The benefits of long term oxygen therapy for patients with Chronic Obstructive Pulmonary Disease (COPD) associated with hypoxemia less than 60 mmHg at rest are well known. It prolongs survival, reduces the frequency of hospitalization, development of pulmonary hypertension, improves patient's quality of life and their ability to do activities of daily living. However, it has not been clearly established whether supplemental oxygen therapy for patients with mild to moderate hypoxemia is beneficial.
Fujimoto et. al. demonstrated that supplemental oxygen resulted in a significant increase in six - minute walking distance covered in a series of patients with chronic airflow obstruction. The increase in distance noted was greater as the airflow obstruction becomes more severe and correlated negatively with % FEV1 but not with PaO2 at rest or exercise hypoxemia. The improvement in pulmonary hypertension in patients with moderate to severe airflow obstruction was found by the investigators to be the result of inhibition of hypoxic vasoconstriction and reduction in pulmonary artery occlusion pressure which positively correlated with the air trapping index.
When long term oxygen therapy is started, the oxygen flow is individually adjusted to increase PaO2 to greater than 60 mmHg. Plywaczewski et. al. showed that almost half of COPD patients eligible for long term oxygen therapy desaturated during sleep despite breathing oxygen at an airflow ensuring good oxygenation at rest and while awake. Several mechanisms are responsible for these nocturnal desaturations. The minute ventilation decreases during sleep in both normal subjects and COPD patients. The majority of desaturations appear during rapid eye movement sleep. Irregular breathing, especially shallow rapid breathing, increases physiologic dead space ventilation. The decreased activity of intercostal muscles and the increase of upper and lower airway resistance additionally decrease alveolar ventilation. The absence of cough reflex during sleep in patients with decreased mucociliary clearance increases the ventilation perfusion imbalance due to mucus retention in the small airways. Hypoventilation and disturbance in the ventilation perfusion ratio results in transient hypoxemic episodes. Desaturators had higher pulmonary artery pressure at rest and during exercise. Nocturnal drop in oxygen saturation was more frequent in the "blue bloater" type of patients. Server et al observed that a 0.5 liter / minute increase in oxygen flow abolished nocturnal desaturation.
Pulmonary hypertension is a common complication of advanced COPD and has been considered as an important prognostic factor. Investigations on the use of long- term oxygen therapy showed some positive results. Stark et. al. observed in five COPD patients a fall in a mean pulmonary artery pressure by 12 mmHg after eight months of oxygen therapy given for 15 hours/day. Cooper et. al. also found a decrease in pulmonary artery pressure by 2.2 mmHg after one year of long term oxygen therapy for 15 hours/day in 40 COPD patients. In the NOTT trial, patients who were treated with continuous oxygen for 18 hours/day demonstrated a fall in pulmonary artery pressure by 3 mmHg, whereas no change in pulmonary artery pressure was observed in patients receiving oxygen for 12 hours. In 1998, Zielinski et. al. investigated the effects of six years of domiciliary oxygen therapy in a large group of patients with COPD. He reported that oxygen therapy for 14 to 15 hours/day resulted in a small reduction in pulmonary hypertension after the first two years followed by a return to initial values and subsequent stabilization of pulmonary artery pressure over six years. The stabilization of pulmonary hypertension occurred despite progression of the airflow limitation and of hypoxemia. When used correctly, long term oxygen therapy, can have a significant impact in the management of hypoxemic patients with chronic airflow obstruction.
Aileen Guzman, MD
Dr. Aileen Guzman is a graduate of the College of Medicine, Manila Central University. She had her residency training in Internal Medicine at the same institution and fellowship training in Pulmonary Medicine and Critical Care at the Philippine Heart Center. She is a fellow of the Philippine College of Physicians (PCP) and Philippine College of Chest Physicians (PCCP). She had a training in Sleep Medicine and Clinical Polysomnography at Stanford University, USA. Currently, she is a consultant and the Training Officer of the Division of Pulmonary and Critical Care, Philippine Heart Center.