Evaluation of blood oxygenation.

Applying critical thinking and problem solving skills when assessing the oxygenation status of the blood of a patient can be considered the cornerstone of respiratory therapy. Every clinical day respiratory therapists treat patients who have hypoxemia. At the same time, respiratory therapists are actively engaged in (1) evaluating the level of hypoxemia, (2) determining the appropriate oxygen therapy device, (3) deciding how much oxygen to administer, and (4) judging when to increase, decrease, or discontinue O2 therapy. Within the context of this complex decision making process, respiratory therapists must understand the physiology underlying how oxygen is transported in the blood.

Oxygen is carried in two compartments in the blood. It is physically dissolved in the plasma, and chemically and reversibly bound to hemoglobin. As respiratory therapists, an understanding of different measurements and calculations related to oxygen transport in these two compartments is imperative.

The indexes used to evaluate the oxygenation status of the blood of a patient often include:

* Fraction of inspired oxygen (FIO2)

* Partial pressure of oxygen in arterial blood (PaO2)

* Arterial oxygen saturation (SaO2)

* Hemoglobin concentration ([Hb])

* Dysfunctional hemoglobin

* Arterial oxygen content (CaO2)

The fraction of inspired oxygen (FIO2) is the amount of oxygen in the inspired gas expressed in decimal form. For example, a patient breathing 40% oxygen is receiving an FIO2 of 0.40. Clinicians often use the terms oxygen percentage (O2%) and FIO2 interchangeably; a practice that is incorrect.

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The FIO2 directly influences the amount of oxygen that physically dissolves in the plasma of the arterial blood. This quantity of oxygen is usually expressed as the arterial partial pressure of dissolved oxygen, or the PaO2. The quantity of oxygen that physically dissolves in the plasma is governed by Henry's law of solubility, which states that the amount of gas that dissolves in a liquid, at a certain temperature, is directly proportional to the partial pressure of that gas above the surface of the liquid. For a clinical example of Henry's law, consider the blood in the pulmonary capillaries as the liquid, and think of the oxygen in the alveolus as the gas above the surface of the liquid. Under room air conditions at sea level, the alveoli contain a partial pressure of oxygen of slightly above 100 mm Hg. This alveolar partial pressure of oxygen (PAO2) produces a partial pressure of oxygen in the pulmonary capillary blood of 100 mm Hg.

Considering Henry's law, if the FIO2 was increased from 0.21 to 0.40, a significant amount of nitrogen molecules would wash out of the lungs, and the PAO2 would increase to about 225 mm Hg, based on the alveolar air equation. Assuming complete equilibration across the alveolar-capillary membrane, the PaO2 would become 225 mm Hg. Therefore, the PAO2 would equal the PaO2. By virtue of elevating the FIO2, more oxygen molecules were deposited in the alveoli (the gas above the surface of the liquid), and more oxygen molecules dissolved in the blood (the liquid).

An oxygen analyzer can measure the FIO2 and the O2%, and the PaO2 can be measured via arterial blood analysis. From a PaO2 standpoint, hypoxemia is defined as a PaO2 of less than 80 mm Hg. Hypoxemia is classified as (1) mild hypoxemia: 60 to 79 mm Hg; (2) moderate hypoxemia: 40 to 59 mm Hg; and (3) severe hypoxemia: < 40 mm Hg.

In addition to providing the PaO2 measurement, an arterial blood gas analyzer furnishes the arterial oxygen saturation (SaO2) and the hemoglobin ([Hb]) concentration, both of which represent further indexes of the oxygenation status. The SaO2 signifies the percentage of oxygen chemically and reversibly bound to hemoglobin. The SaO2 is obtained from the formula (O2 content / O2 capacity) 100 = SaO2. The content is the actual amount of oxygen combined with hemoglobin, and the capacity is the maximum amount of oxygen that the available hemoglobin can carry.

The maximum amount of oxygen that can be transported by the hemoglobin can be calculated by multiplying 1.34 ml O2/gram Hb by the hemoglobin concentration ([Hb]). 1.34 ml O2/gram Hb is the volume of O2 that can be carried by each gram of hemoglobin. For example, if the [Hb] is 15 grams percent (g %), the maximum volume of oxygen that can bind with that amount of hemoglobin would be (1.34 ml O2/gram Hb)(15 g%) = 20.1 volumes percent (vol %). (The unit grams percent (g %) means a certain number of grams of hemoglobin contained in 100 ml of blood. The unit volumes percent (vol %) refers to the volume of O2 contained in 100 ml of blood.)

Carrying this example further, if the SaO2 via arterial blood gas analysis was found to be 97.5%, the O2 content would be 19.6 vol %, or (20.1 vol %)(0.975) = 19.6 vol %. The SaO2 obtained from an arterial blood sample is a calculated value, not a measured quantity.

On the other hand, the oxygen saturation obtained from a pulse oximeter (SpO2) is a measured value, not a calculated one. Based on the oxygen saturation, hypoxemia is defined as an SaO2 of 90% or less. On the oxyhemoglobin dissociation curve, an SaO2 of 90% equals a PaO2 of 60 mm Hg. Therefore, whenever the SaO2, or SpO2 is 90% or less, hypoxemia exists.

