The first response of the RV to a rise in pulmonary arterial load is RV dilatation, which serves as a compensatory response which may assist in the maintenance of RV stroke volume (heterometric autoregulation). The normal RV measures approximately 2.5–3.5 cm at end-diastole, with a planimetered area of 15–18 cm². Of note, the presence of RV dilatation does not always signify RV systolic dysfunction. A dilated RV with normal or dynamic function often signifies excess preload only, with the rise in RV function occurring in response to volume loading and increased sarcomere length. This is often the case in primary tricuspid valve and pulmonic valve regurgitation as well as in the presence of systemic to pulmonary shunt conditions. A practical rule is that in the absence of moderate or greater tricuspid or pulmonic regurgitation, a dilated and hyperdynamic RV should alert the clinician to the presence of a systemic to pulmonary shunt.

A practical way to estimate RV size is to compare RV dimensions to LV dimensions (or area) in the apical four chamber view. RV dimensions are obtained at the RV base, approximately 1 cm apical of the tricuspid annulus. A diastolic RV/LV diameter ratio of 0.5–0.7 is considered normal, with increasing RV:LV ratios in patients with mild (0.8–1.0), moderate (1.1–1.4), and severe (≥1.5) RV dilatation. A useful rule of thumb is that the RV:LV ratio should be <1.0, and any value >1.0 is strongly suggestive of RV dilatation, often coinciding with RV dysfunction (Fig. 9.3) [22].

The apex of the heart is normally formed by the LV. In cases of RV enlargement related to increased RV afterload, the RV will occupy or share the apex of the heart (apex-forming RV). This is associated with widening (opening) of the angle of the RV apex which is normally acute in the apical four chamber view. Lopéz-Candeles showed that a relatively large or “open” RV apical angle was a common finding in the setting of chronic pulmonary hypertension, relating inversely to decreases in TAPSE and RV fractional area change [23]. In our experience, a dilated and open RV apex angle is quite specific for PH related to high afterload conditions such as PAH and CTEPH. In PVH and other PH conditions where the PVR is normal, the RV apex angle will remain rather acute or “closed,” imparting an inverted triangle shape to the RV, where the dimension tapers quickly from base to RV apex (Figure 9.3).

As the RV dilates, the potential space in the pericardium is obliterated leading to pericardial constraint. With increasing constraint of the heart, further dilatation of the RV leads to shifting of the IVS from right to left (an example of diastolic ventricular interdependence), leading to a fall in LV cavity size and impaired LV diastolic filling [24]. As a result, the transmitral Doppler filling pattern will demonstrate reversal of early (E) versus late (A) diastolic waveforms on pulsed wave Doppler mitral inflow (so-called E to A reversal) in patients with PAH or CTEPH [25]. Confusion can sometimes arise if the clinician wrongly assumes that reversal of E/A ratio (often reported as “Grade I diastolic dysfunction”) equates to elevated LA pressure and hence attributes the PH to left sided diastolic dysfunction. In the presence of RV dilation and significant right to left septal shifting, the presence of a reversed E/A ratio is a strong indicator of normal left atrial pressure. Moreover, when septal IVS shift is due to PAH or CTEPH and alleviated by treatment, LV diastolic filling returns to normal [26, 27].

Additionally, RV dysfunction is associated with a delay in the time to peak RV contraction, which leads to mechanical RV/LV dyssynchrony [28]. This RV/LV dyssynchrony leads to systolic septal flattening which is characteristic of high RV afterload states. Magnetic resonance imaging studies have shown relatively strong correlations between the degree of RV systolic septal bowing and pulmonary vascular resistance [26, 28]. Systolic septal flattening is nearly always present in PAH and CTEPH, and thus functions as a reliable and practical indicator of increased pulmonary arterial afterload (Fig. 9.4).

Mild to moderate pericardial effusion is a common finding, occurring in about 50 % of patients with PAH [29]. It has long been recognized as poor prognostic sign in PAH, often signifying relative RV decompensation (Fig. 9.4b). The accumulation of pericardial fluid in PAH likely occurs due to chronically elevated RA pressure, which in turn impairs coronary venous return and results in transudation of fluid into the pericardial space [30]. Importantly, attempts at pericardial drainage (surgical or catheter based) should be avoided unless there is compelling evidence of tamponade, as the effusion is typically the result and not the cause of RV failure in this setting and mechanical drainage of the effusion in some series was associated with unacceptably high mortality rates [31]. Others have achieved success in draining pericardial effusions in PAH with low procedural mortality [32]. In our experience, in the absence of frank tamponade, pericardial effusions may resolve over weeks or months if the PAH and RV failure respond to diuresis and PH specific medical therapy.

RA enlargement is another indirect measure of RV dysfunction. An increased RA area index is predictive of increased mortality in PAH. While RA dilatation might be seen as simply a marker of progressive RV failure and worsening TR, more recent work suggests that RA systolic function may account for upwards of 50 % of total longitudinal RV shortening in patients with PAH [33–35]. Therefore, the RA may play a larger role in maintaining total right heart function than previously appreciated.

Pulmonary arterial load (the afterload of the RV) and pulmonary artery pressure should not be viewed as synonymous. Afterload is the force that opposes flow, whereas pressure results from the interaction between flow and afterload. Since pressure does not oppose flow, it cannot be considered afterload. This fundamental physiologic distinction is supported by the fact that PA pressure correlates poorly with RV function and prognosis in patients with PAH [35]. Moreover, there are conditions where pressure is low and afterload is high (i.e., acute PE) and where pressure is high and afterload is low (i.e., systemic to pulmonary shunt with PH from high pulmonary blood flow). RV afterload is better represented by pulmonary vascular resistance rather than pressure. Thus, practically speaking when a clinician notes PH + RV dysfunction a high PVR should be suspected, whereas when PH is present without RV dysfunction, low PVR PH conditions are far more likely, such as left-sided diastolic HF.

Despite its shortcomings as a measure of RV afterload, PA pressure remains an important component in the evaluation of established or suspected pulmonary hypertension since some elevation is nearly always present in varying degrees in patients with PVD. A pulmonary artery systolic pressure (PASP) of 40 mmHg has been traditionally accepted as the upper limit of normal in most subjects, although the upper limit of PASP may be slightly higher in advancing age. The echocardiographic discovery of an elevated PA pressure is usually, although not ideally, what triggers further workup for PH, and therefore, it is important to review the echocardiographic assessment of PA pressure [36–38].^.

The PASP is most commonly estimated through continuous wave Doppler interrogation of the tricuspid regurgitant jet velocity, which provides an estimate of the pressure gradient between the RV and the RA [39]. This gradient, when added to RA pressure, should equal the systolic RV pressure, which in turn should equal PASP in the absence of pulmonic valve stenosis. In the latter case, the PASP can still be estimated by subtracting the gradient across the pulmonic valve from peak RV systolic pressure. This approach utilizes the modified Bernoulli equation: PASP ≈ 4v ² + estimated RA. The v represents the velocity of the tricuspid regurgitant jet. The use of a high quality TR jet signal with a clearly defined peak velocity ensures the highest chance of an accurate and reproducible PASP estimate (Fig. 9.5).

The most common method of RA pressure assessment utilizes inferior vena cava (IVC) size and collapsibility with inspiration [40, 41]. Unfortunately, this method has not proved very reliable, likely because the size of the IVC is governed not only by the RA pressure, but also by the compliance of the IVC and the degree to which the negative pleural pressure is transmitted to it [42–44]. Another way of estimating RA pressure was recently described. Using the ratio of tricuspid inflow E velocity to tricuspid annular tissue Doppler E _a wave velocity (E/E _a) yielded values that correlated well (r = 0.8) with invasively measured pressures [45]. However, given the significant variation in these techniques, we prefer to use the clinically estimated RA pressure, when possible.

Data on the accuracy of Doppler echo assessment of PA pressure have been disappointing [42, 46]. This is largely due to the fact that in many patients the ideal conditions of a high quality TR jet signal, proper Doppler alignment, and accurate RA pressure estimation do not exist. Fisher et al. examined the correlation between Doppler estimated and invasively measured PA systolic pressure in a cohort of 65 patients with more severe pulmonary hypertension (62 % with pulmonary arterial hypertension) [42]. Using Bland-Altman analysis, 48 % of the patients had an estimated PA systolic pressure that was 10 mmHg or more different from invasively obtained measurements. Pressure overestimations were due to either overestimation of the peak velocity signal, or an overestimation of the RA pressure by IVC size and collapsibility assessment (Fig. 9.6). Underestimations were also common, and more pronounced (−30 mmHg) than overestimations (+19). Suboptimal TR jet signal quality was the most common reason for PASP underestimation (Fig. 9.6). Nonetheless, in the study by Fisher et al., the absence of TR did not necessarily equate to the absence of pulmonary hypertension, as four of the six subjects with no TR had severe pulmonary hypertension by catheterization. In addition, 12 of the 16 patients in whom PA pressure was underestimated by Doppler had evidence of RV enlargement and/or dysfunction on their Doppler echo exam. Thus, if the PASP estimate is <40 mmHg in the presence of significant RV dilation, dysfunction or septal flattening, the clinician should remain highly suspicious for the presence of significant PH and the patient should be referred for right heart catheterization.

