Ascending aorta and arch (pre-ductal)
Great vessels · SaO2 ~65% Pre-ductal
The left ventricle sends the best-oxygenated arterial blood in the fetus into the ascending aorta. The coronary arteries, brachiocephalic trunk, left common carotid and left subclavian arteries take most of it before the ductus joins just beyond the left subclavian.
Traced from the start
- Placenta
- Umbilical vein
- Ductus venosus
- Foramen ovale to left atrium and left ventricle
- Ascending aorta and arch (pre-ductal)
Detail
- Saturation
- ~65% (PO2 ~25-28 mmHg), pre-ductal
- Distribution
- Most left ventricular output leaves through the coronary and arch branches; only ~10% of combined output continues across the isthmus
- Isthmus
- Narrow segment between the left subclavian artery and the ductal insertion carries little fetal flow, which is why it is the site of coarctation
- Pre-ductal sampling
- Right hand pulse oximetry in the neonate reads this blood
When it goes wrong
Newborn pulse oximetry screening (right hand and either foot at 24 h)
Pass is 95% or more with no more than 3% pre/post-ductal difference; lower values or a bigger gap prompt echocardiography for critical congenital heart disease; a gap over 10% with the hand higher means right-to-left ductal shunting, as in PPHN
Coarctation with a still-patent duct in the first days
Lower body perfused with desaturated ductal blood: differential cyanosis and weak, delayed femoral pulses; when the duct closes the neonate collapses in shock
Practise this structure
1 question in the bank tagged Oxygen Content.