Pulmonary Hypertension (PH) is characterized by abnormally high blood pressure within the arteries of the lungs. This elevated pressure strains the heart, leading to long-term damage and dysfunction. Echocardiography (echo) is the primary non-invasive method for screening individuals suspected of having PH. This ultrasound examination provides an initial estimate of the pressure in the lung arteries, guiding the need for more definitive testing.
Defining Pulmonary Hypertension and Echocardiography
Pulmonary hypertension occurs when the blood vessels carrying blood from the right side of the heart to the lungs become narrowed, stiff, or blocked. This creates resistance, forcing the right ventricle (RV) to pump against abnormally high pressure. Historically, PH was defined as a mean pulmonary artery pressure (mPAP) of 25 millimeters of mercury (mmHg) or greater at rest, measured through an invasive procedure.
Echocardiography employs high-frequency sound waves to generate real-time moving images of the heart and its internal structures. The non-invasive procedure uses a probe placed on the chest to visualize the size and movement of the heart chambers, valves, and blood flow. By observing these flow patterns, the echocardiogram provides indirect clues about the pressures within the heart and pulmonary circulation.
Key Pressure Estimates from the Echocardiogram
Assessing pulmonary pressure relies on tricuspid regurgitation (TR), which is the backward flow of blood across the tricuspid valve during the heart’s contraction. This TR jet is particularly strong when the pressure inside the right ventricle (RV) is high. Using Doppler technology, the velocity of this TR jet can be precisely measured.
The peak velocity of the tricuspid regurgitation jet (TRV) is the most important measurement, relating directly to the pressure difference between the right ventricle and the right atrium. This relationship is calculated using the modified Bernoulli equation: the pressure gradient (\(\Delta P\)) equals four times the velocity squared (\(4V^2\)). For example, a TRV of \(3.0\) meters per second translates to a pressure gradient of \(36\) mmHg.
This pressure gradient represents the difference between the Right Ventricular Systolic Pressure (RVSP) and the Right Atrial Pressure (RAP). To calculate the estimated RVSP, which is equivalent to the Pulmonary Artery Systolic Pressure (PASP), the estimated RAP must be added to the pressure gradient. The calculation is expressed as \(PASP \approx 4V^2 + RAP\).
The Right Atrial Pressure (RAP) is not measured directly by the echo. It is instead estimated by evaluating the size and collapsibility of the Inferior Vena Cava (IVC), a large vein that drains into the right atrium. If the IVC is small and collapses significantly with a breath, a low RAP is assumed, typically 3 to 5 mmHg. Conversely, if the IVC is large and non-collapsible, a higher RAP is assumed, which can be 15 mmHg or more, reflecting fluid backup due to high pressure.
The numerical value of the peak TRV is the primary screening indicator for PH. A TRV of \(2.8\) meters per second or less suggests PH is unlikely. Conversely, a TRV greater than \(3.4\) meters per second indicates a high probability of PH. If the TRV falls between these two values, other echocardiographic and clinical signs must be considered to determine the overall probability.
Assessing the Impact on Heart Structure and Function
Chronic pressure overload forces the right side of the heart to work harder, leading to distinct structural and functional changes visible on the echocardiogram. The right ventricle (RV), normally a thin-walled, crescent-shaped chamber, begins to remodel in response to the increased afterload. Early on, the RV wall may thicken (hypertrophy) as the muscle attempts to generate the necessary force to push blood into the pulmonary arteries.
If the pressure remains high, the right ventricle may begin to dilate, or enlarge, as it struggles to maintain pumping effectiveness. This enlargement often causes the right atrium, which receives blood from the body before the RV, to also become distended. The degree of enlargement in both chambers indicates the severity and chronicity of the pulmonary hypertension.
A sign of severe pressure overload is the alteration of the interventricular septum, the wall separating the right and left ventricles. High pressure in the RV can push the septum into the left ventricle (LV), causing the normally circular LV to appear flattened or “D-shaped.” This septal flattening signifies that the RV pressure exceeds that of the LV, indicating advanced pressure strain.
The echo also provides functional metrics of the right ventricle’s pumping ability. A common measurement is the Tricuspid Annular Plane Systolic Excursion (TAPSE). TAPSE measures the longitudinal movement of the tricuspid valve annulus toward the heart’s apex during contraction. A reduced TAPSE value suggests impaired RV contractility, indicating that the right side of the heart is beginning to fail under sustained high pulmonary pressure.
When the Echocardiogram Requires Further Testing
The echocardiogram provides an estimate of pulmonary pressure and acts only as a screening tool, not a definitive diagnostic test for PH. The calculated PASP relies on assumptions regarding the accuracy of the TRV measurement and the estimated Right Atrial Pressure (RAP). These assumptions introduce potential for error, meaning the echo can sometimes over- or underestimate the true pulmonary artery pressure.
Poor image quality due to a patient’s body habitus (a limited acoustic window) can prevent the sonographer from obtaining a clear TR signal, often resulting in pressure underestimation. Furthermore, a very severe TR jet can cause rapid pressure equalization between the right ventricle and right atrium, also leading to an inaccurate velocity. In these instances, the echocardiogram is not sufficient to confirm or rule out the disease.
The definitive diagnostic procedure, considered the gold standard for PH, is Right Heart Catheterization (RHC). This invasive test involves guiding a thin, flexible tube into the pulmonary artery to directly and accurately measure the pressures within the heart chambers and the pulmonary circulation. RHC is necessary to confirm the diagnosis, determine the precise mean pulmonary artery pressure, and classify the specific type of pulmonary hypertension. A high-probability echo, or one that is technically limited but suggests the condition, typically triggers referral for the confirmatory RHC.