First we need some basic definitions:
Supplemental oxygen: Any device that provides more oxygen than what one would get breathing room air.
Hypoxemia: This is when the oxygen in the blood is low, and is generally measured by a PaO2 of 60 or less, or a SpO2 of 90% or less.
PaO2: This is the level of oxygen in the blood. It should be kept at 60 or better to avoid hypoxemia. It’s obtained by invasive Arterial Blood Gas (ABG) or estimated by SpO2.
SpO2: Also called oxygen saturation, pulse ox or sat. This is a non-invasive measurement of the amount of oxygen inspired that gets to the arteries. A normal SpO2 is about 98%. Be aware that a person’s normal SpO2 decreases with age and with some disease processes. The only way it can get to 100% is with supplemental oxygen.
You can use your SpO2 to predict the PO2 using the 4-5-6, 7-8-9 rule as below:
- SpO2 70% = PO2 of 40
- SpO2 80% = PO2 of 50
- SpO2 90% = PO2 of 60 (This is what you want to maintain for most patients)
Fraction of Inspired Oxygen (FiO2): This is the percent of oxygen a patient is inhaling. Room air FiO2 is 21%. By applying supplemental oxygen, the FiO2 can go as high as 100%.
Indications for Oxygen Therapy:
- To correct hypoxemia
- To reduce oxygen demand on the heart
- Suspected or acute marcardial infarction (MI)
- Severe trauma
- Post anesthesia recovery
How much FiO2 is delivered to the patient is dependent on:
- Liter flow set at the flowmeter
- Respiratory rate and pattern of the patient
- Equipment reservoir (stores oxygen)
1. Nasal Cannula: The nasal cannula is the most common oxygen device used and the most convenient for the patient. A nasal cannula at 2lpm is usually a good place to start.
You may at times need to estimate the FiO2. How to estimate FiO2 on a nasal cannula? For every liter per minute, the FiO2 increases by 4% as per the chart below:
- 1 lpm = 24%
- 2 lpm = 28%
- 3 lpm = 32%
- 4 lpm = 36%
- 5 lpm = 40%
- 6 lpm = 44%
A bubbler can be added to humidify the nose to prevent nasal drying and bleeds. This is automatically set up at flows greater than 4lpm, or as ordered by physician.
3. Non-Rebreather Mask (NRB): This is a mask that ideally will bring in 100% Fio2 so long as the liter flow is 15 and there is a good seal between the mask and the patient's face. And all three one-way valves are on the mask to prevent air entrainment.
For legal purposes, however, one flap is always removed just in case the oxygen gets shut off. And therefore the highest FiO2 you can get from an NRB is 75%. The bag acts as a reservoir for oxygen, and therefore allows device to provide higher FiO2s to the patient.
4. Partial Rebreather Mask (PRB): This is basically an NRB with both one-way valves removed from the mask. The estimated FiO2 is 60-65%. Flow should be set at 6-15 lpm.
High Flow Oxygen Devices: These devices meet the inspiratory flow of the patient, and generate accurate FiO2s so long as there is a good seal between the mask and the patient's face. The flows are such that the patient will not be entraining room air that will lower the FiO2. Respiratory rate and tidal volume of the patient have no effect on FiO2 delivered.
Ideally, the larger the entrainment port on the device the lower the FiO2, and the smaller the entrainment port the higher the FiO2. A major disadvantage is a mask is required, and this may be a bit more uncomfortable than a nasal cannula.
1. Venturi Mask: This mask is ideal for patients who are in respiratory distress with high tidal volumes or high respiratory rate to guarantee a certain amount of oxygen.
If a nasal cannula does not provide adequate oxygenation, Venturi Masks set from 28% to 40% are ideal for COPD patients.
Modern Venturi masks come with one or more color coded caps, and whichever one you use the desired liter flow for that particular cap is written right on the cap.
The Venturi Masks used at MMC are set up as follows:
A. White cap:
- 35% FiO2 set lpm at 9
- 40% FiO2 set lpm at 12
- 50% FiO2 set lpm at 15
- 24% FiO2 set lpm at 3lpm
- 26% FiO2 set lpm at 3lpm
- 28% FiO2 set lpm at 6lpm
- 30% FiO2 set lpm at 6 lpm
2. Aerosol set-up: This device will deliver anywhere from 21 to 100% FiO2 depending on how it is set up. The desired flow to set the flow meter at is written write on the cap
Usually a humidity device is connected to the flowmeter, and wide bore tubing connects this to the patient's mask Wide bore tubing acts as a reservoir to obtain higher FiO2s.
These are ideal for patients with tracheotomies because it allows for inspired air to be oxygenated, humidified and even heated if necessary. They can be hooked up to a simple mask, tracheotomy mask, and even a t-piece.
The flow may exceed the required flow, although if it is less the patient may retain CO2, and the FiO2 be lower than desired. On inhalation a mist should be seen coming from mask or reservoir.
3. High flow nasal cannula: An Fio2 of 21% to 100% may be maintained because the flow meets the patient's spontaneous inspiratory demand. This is made possible due to thicker tubing and humidified oxygen.
Other oxygen devices you might see:
1. BiPAP: This is a discussion for another day. Still, pressure can be given by a non-invasive mask over the patient’s face to improve ventilation, and to supply any FiO2 from 21% to 100%. These also have other means of improving oxygenation.
4. Ventilator: This is also a discussion for another day. Yet for patients whose oxygen demands exceed any of the above devices, intubation and ventilation with a ventilator may be required. These can supply any FiO2 from 21% to 100%, and also have other means of improving oxygenation.
Hazards of oxygen therapy:
- Oxygen may suppress the respiratory drive for some COPD patients, and should be used with caution.
- FIO2s greater than 60% for greater than three hours have been linked to increased risk for lung injury and other future consequences.
- For most patients, you will start low and work your way up if needed
- We usually start at 2lpm for most patients and adjust accordingly.
- If you have a patient in respiratory distress, you may want to start at 40%.
- However, if the patient is in severe respiratory distress, or is the victim of a trauma, you may want to simply start at 100% and decrease as appropriate
- All patients suspected of chronic heart failure should be placed on 100% FiO2 and adjusted down from there.
- All patients who are suspected to be CO2 retainers should be started on 2lpm or, if in respiratory distress, on a venturi mask set no higher than 40%.
- Still, a majority of patients do quite well on 2lpm.
Ideally, whatever oxygen device is needed to maintain a SpO2 of 90% or greater or as otherwise specified by a specific oxygen protocol or physician order is indicated.
Oxygen supplementation for uncomplicated acute coronary syndrome is no longer routinely indicated and should only be applied only if the oxyhemoglobin saturation is less than or equal to 94 percent. The old recommendation was to place all patients complaining of chest pain on 4lpm with the belief that it would increase oxygen to the heart and decrease work of breathing. I'm simply noting this here because some physicians prefer to stick with the old recommendations, and that's fine.
Sedatives, analgesics (like Morphine) and anesthesia may also depress respiratory drive, and these patients are often placed on oxygen. The amount used is usually 2-3 lpm via nasal cannula, however this depends on the patient, physician, or protocol.
How to determine if oxygen therapy is working:
You know oxygen therapy is working when:
- SpO2 improved to patient normal (or as determined by physician)
- Respiratory rate improves
- Patient tidal volume is not erratic
- Patient notes improved work of breathing
- Pulse is normal or improved or improving
- Blood pressure is improved or improving
- Underlying condition is improving, or whatever occurred to cause the hypoxemia
So you want to use an e-cylinder to take a patient to x-ray and you want to know if you have enough oxygen in the tank to make it there. You can use the following formula:
e-cylinder time remaining = .30 (PSI) / LPM