Sinoatrial block

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Mahmoud Sakr, M.D. [2]

Synonyms and keywords:; SA nodal exit block; exit block; Sino atrial exit block; Sinoatrial nodal block; sino-auricular block; Sino-auricular heart block


Sinoatrial block is an uncommon dysrhythmia in which the electrical impulse is delayed or blocked on the way to the atria, thus delaying atrial depolarization. It is characterized by the omission of P waves in the setting of a basic regular rhythm. It is found incidentally in normal asymptomatic subjects and in some having pre syncope or syncope. It may occur as an isolated dysrhythmia or in association with sinus bradycardia, tachycardia or sometimes with atrioventricular conduction disorders.[1][2]


First Degree SA Exit Block

There is a lag between the time that the SA node fires and actual depolarization of the atria. This rhythm is not recognizable on an ECG strip because a strip does not denote when the SA node fires. It can only be detected during an electrophysiology study.

Second Degree SA Exit Block

Second degree SA blocks are broken down into two subcategories just like AV blocks.

Type I (Wenckebach Phenomenon) Sinoatrial Exit Block

This rhythm is irregular, and the R-R interval gets progressively smaller until a QRS segment is dropped. Note that this is different from the Wenckebach AV block in which the PR interval gets progressively longer before the dropped QRS segment.

Type II Second Degree Sinoatrial Exit Block

A second degree type II, or sinus exit block, is a regular rhythm which may be normal or slow. It is followed by a pause that is a multiple of the R-R interval. Conduction across the SA node is normal until the time of the pause when it is blocked.

Third Degree Sinoatrial Exit Block

A third degree sinoatrial block looks very similar to a sinus arrest. However, a sinus arrest is caused by a failure to form impulses. A third degree block is caused by failure to conduct them. The rhythm is irregular and either normal or slow. It is followed by a long pause that is not a multiple of the R-R interval. The pause ends with a P wave, instead of a junctional escape beat the way a sinus arrest would.


Life Threatening Causes

Life-threatening causes include conditions which may result in death or permanent disability within 24 hours if left untreated.

Common Causes

Causes by Organ System

Cardiovascular Acute coronary syndrome, acute rheumatic fever, Andersen cardiodysrhythmic periodic paralysis, Brugada syndrome, cardiac lymphoma, cardiac tumor, cardio inhibitory syncope, congenital heart disease, congestive heart failure, coronary reperfusion therapy, dilated cardiomyopathy, hypertensive heart disease, hypertrophic cardiomyopathy, ischemic heart disease, Jervell and Lange-Nielsen syndrome, long QT syndrome, myocardial infarction, myocardial rupture, myocarditis, NSTEMI, pericarditis, Romano-Ward syndrome, sick sinus syndrome, sinus bradycardia, sinus node fibrosis, STEMI, tachycardia-bradycardia syndrome, Timothy syndrome, valvular heart disease
Chemical / poisoning Berberine, grayanotoxin, organophosphate poisoning, parathion poisoning, pyrethroid poisoning, scorpion toxin
Dermatologic No underlying causes
Drug Side Effect Acetylcholine, alfentanil, amiodarone, anthracyclines, barbiturate, beta-blockers, bortezomib, bupivacaine, calcium channel blockers, cholinesterase inhibitors, clonidine, dexmedetomidine, digitalis, digoxin, diltiazem, donepezil, edrophonium, fentanyl, flecainide, granisetron, guanethidine, guanfacine, halothane, ibutilide, idarubicin, lacosamide, lidocaine, lithium[4], magnesium, mepivacaine, mesalamine, methyldopa, mexiletine, neostigmine, nitrous oxide, pentostatin, phenothiazine, phenytoin, procainamide, propafenone, propofol, pyridostigmine, quinidine, remifentanil, rescinnamine, reserpine, rilmenidine, ropivacaine, tacrine, thiamylal, vecuronium, verapamil
Ear Nose Throat No underlying causes
Endocrine Diabetic ketoacidosis, thyrotoxic periodic paralysis, pheochromocytoma, profound hypothyroidism
Environmental Berberine, hypothermia, poisonous spider bites, scorpion toxin
Gastroenterologic No underlying causes
Genetic Andersen cardiodysrhythmic periodic paralysis, Brugada syndrome, congenital heart disease, Emery-Dreifuss muscular dystrophy, Jervell and Lange-Nielsen syndrome, Kearns-Sayre syndrome, limb-girdle muscular dystrophy type 1B (LGMD1B), muscular dystrophy, myotonic dystrophy, Romano-Ward syndrome, Timothy syndrome
Hematologic Hemochromatosis, multiple myeloma
Iatrogenic Cardiac catheterization, cardiac transplantation, coronary artery bypass grafting, Fontan procedure, heart surgery, infraclavicular brachial plexus block, Maze procedure, post catheter ablation for arrhythmias
Infectious Disease Acute rheumatic fever, Chagas disease, diptheria, Lyme disease, myocarditis, pericarditis, septic shock, sarcoidosis, systemic lupus erythematosus, tuberculosis
Musculoskeletal / Ortho Muscular dystrophy, myotonic dystrophy, Timothy syndrome
Neurologic Carotid sinus hypersensitivity, lateral medullary syndrome, vagal reaction
Nutritional / Metabolic Hypermagnesemia, metabolic acidosis
Obstetric/Gynecologic No underlying causes
Oncologic Cardiac lymphoma, cardiac tumor, multiple myeloma, pheochromocytoma
Opthalmologic Sjogren's syndrome
Overdose / Toxicity Acetylcholine, amiodarone, anthracyclines, barbiturate, bortezomib, cholinesterase inhibitors, digitalis, edrophonium, nitrous oxide, phenytoin, propofol
Psychiatric Takotsubo cardiomyopathy, severe anorexia nervosa
Pulmonary Hypoxia, sleep apnea
Renal / Electrolyte Acute renal failure, hyperkalemia
Rheum / Immune / Allergy Acute rheumatic fever, sarcoidosis, Sjogren's syndrome, scleroderma
Sexual No underlying causes
Trauma Myocardial contusion, myocardial rupture, severe brain injury
Urologic No underlying causes
Dental No underlying causes
Miscellaneous Amyloidosis

