Pulmonary Hypertension Evaluation: Hemodynamic Criteria, PVR Calculation, and Fick-Based Assessment

The 2022 European Society of Cardiology (ESC) and European Respiratory Society (ERS) guidelines redefined pulmonary hypertension as a mean pulmonary artery pressure (mPAP) greater than 20 mmHg at rest, lowered from the previous threshold of 25 mmHg that had been used since 1973. This revised threshold was adopted because population studies demonstrated that the upper limit of normal resting mPAP is approximately 14 ± 3 mmHg, and an mPAP of 21–24 mmHg is associated with increased morbidity and mortality in epidemiological cohorts. The hemodynamic definition requires confirmatory right heart catheterization — echocardiographic estimates of PA systolic pressure are valuable screening tools but cannot establish the diagnosis definitively.

Beyond the mPAP threshold, the hemodynamic characterization of PH requires measurement of PCWP and calculation of pulmonary vascular resistance (PVR). Pre-capillary PH is defined by mPAP > 20 mmHg, PCWP ≤ 15 mmHg, and PVR > 2 Wood units. Isolated post-capillary PH (IpcPH) is defined by mPAP > 20 mmHg with PCWP > 15 mmHg and PVR ≤ 2 Wood units. Combined pre- and post-capillary PH (CpcPH) is defined by mPAP > 20 mmHg, PCWP > 15 mmHg, and PVR > 2 Wood units. This tripartite classification replaced the earlier dichotomous system and requires accurate cardiac output measurement, making the Fick method essential for correct PH phenotyping.

The five WHO clinical groups of PH further categorize disease etiology: Group 1 (pulmonary arterial hypertension, PAH), Group 2 (PH due to left heart disease), Group 3 (PH due to lung diseases or hypoxia), Group 4 (chronic thromboembolic PH, CTEPH), and Group 5 (PH with unclear or multifactorial mechanisms). Treatment strategies differ fundamentally between groups — pulmonary vasodilator therapy is indicated for Group 1 PAH and Group 4 CTEPH but may be harmful in Group 2 PH. Accurate hemodynamic classification during right heart catheterization, including Fick-based PVR calculation, is therefore the critical step that determines the treatment algorithm for each patient.

Pre-capillary pulmonary hypertension encompasses WHO Groups 1, 3, 4, and some Group 5 etiologies, all characterized by primary elevation of pulmonary vascular resistance with normal or low left-sided filling pressures (PCWP ≤ 15 mmHg). In PAH (Group 1), the pathology involves progressive obliterative vasculopathy of small pulmonary arteries with intimal proliferation, medial hypertrophy, plexiform lesion formation, and in-situ thrombosis. The hemodynamic hallmark is a markedly elevated PVR (often > 5–10 Wood units in advanced disease) with a wide transpulmonary gradient (TPG = mPAP − PCWP, normally < 12 mmHg) and elevated diastolic pressure gradient (DPG = PA diastolic − PCWP, normally < 7 mmHg).

Post-capillary PH (WHO Group 2) is the most common form of pulmonary hypertension, caused by passive backward transmission of elevated left atrial pressure to the pulmonary vasculature. The hallmark is PCWP > 15 mmHg with PVR ≤ 2 Wood units in isolated post-capillary PH. Causes include left ventricular systolic or diastolic dysfunction, mitral or aortic valve disease, and atrial fibrillation with elevated LA pressure. The distinction from pre-capillary PH is clinically critical because PAH-specific therapies are not indicated in Group 2 PH and may worsen outcomes by increasing pulmonary blood flow into a congested left heart.

Combined pre- and post-capillary PH (CpcPH) represents an important intermediate phenotype where longstanding elevation of left-sided filling pressures has induced secondary pulmonary vascular remodeling, resulting in a PVR that exceeds 2 Wood units despite PCWP > 15 mmHg. The DPG is particularly useful for identifying CpcPH: a DPG ≥ 7 mmHg in the setting of elevated PCWP suggests a fixed pulmonary vascular disease component superimposed on the post-capillary process. CpcPH carries a worse prognosis than IpcPH, and research is ongoing regarding whether pulmonary vasodilator therapy may benefit selected CpcPH patients. Accurate Fick cardiac output is indispensable for calculating PVR and making this hemodynamic distinction.