An alternative or complementary method of pulmonary artery pressure assessment utilizes the acceleration time (AcT) (Fig. 9.7) of the pulsed wave Doppler flow velocity envelope in the right ventricular outflow tract (RVOT Doppler) [mean PAP = 79 − 0.45 (RVOT AcT)] [47]. This measure is simple, reproducible, and inversely correlates with mean PA pressure (mPAP). Moreover, AcT may be a better marker of pulmonary artery impedance/right ventricular afterload than pressure; thus, in two patients with equal mPAPs, the patient with a high AcT (i.e., >100 ms) will have a relatively normal PVR, whereas a patient with an AcT less than 100 ms is more likely to have an increased PVR and right ventricular afterload.

In addition to systolic and mean PA pressures, the diastolic PA pressure (PADP) can be derived from the end-diastolic point of the pulmonary regurgitation signal, again added to an estimate of right atrial pressure [48]. Alternatively, PADP can be estimated from the velocity obtained from the TR jet at the time of pulmonic valve opening [49, 50]. In either case, the accuracy of the method is contingent on the presence of PR or TR and the quality of the respective Doppler envelopes.

The most important thing to remember about Doppler derived PA pressure assessment is that it should be taken in the context of the clinical condition and, importantly, in the context of other echocardiographic features such as RV size, RV apex morphology, RV systolic function and septal position. Using this more integrated approach provides a far more comprehensive PH assessment and greatly offsets the potential for PH misdiagnosis by PASP Doppler estimation alone.

As discussed earlier, an elevated PVR is the hallmark of PH_PVD. Doppler echocardiography can also be used to estimate PVR. Abbas et al. showed that the ratio of trans-tricuspid flow velocity (surrogate of pressure) to the velocity-time interval obtained from the RV outflow tract (RVOT VTI) (surrogate of flow) closely correlates to PVR [51]. This approach estimates PVR well when PVR is low (<2 WU), but does not correlate well in the setting of elevated PVR (>8 WU) [52]. Additionally, as this ratio correlates with total pulmonary resistance (mPAP/cardiac output) rather than PVR alone, this measure could lead one to believe that an elevated ratio is related to elevated PVR (i.e., PVD), when the total pulmonary resistance is actually elevated secondary to left atrial hypertension. Another formula, proposed recently, derives PVR from the ratio of PASP to RVOT VTI, with a corrective constant (+3) in the setting of mid-systolic notching of the RVOT Doppler signal (vide infra). This method (PVR = [(PASP/RVOT VTI) + 3]) demonstrated an area under the curve (AUC) of 0.95 and 0.92 for PVR more than 3 and 5 WU, respectively, and was 98 % sensitive and 61 % specific for predicting PVR more than 3 WU [53].

A simpler approach to estimating PVR relies on the visual inspection of the pulsed wave Doppler profile in the RVOT (RVOT Doppler). In the setting of normal RV afterload (normal PVR), the RVOT Doppler signal is rounded and parabolic in shape, with no evidence of systolic deceleration or “notching” of the Doppler profile (Fig. 9.7a). In the setting of an increased PVR and decreased pulmonary artery compliance, the RVOT Doppler signal will demonstrate evidence of “notching,” that can occur in late systole (Fig. 9.7b) or mid-systole (Fig. 9.7c). RVOT Doppler notching arises from alteration of the flow pattern exiting the RV due to early arrival of reflected pressures waves from a constricted and noncompliant pulmonary circulation [54–56]. Mid-systolic notching is 96 % specific for a PVR > 5 WU, a mean PVR of 9 WU, and is associated with a greater degree of right heart dysfunction than the late notch pattern, which is typically associated with a PVR of 3–6 WU and less severe right heart dysfunction [57]. In this study, a notched RVOT Doppler profile was strongly associated with a PVR > 3 WU (odds ratio 29:1), and was present in 100 % of patients with incident PAH. On the other hand, subjects with PH in the absence of Doppler notching are far more likely to have pulmonary venous congestion and left heart disease as the source of their PH (odds ratio 33:1). Visual inspection of the RVOT Doppler profile for the presence of absence of “notching” is a very simple, powerful, and practical way to detect PH_PVD and should be incorporated into the routine echo-Doppler examination of every patient with known or suspected PH.