Echocardiographic evaluation of the pulmonic valve and pulmonary artery

INTRODUCTION — Echocardiographic imaging of the pulmonic valve and Doppler measurement of transpulmonary flow are potent tools in the clinical evaluation of disorders of the pulmonic valve and pulmonary arteries.

The majority of clinically important lesions at the level of the valve, both stenotic and regurgitant, are associated with congenital heart disease. Rarely, acquired lesions, including endocarditis, rheumatic heart disease, and carcinoid heart disease, involve the pulmonic valve. Identification and characterization of these pathologies requires thorough echocardiographic interrogation and consideration of clinical context. Finally, evaluation of flow through the pulmonic valve and right ventricular outflow tract (RVOT) are essential elements for evaluating hemodynamics. Careful measurements can yield significant information regarding flow, pressure and resistance in the pulmonary circulatory bed.

VIEWS FOR EVALUATION OF THE PULMONIC VALVE

Basic transthoracic views — Interrogation of the pulmonic valve by transthoracic echocardiography (TTE) begins with the parasternal short axis at the level of the aortic valve, where pulmonic valve anatomy can be examined for thickening, doming, or vegetation. Color flow Doppler is placed over the RVOT to detect flow acceleration or regurgitation. Then, using both continuous wave and pulse wave Doppler at the level of the pulmonary valve, velocities across the pulmonary valve in systole and diastole and peak velocities around the RVOT can be measured (figure 1 and image 1 and image 2). Several cycles of these views should be recorded to account for small variations in velocities during the respiratory cycle. M-mode through the pulmonary valve can also be obtained in this view. Tilting the probe in a slight cranial direction gives a clearer view of both the pulmonic valve and the proximal pulmonary artery (image 3). Color Doppler demonstrates flows proximal and distal to the valve and pulsed and continuous wave Doppler records the waveforms from that flow. The valve and artery may also be viewed from an orthogonal plane to assure complete interrogation of the structures and the most axial flow signals. Although the apical and subcostal views can be useful in imaging the right ventricle as it relates to the pulmonic valve and arteries (and occasionally allows for visualization of the pulmonic valve), it is often difficult to adequately image the pulmonary arterial system in these views. For further imaging of the branch pulmonary arteries, the suprasternal notch view with visualization of the aortic arch may be used (image 4).

Three-dimensional transthoracic views — If there is excellent visualization of the pulmonic valve in two dimensions, a three-dimensional reconstruction may be helpful in further defining valvular and proximal arterial anatomy. This may be especially useful when considering interventions on prosthetic valves [1]. Three-dimensional echocardiography may also allow more accurate measurement of the pulmonic valve annulus area [2], which can be useful in calculating shunts.

Transesophageal echocardiography — The pulmonic valve can be imaged from several views [3]. First, it can be obtained by turning the probe counterclockwise from the mid-to-high esophageal ascending aorta long-axis view (approximately 110 to 130°). At this position, the main pulmonary artery is seen with the pulmonic valve in the far field. Spectral Doppler interrogations can be performed here. At the mid-to-high esophageal ascending aortic short-axis view (approximately 40 to 60°), both the RV inflow and outflow (including the pulmonic valve) can be visualized. Here, two of the pulmonic leaflets can be seen. In the presence of calcific aortic stenosis or aortic prosthesis, acoustic noise can limit visualization of the pulmonic valve, which is immediately anterior to the aortic valve.

Transgastric views of pulmonic valve are used primarily for spectral Doppler interrogations (such as when calculating Qp:Qs), as the pulmonic flow is most parallel to the Doppler beam. With the transducer in the stomach, clockwise rotation results in pulmonic valve visualization, where Doppler measurements can be made.

Intracardiac echocardiography — Intracardiac echocardiography (ICE) is being increasingly used to image the pulmonic valve during transcatheter pulmonic valve replacement. In a study of 35 patients undergoing transcatheter pulmonic valve replacement, there was reasonable correlation of transpulmonic gradients between ICE and TTE, with ICE-derived gradients lower than TTE-derived gradients, possibly due to patient’s sedation status during ICE image acquisition, high-frequency sound waves with ICE, as well as suboptimal Doppler alignment [4].

PULMONIC VALVE STENOSIS — Pulmonic valve stenosis is primarily diagnosed using Doppler echocardiography, although traditional two-dimensional and M-mode echocardiography can be helpful in confirming the diagnosis.

Two-dimensional echocardiography — Basic two-dimensional TTE views can show the anatomic features associated with isolated valvular pulmonic stenosis, including [5]:

●Right ventricular hypertrophy

●Systolic doming of the valve

●Valve thickening – Thickening can range from mild in the young patient to severe calcification in older adults

●Poststenotic dilation of the main pulmonary artery

Color Doppler echocardiography — Color flow Doppler signals around the point of stenosis will show aliased flow and are used to identify accelerating blood flow across the stenotic lesion. These images can also help target specific areas for interrogation by spectral Doppler.

Continuous wave Doppler — A continuous wave Doppler signal across the area of stenosis is a key component of any examination in pulmonic stenosis. The peak flow velocity across the valve is measured and converted to a peak pressure gradient via the modified Bernoulli equation (figure 1 and waveform 1). Of note, although pulsed wave Doppler can be used to quantify velocities, due to the Nyquist limit, it can quantify only the mildest degrees of obstruction.

Unlike aortic stenosis, where velocities are used to generate peak pressure, mean pressure and valve area that determine severity and drive management, the hemodynamic severity of pulmonic stenosis is usually defined solely by the peak pressure gradient across the valve. To calculate this pressure drop, peak velocities (V) obtained from continuous wave Doppler signals are converted into a pressure gradient by the simplified Bernoulli equation:

Note that prestenotic RVOT velocity is ignored as it is usually less than 1 m/s. However, in so-called "double chamber right ventricle" when a dynamic prevalvular gradient is present in the distal RVOT, this accelerated velocity must be factored into the equation.

Severe pulmonic stenosis is defined as peak velocity >4 m/s and mean gradient >35 mmHg, as noted in the 2018 American College of Cardiology/American Heart Association guidelines for management of adults with congenital heart disease [6]. Anatomically, the pulmonic valve may be thickened, distorted, or calcified leaflets, with reduced leaflet excursion. Right ventricular hypertrophy, poststenotic dilatation of the pulmonary artery, and sometimes right ventricular or atrial enlargement are hemodynamic consequences of severe pulmonic stenosis [7]. Although only mentioned in passing in the American Society of Echocardiography/European Association of Echocardiography guidelines [5], RV enlargement is considered by some clinicians as an important means of evaluating the significant of the systolic transpulmonic gradient.

M-mode echocardiography — M-mode findings in pulmonic stenosis are usually limited to the occurrence of an exaggerated "A wave" during diastole. The A wave is due to a powerful right atrial contraction that because of low diastolic pulmonary artery pressure, is able to open (or at least dome) the stenotic pulmonary valve in late diastole. The A wave is most pronounced during inspiration and accounts for the disappearance of the pulmonic ejection sound at that point in the respiratory cycle. This behavior makes the pulmonic ejection sound the only right-sided event that diminishes with inspiration.

Transesophageal echocardiography — When rare anatomic variants involving the right ventricular outflow tract are suspected, TEE can provide significant information about the right ventricular anatomy. Case reports of thrombosis after a gunshot wound, sinus of Valsalva aneurysm, subarterial ventricular septal defects, and membranous ventricular septal aneurysm in the setting of L-transposition have been reported [8] and visualized primarily by TEE.

Other considerations — The presence of a pressure gradient across the pulmonic valve is strongly suggestive of pulmonic stenosis. However, it is important to eliminate other causes of increased flow velocity. When high RVOT velocities are encountered, examine the RVOT carefully to seek muscular subvalvular stenosis (right ventricular outflow obstruction) that provides evidence of conditions such as a double chambered right ventricle. A ventricular septal defect (with or without evidence of a double chamber right ventricle) can introduce flow acceleration into the right ventricular outflow tract. Interrogating the septum in the apical two and four chamber can help to exclude this diagnosis. Right ventricular outflow tract obstruction may also result from extrinsic compression due to postoperative hematoma, thoracic tumors or a pericardial cyst [8], and these possibilities should be entertained in the appropriate clinical situation. (See "Pulmonic valve stenosis in adults: Management" and "Clinical manifestations and diagnosis of pulmonic stenosis in adults".)

PULMONIC VALVE REGURGITATION — Pulmonic regurgitation is most frequently diagnosed with two-dimensional color Doppler in combination with pulse or continuous wave Doppler signals.

Two-dimensional echocardiography — In the parasternal short axis view, examine the valve for thickening, vegetation or prolapse prior to interrogation with color as valvular anatomy can be a first clue to the etiology of the regurgitant lesion. Pulmonic valve carcinoid, for example, manifests as a short, thickened valve with both stenosis and regurgitation [9]. Endocarditis is suggested by a vegetation-like mobile mass, and congenital dysplasia of the pulmonic valve is associated with thickening of the leaflets [5,7,10,11]. Additionally, dilation of the right ventricle is an important adjunctive clue to hemodynamically significant PR [11].

Color Doppler — Examination of antegrade and retrograde flow in the short axis view across the pulmonic valve with visualization of both the RVOT and the proximal pulmonary artery gives the most information about PR (movie 1).

Trivial or trace pulmonary regurgitation is normally present in a high percentage of healthy people and is almost never clinically significant [12,13]. A small amount of diastolic flow into the RVOT is consistent with this diagnosis. Distinguishing between mild and more significant (moderate to severe) regurgitation can be more challenging and requires closer attention to the morphology of the pulmonic regurgitation jet. Jet characteristics important in delineating regurgitation severity include:

●The vena contracta – The diameter of the narrowest portion of the pulmonic regurgitation jet as it crosses the pulmonary valve varies with severity proportionally.

●Volume of RVOT occupied with regurgitation and the degree of retrograde jet penetration.

●Percentage of time in diastole occupied with regurgitation – Severe pulmonic regurgitation will have a dense jet that rapidly decelerates so that regurgitant flow ends around mid-diastole while mild to moderate pulmonic regurgitation flow persists throughout diastole; occasionally, a jet of mild pulmonic regurgitation will decelerate quickly, but its signal density is low.

●By contrast, velocity of flow is not proportional to severity of regurgitation. Higher velocities suggested by aliasing of the color Doppler signal or by high continuous wave Doppler peak velocity reflect higher pressure gradients between RVOT and pulmonary circulation. Higher velocities are associated with pulmonary hypertension.

The following are characteristics of severe PR: abnormally distorted or absent valve leaflets, annular dilatation, color jet filling the RVOT, dense laminar continuous wave Doppler jet that descends steeply and may terminate abruptly, and the associated hemodynamic consequence of right ventricular enlargement [7]. Symptoms may be none or variable depending on cause of PR and right ventricular function.

Pulmonic to aortic flow ratio by pulsed wave Doppler is greatly increased with severe pulmonic regurgitation (as flow across the pulmonic valve consists of both forward cardiac output and regurgitant flow), as recognized in the European Association of Cardiovascular Imaging Recommendations [14]. The pressure half time cutoff reported for severe PR is <100 ms, and the jet width ratio is categorized as >50 to 65 percent.

In our experience, color Doppler is not the best indicator of severe PR, as the regurgitant flow may terminate early in diastole. Doppler will show the dense regurgitant flow with a steep slope that terminates abruptly.

Continuous or pulsed wave Doppler — Most regurgitant lesions can be imaged with continuous or pulse wave Doppler signals across the pulmonic valve. Density of the pulmonic regurgitation signal increases with increasing regurgitation (image 5 and image 6 and image 2). Additionally, in severe pulmonic regurgitation, rapid equalization of pressures between the pulmonary artery and the RVOT early in diastole results in rapid termination of regurgitant flow around mid-diastole. On Doppler this signature of severity is seen as an abrupt return to baseline of the signal well before the onset of the next systole (image 2). The rapid decrease in slope of the PR signal that is typical of severe pulmonic regurgitation may be masked by fast heart rates that shorten diastole and, in doing so, diminish regurgitant fraction.

Three-dimensional echocardiography — Three-dimensional assessment of pulmonic valve anatomy can be helpful, although the technical factors limiting visualization in two dimensions affect three dimensions to a greater extent. Quantification of the pulmonary regurgitant jet and specifically the cross sectional area of its vena contracta may be better achieved with three-dimensional Doppler [15]. Three-dimensional echocardiography may also have a role in measuring the diameter of the pulmonary artery.

Transesophageal echocardiography — Because of the relatively distal position of the pulmonary valve with respect to the esophagus, it can be difficult to accurately determine the severity of pulmonary regurgitation with this modality. Additionally, visualization of the pulmonary artery is almost always more complete with transthoracic studies. As such, TEE has limited utility in the evaluation of pulmonic regurgitation. However, for supplemental anatomic information about the pulmonic valve, TEE can be helpful.

Clinical considerations — Clinically significant pulmonary regurgitation is rare and the differential diagnosis is fairly limited. As such, understanding the clinical context prior to performing the echo is extremely important, and the exam should be guided by the specific clinical question being asked.

In adults, the most common cause of moderate PR is pulmonary hypertension. (See 'Pulmonary hemodynamics' below.)

Severe pulmonic regurgitation is most commonly caused by the following:

●Tetralogy of Fallot (or other conotruncal abnormality) repair. Specific recommendations on multimodality imaging after Tetralogy repair have been published [16]. (See "Tetralogy of Fallot (TOF): Management and outcome".)

●Endocarditis. Vegetations are difficult to detect on TTE and TEE, and both modalities are frequently required to confirm the diagnosis. Most of the TEE literature on this condition is limited to single-case reports, but the availability of multiplane imaging capability has led to an increased frequency of this diagnosis [17].

●Carcinoid. (See "Carcinoid heart disease".)

Percutaneous pulmonary valve prosthesis — Pulmonic regurgitation or right ventricular outflow tract obstruction associated with congenital heart disease can be treated percutaneously, as discussed separately. In general, normally functioning pulmonic bioprosthetic valves have a peak velocity <3.2 m/s and a mean gradient <20 mmHg [18]. Longitudinal studies have shown that these valves have stable mean gradients <20 mmHg over the near term (18 months for both Sapien and Melody valves) [19] and long term (10 years for Melody valves) [20]. (See "Transcatheter pulmonary valve implantation".)

PULMONARY ARTERY — The main pulmonary artery and the proximal branches are routinely examined as part of a standard TTE.

Two-dimensional echocardiography — The main pulmonary artery is seen in the parasternal short axis view through the base of the heart. A slight cranial tilt to the probe (RVOT view) reveals the main pulmonary artery and proximal branches longitudinally from the pulmonic valve (image 3). In some patients, an anteriorly angulated four chamber apical view, a precordial long axis view (image 7), or a subcostal short axis view will better demonstrate the pulmonary artery. If there is significant difficulty with visualization, a suprasternal notch view examining the transverse aortic arch will often show the right pulmonary artery (image 4) or main pulmonary artery. Of note, in the parasternal view in adults, the lateral wall of the main pulmonary artery is usually not well visualized because of overlying lung. In children, the success rate of imaging the lateral wall and measuring the transverse diameter of the main pulmonary artery is much higher. Since the right pulmonary artery is readily imaged, doubling its area has been used as a rough substitute measurement for the main pulmonary artery. However, this method awaits further confirmation.

Anatomic evaluation of the pulmonary artery is important in several disease processes. In the presence of an atrial septal defect, the size of the main artery gives some indication of the duration or magnitude of the shunt. Severity of pulmonary artery dilation can also give clues as to the severity or duration of pulmonary hypertension (image 4).

With the caveat that the lateral wall of the pulmonary artery is difficult to image, pulmonary artery enlargement relative to aortic diameter can be an indicator of pulmonary hypertension [21]. Pulmonary artery dilation may be recognized even when views are limited and the lateral wall is not well seen.

Color Doppler — Normal systolic color flow by Doppler in the pulmonary artery is best seen in a short axis view through the base of the heart; a diastolic color flow Doppler signal is not usually detectable (image 8). In patients with a patent ductus arteriosus (PDA), a high velocity jet coming from the left main branch of the pulmonary artery and travelling retrograde to the pulmonic valve can be detected during both phases of the cardiac cycle (image 9). The high-velocity PDA signal may pollute the pulmonary regurgitation signal and confound its use in hemodynamic evaluation.

Pulmonary embolism — The role of echocardiography in patients with suspected or known pulmonary embolism is discussed separately. (See "Clinical presentation, evaluation, and diagnosis of the nonpregnant adult with suspected acute pulmonary embolism", section on 'Echocardiography' and "Treatment, prognosis, and follow-up of acute pulmonary embolism in adults", section on 'Prognostic factors'.)

PULMONARY HEMODYNAMICS — A noninvasive survey of hemodynamics is crucial to the management of patients with heart failure and pulmonary hypertension as it may obviate the need for repeated catheterizations. Calculations can be obtained using measurements in the right ventricle, tricuspid valve and pulmonic valve. Measurements obtained at the level of the pulmonic valve will be discussed here. Measurements utilizing the tricuspid valve and right ventricle are discussed in the following topic review. (See "Echocardiographic assessment of the right heart", section on 'Hemodynamics'.)

