Assessment of PPHN by Echo
This is arguably one of the most immediately valuable uses of functional echo.
Differentiating PPHN and congenital heart disease in the middle of the night is not easy. At this point we refer you again to this Disclaimer. If you are in any doubt start Prostin, and get an urgent cardiology review.
Once structural heart disease has been excluded, assess the severity of PPHN. As ever with echo, there is no single measure that ensures the correct assessment is made. We would argue that an echo assessment of PPHN severity requires 3 key components:
1 - Direction of ductal shunt. After the first hour or two of life any PDA should have systemic-to-pulmonary shunting. This is generally visible as a jet of orange flow on the ductal view. However in PPHN the shunt is pulmonary-to-systemic, and therefore the ductal flow appears blue:
Keep this mental image in mind. it is ALWAYS abnormal. It is often referred to as the 'three-legged stool':
2 - Quantification of TR jet velocity. A simplified version of the Bernoulli equation suggests that the pressure gradient between two chambers across a narrow orifice is equal to 4 times the velocity squared. Since the tricuspid valve is closed during ventricular systole any regurgitant jet is occuring through a very narrow orifice:
By estimating the velocity of the jet (if present) the pressure gradient across the valve can be estimated as 4 times the velocity squared. For example a velocity of 2 m/s suggests a gradient of 16mmHg. A velocity of 4 m/s suggests a gradient of 64mmHg. Remember to add around 5mmHg (an estimate of right atrial pressure) to the gradient to calculate the pulmonary artery pressure, and to compare this pressure level to systemic systolic blood pressure (not the mean systemic blood pressure).
3 - Evidence of ventricular septal distortion. In the physiological situation the left ventricular pressure is higher than the left. Therefore the intraventricular septum will 'bow' into the right ventricle - producing the classic circular left ventricular shape (A, bleow). However in PPHN the right ventricular pressure is often equal to, or even above the left. Therefore the intraventricular septum will be flattened, or even bow into the left ventricle (B, below):
As well as these 3 components 2 other patterns are worthy of mention. The most severe, and a rare but important diagnostic trap, is functional pulmonary atresia. In these infants the pulmonary resistance is so high, and right ventricular function so poor, that the contracting right ventricle cannot open the pulmonary valve. So the long axis view looks like this:
This can obviously be mistaken for a structural atresia, resulting in unnecessary referral for a cardiology opinion, and insufficient focus on therapies to decrease pulmonary vascular resistance. The two clues that the atresia is functional rather than structural is that the valve leaflets here look thin and crisp, rather than the thickened valves seen in pulmonary atresia/stenosis:
In addition there is often regurgitation through the immobile pulmonary valve in PPHN, which clearly would not be the case in a structural atresia: