Five-Week-Old With a Rate of 268
A 5-week-old, 4.2 kg, is brought in at 03:00 for 2 hours of poor feeding and inconsolable fussiness. The mother says the baby 'felt like a hummingbird' against her chest. HR 268, narrow complex, no visible P waves, RR 60, BP 68/40, SpO₂ 97%, temperature 37.1 °C, capillary refill 3 seconds with mottled, cool extremities. Point-of-care glucose 5.4 mmol/L. Two peripheral IV attempts have failed and the physician is scrubbing for a third. The infant is awake, fussy and sucking a pacifier. There is no in-house paediatric cardiology; adenosine is stocked in the automated dispensing cabinet.
1. How do you distinguish this rhythm from sinus tachycardia, and what will you do before the IV is in?
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Tachycardia discrimination
- Rate: 268 bpm. Sinus tachycardia in an infant rarely exceeds 220 bpm; SVT is typically 220–300.
- Morphology: no visible P waves, narrow and fixed QRS, no beat-to-beat variability.
- Behaviour: abrupt onset and offset. Sinus tachycardia drifts with the cause (fever, pain, dehydration, sepsis) and shows variability.
- Look for a cause of sinus tachycardia - there isn't an obvious one here, which itself supports SVT.
Before intravenous access
- Apply continuous ECG, BP and SpO₂ monitoring; run a rhythm strip for the chart.
- Vagal manoeuvre immediately: a bag of ice and water wrapped in a cloth applied to the forehead, eyes and bridge of the nose for 15–20 seconds (diving reflex); repeat once. Do not occlude the airway or apply pressure to the eyes.
- Keep trying for access in a warm, well-lit limb, but do not burn time. This infant has been symptomatic for 2 hours and has two failed attempts - intraosseous access in the proximal tibia is appropriate and faster than a third attempt.
- Have the defibrillator at the bedside. If perfusion worsens (altered mental status, hypotension, further mottling), move to synchronised cardioversion at 0.5–1 J/kg with sedation if feasible.
- Draw up adenosine 0.1 mg/kg (0.42 mg) and a saline flush in a two-syringe set-up, ready for the moment access exists.
Judgement being tested: the infant is currently compensated, so vagal manoeuvres and rapid access come before electricity - but the ceiling for delay is short.
2. The first adenosine dose of 0.1 mg/kg produces no change in rhythm. What could have gone wrong, and what is your next step?
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First, question the delivery before the drug.
- Adenosine has a half-life under 10 seconds. It must be given as a rapid push in a large, proximal vein, ideally through a stopcock, followed immediately by a 5–10 mL saline flush in the same line.
- Common failures: slow administration, a flush given too late or through a different port, a small distal vein, or a tourniquet left in place.
Next step
- Repeat at 0.2 mg/kg (0.84 mg, maximum 12 mg) using a verified two-syringe rapid technique and the largest available vein.
- Observe the monitor during the push - a brief period of asystole or bradycardia lasting 5–10 seconds is expected and self-limiting; warn the parents so they are not frightened.
- If two adequate doses fail, proceed to synchronised cardioversion at 0.5–1 J/kg (escalating to 2 J/kg), with sedation if access and time permit.
- Avoid verapamil in infants. Have the defibrillator, oxygen and suction ready throughout.
Reasoning. The first dose failing is most often a technique problem, so the answer is a corrected technique plus a higher dose, not an immediate jump to electricity in a compensated infant.
3. Twenty minutes later the rhythm converts to sinus at 148 bpm with visible P waves and perfusion improves. What are your priorities now?
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Immediate post-conversion care
- Continue cardiorespiratory monitoring and keep the defibrillator available; recurrence is common.
- Obtain a 12-lead ECG and examine for a short PR and delta wave (Wolff-Parkinson-White). Save the pre-, during- and post-conversion rhythm strips for the record.
- Labs: electrolytes including potassium, magnesium and calcium, glucose, CBC, and thyroid function.
- Anticipate echocardiography to look for structural abnormality.
- Admit for continuous cardiac monitoring, generally at least 24 hours.
Family communication and teaching
- Explain what happened in plain language; acknowledge the mother's observation ('hummingbird') as the reason the baby was brought in.
- Teach how to count a brachial pulse or use a home monitor, how to perform the ice-to-the-face manoeuvre, and when to call emergency services.
- Explain that SVT in an infant with a structurally normal heart often resolves by 12 months, and that cardiology follow-up will be arranged.
Reasoning. Conversion is the midpoint, not the endpoint: the infant is now at risk of recurrence and of an underlying substrate that changes long-term management.