Supraventricular tachycardia — formative SAQs

SAQ 1 (10 marks)

A four-month-old boy is brought to the emergency department by his mother, who says he has been "fussy and not feeding well" for a day. On examination he is pale and irritable, with a respiratory rate of 50 and capillary refill of three seconds. The nurse reports a heart rate of 250 per minute on the monitor. A 12-lead ECG shows a regular narrow-complex tachycardia at 250 per minute with no visible P waves. ^{[3] [5]}

1. Describe your immediate assessment and management of this infant, including the specific technique and dose of each pharmacological agent. (5) ^{[1] [5]}
2. Explain how you would distinguish this rhythm from sinus tachycardia, and why that distinction matters for management. (2) ^{[1] [3]}
3. Outline the long-term management plan after the acute episode, including medication choices and the expected natural history. (3) ^{[3] [5]}

Model answer — SAQ 1

(1) Immediate management (5). This infant is in SVT and is showing early signs of haemodynamic compromise — pallor, poor perfusion, irritability — but is not yet in overt shock with hypotension. He is stable enough for a stepwise approach. First, apply a 12-lead ECG and cardiac monitoring, establish intravenous access, and give high-flow oxygen. Attempt a vagal manoeuvre: the diving reflex, using a sealed ice pack or cold gel pack applied to the face for 15 to 30 seconds, is the most reliable technique in an infant. If the vagal manoeuvre fails, administer adenosine at 0.1 milligramme per kilogramme as a rapid intravenous bolus through the largest-bore cannula closest to the heart, followed immediately by a 5 to 10 millilitre saline flush. The maximum first dose is 6 milligrammes. If the first dose does not terminate the tachycardia, give 0.2 milligramme per kilogramme (maximum 12 milligrammes) by the same rapid technique. If adenosine fails or the infant deteriorates, proceed to synchronised DC cardioversion at 0.5 to 1 joule per kilogramme. A defibrillator must be available at the bedside throughout. ^{[1] [5]}

(2) Distinguishing SVT from sinus tachycardia (2). Sinus tachycardia has a normal P wave axis with visible P waves preceding each QRS, and the rate varies with stimulation — crying, fever, pain. The infant heart rate in sinus tachycardia is usually below 220 per minute, and a treatable cause (fever, dehydration, sepsis, anaemia) is present. SVT has a fixed rate that does not vary, absent or retrograde P waves, and a rate typically above 220 in infants. The distinction matters because sinus tachycardia is managed by treating the underlying cause, while SVT requires active termination with vagal manoeuvres or adenosine. Giving adenosine to a child in sinus tachycardia is unnecessary and may cause transient asystole. ^{[1] [3]}

(3) Long-term management (3). After successful termination, obtain a 12-lead ECG in sinus rhythm to check for the Wolff-Parkinson-White pattern (delta wave, short PR interval). Start prophylactic antiarrhythmic therapy because recurrence is common — first-line is propranolol at 2 to 4 milligrammes per kilogramme per day in three divided doses, or digoxin at 10 microgrammes per kilogramme per day. An echocardiogram excludes structural heart disease. The family is educated on recognising recurrence and performing simple vagal manoeuvres. Most infants (60 to 80 per cent) have spontaneous resolution by one year of age, so medication is typically weaned at 12 months with monitoring for recurrence. ^{[3] [5]}