Causes in Alphabetical Order

Differentiating Sinoatrial block from other Diseases

Other types of SA nodal dysfunction are discussed in detail in other chapters on wikidoc. Follow the hyperlinks for details and those include:



  • First degree SA block: Delay between impulse generation and transmission to the atrium[10]
    • This abnormality is not detectable on the surface ECG.
  • Second degree SA block, type 1(Wenchebach): Progressive lengthening of the interval between impulse generation and transmission, culminating in failure of transmission.
    • The P-P interval progressively shortens prior to the dropped P wave.

  • Second Degree SA block, Type II: Intermittent dropped P waves with a constant interval between impulse generation and atrial depolarisation.
    • Intermittent dropping of the P wave, while other P waves are normal.
  • Third Degree SA Block: None of the sinus impulses are conducted to the right atrium.
    • There is a complete absence of P waves.


  • Sinoatrial block principles of treatment are the same as sinus pause or sick sinus syndrome.
  • Usually no treatment is indicated if the patient is asymptomatic.
  • Stopping the offending drug is generally reasonable.
  • When symptoms occur and become intolerable or life-threatening, then a permanent pacemaker would be indicated.


  1. GREENWOOD RJ, FINKELSTEIN D, MONHEIT R (1961). "Sinoatrial heart block with Wenckebach phenomenon". Am J Cardiol. 8: 140–6. PMID 13708372.
  2. Dighton DH (1975). "Sinoatrial block. Autonomic influences and clinical assessment". Br Heart J. 37 (3): 321–5. PMC 483972. PMID 1138735.
  3. Boujnah MR, Jaafari A, Boukhris B, Boussabah I, Thameur M (2000). "[Sinoatrial block induced by therapeutic doses of diltiazem. Report of 3 cases]". Tunis Med. 78 (12): 735–7. PMID 11155380.
  4. Eliasen P, Andersen M (1975). "Sinoatrial block during lithium treatment". Eur J Cardiol. 3 (2): 97–8. PMID 1183468.
  5. Bailey PL (1990). "Sinus arrest induced by trivial nasal stimulation during alfentanil-nitrous oxide anaesthesia". Br J Anaesth. 65 (5): 718–20. PMID 2248851.
  6. 6.0 6.1 6.2 Mills TA, Kawji MM, Cataldo VD, Pappas ND, O'Meallie LP, Breaux DM; et al. (2004). "Profound sinus bradycardia due to diltiazem, verapamil, and/or beta-adrenergic blocking drugs". J La State Med Soc. 156 (6): 327–31. PMID 15688675.
  7. 7.0 7.1 Lines D, Shipton EA (1991). "Severe bradycardia and sinus arrest after administration of vecuronium, fentanyl and halothane. A case report". S Afr Med J. 80 (4): 200–1. PMID 1678901.
  8. Bonvini RF, Hendiri T, Anwar A (2006). "Sinus arrest and moderate hyperkalemia". Annales De Cardiologie Et D'angéiologie. 55 (3): 161–3. PMID 16792034. Unknown parameter |month= ignored (help)
  9. Koay S, Dewan B (2013). "An unexpected Holter monitor result: multiple sinus arrests in a patient with lateral medullary syndrome". BMJ Case Rep. 2013. doi:10.1136/bcr-2012-007783. PMID 23386489.
  10. "Sinoatrial Exit Block - Life in the Fastlane ECG Library".

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