Describe the hypercyanotic / 'Tet' Spell of Cyanotic Spell.

Definition

• A hypercyanotic (or 'Tet') spell is a paroxysmal episode of acutely worsening cyanosis with hyperpnoea, occurring most classically in Tetralogy of Fallot.
• It is a medical emergency that can lead to syncope, seizures and death.

Precipitating Factors

• Typically occur in the morning on waking, and are triggered by crying, feeding, defecation, or exertion.
• Anything that reduces systemic vascular resistance or increases infundibular spasm.

Mechanism

Trigger → infundibular (RV outflow) spasm + fall in systemic resistance → Increased right-to-left shunt across the VSD → Sudden fall in pulmonary blood flow → severe hypoxaemia → Hyperpnoea (worsens the cycle) → deep cyanosis, limpness

Clinical Features

• Sudden deep cyanosis, rapid deep breathing (hyperpnoea), irritability and inconsolable crying.
• The murmur becomes softer/disappears during a spell (less flow across the pulmonary outflow).
• Severe spells → limpness, syncope, convulsions.

Management (stepwise)

• Knee-chest position (or hold the infant with knees flexed onto the chest) — the first and simplest step.
• Oxygen; calm the child; morphine (reduces hyperpnoea and infundibular spasm).
• IV fluids (volume); beta-blocker (propranolol/esmolol) to relieve infundibular spasm.
• Phenylephrine (or squatting/knee-chest) to raise systemic resistance; sodium bicarbonate for acidosis.

Prevention

• Oral propranolol prophylaxis; maintain hydration; correct anaemia; and early surgical correction of the underlying TOF.

Key Difference from Respiratory Causes

Unlike respiratory cyanosis, a Tet spell is not relieved by oxygen alone and is dramatically helped by the knee-chest position, which raises systemic resistance and reduces the right-to-left shunt.

A Note on WHY the Spell Feeds on Itself

• Understanding why a hypercyanotic spell escalates so rapidly — and why it must be broken urgently — reveals a classic vicious cycle.
• A spell begins with a trigger (crying, feeding, defaecation, fever, dehydration, waking) which either causes infundibular spasm (constricting the already-narrow right ventricular outflow) or lowers the systemic vascular resistance (making the aorta an easier escape route).
• Either way, more blood is shunted right-to-left, so the arterial oxygen falls and the child becomes deeply cyanosed.
• Now the cycle begins.
• The hypoxia and the resulting acidosis stimulate the respiratory centre — so the child begins to hyperventilate (hyperpnoea).
• But hyperventilation increases venous return to the right heart and, by increasing the work of breathing, further lowers the systemic vascular resistance — both of which increase the right-to-left shunt still further.
• So the hypoxia deepens, the hyperpnoea intensifies, and the spiral tightens — ending in syncope, seizures, stroke or death.
• Every element of the treatment is designed to break this cycle: knee-chest position (raises systemic resistance), oxygen, morphine (abolishes the hyperpnoea and calms the child — the key drug), IV fluids (increases preload), propranolol (relieves the infundibular spasm) and phenylephrine.