Another great podcast from PedsCases: Cyanotic Congenital Heart Disease, Aug 29, 2019. [Link to the podcast] [Link to the transcript] by Shealynn Carpenter and Brie Cawston-Grant, medical students at the University of Alberta, in collaboration with Dr. Jessica Foulds, a pediatrician, and Dr. Lily Lin, a pediatric cardiologist at the University of Alberta.
Here are excerpts:
A congenital heart disease is defined as a structural or functional malformation of the heart or great vessels that occurs during gestational development. Congenital heart diseases are the most common type of congenital defect and occur in approximately 1% of the general population .
Congenital heart disease is a broad topic that includes several conditions that are typically categorized as either acyanotic or cyanotic.
This podcast will focus on cyanotic heart diseases.
Cyanotic heart defects include several different anatomic malformations that are characterized by the presence of a right-to-left shunt, reduced blood flow to the lungs, or a combination of both. A shunt is essentially blood flowing through an atypical pathway.Specifically, a right-to-left shunt allows the mixing of deoxygenated blood (from the right side of the heart) with the oxygenated blood (from the left side of the heart), which will
enter the systemic circulation.
Next is presented the case of a four hour old infant who appeared well at birth.
The infant is now noted to have blue fingers and lips and to have an oxygen saturation of 68% measured in the right hand.
This could be a critically ill child and you need to be prepared to support their ABCs. At this point we only know that John has blue lips and a low O2 saturation. Remember that central cyanosis presents as a blue-purple-ish tinge on the lips, tongue, oral mucosa or trunk. It is easiest to see cyanosis where the skin is thin.
Central cyanosis is a common presentation for many pulmonary conditions, so your differential diagnosis must include both pulmonary and cardiac causes. At his point, differentiating between a cardiac and pulmonary cause is critical and should guide your history taking, physical exam and further investigations.
Finally, you’ll note that we mentioned John’s oxygen saturation was measured from the right hand. Taking a right-hand saturation (or a preductal sat) and taking one in a lower limb (a postductal sat) can also help you in your assessment of a newborn with cyanosis.
The terms pre and postductal refer to locations relative to the ductus arteriosus, which is a connection between the main pulmonary artery and the proximal descending aorta. It is important for fetal circulation before the lungs are functioning.
We would generally expect the preductal sat to be the same as the postductal. A difference of greater than 3% as per the Canadian Pediatric Society (or greater than 5% clinically)
is considered significant. If a preductal sat is higher than the postductal there’s a right to left shunt through a patent ductus arteriosus to the lower body.
What’s interesting is that if transposition of the great arteries (TGA) also has either coarctation or pulmonary hypertension, the preductal sat is LOWER than the postductal! 
HPI and Gestational History:
[The nurse informs the doctor that the infant] was feeding well
and had been active until just 5 minutes ago when his parents noticed that his lips were blue, and he felt cold. The nurse provided oxygen therapy but there was no improvement of John’s O2 saturation. On prompting, [the infant’s] mother states that she had routine prenatal screening with a normal 18-week ultrasound.