SIMPLIFIED APPROACH TO BLOOD GAS INTERPRETATION
Arterial blood gas analysis will provide two separate but related pieces of clinical information. First, it provides information
on the respiratory status of the patient (PaO2, PaCO2, A-a). Second, it gives you information on the acid-base status of the patient (pH, HCO3
-, base excess).
Oxygen is less diffusible than carbon dioxide, so impairments to diffusion will cause the A-a gradient to increase (Table 2). A more common cause of increased AaDO2 during anesthesia is the mixing of nonoxygenated blood with oxygenated blood (e.g. shunting or ventilation-perfusion [V/Q] mismatch). Unoxygenated hemoglobin will mix with oxygenated hemoglobin and steal
oxygen away until both are equally saturated. Because of the shape of the oxyhemoglobin dissociation curve, the PaO2 will not directly tell you the magnitude of the arterial-venous mixing. During anesthesia, it is common to have increased
V/Q mismatch and A-a gradients, especially in large animals.
Table 2: The Five Causes of Hypoxemia in Anesthetized Patients
Carbon dioxide production is usually relatively constant during anesthesia. Since most of the carbon dioxide produced is excreted
through the lungs, changes in minute ventilation will alter measured PaCO2. When minute ventilation is doubled, the PaCO2 will decrease by about 50%. When minute ventilation is halved, the PaCO2 will about double. This relationship is worth knowing because it allows an anesthetist to adjust a ventilator to maintain
a desired PaCO2 during anesthesia.
The traditional approach to assessing acid-base status is based on the Henderson-Hasselbalch equation. The PaCO2 is an independent measure of the respiratory component of the acid-base balance, while extracellular base excess is an independent
measure of the nonrespiratory (metabolic) component of acid-base balance, and the pH is used as a measure of overall acid-base
Four primary acid-base disturbances are possible: respiratory acidosis with increased carbon dioxide, respiratory alkalosis
with decreased carbon dioxide, metabolic acidosis with decreased base excess, and metabolic alkalosis with increased base
excess and HCO3
- (Table 3). Compensation may occur to reduce the magnitude of the primary abnormality, but it never results in overcorrection.
Table 3: Characteristics of Primary Acid-Base Disturbances
Acid-base disturbances are often corrected before or are tolerated during anesthesia. It may be less problematic to correct
chronic abnormalities slowly rather than correcting them during a short anesthetic-surgical procedure. Acute respiratory depression
or hypoxia is common during anesthesia and is usually corrected rapidly.
Three steps can be used to quickly assess the acid-base status of anesthetized patients.
Step 1. The first step is determining the primary disturbance. The pH indicates the direction of the primary condition. If the pH
decreases, the primary condition is causing acidosis. If the pH increases, the primary condition is causing alkalosis.
Step 2. The second step is to determine if the primary condition has a respiratory component. If the primary condition is acidosis
with a respiratory component, the PaCO2 should increase. If the PaCO2 were decreased, you could conclude that there is respiratory compensation, which can be partial or complete depending on
the duration of the condition. Compensation takes some time and may not be obvious if the sample is collected early in the
If the primary condition is alkalosis with a respiratory component, the PaCO2 should be decreased. If PaCO2 is increased, it indicates respiratory compensation.
Step 3. The next step is to determine if the primary condition has a metabolic component. If the primary condition is acidosis with
a metabolic component, the base excess and HCO3
- should be decreased. If it were increased, that would indicate metabolic compensation.
Conversely, if the primary disturbance is alkalosis with a metabolic component, then the base excess should be increased.
A base excess and HCO3
- decrease would indicate metabolic compensation. Again, sufficient time is necessary for compensation to occur.