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Estimation of Pulmonary Artery Pressure (PAP) is an important aspect of the diagnosis and treatment of Persistent Pulmonary Hypertension of the Newborn (PPHN).  There are a number of echocardiographic indicators which help to directly or indirectly measure PAP.

• It is important to perform echocardiography to exclude cyanotic cardiac disease and to assess cardiac function.
• Infants with severe persistent hypoxaemia should be assessed by an experienced paediatric echocardiographer.

Tricuspid Regurgitation

• This is the most accurate way of determining PAP.
• The jet of blood leaking through the tricuspid valve is interrogated with Doppler.  The peak velocity of the tricuspid regurgitation (TR) jet is a direct indicator of the right ventricular pressure (and therefore PAP).
• Using the Bernoulli equation (p = 4v², where p is the pressure drop (mmHg) and v is the velocity of blood flow (m/sec)), the pressure in the RV can be calculated by

RV pressure = RA pressure + (4 x (TR jet velocity)²).

Click on the thumbnail images to see a larger image
	The image to the left demonstrates an apical 4-chamber view of the heart. The chambers and valves are labelled (LA Left Atrium, MV Mitral Valve, LV Left Ventricle, IVS Interventricular septum, RA Right Atrium, TV Tricuspid Valve, RV Right Ventricle).  The green wedge shape is the outline of the colour Doppler field seen in the next image.
	In this image, colour Doppler has been applied. The TR jet is seen as the blue-yellow streak heading upwards away from the tricuspid valve.
	In this image, Doppler interrogation has been applied to determine the velocity of the TR jet. The velocity is in the region of 2.9 m/sec, giving a gradient across the tricuspid valve of 34mmHg.  This indicates that that PAP is at least 34mmHg above RA pressure.  The RA pressure is generally assumed to be 5-10mmHg in ventilated infants. It is important to know the systemic blood pressure to determine whether PAP is above systemic BP.

Ductal Flow

• The direction and velocity of ductal blood flow can give useful information on PAP.
• Pure right-to-left flow indicates that PAP is higher than aortic pressure throughout the cardiac cycle.
• Bidirectional flow occurs when the aortic and pulmonary pressures are approximately equal.  Flow is left-to-right during diastole and right-to-left during systole (as the pulmonary arterial pressure wave reaches the duct before the aortic pressure wave).
• Bidirectional flow is common in healthy babies in the first 12 hours but changes to pure left-to-right when aortic pressures become higher than pulmonary pressures.

This baby had severe PPHN (with RV pressures calculated to be 74mmHg above RA pressure), although his requirement for ventilatory support was minimal.  The Doppler gate is in the PDA.  Left-to-right flow (above the line) and right-to-left (below the line) are seen.

TPV/RVET Ratio and RPEP/RVET Ratio

• This ratio of Time to Peak Velocity (TPV) of pulmonary blood flow and the Right Ventricular Ejection Time (RVET) can indirectly indicate PAP.
  • With PPHN, the TPV/RVET ratio falls as there is a rapid acceleration of blood flow into the pulmonary artery followed by an early deceleration of pulmonary blood flow from increased pulmonary resistance.  In severe cases, there can be a secondary "notching" of the flow profile.
• In PPHN, the period of isovolumetric contraction (between closure of the tricuspid valve and the opening of the pulmonary valve - RPEP, Right Ventricular Pre-Ejection Period) is prolonged as the RV generates enough pressure to open the pulmonary valve, resulting in an elevated RPEP/RVET ratio.
• There can be poor repeatability of these measures but they do have a role when you cannot measure the velocity of the TR jet.

Atrial Shunting

• Some degree of right-to-left atrial shunting through the patent foramen ovale is common, although it is rare for this to be purely right-to-left (pure right-to-left flow is Totally Anomalous Pulmonary Venous Drainage [TAPVD] till proven otherwise!).
• Bowing of the interatrial septum to the left is commonly seen.
• Right-to-left atrial shunting reflects right atrial filling (diastolic) pressure or ventricular filling more than right ventricular systolic pressures.

Other Shunts

Click on the thumbnail image to see a larger image

Other shunts can be used to estimate RV pressure. The image to the left demonstrates a subcostal view of the heart focusing on the interventricular septum.  This baby did not have a duct, nor any evidence of tricuspid regurgitation from which to estimate RV pressure.  However, he was found to have a small VSD (seen as the reddish shape in the middle of the "wedge").  The redness of the image demonstrates left-to-right flow through the small VSD at the point in time that the image was taken. Doppler interrogation of the flow through the VSD demonstrated pure left-to-right flow, and the peak velocity was 2.2 m/sec.  This indicated that the peak LV pressure was at least 19 mmHg above RV pressure.

Cardiac Function and Output

• Elevated PAP is generally associated with decreased pulmonary blood flow and increased pulmonary vascular resistance.
• Not uncommonly, there is enlargement of the RV and RA, as well as the main pulmonary artery.
  • There may be flattening or even bowing of the interventricular septum to the left if RV pressures exceed LV pressures.
• As cardiac output is dependent on venous return to the RA and LA, cardiac output (both RVO and LVO) is frequently reduced with PPHN.
  • Severe PPHN may be associated with LVO below 100ml/kg/min (normal 150-300ml/kg/min)
• Quantitative assessment of cardiac function may assist with decisions and assessments of the roles of inotropes, inhaled nitric oxide, and other interventions affecting cardiac output.
• If the LA and LV appear under-filled, it is critical to exclude TAPVD.

Click on the images to the left to see larger images TAPVD can be missed when assessing infants with presumed PPHN. The top images on the left are taken in the parasternal long axis view.  The RV is anterior to the LV.  The ascending aorta is labelled as AO.  The RV is enlarged and the LV is underfilled and "squashed".  This is common in severe PPHN. However, the lower image shows the apical 4-chamber view.  The RA and RV are grossly dilated, the LV is small, and another structure - the pulmonary venous chamber, PVC,  - is seen (usually not as obviously as in this image). The PVC represents the confluence of the abnormal pulmonary veins draining into the systemic venous circulation (which can be at the level of portal vein, coronary sinus, RA directly, or to an ascending vertical vein and then innominate vein). Demonstration of a left-to-right shunt at atrial level essentially excludes TAPVD

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