Supplemental oxygen is a commonly used medical treatment administered to patients who are not taking in enough oxygen from the air. Oxygen therapy is considered a medication, and its correct dosage must be carefully managed by healthcare providers. The goal is to raise blood oxygen levels to a safe range without causing harm from excessive administration. This process requires continuous monitoring and adjustment based on the patient’s individual needs.
Measuring the Need for Supplemental Oxygen
The decision to administer supplemental oxygen and the amount needed is primarily guided by a simple, non-invasive measurement called pulse oximetry. This device, often clipped to a finger or earlobe, estimates the percentage of hemoglobin in the blood that is saturated with oxygen, which is referred to as the peripheral oxygen saturation, or SpO2. Normal SpO2 in a healthy person is generally between 95% and 100%.
For most acutely ill patients, the standard target range for SpO2 is set between 94% and 98%. The concept of “titration” involves adjusting the oxygen flow rate up or down to keep the patient’s SpO2 within this defined goal range. Titration ensures that a patient receives just enough oxygen to meet their physiological needs, but not more than necessary.
The goal is not to achieve a saturation of 100%. An SpO2 reading of 100% can mask an underlying issue or indicate that the patient is receiving an unnecessarily high concentration of oxygen. Healthcare providers continually monitor the pulse oximetry reading and adjust the flow rate to maintain the patient within their specific target range.
Methods of Oxygen Delivery and Flow Rates
Oxygen is delivered through various devices, with the choice depending on the required concentration and the patient’s condition. The amount of oxygen flowing from the source is measured in Liters Per Minute (LPM). However, the actual concentration of oxygen a patient breathes, known as the Fraction of Inspired Oxygen (FiO2), is what truly matters, and this is not always directly proportional to the LPM setting.
The nasal cannula is the most common low-flow device, delivering oxygen through two small prongs placed in the nostrils. It is used for flow rates of 1 to 6 LPM, which corresponds to an FiO2 increase of about 4% per liter above room air’s 21%. For instance, 1 LPM provides an estimated FiO2 of about 24%, while 6 LPM can reach approximately 44%.
For patients requiring a higher concentration, a simple face mask may be used, which offers a flow rate between 6 and 10 LPM. This mask can deliver an FiO2 ranging from approximately 30% to 60%, depending on the flow rate and the patient’s breathing pattern. The flow rate must be set to at least 5 LPM to prevent the rebreathing of exhaled carbon dioxide that can accumulate within the mask.
Understanding the Risks of Too Much Oxygen
The misconception that more oxygen is always beneficial can be dangerous, as excessive oxygen, a state known as hyperoxia, carries specific physiological risks. Oxygen is a chemically reactive molecule, and when its partial pressure is too high, it can lead to the formation of reactive oxygen species that overwhelm the body’s natural defenses. This can result in oxygen toxicity, which primarily affects the lungs because they are exposed to the highest concentration of the gas.
Prolonged exposure to a high FiO2 (greater than 60%) can damage the respiratory epithelium, leading to retrosternal burning, chest tightness, and acute lung injury. This pulmonary toxicity is a form of oxidative stress that causes inflammation and cellular damage in the lung tissue. Hyperoxia can also affect the central nervous system, causing symptoms that range from nausea and dizziness to convulsions, particularly under hyperbaric conditions.
Another risk is absorption atelectasis, which is the collapse of small air sacs, or alveoli, in the lungs. Room air contains about 79% nitrogen, an inert gas that helps keep the alveoli open. When a patient breathes a very high concentration of oxygen, the nitrogen is washed out and replaced by oxygen. If the small airways are partially blocked, the pure oxygen behind the blockage is quickly absorbed into the bloodstream, causing the alveoli to collapse.
Adjusting Oxygen for Specific Patient Conditions
Standard oxygen targets must be modified for patients with certain underlying chronic respiratory conditions. Individuals at risk for hypercapnic respiratory failure, most commonly those with severe Chronic Obstructive Pulmonary Disease (COPD), require a lower, more conservative SpO2 target range. For these patients, the recommended goal saturation is 88% to 92%.
The reason for this lower target is that excessive oxygen can lead to the retention of carbon dioxide (CO2) in the blood, a condition called hypercapnia. While the traditional explanation involved the suppression of the respiratory drive, the primary mechanism is related to changes in the balance of ventilation and blood flow in the lungs and the Haldane effect. High oxygen levels counteract the body’s mechanism of diverting blood flow away from poorly ventilated areas, leading to a worsening mismatch between ventilation and perfusion.
This increased CO2 retention can cause worsening respiratory acidosis and altered mental status. Therefore, oxygen therapy in these individuals must be started at low flow rates, such as 1 to 2 LPM via nasal cannula, and carefully titrated to prevent the SpO2 from exceeding the 92% upper limit.