Dr.S.Venkatesan MD

Differential cyanosis classically occurs in PDA with reversal of shunt when raised PA pressures /PVR is able to supersede the systemic Aortic pressure and drive the blood from LPA to descending Aorta bringing down the lower limb saturation.

Of course,  this can be undone by the presence of any other intra-cardiac shunts or aberrant left subclavian that arising from the desaturated descending aorta.

Other causes of reversed differential cyanosis 

Where the upper body is cyanosed (desaturated) and the lower half is not. There is a conventional list of conditions.

1. Transposition of the great arteries (TGA) with patent ductus arteriosis (PDA) and elevated pulmonary vascular resistance
2. TGA with PDA and pre-ductal aortic interruption or coarctation
3. Supracardiac TAPVC* + PDA
4. Anomalous right subclavian artery connected to hypertensive ductus through RPA

(*This occurs due to streaming effect ) Highly saturated superior vena cava (SVC) blood into the right ventricle, reach MPA / through a PDA, and to the descending aorta, with streaming of more desaturated blood from the inferior vena cava (IVC) into the LA through PFO (Ref Yap S H Pediatr Cardiol. 2009 )

Now let us add one more cause for  reversed differential cyanosis in the Modern Era

It is seen with ECMO in VA connection (Often reported in babies ) . The Aorta has high oxygen content entering from the femoral cannula going up into the Aortic arch., while deoxygenated blood from LV (because of failing lungs) reach antegradely to the Aorta. Ideally, the ECMO is expected to supply the entire aortic arch and hence oxygenation is uniform all over the body. It rarely happens as some amount of flow will come from LV unless its in asystole. However, If the severely dysfunctional heart tends to recover & lung oxygenation is very poor as well, the LV stroke volume competes with highly oxygenated blood coming from below ( femoral inflow ) into the Aorta , creating a watershed zone . This makes the deoxygenated blood perfusing upper half of the body and hyper oxygen saturation lower half. This is been referred to as North-south syndrome or (Harlequin syndrome the famous Italian comical character)

How to manage North-South syndrome?

• Try to Improve the oxygen perfusion with high-frequency ventilation(This is logical first step , to improve the native lung function)
• ECMO flow rate may be increased and overdrive the LV ejection .(This can be counter-productive as we are hitting a recovering ventricle)
• Converting to VV ECMO if the hemodynamics allows. This is possible as North-south syndrome is a sign of recovering cardia function VV ECMO will convert it into a primary lung support

Reference