The pulmonary vascular load in patients with lung disease has a component driven by hypoxic pulmonary vasoconstriction (HPV) and a structural component driven by remodeling, vessel loss, or thrombotic obstruction. Acute vasoconstriction resolves with correcting hypoxia, whereas structural change may regress over months to years, or not at all. In order to effectively manage the disease, it’s important to recognize which component dominates in a patient.
HPV is an intrinsic vasomotor response of small intrapulmonary arteries to a fall in alveolar oxygen tension. A mitochondrial sensor in pulmonary artery smooth muscle transduces the change through reactive oxygen species and redox couples; voltage-gated potassium channels close, the cell depolarizes, and L-type calcium channels open. When hypoxia is regional, the response diverts perfusion from poorly ventilated segments and preserves arterial oxygen tension without changing pulmonary artery pressure. When hypoxia is diffuse, pressure rises.1
Three situations illustrate the pulmonary vasculature’s physiologic response to hypoxia: chronic lung disease, high-altitude pulmonary hypertension (PH), and pulmonary arterial hypertension (PAH) in patients living at high altitude.
Hypoxia in chronic lung disease
Inchronic lung disease, hypoxia at the alveolar level is heterogeneously distributed throughout the lung, and the resulting vasoconstriction preserves gas exchange. Nonselective vasodilation can improve hemodynamics but worsen gas exchange. Melot and colleagues gave sublingual nifedipine to six patients with PH complicating advanced COPD: pulmonary vascular resistance (PVR) fell 28% with no change in mean pulmonary artery pressure, about 20% of blood flow shifted to units with low ventilation-perfusion (V/Q) ratios, and arterial oxygen tension fell from 52 to 47 mm Hg.2 Systemic riociguat in PH associated with idiopathic interstitial pneumonia was stopped early for excess serious adverse events and mortality.3 Route of delivery also matters, since inhaled agents reach ventilated units preferentially and spare V/Q matching. Consistent with this, inhaled treprostinil improved 6-minute walk distance results by 31 meters and reduced clinical worsening in PH due to interstitial lung disease in the INCREASE trial.4
High-altitude PH
In high-altitude PH, hypoxia drives both vasoconstrictor and structural components. In one study by Sime and colleagues, 11 healthy men native to the Peruvian Andes, living above 4,000 meters, underwent right heart catheterization at baseline and then again after two years of living in Lima at sea level. Heart rate fell, cardiac index rose, and eventually PVR normalized. Breathing 35% oxygen at altitude for 15 minutes lowered pressure and resistance only partially, highlighting the role of the structural component.5
PAH at high altitude
In patients withPAH residing at high altitudes, the structural component dominates. In a recent article published in the journal CHEST®, Ulrich and colleagues transported 28 patients with PAH living near 2,850 meters in Quito, Ecuador, to sea level for two nights.6 Arterial oxygen tension (Pao2) rose from 7.6 to 10.5 kPa (57 to 79 mm Hg). Systolic pulmonary artery pressure fell after the first night, and cardiac output fell after the second night. Oxygen delivery and the ratio of systolic pressure to cardiac output, a noninvasive surrogate for total pulmonary resistance, were unchanged. The 41-meter gain in the 6-minute walk distance on day 2 was lost by day 3. Unlike the partial response to oxygen in the Sime cohort, the resistance surrogate did not change, which may be explained by a remodeled vascular bed that has little hypoxic vasoconstriction left to relieve.
In high-altitude PH, two years of sea level conditions normalized resting hemodynamics, although the pressure response to exercise remained abnormal. Whether sustained correction of hypoxemia would do the same in PAH, a condition with distinct pathogenesis, is unknown—and the Quito study was not designed to answer it. Ulrich and colleagues suggest that for patients with PAH living at high altitudes, access to PAH-targeted therapy is likely more important than permanent relocation to low altitude.6
References
1. Dunham-Snary KJ, Wu D, Sykes EA, et al. Hypoxic pulmonary vasoconstriction: from molecular mechanisms to medicine. Chest. 2017;151(1):181-192. doi:10.1016/j.chest.2016.09.001
2. Melot C, Hallemans R, Naeije R, Mols P, Lejeune P. Deleterious effect of nifedipine on pulmonary gas exchange in chronic obstructive pulmonary disease. Am Rev Respir Dis. 1984;130(4):612-616. doi:10.1164/arrd.1984.130.4.612
3. Nathan SD, Behr J, Collard HR, et al. Riociguat for idiopathic interstitial pneumonia-associated pulmonary hypertension (RISE-IIP): a randomised, placebo-controlled phase 2b study. Lancet Respir Med. 2019;7(9):780-790. doi:10.1016/S2213-2600(19)30250-4
4. Waxman A, Restrepo-Jaramillo R, Thenappan T, et al. Inhaled treprostinil in pulmonary hypertension due to interstitial lung disease. N Engl J Med. 2021;384(4):325-334. doi:10.1056/NEJMoa2008470
5. Sime F, Peñaloza D, Ruiz L. Bradycardia, increased cardiac output, and reversal of pulmonary hypertension in altitude natives living at sea level. Br Heart J. 1971;33(5):647-657. doi:10.1136/hrt.33.5.647
6. Ulrich S, Müller J, Furian M, et al. Effects of a sea-level stay on pulmonary hemodynamics in patients with pulmonary arterial hypertension living in Quito near 2,850 m. Chest. 2026;170(2):512-523. doi:10.1016/j.chest.2026.02.043
