Supplemental oxygen, defined as fraction of inspired oxygen (Fio2) in concentrations greater than 21%, is usually administered to patients throughout the perioperative period. The most common devices utilized in the operating room are attached to the anesthesia work station. Patients arriving in the post-anesthesia care unit (PACU), intensive care unit (ICU), and postsurgical floors will often require oxygen delivery by other means. Upon arriving in the PACU, 20% of patients aged 1-3 years, 14% of patients aged 3-14 years, and 8% of adults will experience arterial oxyhemoglobin desaturation on room air to SaO2 < 90%.
The goal of oxygen administration is to prevent tissue hypoxia. Supplemental oxygen does not address the cause of hypoxemia, often does not eliminate tissue hypoxia, and may mask hypoventilation. It is one step in the treatment of hypoxemia that is coupled with other interventions such as incentive spirometry, pain control, positioning, and diuresis to insure a good outcome. With the standardization of pulse oximetry in the perioperative setting, administration of supplemental oxygen has increased.
Oxygen delivery systems are categorized as either low-flow or high-flow systems.
When supplemental oxygen is delivered via low-flow devices, Fio2 can only be approximated due to the entrainment of room air and variation in minute ventilation. When a patient’s minute ventilation exceeds the flow rate, more room air is inspired.
Nasal cannulas are the most frequently used device. Flow can be increased from 1 to 6 liters per minute (L/min) after which increasing flow no longer increases Fio2. With this device, Fio2 increases approximately 4% above room air (21% Fio2) per liter per minute increase in oxygen flow. The maximum Fio2 obtainable with a nasal cannula is 44%.
Humidification of inspired oxygen is necessary to prevent drying of mucous membranes when flows become greater than 4 LPM. The amount of room air inhaled through nose and mouth mixes with supplemental oxygen delivered via the nasal cannula. Fraction of inspired oxygen decreases as minute ventilation increases (minute ventilation [VE] = respiratory rate [RR] × tidal volume [Vt]). The actual Fio2 with nasal oxygen varies with minute ventilation.
|Fio2 ||Minute Ventilation (L/min) ||Oxygen Flow (L/min) |
|0.60 ||5 ||6 |
|0.44 ||10 ||6 |
|0.32 ||20 ||6 |
Simple face masks are loosely fitted devices that allow entrainment of room air in addition to supplemental oxygen. Flow rates of at least 5 L/min are required for all masks (simple, partial nonrebreather, and nonrebreather) to flush the expired CO2 from the mask and prevent rebreathing. Like the nasal cannula, each increase ...