Pulmonary Hypertension

Pulmonary hypertension carries a genuinely sobering diagnostic delay — one study found an average of 12 months to first medical contact, five general practitioner visits, three specialist visits, and over two alternative diagnoses along the way, with a median of 44 months from symptom onset to definitive diagnosis by right heart catheterization. Echocardiography is the tool best positioned to close that gap, and this page covers both how it screens for the diagnosis and — just as importantly — how it points toward the underlying cause.

Current Definitions

Pulmonary hypertension is defined, following the 2018 World Symposium on Pulmonary Hypertension’s revision, as an invasively measured mean pulmonary artery pressure (mPAP) over 20 mmHg — down from the original 1973 threshold of 25 mmHg or greater. This change followed large cohort and machine-learning analyses showing a genuinely normal mPAP of 14.0 ± 3.3 mmHg, with a linear rise in adverse outcomes even within the previously “normal” 20–25 mmHg range.

Hemodynamic subcategories are defined by combining mPAP with pulmonary artery wedge pressure (PAWP) and pulmonary vascular resistance (PVR) — and the PVR threshold has itself been updated:

Category	Definition
Precapillary PH	mPAP >20 mmHg, PAWP ≤15 mmHg, PVR >2 Wood units
Isolated postcapillary PH	mPAP >20 mmHg, PAWP >15 mmHg, PVR ≤2 Wood units
Combined pre- and postcapillary PH	mPAP >20 mmHg, PAWP >15 mmHg, PVR >2 Wood units

The PVR threshold itself has been lowered from the older 3 WU cutoff to 2 WU in current guidelines — worth knowing explicitly, since older material (including some still-circulating echocardiography texts) may still cite the earlier figure. A further category, exercise PH, is now recognized: an mPAP/cardiac output slope between rest and exercise exceeding roughly 3–3.5 mmHg·min/L, relevant specifically to precapillary and postcapillary disease groups.

High cardiac output states can raise mPAP above 20 mmHg with entirely normal PVR — this does not meet current criteria for pulmonary hypertension, a genuinely important exclusion worth actively considering before labeling an elevated mPAP as true PH.

The Echocardiographic Probability Algorithm

Rather than a single cutoff, current guidelines assign a probability of pulmonary hypertension using peak tricuspid regurgitation velocity (TRVmax) combined with additional echo findings:

Peak TR velocity	Other echo signs of PH	Probability
≤2.8 m/s or not measurable	No	Low
≤2.8 m/s or not measurable	Yes	Intermediate
2.9–3.4 m/s	No	Intermediate
2.9–3.4 m/s	Yes	High
>3.4 m/s	Not required	High

The wide gray zone from 2.9 to 3.4 m/s is exactly where the additional echo signs do the real diagnostic work. These fall into three categories:

• The ventricles: RV/LV basal diameter ratio over 1.0; flattening of the interventricular septum (LV eccentricity index over 1.1 in systole and/or diastole).
• The pulmonary artery: RVOT acceleration time under 105 ms and/or mid-systolic notching; early diastolic pulmonary regurgitation velocity over 2.2 m/s; PA diameter over 25 mm.
• The IVC and right atrium: IVC diameter over 21 mm with decreased inspiratory collapse (under 50% with a sniff, or under 20% with quiet inspiration); RA end-systolic area over 18 cm².

Measuring TR Velocity Correctly: The “Chin vs. Beard” Principle

This is a genuinely memorable, practical technical point worth internalizing: measure the dense, modal “chin” of the CW Doppler TR envelope, not the fainter, higher-velocity “beard” that spreads at its edge. The beard reflects noise and the transit-time effect, not true peak velocity, and including it produces a falsely elevated — and falsely alarming — RVSP estimate. Getting this distinction right meaningfully improves estimation accuracy.

A second, equally important point: up to 50% of patients with confirmed pulmonary hypertension have inadequate TR for reliable Doppler assessment. The absence of a measurable jet never rules out the diagnosis — when this happens, the report should state that the data are inadequate rather than implying normal pulmonary pressures, and alternate diagnostic approaches should be pursued.

Practical steps when the TR signal is poor — genuinely useful technique tips worth trying before concluding a study is non-diagnostic: measure during inspiration (TR is typically more prominent); a passive leg raise to increase venous return and the regurgitant signal; atypical windows (one intercostal space higher, a parasternal long-axis or subcostal RV view); and agitated saline contrast injection to enhance the Doppler signal itself.

