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Cardiovascular System (CVS)

Supraventricular tachycardia (SVT)

Dysrhythmia originating at or above the atrioventricular (AV) node and defined by a narrow complex (QRS < 120 ms) at a rate of > 100bpm.

  • M/C symptomatic dysrhythmia in infants in children

Classification

SVT is caused by 1 of 3 mechanisms: reentry, increased automaticity, or triggered activity.

Subtypes of supraventricular tachycardia. The figures demonstrate the mechanisms of arrhythmia. Atrioventricular nodal reentrant tachycardia (far left) and atrioventricular reciprocating tachycardia (middle) are exclusively reentrant in mechanism. Atrial arrhythmias include atrial tachycardia, atrial flutter, and fibrillation and may be because of reentry, automaticity (star), or triggered activity (T) (far right). Enhanced automaticity and triggered activity may cause the initiation of reentry. | Sohinki, D., & Obel, O. A. (2014). Current Trends in Supraventricular Tachycardia Management. Ochsner Journal, 14(4), 586 LP – 595. http://www.ochsnerjournal.org/content/14/4/586.abstract

Atrioventricular nodal reentrant tachycardia (AVNRT):

Patients with AVNRT demonstrate dual atrioventricular nodal inputs with differing electrophysiologic properties, the fast and slow pathways that act as the 2 limbs of the reentrant circuit.
  • M/C paroxysmal SVT

Atrioventricular reentrant tachycardia (AVRT):

Accessory pathways are anomalous, extranodal conduction pathways between atrium and ventricle. The accessory pathway and the atrioventricular node can act as the 2 limbs of the AVRT reentrant circuit.

Accessory pathways are classified based on their location along the mitral or tricuspid annuli, conduction characteristics, and directionality of conduction (antegrade, retrograde, or both).

  • Antegrade conduction:
    • Orthodromic: Antegrade conduction occurs via AV node
    • Antidromic: Antegrade conduction occurs via accessory pathway
  • Retrograde conduction

Wolff-Parkinson-White (WPW) pattern: Accessory pathways capable of antegrade conduction demonstrate a delta wave on ECG

Accessory pathways capable only of retrograde conduction do not exhibit a delta wave on the surface ECG (concealed pathways) but can still form a reentrant loop with the atrioventricular node and result in AVRT.

Atrial tachycardia:

Group of atrial arrhythmias whose mechanisms may be focal or macroreentrant.
  • Focal atrial tachycardia: Results in a centrifugal wavefront spread throughout the atria from a focus of (1) enhanced automaticity, (2) triggered activity, or (3) a small area of reentry (microreentry).
  • Macroreentrant atrial tachycardia: Results from a large intraatrial reentrant loop >2 cm in length. Note that the term macroreentrant atrial tachycardia is synonymous with the term atrial flutter.

Both focal and macroreentrant atrial tachycardia may result from prior structural heart disease or surgery for congenital heart disease, may follow surgical or catheter ablation of atrial fibrillation, or may be due to idiopathic phenomena.

  • Right atrial tachycardia often arises from the crista terminalis but can also arise from the tricuspid annulus, compact atrioventricular node, and coronary sinus ostium
  • Left atrial tachycardias often arise from the pulmonary veins, as well as the mitral annulus and left atrial appendage

Atrial flutter:

Form of macroreentrant atrial tachycardia that may result in stroke and cardiomyopathy, and is highly amenable to catheter ablation.
Mechanisms of supraventricular tachycardias | By CardioNetworks: Drj – CardioNetworks: SVT_overview.svg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=24373462

Aetiology

Risk factors:

  • Congenital heart disease (children)
    • In children younger than 12 years old, an accessory atrioventricular pathway causing reentry tachycardia is the most common cause of SVT.

Triggering factors:

  • Medications
  • Caffeine
  • Alcohol
  • Physical/emotional stress
  • Cigarette smoking

Clinical features

The clinical presentation of SVT is variable- ranging from asymptomatic to severe palpitation.

  • Mild cases: Anxiety, palpitations, chest discomfort, lightheadedness, syncope, or dyspnea
  • Severe cases: Shock, hypotension, signs of heart failure, lightheadedness, or exercise intolerance
  • Signs of hemodynamic instability: Hypotension, hypoxia, shortness of breath, chest pain, shock, evidence of poor end-organ perfusion, or altered mental status.

Diagnosis

ECG:

  • Narrow QRS complex, regular tachycardia of 180-220 bpm rate
  • P waves not detectable (if detectable, consider sinus tachycardia or atrial fibrillation/flutter as a potential etiology)

Management

Immediate synchronized cardioversion: For haemodynamically unstable cases

Defibrillator synchronized with the QRS complex, to prevent the shock from being delivered during the T-wave, while the heart is depolarized.

Vagal maneuvers: For haemodynamically stable patient

Act to stimulate the parasympathetic system. This slows impulse formation at the sinus node, slows conduction velocity at the AV node, lengthens the AV node refractory period, and decreases ventricular inotropy.
  • Valsalva maneuver: Expiring against a closed glottis, and needs to be held for 10 seconds to 15 seconds.
  • Carotid massage: Placing patient in supine position with neck extended, and applying pressure to one carotid sinus for approximately 10 seconds.

Chemical cardioversion:

If vagal maneuvers prove ineffective
  • Adenosine 6 mg IV (pediatric dose 0.1 mg/kg, maximum of 6 mg)
    • If initial dose ineffective, 2nd dose at 12 mg IVP (pediatric dose 0.2 mg/kg, maximum dose 12 mg)
  • Second line medfications:
    • Diltiazem (0.25 mg/kg IV loading dose followed by 5mg/hr to 15 mg/hr infusion)
    • Esmolol (0.5 mg/kg IV loading dose, then 0.5 mg/kg/min up to 0.2 mg/kg/min, will need to repeat bolus for every up-titration)
    • Metoprolol (2.5-5 mg IV every 2-5 minutes, not exceeding 15 mg over 10-15 minutes)

Overdrive pacing:

Pacing the heart at a faster rate than its native rhythm, can help discontinue SVT. Performed if chemical cardioversion still prove ineffective

Maintenance therapy:

Indicated in recurrent SVT without a .pre-excitation syndrome
  • Oral beta-blockers or CCBs to maintain sinus rhythm
  • Radio-frequency ablation (if accessory pathway identified)
  • Patients should be counseled on how to perform vagal maneuvers on their own for long-term management of recurrent SVT
Fluoroscopic catheter positions during typical atrial flutter ablation. A: around the tricuspid annulus (Halo catheter). B: in the coronary sinus. C: tip of the ablation catheter positioned on the cavotricuspid isthmus. Catheters A and B measure isthmus block. Ablation is performed with the catheter looping back on itself as in this example or with the catheter flat against the isthmus. | Sohinki, D., & Obel, O. A. (2014). Current trends in supraventricular tachycardia management. The Ochsner journal, 14(4), 586–595.

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