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 breath 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).

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:

Vlog Brothers via GfyCat

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.

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)

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.