The Formulas

RVSP (= PASP, absent pulmonic stenosis) = 4 × TRVmax² + RAP

Right atrial pressure is estimated from IVC diameter and its respiratory collapse — see Evaluation of the Right Atrium for the standard approach.

Diastolic pulmonary artery pressure is estimated from the end-diastolic pulmonary regurgitation velocity:

PAEDP = 4 × (end-diastolic PR velocity)² + RAP

Mean PA pressure can be derived several ways, though current guidance treats calculated mPAP as less favored than direct RVSP reporting, given its reliance on multiple indirect measurements:

• mPAP = ⅓(PASP) + ⅔(PAEDP)
• mPAP = 4 × (early PR velocity)² + RAP
• Mahan’s equation, using pulmonary artery acceleration time

Pulmonary vascular resistance can be estimated as PVR (Wood units) = [(TRVmax ÷ RVOT VTI) × 10] + 0.16, performing best in less severe disease (PVR under 8 WU); a TRVmax/RVOT VTI ratio over 0.275 is highly suggestive of a PVR over 6 WU.

Finding the Etiology: Precapillary vs. Postcapillary

This distinction is the central diagnostic question in pulmonary hypertension, and it genuinely changes treatment: modern pulmonary vasodilator therapy is actively harmful in postcapillary PH from left heart disease — commonly from longstanding systemic hypertension itself, via the diastolic dysfunction it produces — where standard heart failure therapy and diuretics are the correct approach instead. Getting this right from echo alone, before any catheterization, is exactly the problem the tools below are built to solve.

ePLAR: A Single Number That Separates the Two

The echocardiographic pulmonary-to-left-atrial ratio (ePLAR) is a genuinely elegant non-invasive surrogate for the invasive transpulmonary gradient and its relationship to left atrial pressure, built on the same E/e′ estimate of filling pressure covered on the LV Diastolic Function page:

ePLAR (m/s) = TRVmax (m/s) ÷ mitral E/e′

The logic is direct: in precapillary disease, PAP (and TRVmax) rises while LAP (and E/e′) stays normal — so ePLAR rises. In postcapillary disease, LAP (and E/e′) rises while the pressure gradient across the lung stays comparatively normal — so ePLAR falls. Validated across multiple populations, values above roughly 0.3 m/s point toward precapillary physiology, and values below 0.3 m/s toward postcapillary physiology.

	Normal	LHF, normal PAP	Postcapillary PH	Precapillary PH
TRVmax	~2.4 m/s	~2.4 m/s	~4.0 m/s	~4.0 m/s
E/e′	~8	~20	~20	~8
ePLAR	~0.30 m/s	~0.12 m/s	~0.20 m/s	~0.50 m/s

A newer variant, ePLAGS (using global longitudinal strain instead of E/e′), has shown similarly strong performance and particular value in mixed physiology cases where ePLAR alone is less discriminating.

Anatomic and Hemodynamic Patterns Worth Recognizing Together

Feature	Precapillary PH	Postcapillary PH
RA/RV size	Severely enlarged	May be enlarged
LA/LV size	Small, underfilled	Enlarged
Interatrial septum	Bows right to left	Bows left to right
Mitral E/A ratio	Under 1	Over 1
RVOT Doppler notching	Common	Rare
PCWP	Normal	Elevated
PADP − PCWP gradient	Markedly increased	Minimal or absent

The LA-to-RA volume ratio has also been shown to discriminate the two, higher in precapillary disease (roughly 1.03) than postcapillary disease (roughly 0.50). A genuinely important caveat: this same ratio, in its reciprocal form (RA area ÷ LA area), can also be elevated by an interatrial shunt — ruling out a shunt is a necessary step before interpreting atrial size ratios as evidence of precapillary physiology specifically.

Confirming and Characterizing the Mechanism: Septal Motion

Beyond pressure estimation, the pattern of interventricular septal motion itself distinguishes pressure overload from volume overload — a mechanistic distinction, not just a descriptive one:

• RV pressure overload (as in PH) shifts the heart’s center of mass anteriorly, reversing septal curvature in both systole and early-to-mid diastole — the septum moves toward the center of the RV rather than the LV.
• RV volume overload produces diastolic flattening only, with normal systolic curvature, reflecting increased diastolic flow into the RV without the systolic pressure abnormality seen in true PH.

