Oxygen is a fundamental component of medical care, but its administration is a measured intervention, not a simple supplement. Like any therapeutic agent, oxygen must be dosed carefully; the idea that more is always better is a misconception. Oxygen therapy’s primary goal is to correct hypoxemia, or low oxygen levels in the blood. This ensures tissues receive the necessary supply for survival, using supplemental oxygen at concentrations higher than the 21% found in room air.
Determining the Need and Target Saturation
The decision to administer oxygen is based on a patient’s oxygen saturation level, monitored using a pulse oximeter. This non-invasive device, clipped to a finger or earlobe, measures the percentage of hemoglobin carrying oxygen, known as the peripheral oxygen saturation (\(\text{SpO}_2\)). Normal saturation levels for a healthy individual generally fall between 95% and 100%.
A saturation reading below 95% indicates hypoxemia. The goal of treatment is not necessarily to achieve 100% saturation, which can be harmful. For most acutely ill patients not at risk of carbon dioxide retention, the recommended target saturation range is 94–98%.
This target range provides sufficient oxygen to the body’s tissues without leading to excessive oxygen levels. The range is significantly lower for patients with chronic conditions like Chronic Obstructive Pulmonary Disease (COPD) who are at risk of hypercapnic respiratory failure. For these individuals, the target is set to a more conservative 88–92% to prevent complications.
Common Delivery Methods and Associated Flow Rates
The “dose” of oxygen is determined by the flow rate, measured in Liters Per Minute (LPM), and the delivery device used. These factors together determine the concentration of oxygen the patient breathes, known as the Fraction of Inspired Oxygen (\(\text{FiO}_2\)). The choice of device depends on the patient’s condition and the required oxygen concentration.
The simplest method for mild hypoxemia is the Nasal Cannula, a low-flow device set between 1 and 6 LPM. Each liter increases the \(\text{FiO}_2\) by about 4% above room air’s 21%, though the exact concentration is variable based on the patient’s breathing pattern. A Simple Face Mask is used when a higher flow is needed, generally delivering oxygen at 6 to 10 LPM.
This mask provides a moderate concentration, estimated between 35% and 60% \(\text{FiO}_2\). The flow must be at least 5 LPM to prevent the patient from rebreathing exhaled carbon dioxide. For severe hypoxemia, the Non-rebreather Mask is used, which features a reservoir bag and one-way valves.
Flow rates for the Non-rebreather Mask are set high, between 10 and 15 LPM, ensuring the reservoir bag remains inflated during inhalation. This device can deliver an \(\text{FiO}_2\) of 60% up to nearly 90% or higher, making it the most aggressive option for non-intubated patients.
The Importance of Titration and Avoiding Hyperoxia
The core principle of safe oxygen administration is titration, meaning adjusting the flow rate based on continuous \(\text{SpO}_2\) monitoring. This adjustment ensures the patient stays within the target saturation range and avoids over-oxygenation, known as hyperoxia. Hyperoxia, an excessive supply of oxygen in the tissues, can be harmful if administered without careful dosing.
Breathing high concentrations of oxygen for an extended time can lead to oxygen toxicity, involving oxidative damage to the lungs. This can manifest as pulmonary effects, such as damage to the airways and alveoli, potentially leading to pulmonary edema. Another danger is absorption atelectasis, where high oxygen concentrations dilute nitrogen gas in the small airways, causing them to collapse as oxygen is rapidly absorbed into the bloodstream.
A safety consideration involves patients with COPD, for whom excessive oxygen can lead to a dangerous buildup of carbon dioxide (hypercapnia). High oxygen levels cause hypercapnia primarily by disrupting the efficient matching of ventilation and blood flow within the lungs. For this reason, oxygen is carefully titrated to the lower 88–92% \(\text{SpO}_2\) target in these patients.