attacks resembling Adams-Stokes syndrome), or symptomatic bradycardia require searching for a reversible cause: discontinuation or dose adjustment of sinus node-slowed drugs, ischemia treatment, or electrolyte correction. The decision for permanent cardiac pacing is based primarily on symptomatic sinus node dysfunction, especially with documented correlation of symptoms with pauses or bradycardia; there is no fixed minimum pause interval for indications. Asymptomatic documentation of sick sinus syndrome is not an indication for pacing by itself. If symptomatic sinus node dysfunction persists after eliminating reversible causes, permanent pacing is considered according to current guidelines. It is important to record the correlation of symptoms with pauses, preferably using Holter monitoring.
Common interpretation errors
- Confusing SA block with a regular sinus pause, forgetting that on a surface ECG these conditions are often indistinguishable and PP multiples are only a guideline, not proof
- Mistaking a blocked atrial premature complex for SA block by failing to spot the premature, deformed P wave at the beginning of the pause
- Failing to measure PP intervals accurately with a ruler or caliper, assessing them "by eye"
- Confusing it with high-grade AV block by ignoring the presence or absence of the P wave during the pause
- Diagnosing first- or third-degree SA block on a surface ECG, although both can only be reliably established by invasive or special methods
1. Which sign supports the diagnosis of Mobitz type II second-degree SA block when evaluating an ECG pause?
A pause that is a multiple of the baseline PP supports exit block, although it does not absolutely prove it and requires ruling out sinus arrest.
2. What happens to the expected sinus cycle during a pause in SA block?
Since the impulse fails to reach the atria from the sinus node, neither the expected P nor the subsequent QRS and T complexes occur, although escape complexes are possible.
3. Which sign allows distinguishing SA block from second-degree AV block?
In AV block, the P wave persists during the pause and simply fails to conduct to the ventricles; in SA block, the expected P wave is absent along with the QRS complex.
Frequently asked questions
How to distinguish SA block from a sinus pause?
A pause equal to a whole number of basic PP intervals (most often ×2, less commonly ×3) supports second-degree SA block, whereas a non-multiple pause points more toward sinus arrest. This is a guideline rather than an absolute criterion: on a surface ECG, these conditions are often indistinguishable, and when in doubt, it is more accurate to describe the pause or sinus arrest, taking the clinical context into account.
How to distinguish SA block from AV block?
In SA block, the expected sinus P wave and its corresponding QRS complex are absent, although escape beats may occur. In second-degree AV block, the P wave appears on time, but is not followed by a QRS complex.
Is sinoatrial block dangerous?
The risk depends on the duration of the pauses and symptoms. Short, asymptomatic pauses, especially of vagotonic origin, are usually not dangerous. Prolonged pauses with syncope require investigation and may be an indication for cardiac pacing in symptomatic sick sinus syndrome.
Can first-degree SA block be diagnosed on a standard ECG?
No, first-degree block does not alter PP intervals on a surface ECG, as only intra-nodal conduction is slowed without dropped beats. It is detected using electrophysiological study methods.
Which degree of SA block is easiest to recognize on a tracing?
Second-degree type II is the easiest to recognize due to the fairly distinct multiple pause. Type I (Wenckebach) is harder to recognize due to the gradual shortening of PP intervals, and the frequency of different types in the general population cannot be reliably estimated from routine interpretation practice—it strongly depends on the sample and recording conditions.
ECG Course
Interpretation from basics to myocardial infarction — learn to read any ECG using a systematic algorithm.
- Video lessons and theory for every topic
- Simulator case studies under every lesson
- Unlimited ECG simulator practice
- Study guide: "All ECGs in 25 minutes"
ECG Strip Simulator
ECG, radiology, histology, pathomorphology, pharmacology — thousands of flashcards with detailed explanations.
- Real ECG strips, not textbook drawings
- Instant answers and explanations
- Streaks and progress tracking in your dashboard
Related terms
More in the «Sinus Node» section
Sources
- Goldberger's Clinical Electrocardiography, 9th ed.
- Surawicz, Knilans — Chou's Electrocardiography in Clinical Practice, 6th ed.
- Orlov V.N. Guide to Electrocardiography
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.