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Supraventricular Tachycardia - SVT

The term SVT encompasses any tachycardia arising from the atrioventricular (AV) node and above, but is most commonly used to refer to Atrioventricular Nodal Reentrant Tachycardia (AVNRT) and Atrioventricular Reentrant Tachycardia (AVRT), the commonest of these being AVNRT. SVTs include inappropriate sinus tachycardia, AT (including focal and multifocal AT), macroreentrant AT (including typical atrial flutter) and various forms of accessory pathway-mediated reentrant tachycardias.

In AVNRT there is a functional re-entry circuit within the AV node. In AVRT there is an anatomical re-entry circuit between the atrium and ventricle, bypassing the AV node (e.g. Wolff-Parkinson-White Syndrome).

Clinical presentation and treatment are very similar with the exception of a broad complex tachycardia arising from an AVRT. Broad complex tachycardias should be considered separately.

SVT is a common cardiac dysrhythmia, affecting people of all ages, often starting at a young age. It affects women more commonly than men. Whilst it is rarely life-threatening it presents with recurrent episodes of palpitations at a rate of 140-200bpm. Whilst this may be tolerated well for hours or days, it may produce heart failure / cardiomyopathy.

Common presentations include palpitations, chest pain, anxiety, shortness of breath and light headedness. Rarely, it presents with syncope.

SVT - Assessment

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Initial Assessment

Initial assessment should be made as to the patient's haemodynamic status.

Features suggesting haemodynamic compromise include:

  • Decreased LOC
  • Chest pain
  • Shortness of breath (due to heart failure)
  • Systolic BP of less than 90mmHg (or 30-40mmHg less than the patient's usual SBP)
  • Diaphoresis

Signs of shock are rare in SVT if structurally normal heart. Consider underlying cause and treat as appropriate.

Investigations

ECG

The ECG should be evaluated for the presence of abnormal rhythm, pre-excitation, prolonged QT interval, sinus tachycardia, segment abnormalities, or evidence of underlying heart disease. The presence of preexcitation on the resting ECG in a patient with a history of paroxysmal regular palpitations is sufficient for the presumptive diagnosis of AVRT.

ECG findings will typically include:

  • Regular tachycardia normally 140-180bpm
  • QRS complexes usually narrow (<120msecs) unless pre-existing BBB or accessory pathway
  • (Broad complex tachycardia exit pathway and follow ALS protocol)

P- waves often not visible (obscured by the t-wave), if visible may be retrograde with inversion in leads II, III, aVF.

CXR only if clinically indicated

Blood testing is rarely useful and should only be performed when the patient has specific co-morbidities likely to result in electrolyte abnormality (eg renal, metabolic or endocrine disorders, alcoholism, diuretic or antihypertensive medications affecting renal function)

SVT - Treatment

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The treatment for AVRT and AVNRT is the same for narrow complex tachycardias. Do not use these guidelines for wide complex tachycardias.

Increased Vagal Tone

Modified valsalva – REVERT trial shows improved response to Valsalva manoeuvers if done sitting, sustained Valsalva for 15 secs(attempt to blow into 10ml syringe), then positioning flat and raising legs for 15 secs. This increased rates of cardioversion from 17% to 40%.

Carotid sinus massage – Patient supine with neck hyperextended, apply gentle pressure for 10-15secs.

Check for bruits first, do not perform if bruits present or at risk of stroke (age >65, Hx of TIA / CVA or risk factors present). Do not do bilateral massage simultaneously.

Drugs

Ensure patient is on cardiac and BP monitor. Turn on ECG trace recorder.

Mainstay of treatment is chemical cardioversion using intravenous Adenosine. This is recommended to be given at increments of 6mg, 12mg and then a further 12mg dose.

Adenosine has a very short half life (10secs) and should be given via a large vein, ideally in the antecubital fossa. Give as a rapid bolus using a 3 way tap, then a 20mL rapid 0.9% saline flush.

Side effects are very short lasting but patients should be warned of these:

  • Feeling of apprehension/impending doom
  • Flushing/sweating
  • Chest discomfort
  • Shortness of breath
  • Headache/dizziness

Adenosine is contraindicated in patients with severe asthma. In patients taking theophylline higher doses may be needed, and in patients taking dipyridamole lower doses may be needed.

Note - Adenosine is not contraindicated in patients with WPW where there is a narrow complex tachycardia.

Verapamil IV 1mg/minute up to 15mg.

Avoid in patients on B-blockers.

Pre-treatment with calcium – e.g. 1g calcium gluconate(10mLs 10%) over 2 to 3 minutes can reduce the hypotensive effect without affecting the antiarrhythmic effects of verapamil.

DC cardioversion

In unstable patients DC cardioversion may be the most appropriate management .

Give a synchronised shock at 50-100J (ACLS guidelines 2010).

Procedural sedation required.

Long term

Catheter ablation

Single dose “pill in the pocket” oral therapy propranolol or diltiazem may be instigated by patient’s cardiologist for recurrent prolonged episodes

SVT - Discharge Criteria

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Admit / discuss with cardiologist all patients with signs of shock, chest pain, prolonged SVT or in whom the diagnosis is unclear.

Discharge criteria

  • No signs of LVF or ACS
  • Full recovery post sedation
  • Follow up with GP or cardiologist – the urgency for follow up depends on likelihood of recurrence, risk of complications and planned interventions eg catheter ablation.

SVT - Further References and Resources

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  • MJA review article - Medi C, Kalman J, Freedman SB, Supraventricular tachycardia, MJA, 2009; 190(5): 255-260.
  • BMJ clinical review - Whinnett ZI, Afzal Sohaib SM, Davies DW, Diagnosis and management of supraventricular tachycardia, BMJ, 2012; 245: e7769.
  • Bestbets - Are troponin levels indicated for the routine management of SVT?
  • ACC / AHA / ESC - 2015 Guidelines for the Management of Patients with Supraventricular Arrhythmias, JACC, 2016; 67(13): e27-115.
  • REVERT Trial - Appelboam A, Reuben A. et al. Postural modification to the standrd Valsalva manoeuvre for emergency treatment of supraventricular tachycardias (REVERT): a randomised controlled trial. The Lancet, 2015 (386): 10005: 1747-1753.
  • RCH - Guidelines for managing paediatric SVT

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