Indirect M-mode and Doppler signs of PH are worth knowing even though they indicate presence rather than exact severity: a pulmonic valve M-mode showing a reduced a-wave and mid-systolic closure has over 90% specificity but only 30–60% sensitivity; the paralleling Doppler finding is an abrupt mid-systolic deceleration (notching) of the RVOT flow curve.

Right Ventricular Consequences

With long-standing pressure overload, RV systolic dysfunction can develop, with secondary dilation as a compensatory mechanism — but RV dilation itself causes tricuspid annular dilation and papillary muscle malalignment, producing secondary tricuspid regurgitation that adds volume overload on top of the existing pressure overload, further dilating the RV in a genuine feedback cycle. See Evaluation of the Right Ventricle and Tricuspid Regurgitation for the general assessment frameworks this fits into. Before attributing TR to pulmonary hypertension, tricuspid valve anatomy should be specifically evaluated to exclude other causes — vegetation, rheumatic disease, carcinoid, or Ebstein anomaly — rather than assuming a secondary mechanism by default.

Pulmonary Embolism: A Specific Precapillary Cause Worth Separate Mention

Acute PE raises pulmonary vascular resistance suddenly, and echocardiography genuinely helps assess PA pressure and RV function in this setting — see Intracardiac Thrombus and Sources of Embolism for the right-heart thrombus-in-transit picture this can produce — though its sensitivity for directly visualizing pulmonary artery thrombus itself is low, since clot is usually lodged too distally in the pulmonary vasculature, and the PA bifurcation itself is often obscured by the air-filled trachea and bronchi. TEE can occasionally show thrombus in the main, right, or left pulmonary artery directly.

Indirect signs are more often what’s actually seen, and are worth actively considering even when PE wasn’t the original reason for the study — patients later diagnosed with PE are frequently referred initially for nonspecific indications like “chest pain,” “dyspnea,” or “heart failure”:

• Elevated pulmonary artery pressures
• Evidence of acute RV pressure overload
• RV dilation and dysfunction
• Tricuspid regurgitation

How to Approach the Patient With Suspected Pulmonary Hypertension: A Practical Sequence

1. Measure TRVmax carefully, using the “chin” of the modal envelope rather than the “beard,” and from multiple windows if the initial signal is suboptimal.
2. Assign an echo probability using the three-tier framework, actively screening the ventricular, pulmonary artery, and IVC/RA signs when TRVmax falls in the 2.9–3.4 m/s gray zone.
3. Never conclude “normal pressures” from an absent TR jet — report inadequate data explicitly and pursue alternate approaches, given how common inadequate TR genuinely is in confirmed PH.
4. Calculate ePLAR (TRVmax ÷ mitral E/e′) once PH is suspected, to non-invasively estimate precapillary versus postcapillary physiology before any catheterization.
5. Cross-check with the anatomic pattern — atrial size, septal bowing direction, mitral E/A ratio, and RVOT notching — rather than relying on ePLAR alone, particularly in mixed or ambiguous cases.
6. Actively exclude an interatrial shunt before interpreting an elevated RA/LA size ratio as evidence of precapillary disease.
7. Evaluate tricuspid valve anatomy specifically before attributing TR to pulmonary hypertension by default.
8. Consider pulmonary embolism actively when RV pressure overload and dysfunction are found in a patient referred for a nonspecific indication, rather than only when PE was the explicit clinical question.
9. Remember that treatment hinges on this distinction — confirming precapillary versus postcapillary physiology before any vasodilator therapy is considered isn’t a formality, since the wrong classification can lead to genuinely harmful treatment.

Clinical Importance

Pulmonary hypertension rewards the same discipline as nearly every other topic in this section: a single number (TRVmax) is a starting point, not an answer, and the real diagnostic value comes from combining it with a structured probability framework, a non-invasive marker of physiology (ePLAR), and the anatomic pattern the heart itself has adopted in response to years of altered pressure. Given how long these patients typically wait for a diagnosis, and how differently precapillary and postcapillary disease are actually treated, getting this right — not just detecting that pressures are elevated — is where echocardiography earns its central role in this disease.