Pulmonary pressures — Doppler interrogation of the pulmonic valve provides a wealth of information about pulmonary hemodynamics. First, pulsed wave Doppler velocities across the pulmonic valve when integrated over time provide a velocity-time integral, or velocity time integral (VTI) (image 1). This well-validated technique gives an estimation of the stroke distance during one beat through the pulmonic valve [12,22]. If multiplied by the area of the RVOT, a stroke volume can be calculated. Multiplying this value by heart rate provides a cardiac output in L/min. In practice, as the area of the RVOT is similar in most people, the stroke distance (as calculated by the VTI) is a reasonable estimate of right ventricular cardiac output (over a range of normal heart rates). Reduced VTI across the pulmonic valve (<17 cm) has been shown to predict heart failure hospitalization and mortality independent of clinical and other echocardiographic parameters among individuals with coronary artery disease [23]. Technically, proper positioning of the Doppler sample volume can be assured by choosing a spot across the pulmonary valve where the opening valve signal is more intense than the closing. If only a closing signal is seen, the sample volume is not across the valve but in the RVOT. This malpositioning may result in an artificially low VTI because the RVOT is wider than the diameter of the passage between the PV leaflets.

Noninvasive measurement of pulmonary arterial pressures requires a continuous wave Doppler of the pulmonary regurgitant jet. The pulmonary arterial end diastolic pressure (PA EDP) is estimated using the velocity of the PR jet at the end of diastole (immediately after the atrial notch) which generally coincides with the first peak deflection of the QRS complex (usually the R wave) (image 6). Evaluation of this end diastolic pulmonary regurgitation gradient provides prognostic value and indicates an increased risk of heart failure and mortality when the gradient exceeds 5 mmHg [13] or 9 mmHg [24]. If more precise calculation of the PA end diastolic pressure is desired, the Bernoulli equation (pressure gradient = 4v²) is used to estimate the PA diastolic pressure: pressure gradient + baseline diastolic pressure in the right ventricle (which is approximately right atrial pressure) = PA diastolic pressure.

To calculate mean pulmonary pressures, the peak pulmonary regurgitant velocity and right atrial pressure may be used [22]. By again using Bernoulli’s principle (4 x peak PR velocity) and adding the right atrial pressure, a close approximation of pulmonary artery mean pressure is obtained. Pulmonary arterial pressure measurements are more commonly obtained using the tricuspid regurgitant jet.

To calculate the mean pulmonary artery systolic pressure in the physiologic range of heart rates, the following equation can be used:

Solving for PA systolic pressure:

Pulmonary vascular resistance — Calculations of pulmonary vascular resistance (PVR) using Doppler-derived pulmonary systolic flow are especially useful in monitoring patients with heart failure due to left ventricular systolic dysfunction [17]. Measurements that have been used include:

●Pre-ejection period – The time from the onset of the QRS interval on the ECG to the onset of pulmonary artery systolic flow.

●Acceleration time – The time between onset of flow and peak systolic flow.

●Ejection time – The interval from the onset to the cessation of flow.

A composite calculation putting these measurements together estimated PVR with a strong correlation coefficient in one study (0.96):

PVR (Wood units)  =  -0.156  +  1.154  x  (Preejection period  /  acceleration time)  /  total systolic time [22]

Another technique utilizes a ratio of peak tricuspid valve velocity (in m/s) to pulmonary artery VTI [22,25] to estimate PVR. The upper normal of this ratio is 0.15 units. (See "Echocardiographic assessment of the right heart", section on 'Pulmonary vascular resistance'.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Cardiac valve disease".)

SUMMARY

●Most pulmonic valve stenosis and regurgitation is associated with congenital heart disease. Other causes include endocarditis, rheumatic heart disease, and carcinoid heart disease. (See 'Introduction' above.)

●Pulmonic valve stenosis is primarily diagnosed using Doppler echocardiography, although traditional two-dimensional and M-mode echocardiography can be helpful in confirming the diagnosis. (See 'Pulmonic valve stenosis' above.)

●Pulmonary regurgitation is most frequently diagnosed with two-dimensional color Doppler in combination with pulse or continuous wave Doppler signals.

●In adults, the most common cause of moderate pulmonic regurgitation is pulmonary hypertension; severe pulmonic regurgitation is most commonly seen in Tetralogy of Fallot following repair, and less commonly endocarditis or carcinoid heart disease.

●Doppler interrogation of the pulmonic valve provides prognostically important measurements of cardiac output and pulmonary artery pressure.

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Ameya Kulkarni, MD, who contributed to earlier versions of this topic review.