Continuous Positive Airway Pressure (CPAP) is a mechanical treatment for breathing disorders, not oxygen therapy. CPAP machines primarily use filtered ambient air, which contains approximately 21% oxygen, to create a pressurized airflow. The confusion often arises because both CPAP and supplemental oxygen help patients breathe more effectively. The fundamental difference is that CPAP manages pressure, while oxygen therapy manages gas concentration.
How CPAP Works
CPAP functions by delivering a steady stream of pressurized air through a mask worn during sleep. The primary purpose of this pressure is to manage the physical structure of the upper airway. The air is drawn from the room, filtered, and then pressurized before reaching the patient.
The mechanism is commonly described as “pneumatic splinting,” where the air pressure acts like an internal support structure. This continuous force prevents the soft tissues of the throat, tongue, and palate from collapsing inward. Preventing this collapse is the direct treatment for obstructive sleep apnea (OSA).
The pressure setting, measured in centimeters of water (cmH2O), is precisely calibrated for each patient by a medical professional. This titration ensures the minimum pressure necessary to keep the airway open is delivered. The machine’s output is purely mechanical, focused on maintaining the patency, or openness, of the respiratory passages.
CPAP manages airflow mechanics rather than altering the gas composition being breathed. It is designed to overcome physical obstructions that impede breathing, categorizing CPAP as a non-invasive ventilation device.
The Purpose of Supplemental Oxygen
Supplemental oxygen therapy focuses on the chemical composition of the inspired air. Oxygen is classified as a medical gas, and its administration is treated similarly to prescribing a drug. The therapy increases the fraction of inspired oxygen (FiO2) above the 21% concentration found in ambient air.
The goal of oxygen therapy is to treat hypoxemia, defined by abnormally low levels of oxygen in the blood. Hypoxemia can result from conditions including chronic obstructive pulmonary disease (COPD), pneumonia, or heart failure. Increasing the FiO2 makes more oxygen molecules available for gas exchange within the lungs’ alveoli.
This process directly raises the saturation of oxygen carried by hemoglobin in the red blood cells, addressing the underlying deficit. Delivery systems, such as nasal cannulas or simple masks, are designed purely to enrich the gas mixture. Unlike CPAP, supplemental oxygen does not provide mechanical pressure support to keep the airway from collapsing.
Oxygen prescription is based on measured blood oxygen saturation levels, typically requiring a saturation below 88% or 89%. The therapy directly modifies the concentration of the gas being inhaled, separate from the mechanical pressure provided by CPAP.
When CPAP and Oxygen Are Used Together
In some clinical situations, patients require both mechanical airway support and enriched gas concentration, necessitating the combined use of CPAP and supplemental oxygen. This combination is common for individuals who have obstructive sleep apnea but also suffer from concurrent underlying disorders that cause chronic low blood oxygen levels. Examples of such disorders include severe COPD, pulmonary fibrosis, or certain neuromuscular diseases.
When used together, oxygen is typically introduced into the CPAP circuit after the air has been pressurized. This integration is managed through a designated port located on the CPAP machine’s tubing or mask assembly. The oxygen flow is added to the pressurized room air stream, enriching the gas mixture before it reaches the patient.
The CPAP machine still performs its function of pneumatic splinting to prevent airway collapse during sleep. Simultaneously, the added oxygen raises the FiO2, ensuring the patient receives a concentration higher than 21% to combat hypoxemia. This combined approach addresses both the anatomical obstruction and the physiological gas exchange inefficiency.
The concentration of oxygen added is determined by the physician and is independent of the CPAP pressure settings. The combined therapy ensures that the pressurized air stream carries the enriched gas necessary to maintain therapeutic blood oxygen saturation throughout the night.
Safety and Prescription Requirements
Both CPAP and supplemental oxygen are regulated medical therapies that require a doctor’s prescription and ongoing supervision. CPAP machines are regulated as medical devices, and their pressure settings must be carefully determined through sleep studies and subsequent titration. The main safety considerations for CPAP involve ensuring the correct pressure is used and monitoring for mask fit issues.
Supplemental oxygen, however, is regulated as a drug because it directly affects blood chemistry and carries specific risks. Over-administering oxygen can lead to oxygen toxicity, which damages lung tissue. In patients with certain types of advanced COPD, excessive oxygen can also suppress the respiratory drive, leading to dangerous levels of carbon dioxide retention.
Oxygen also presents a heightened fire hazard, as it is an accelerant, making proper safety protocols a necessary part of its use. The prescription for oxygen is highly specific, detailing the flow rate and duration of use. When CPAP and oxygen are combined, the medical team must monitor both the mechanical pressure and the gas flow to ensure patient safety and therapeutic effectiveness.