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Author: Thong Yi Kun
Editor: Dr. Suneet Sood
Narrators: Thong Yi Kun, Alan Koay

Transcript

Aris was sitting in Dr Basir’s clinic.

He was a 4th year medical student doing his Paediatrics rotation. While attending one of the pediatric ICU rounds, he noticed a most peculiar thing – two nurses were measuring a baby’s oxygen saturations through both their right hand and right foot. His interest piqued, Aris approached the nurses and asked why they were measuring the saturation in two places.

“We are looking for congenital cardiac defects,” the senior of the two nurses said.

Aris planned to go back and read, but ran into the consultant, Dr. Basir. He was a very friendly soul, and Aris had no hesitation in asking him about the dual monitoring.
Dr. Basir just gave him the smallest of smiles and said, “Tell you what. Come to my clinic tomorrow and I’ll explain why”.

So here Aris was, sitting in clinic with Dr. Basir. An 8 year old boy walked through the door accompanied by his parents.

“Look at his hands and feet,” said Dr Basir. “Tell me what you see.”

Aris looked closely at the boy’s hands. They were normal as far as he could see. But when he looked at the feet, it became obvious what Dr. Basir was demonstrating.

“The toes on both his feet are clubbed, but not on his hands! How is that possible?”

“Well, chronic cyanosis is a well-known cause of clubbing. Do you know what can cause cyanosis in the lower limbs but not the upper?”

Aris froze. He had long forgotten the anatomy and physiology of human circulation.

“This is called differential clubbing, and is a consequence of differential cyanosis” explained Dr. Basir. “It occurs when deoxygenated blood from the pulmonary artery flows through a patent ductus arteriosus into the aorta and down to the lower extremities. The right and left subclavian arteries branch off before the ductus arteriosus. This is why the lower half of the body is cyanotic but the upper limbs are normal.”

“But, but, I thought blood in a PDA flows from the aorta to the pulmonary artery,” objected Aris, a little hesitantly.

“Blood flows from high pressure to low pressure down a gradient. Once you understand that principle, the causes of differential cyanosis becomes easy to remember.”

“This 8 year old boy has a patent ductus arteriosus. You are right: initially, blood from the aorta has a higher pressure than the pulmonary artery, causing a left-to-right shunt. Over time, the pulmonary vascular resistance increases, and the right ventricle hypertrophies. The pulmonary artery pressure rises, and the patient develops a right-to-left shunt. We call this a shunt reversal.”

“So the nurses were looking for a shunt reversal?” asked Aris, a little doubtful.

“In neonates, the causes of differential cyanosis are quite different. Their right ventricle often has not had time to hypertrophy, thus NO shunt reversal. Their differential cyanosis often a result of LOW aortic pressure. Examples include an aortic coarctation proximal to the ductus, or severe aortic stenosis. Of course, persistent pulmonary hypertension with a patent ductus may cause differential cyanosis as well.

“Ah, that makes sense,” said Aris. Blood from the pulmonary artery may enter the aorta from a patent ductus if there is pulmonary hypertension, or if the pressure in the aorta is very low. And most of the de-oxygenated blood will go to organs distal to the entry of the patent ductus, such as the legs.”

References

Singh J, Singh A. Differential Cyanosis. The American journal of medicine. 2013 Oct 1;126(10):e9.

Anoop TM, George KC. Differential clubbing and cyanosis. New England Journal of Medicine. 2011 Feb 17;364(7):666-.