Pulmonary vascular resistance (PVR) is calculated as: PVR = (mPAP − PCWP) / CO, where mPAP is mean pulmonary artery pressure in mmHg, PCWP is pulmonary capillary wedge pressure in mmHg, and CO is cardiac output in L/min. The result is expressed in Wood units (mmHg/L/min). To convert to metric resistance units (dyn·s·cm⁻⁵), multiply by 80. Normal PVR is less than 2 Wood units (160 dyn·s·cm⁻⁵). The 2022 ESC/ERS guidelines adopted a PVR threshold of > 2 Wood units (previously > 3 Wood units) for defining the pre-capillary component of PH, aligning with the revised mPAP threshold.

The accuracy of PVR calculation depends critically on reliable cardiac output measurement. The Fick method is strongly recommended in pulmonary hypertension evaluation for several reasons: many PH patients have low cardiac output states where thermodilution becomes inaccurate; tricuspid regurgitation is common in advanced PH and distorts the thermodilution curve; and PAH patients frequently have intracardiac shunts (e.g., patent foramen ovale) that invalidate thermodilution assumptions. When using the Fick method, direct measurement of VO₂ by indirect calorimetry is preferred over assumed VO₂ values, particularly in PH patients who may have abnormal metabolic rates due to right ventricular hypertrophy, deconditioning, or concomitant lung disease.

The pulmonary vascular resistance index (PVRI) normalizes PVR to body surface area: PVRI = (mPAP − PCWP) / CI, expressed in Wood units·m² or dyn·s·cm⁻⁵·m². Indexing is particularly important in pediatric patients and in adults with extreme body habitus, where absolute PVR may be misleading. The transpulmonary gradient (TPG = mPAP − PCWP) and diastolic pressure gradient (DPG = PA diastolic − PCWP) provide additional hemodynamic markers that, combined with PVR, facilitate comprehensive characterization of the pulmonary vascular bed. All of these calculations require accurate, simultaneous measurement of PA pressures, PCWP, and cardiac output during right heart catheterization.

Vasoreactivity testing during right heart catheterization determines whether the elevated PVR in pulmonary arterial hypertension (Group 1 PAH) has a reversible vasospastic component responsive to calcium channel blocker (CCB) therapy. Testing is performed by administering a short-acting pulmonary vasodilator — most commonly inhaled nitric oxide (10–20 ppm for 5–10 minutes), intravenous epoprostenol, or inhaled iloprost — and repeating hemodynamic measurements. A positive vasoreactivity response is defined as a decrease in mPAP ≥ 10 mmHg to an absolute mPAP ≤ 40 mmHg with a maintained or increased cardiac output.

Approximately 10–15% of idiopathic PAH patients demonstrate a positive vasoreactivity response, and this subgroup may be treated with high-dose calcium channel blockers (nifedipine, diltiazem, or amlodipine) with favorable long-term survival. Patients who do not meet the positive response criteria should not receive CCB therapy, as these agents can cause systemic hypotension and worsening hemodynamics in non-responsive PAH. Instead, non-responders are treated with PAH-specific therapies targeting the endothelin, nitric oxide-cGMP, and prostacyclin pathways, selected based on risk stratification using hemodynamic parameters (CI, RAP, SvO₂), exercise capacity, and biomarkers.

Accurate cardiac output measurement during vasoreactivity testing is essential because a decrease in mPAP with a concurrent fall in cardiac output may represent drug-induced systemic hypotension rather than true pulmonary vasodilation. The Fick method provides reliable cardiac output before and after vasodilator administration, ensuring that the observed hemodynamic changes reflect genuine pulmonary vascular responses. Follow-up hemodynamic assessment at 3–6 months after initiating PAH therapy evaluates treatment response and guides escalation: achieving a CI > 2.5 L/min/m², RAP < 8 mmHg, and SvO₂ > 65% defines a low-risk hemodynamic profile associated with favorable long-term prognosis.