Measuring the SpO2 when dysfunctional hemoglobin, i.e., carboxyhemoglobin or methemoglobin, is present will lead to erroneously high SpO2 values because a pulse oximeter cannot differentiate between either carboxyhemoglobin or methemoglobin and oxyhemoglobin. A pulse oximeter's red and infrared light can only differentiate oxyhemoglobin from deoxyhemoglobin.

When SaO2 falls below 90%, the steep portion of the oxyhemoglobin dissociation curve is encountered. Along this part of the curve, hemoglobin's affinity for O2 diminishes, and unloading of O2 to the tissues is facilitated. Furthermore, a given change in the PO2 (x-axis of the curve) of the blood along this segment of the curve is associated with a greater change in SaO2 (y-axis of the curve). For example, if the PO2 of the blood dropped from 60 mm Hg to 40 mm Hg, i.e., a 20 mm Hg change, the SO2 would fall from 90% to 84%, which reflects a 16% change.

On the other hand, compare that PO2 change to a PO2 change of the same magnitude along the flat portion of the oxyhemoglobin dissociation curve. If the PaO2 decreased from 100 mm Hg to 80 mm Hg, the corresponding SaO2 decrease would be from 97.5% to 95%, or a mere 2.5%. The flat segment of the curve represents the diffusion of O2 from the alveoli to the pulmonary capillary blood. This activity coincides with an increase in the affinity hemoglobin has for O2, which enhances the binding of O2 to hemoglobin.

The status and quality of the hemoglobin must be considered when assessing a patient's oxygenation. Patients who are anemic ([Hb] [less than or equal to] 12 g%) may have a normal oxygenation status at rest, but could become hypoxemic when their O2 demand increases disproportionately to its supply. Conversely, patients who are polycythemic ([Hb] [greater than or equal to] 16 g%) have an increased [Hb] because of chronic hypoxemia, as the body attempts to increase the O2 carrying capacity of the blood by producing more red blood cells.

The quality of the hemoglobin is a crucial factor in evaluating the status of a patient's blood oxygenation because hemoglobin is the major vehicle for O2 transport in the body. The presence of dysfunctional hemoglobin can significantly interfere with O2 transport from the lungs to the tissues. A common form of abnormal hemoglobin is carboxyhemoglobin (HbCO). The HbCO results from the binding of carbon monoxide (CO) with hemoglobin. Hemoglobin has an affinity for CO approximately 210 times greater than it has for O2.

Other relatively common forms of dysfunctional hemoglobin include sulfhemoglobin and methemoglobin. The use certain drugs such as sulfonamides are associated with the development of sulfhemoglobinemia. A sulfur atom attaches to the hemoglobin molecule, preventing the molecule from binding with O2 and causing the dissociation curve to shift to the right.

Methemoglobin can be congenital or acquired. The congenital form is usually benign, while the acquired form can impair tissue O2 delivery. Drugs such as benzocaine and lidocaine can produce methemoglobinemia. Methemoglobin cannot carry oxygen because the ferrous ion (Fe2+) in the center of the heme portion of the molecule becomes oxidized to the ferric ion (Fe3+). The presence of methemoglobin also causes the oxyhemoglobin dissociation curve to shift to the left.

The CaO2 is another measure of blood oxygenation. It incorporates the PaO2, SaO2, and the [Hb]. It is the sum of the O2 dissolved in the arterial plasma and the volume of O2 combined to hemoglobin. The quantity of O2 in both the dissolved compartment and the combined form are expressed in volumes percent (vol %). Quantitatively, the CaO2 is calculated according to the following formula:

(PaO2 X 0.003 vol %/mmHg)(1.34 ml O2/g Hb X [Hb] X SaO2) = CaO2

With normal physiologic values inserted into the equation, the normal CaO2 can be calculated. That is, (100 mmHg X 0.003 vol %/mmHg)(1.34 ml O2/g Hb X 15 g% X 97.5%) = 19.9 vol %

A CaO2 of 19.9 vol % may not be easy to conceptualize because of the unfamiliarity associated with the unit volumes percent. Again, volumes percent means some volume of O2 in milliliters contained in every 100 milliliters of blood. Consider the fact that a normal size adult has approximately 5,000 ml of blood in circulation. Every 100 ml of that 5,000 ml contains 19.9 ml of O2. In other words, think of the total circulating volume of 5,000 ml as segmented into 50 separate "packets" of blood with each "packet" containing 19.9 ml of O2, or 50 "packets."

Other measurements can be used to evaluate the oxygenation status of a patient's blood, e.g., the alveolar-arterial O2 tension gradient (P[A - a]O2), and the shunt fraction. Keep in mind, however, that the ultimate purpose for assessing the oxygenation status of the blood of a patient is to determine that an adequate supply of oxygen is available for tissue oxygen delivery.

by Bill Wojciechowski, MS, RRT