Nursing care
Why children with tetralogy of Fallot squat: raising resistance to send blood to the lungs
Written and reviewed by Dana Whitfield, RN, MSN · 4 min read · Updated October 2026
Short answer
In tetralogy of Fallot, a narrowed right ventricular outflow tract and a ventricular septal defect let deoxygenated blood shunt right to left into the aorta. Squatting or the knee-chest position compresses leg arteries and raises systemic vascular resistance, so less blood crosses the defect and more goes to the lungs. Oxygen saturation rises, which is why it eases a tet spell.
Follow the blood through the four defects
Tetralogy of Fallot combines a large ventricular septal defect, obstruction of the right ventricular outflow tract with pulmonary valve stenosis, right ventricular hypertrophy and an aorta that overrides the septal defect. The outflow obstruction makes it hard for the right ventricle to push blood into the pulmonary artery, so the path of least resistance is across the defect into the aorta.
Blood that bypasses the lungs carries no fresh oxygen, so it lowers the oxygen saturation of blood reaching the body. How much blood shunts right to left depends on a balance: the more resistance in the pulmonary outflow and the less resistance in the systemic circulation, the more deoxygenated blood crosses into the aorta and the bluer the child becomes.
What tips a child into a tet spell
A hypercyanotic or tet spell is a sudden worsening of that balance. Increased outflow obstruction, rising pulmonary resistance or a drop in systemic vascular resistance all increase right-to-left shunting. Triggers include crying, feeding and defecating, which lower saturation, and activity such as playing or sudden leg movements, which can drop systemic resistance.
During a spell the child may breathe rapidly and deeply, become irritable with prolonged crying, grow increasingly cyanotic, and the heart murmur may soften or disappear because less blood is flowing across the narrowed outflow. A prolonged spell can lead to loss of consciousness or cardiac arrest, so it is treated as an emergency.
Why squatting and the knee-chest position help
Squatting flexes the hips and knees tightly, kinking the large arteries of the legs and raising systemic vascular resistance. With higher resistance on the left side of the circulation, less blood escapes across the ventricular septal defect into the aorta. More of the right ventricle's output is pushed through the narrowed outflow to the lungs, so arterial oxygen saturation improves.
Older children with uncorrected tetralogy may squat instinctively after exertion. Infants cannot, so the nurse or parent holds the baby in a knee-chest position, knees drawn up to the chest, often over the shoulder. Calming the child also matters, because crying worsens hypoxia and agitation drives the spell. A child who squats after play is compensating, not misbehaving.
Turn the mechanism into nursing actions
When a spell begins, place the infant in the knee-chest position, keep the child as calm as possible, and give oxygen as prescribed. Call for help early and monitor colour, saturation, respiratory pattern and heart rate. If the spell does not resolve, prepare for treatment under the team's direction, such as intravenous fluids, morphine, phenylephrine or a beta blocker.
Between spells, minimise triggers by organising care to reduce prolonged crying, feeding calmly, and preventing dehydration. Teach parents to recognise a spell, place the baby knee-to-chest, stay calm and seek emergency help if it does not settle quickly. Document the trigger, duration, interventions and response, because spells are a sign that surgical review may be needed.
Work through a hypothetical exam-style scenario
Imagine a hypothetical 5-month-old with unrepaired tetralogy of Fallot who becomes deeply blue and breathless while crying during a blood test. The options are to lay her flat to examine her, to give a feed to calm her, or to hold her in a knee-chest position and give oxygen as prescribed while calling for help.
The knee-chest position with oxygen and escalation is the strongest answer. Lying flat does not raise systemic resistance, and feeding during a spell can worsen hypoxia and adds aspiration risk. The tested reasoning is that flexing the legs increases systemic vascular resistance, reducing right-to-left shunting through the ventricular septal defect and sending more blood to the lungs.
Sources and further reading
MSD Manual Professional: Tetralogy of Fallot. Four components, triggers and signs of hypercyanotic spells, shunt mechanism, squatting and knee-chest raising systemic resistance, and acute treatment options.
Europe PMC: A primer for the student joining the congenital cardiac surgery service, primer 3 of 7. Outflow obstruction forcing right-to-left shunting, tet spells during stress, relief by squatting or knee-to-chest, and risk of loss of consciousness or arrest.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our pediatrics practice questions are the closest set to what this page covers.
Common questions
Why does squatting improve oxygenation in tetralogy of Fallot?
Squatting raises systemic vascular resistance by compressing leg arteries. Less deoxygenated blood crosses the ventricular septal defect into the aorta and more flows to the lungs.
What are the signs of a tet spell?
Rapid deep breathing, irritability and prolonged crying, increasing cyanosis, and a heart murmur that softens or disappears. A prolonged spell can cause loss of consciousness.
What is the first nursing action during a tet spell?
Place the child in the knee-chest position and keep them calm, then give oxygen as prescribed and call for help. Further treatment follows the team's orders if the spell persists.