From Respiratory Wiki
One of the primary treatment modalities that is used as a Respiratory Therapist is oxygen therapy. Therefore, it is important to understand how much \(FiO_2\) we are giving our patients depending on what delivery system we are using. An oxygen delivery system is a device used to administer, regulate, and supplement oxygen to a patient in order to increase the arterial oxygenation. The amount of oxygen delivered to a patient is measured by \(FiO_2\), or the fraction of inspired oxygen, this article will serve to give the range of estimated \(FiO_2\) by each oxygen delivery device.
Hazards of Oxygen Therapy
Before beginning, it is important to note that oxygen therapy can put patients at risk for injury. Though side effects are rare, it is considered best practice to only use whatever amount of oxygen that is needed to maintain a \(S_pO_2\) that is greater than 90 unless otherwise noted by a physician or hospital policy.
Potential side effects of excessive oxygen use include but are not limited to:
Oxygen induced hypercapnia
Other vasoconstrictive diseases(ie. stroke, blindness)
Low Flow Devices
Low flow oxygen delivery systems are considered open, which means that \(FiO_2\) calculations are not 100% accurate. The only way to get an accurate \(FiO_2\) is with the use of an oxygen sensor, as there are many factors that can both increase and decrease the amount of oxygen given.
The nasal cannula is the most common oxygen delivery device used by patients both at home and in the hospital setting. Unless using a wide bore high flow nasal cannula, the flow rate on a nasal cannula should not exceed 6L/m. The formula to determine the \(FiO_2\) for a nasal cannula is\[FiO_2 = 20+(Flow Rate(L/m)\times4)\]
Or more simplified
1 L/m = 24%
2 L/m = 28%
3 L/m = 32%
4 L/m = 36%
5 L/m = 40%
6 L/m = 44%
A non-rebreather mask will give a patient, at a flow of 12-15L/m, a \(FiO_2\) of between 60% and 100%. This range is wide because of a number of factors which include the quality of the seal on the patient's face and the integrity of the one way valves.
Partial Rebreather Mask
This device is identical to a NRB with the exception that the one-way valves are removed from the mask. The flow should be set at 6-10 lpm. and the FiO2 is estimated at 40-70%.
High Flow Oxygen Devices
A high flow oxygen delivery system is defined as a device that produces an accurate \(FiO_2\) and meets or exceeds a patient's inspiratory flow demand.
The Venturi mask, or Venti mask, is an air entrainment device that can provide either 24%, 26%, 28%, 30%, 35%, 40%, or 50% \(FiO_2\). Although the flow rate on these devices are set between 3-15L/m(depending on the manufacturers recommendation), the actual flow delivered to the patient is much higher, but suffers from diminishing returns due to lower air entrainment. As a rule, the higher the \(FiO2\) delivered, the lower the flow delivered. The exact amount of flow can be calculated by finding the air to oxygen ratio using the magic box method. (I will add an explanation for this at a later time).
The aerosol mask delivery system works in the same way as the Venturi mask except that the Venturi device is attached directly to the flow meter as well as a humidification reservoir. In addition, this device uses an adjustable \(FiO_2\) mechanism that can produce between 21%-100% \(FiO_2\). The set flow rate for this device is again dependant on manufacturers recommendation but is typically 10-15L/m. The same principle of total flow to patient applies as in the Venturi mask. A visual indication that total flow meets the patients demand is the presence of mist upon inhalation near the exhalation ports of the mask.