Ventricular septal defect: direction of shunt
An uncomplicated ventricular septal defect (a hole in the wall between the right and left ventricles) usually shunts blood, meaning diverts it, from left to right.
Remember
After the resistance of the lung vessels (pulmonary vascular resistance) falls, an uncomplicated ventricular septal defect usually shunts blood from left to right, because left ventricular pressure is higher.
A ventricular septal defect (VSD) is a hole in the wall (septum) between the right and left ventricles. Blood moves down its pressure gradient, which means from the chamber with the higher pressure to the chamber with the lower pressure.
The left ventricle pumps blood around the whole body, so it generates much higher pressure than the right ventricle, which supplies only the lungs. Blood is therefore shunted (diverted) from left to right through the hole, and the lungs receive extra blood. This is a left-to-right shunt.
In the first days of life the resistance of the lung vessels (pulmonary vascular resistance) is still relatively high, which limits the shunt. As this resistance falls over the following weeks, more blood crosses the hole. With a large defect the baby may then develop breathlessness, poor feeding and poor weight gain at about 4 to 6 weeks.
The baby is typically not blue, so the defect is called acyanotic. A small restrictive defect (one narrow enough to limit flow and the transmission of high pressure to the right side) gives a relatively small shunt, and children with it are typically well.
If a large defect is left untreated, the lung vessels can be damaged and resistance rises until the shunt may reverse. This is Eisenmenger syndrome, a late complication of a large, long-standing defect, and the patient becomes blue.
Key facts
- Defect
- a hole in the wall (septum) between the right and left ventricles.
- Direction
- in an uncomplicated defect, blood usually goes from the left ventricle to the right ventricle, because left ventricular pressure is higher.
- Effect on the lungs
- more blood passes through the lungs, so pulmonary blood flow increases.
- Colour
- the baby is typically not blue (acyanotic).
- Timing
- the shunt grows as pulmonary vascular resistance falls, and symptoms from a large defect often appear at about 4 to 6 weeks.
- Small restrictive defect
- it limits flow and pressure transmission, so the shunt is small and pulmonary pressure stays normal or near normal.
- Late complication
- a large untreated defect can raise pulmonary resistance and reverse the shunt (Eisenmenger syndrome).
Exam traps
- A right-to-left shunt is not the usual pattern in an uncomplicated ventricular septal defect, because the left ventricle has the higher pressure; reversal is a late complication of a large untreated defect.
- Equal flow in both directions is not the usual pattern in an uncomplicated ventricular septal defect, because the net shunt is left to right while pulmonary resistance is low, even when the pressures come close together in a large defect.
- The fall in pulmonary vascular resistance does not close the defect: some small defects do close on their own in infancy, but that is the hole shrinking over time, and the fall in resistance actually increases left-to-right flow.
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