11.1 Cyanotic Congenital Heart Disease

Cyanotic congenital heart disease (CHD) refers to structural heart defects that allow deoxygenated (systemic venous) blood to bypass the lungs and enter the systemic circulation directly — a right-to-left shunt. The result is central cyanosis: a bluish discoloration of the lips, tongue, and mucous membranes reflecting an oxygen saturation typically below 85–90%, in contrast to acrocyanosis (blue hands and feet only), which is common and benign in the first 24–48 hours of life.

The "5 Ts": High-Yield Cyanotic Lesions

Five lesions account for the overwhelming majority of cyanotic CHD tested on written exams, remembered as the 5 Ts:

Timing of Presentation

Timing is a favorite exam discriminator. TGA presents with profound cyanosis in the first 24 hours of life because it depends entirely on mixing (via a patent ductus arteriosus, patent foramen ovale, or VSD) to survive — without a mixing site, TGA is incompatible with life. Obstructed TAPVR and truncus arteriosus typically present in the first days to weeks with severe cyanosis and respiratory distress. TOF is more variable: mild forms may be pink at birth and become progressively cyanotic over weeks to months as dynamic infundibular (RVOT) obstruction worsens, while severe forms are cyanotic from birth. Tricuspid atresia usually presents in the first days of life as well.

The Hyperoxia Test

When a cyanotic neonate cannot be immediately characterized by echocardiography, the hyperoxia test helps distinguish a cardiac from a pulmonary or respiratory cause of cyanosis. The infant breathes 100% oxygen for approximately 10 minutes, and an arterial blood gas is drawn (pre-ductal, typically right radial). If the PaO2 rises above roughly 250–300 mmHg, the lungs can oxygenate blood normally and a primary pulmonary or central nervous system cause is more likely. If the PaO2 stays low — often under 100 mmHg — despite 100% oxygen, a fixed right-to-left cardiac shunt is likely, because no amount of supplemental oxygen can raise the saturation of blood that never traverses the lungs. Any infant whose PaO2 fails to rise on the hyperoxia test (suggesting a duct-dependent cyanotic lesion) should be started on prostaglandin E1 (PGE1) to maintain ductal patency while awaiting definitive imaging and repair, since the ductus arteriosus is often the only source of adequate pulmonary or systemic blood flow in these lesions.

Tet Spells (Hypercyanotic Spells)

"Tet spells" are acute, episodic worsenings of cyanosis classically seen in Tetralogy of Fallot, triggered by crying, feeding, defecation, or agitation. The trigger causes increased infundibular spasm and/or decreased systemic vascular resistance (SVR), which increases right-to-left shunting across the VSD and acutely worsens hypoxemia. Recognizing and managing a tet spell is a high-yield exam scenario:

1. Knee-chest position (or squatting in older children) — increases SVR and systemic afterload, reducing the right-to-left shunt
2. Supplemental oxygen and calming the infant to minimize crying and agitation
3. Morphine — reduces infundibular spasm and respiratory drive
4. IV fluid bolus — increases preload and pulmonary blood flow
5. Phenylephrine — a pure alpha-agonist that raises SVR
6. Beta-blockers (e.g., propranolol) — relax the infundibular spasm and can also be used for chronic spell prevention

Exam traps worth memorizing: a single S2 points toward tricuspid atresia, truncus arteriosus, or TGA, because only one functional semilunar valve closure is audible or the great vessels are malpositioned; 22q11.2 deletion is classically linked to truncus arteriosus (and also to TOF and interrupted aortic arch); and unlike most cyanotic lesions, which produce right ventricular hypertrophy, tricuspid atresia produces left axis deviation and LVH because the dominant, functioning ventricle is the left ventricle.

Newborn Pulse Oximetry Screening

Most cyanotic CHD is now first detected before a murmur is ever heard, through routine critical congenital heart disease (CCHD) pulse oximetry screening, performed after 24 hours of age (to avoid false positives from normal fetal-to-neonatal transition). A right hand (pre-ductal) and either foot (post-ductal) saturation are measured. The screen fails — prompting urgent echocardiography — if either saturation is below 90%, if both are 90–94% on repeat testing, or if there is more than a 3% absolute difference between the pre-ductal and post-ductal readings. This last criterion, a pre-/post-ductal saturation gradient, can also flag duct-dependent left-sided obstructive lesions such as severe coarctation even when the infant is not yet visibly cyanotic, making the screen a broader safety net than cyanosis recognition alone.