Supplemental Oxygen and Mechanical Ventilation
Before going into the modes of ventilation, it is important to have a basic understanding of oxygen delivery and the goals of treatment. In the atmosphere, we all breathe 21% oxygen, with the balance being mostly nitrogen (approximately 78%) with some other trace gases. This level of 21% oxygen is high enough for a healthy person to take the amount of oxygen they need out of the air. This level can be increased with oxygen delivery up to 100% or pure oxygen. If lungs are unhealthy or compromised, they need higher percentages of oxygen given to get enough oxygen into the body.
When you give oxygen to a patient, it can be expressed in multiple ways. When the entire breath is being given via an oxygen delivery device and the patient does not breathe any air other than what is given, it can be expressed as a percentage (%) from 21% to 100%, or a Fraction of Inspired Oxygen (FiO2), which is expressed as a decimal from 0.21 to 1.00. This is a pre-mixed amount of oxygen blended with regular room air to deliver the percentage you desire from the lowest (0.21 or room air) to pure oxygen (1.00).
Key Takeaway
Mechanical ventilation is a sealed system and every breath is fully delivered by the ventilator. The medical provider must decide the percentage of oxygen the patient needs to breathe. Oxygen is expressed in FiO2 on ventilators and can range from 0.21-1.00.
When pure oxygen is given in small doses to a spontaneously breathing patient, but the patient also breathes in room air around the oxygen to make up some of their breath, it is termed supplemental oxygen, and delivery is usually expressed in liters per minute (Lpm) of O2 delivery. Supplemental oxygen can be increased to almost equal 100% oxygen depending on the interface it is supplied with (nasal prong or face masks).
When a person is sick and has an acute disease process happening in their body, the oxygen demand of their cells and vital organs is increased. Often, supplemental oxygen delivery can meet this need without needing to go the mechanical ventilation route. But how do you know if your patient needs more oxygen?
The most direct way to monitor oxygen level is through the saturation of hemoglobin in the blood (SaO2). A healthy individual will have their hemoglobin ( Hb) very close to fully saturated: which means 95-100% attachment to oxygen. This percent saturation reflects the total binding sites available for oxygen on the hemoglobin in the blood and compares the amount bound with oxygen to the total.
Hemoglobin has four binding sites available for oxygen to bind, as you can see in the following animation:
Many factors can affect the oxygen’s ability to bind to the hemoglobin. The most important concept to remember is that increasing the amount of oxygen available to bind (increasing oxygen delivery) usually can increase the binding of the oxygen to the hemoglobin.
If SpO2 is lower than 92%, this usually suggests the patient will require some amount of supplemental oxygen. Supplemental oxygen can be started and increased to attempt to increase the SpO2 to above 92%. Mechanical ventilation would be a subsequent step when high levels of supplementary oxygen is not adequate to support the patient’s oxygen needs, in order to prevent hypoxic failure.
Key Takeaways
If the patient’s SpO2 is less than 92%, this is evidence of lower oxygen levels in the body, and the patient may benefit from supplemental oxygen.
Total oxygen content in the blood
Oxygen saturation is not the whole picture of oxygenation. The total oxygen content (CaO2) in arterial blood—or, oxygen being delivered to the vital organs—is the sum of two distinct factors. Primarily, oxygen is attached or “bound” to hemoglobin (represented by the SpO2) and secondly, a small amount diffuses through the alveolar-capillary membrane and dissolves into the blood plasma because of the high amount of oxygen in the alveoli compared to the blood. This is the same principle of air flowing from a high pressure to a low pressure that we learned about in Chapter 1 , only this time it is referring to oxygen flowing from an area of high “density” of oxygen in the alveoli to lower “density” of oxygen in the blood. This description of density is referred to as a partial pressure. It is expressed as a PAO2 (partial pressure of oxygen in the alveoli) and PaO2 (partial pressure of oxygen in the artery)
The formula to determine total oxygen content in the blood (CaO2), is as follows (units are omitted for simplicity). Note: This is not math that you need to do on a regular basis. It is not essential to do this calculation to ventilate patients safely. It is more important that you understand the concepts here and be able to apply them to oxygenation.
| Oxygen Content (CaO2) | = | (Hb)(1.34)( %SaO2/100) | + | (0.003)(PaO2) |
| Oxygen bound to Hb | Diffused Oxygen |
The 1.34 is constant and represents the maximum amount of oxygen that can bind to 1 gram of Hb. The 0.003 represents a constant that the partial pressure of oxygen dissolved in the blood (PaO2) is multiplied against. Normal or “targeted” PaO2 are 80-100mmHg.
Let’s look at the formula in action. If Patient A has a Hgb 120, SaO2 99% and a “normal” PaO2 of 100mmHg, here is the formula:
Oxygen Content (CaO2) = (120)(1.34)(0.99) + (0.003)(100)
= 159.1 + 0.3
= 159.4
For more information on the Oxygen Content Formula, check out this video: “Easy Ways to Calculate Oxygen Content of Blood.”
Looking at the calculated oxygen content of bound oxygen and diffused oxygen, it is obvious that diffused oxygen is a negligible amount when comparing to the amount of oxygen that is bound to hemoglobin. This is true in most cases and is why SaO2 can be used to approximate the overall oxygen content in most standard cases. However, some distinct situations where diffused oxygen might have more of an impact on overall oxygenation include cases of inadequate or abnormal hemoglobin. In these cases:
- the hemoglobin is at dangerously low levels (anemia),
- the hemoglobin is attached to carbon monoxide and not available for oxygen to attach (carbon monoxide toxicity), or
- the oxygen is literally sticking to the hemoglobin and not unloading to the tissue (shifts in the oxy-hemoglobin curve).
These situations might require high PaO2s well above normal targeted ranges to compensate for lack of oxygen delivery via hemoglobin, allowing the body additional dissolved oxygen to be available in these rare cases until the problem is fixed. Remember, these are the exceptions and not the normal.
Other than these special circumstances, the diffused oxygen portion of the formula represents a very small portion of the oxygen content available in the blood. Therefore, for general understanding, SaO2 can be used as a general overview of the oxygenation status of the patient. As previously discussed, SaO2 and SpO2 are usually the same. We will use SpO2 as our primary method of determining oxygen requirements for the purpose of this book.
Key Takeaway
SpO2 is usually a good indicator of SaO2. SaO2 is the primary impact on oxygen content for a patient. Therefore, SpO2 can usually be used to monitor a patient’s overall oxygenation status.
