CRIT BIT CORNER

Pathophysiology

Dynamic Vasoconstriction

Hypoxemia, hypercapnia, and acidosis tighten pulmonary arterioles. In PAH, vasoconstrictor pathways (e.g., endothelin-1) are overactive, while vasodilator/antiproliferative signals (nitric oxide, prostacyclin) are relatively deficient, immediately increasing PVR [2].

Structural Remodeling

Persistent stress converts temporary narrowing into fixed resistance: endothelial injury, smooth-muscle hypertrophy, intimal fibrosis, and, in advanced PAH, plexiform lesions in small pulmonary arteries. These fixed lesions maintain high PVR even when oxygenation improves [2].

Vascular Loss (“pruning”)

In chronic lung diseases and fibrosis, destruction of the capillary bed removes parallel channels. Fewer perfusion pathways mean higher resistance at any given flow [4].

Obstruction and Shear

In chronic thromboembolic PH (CTEPH), organized thrombus and fibrotic webs narrow or occlude large and segmental vessels. A secondary small-vessel arteriopathy develops downstream, compounding PVR elevation [5].

From Venous to Combined Disease.

In PH due to left heart disease, chronic elevation of left-sided filling pressures transmits backward to the pulmonary veins (post-capillary PH). Over time, reactive arteriolar remodeling adds a genuine pre-capillary component (combined pre-/post-capillary PH) with true PVR rise [6,7].

PH is Not Just Resistance—Stiffness Matters

The normal pulmonary tree is highly compliant, so the RV faces a “soft,” low-pulsatility load. Remodeling stiffens vessels and increases wave reflections, raising pulsatile afterload even when mean pressures have not skyrocketed—another hidden tax on RV work [2].

Causes and Classifications of PH

Group 1 – Pulmonary Arterial Hypertension (PAH)

Idiopathic, heritable, drug/toxin-induced, or associated with conditions such as connective tissue disease, portal hypertension, or HIV. Mechanism: small-artery vasoconstriction and proliferative remodeling → pre-capillary physiology [2].

Group 2 – PH Due to Left Heart Disease

HFpEF/HFrEF and valvular disease raise left atrial/venous pressure (post-capillary PH). Chronicity can evolve to combined pre-/post-capillary PH with elevated PVR from superimposed arteriolar remodeling [6,7].

Group 3 – PH Due To Lung Disease And/Or Hypoxemia

COPD, interstitial lung disease, sleep-disordered breathing, hypoventilation, high altitude. Drivers: hypoxic vasoconstriction, vascular remodeling, and capillary bed loss; polycythemia can add viscosity burden [4].

Group 4 – Chronic Thromboembolic PH (CTEPH)

Persistent organized thromboembolic obstruction with secondary small-vessel arteriopathy elevates PVR; hemodynamic definition is pre-capillary PH with mPAP >20 mmHg, PAWP ≤15 mmHg, and elevated PVR on catheterization [5].

Group 5 – Multifactorial/Unclear Mechanisms

Hematologic, systemic, and metabolic disorders where multiple mechanisms contribute; workup is tailored to the suspected driver(s) [1,3].

Management and Treatment

The Big Picture

Start by fixing the reason pulmonary pressures are high, and protect the right ventricle (RV) while you do it. Oxygen helps, extra fluid often hurts, and not every patient with PH should get PAH-specific drugs.

Treatment depends on the group of PH [1,4–5].

Universal Supportive Care (for any PH group)

Oxygen:

Keep saturations up—hypoxemia tightens pulmonary arteries and raises RV afterload. Correcting low O₂ lowers resistance and eases RV work [4].

Fluids:

Aim for euvolemia. Too much fluid stretches the RV, worsens tricuspid regurgitation, and pushes the septum into the LV. Gentle diuresis often improves symptoms if the patient is congested [1].

Ventilator Strategy:

Use lung-protective settings. Apply PEEP “just enough” to recruit—excess PEEP can reduce venous return and drop RV output. Reassess hemodynamics after vent changes [4].

Treat Triggers:

Manage pneumonia/COPD or ILD flares, pulmonary embolism, arrhythmias, fever, and pain; all can spike pulmonary vascular resistance (PVR) [4–5].

Medication Safety:

If a patient is on a continuous prostacyclin (e.g., epoprostenol/treprostinil), never interrupt the infusion—abrupt stop can cause dangerous rebound vasoconstriction and RV collapse [8].

RV Rescue Checklist

Stabilize Physiology

Oxygenate to target; treat fever and pain; correct acidosis and hypercapnia if possible (each increment increases PVR) [4].

Choose vasopressors that preserve diastolic pressure and RV perfusion (norepinephrine typically first) [7].

Unload The RV (Without Starving Preload)

Gentle diuresis to reduce RVEDP and TR if volume-overloaded; avoid large fluid boluses once RV dilation and septal shift are present [3].

Lung-protective ventilation; titrate PEEP to recruit but re-check MAP/JVP/echo after changes [3,4].

Optimize Pulmonary Circulation

Continue PAH-specific therapy if prescribed; ensure prostacyclin pumps, cassettes, and backups are functioning and available [8].

Treat precipitating factors (PE, pneumonia, COPD/ILD flare, arrhythmia).

Know When To Escalate

Worsening syncope, rapidly rising JVP, cool mottled extremities, or echo evidence of acute RV failure → urgent team huddle to discuss advanced options (e.g., inhaled pulmonary vasodilators, mechanical support pathways) [3,8].