In this article, I focus on interpreting the PaCO2, PaO2, bicarbonate (HCO3
-), base excess, and alveolar-arterial oxygen difference (AaDO2) values. Normal values for several of these parameters in dogs, cats, and people are given in Table 1.
Table 1: Normal Values and Ranges for Arterial Blood Gas Analysis in Dogs, Cats, and People
Additionally, many blood gas analyzers also measure sodium, potassium, and calcium; total plasma protein can be measured with
a refractometer or other technique. This additional information allows the calculation of the anion gap or other ionic differences
that can provide insight into the metabolic origin of some acid-base disturbances.5,6 These nontraditional approaches to assessing acid-base balance are not routinely used to manage anesthetized patients during
surgery, and, thus, they will not be covered here. However, these approaches will be encountered in the context of metabolic
acid-base disturbances in critical care and internal medicine cases. A discussion on anion gap and strong ion difference theory
is beyond the scope of this article.
The PaCO2 increases when alveolar minute ventilation is decreased and vice versa. When the PaCO2 increases, ventilation is said to be depressed (i.e. hypoventilation). Most anesthetic drugs (e.g. opioids, inhalant anesthetics, propofol) are respiratory depressants; thus, the PaCO2 usually is increased during anesthesia unless ventilation is controlled. Carbon dioxide—not oxygen—is the main stimulus for
respiration during anesthesia in normal patients.
The PaO2 is the partial pressure of oxygen dissolved in the arterial plasma. Alone, this value does not tell you the oxygen content
in the blood. Hemoglobin is the major carrier of oxygen in blood but is not the carrier of dissolved oxygen in plasma; thus,
a hematocrit or hemoglobin concentration is also required before estimating the oxygen content. The relationship between the
PaO2 and oxygen content is estimated by the equation7:
Oxygen content (ml/dl) =
(hemoglobin concentration [g/dl] X
hemoglobin saturation [%] X 1.3) +
(0.003 X PaO
The value 1.3 in this equation is the amount of oxygen that can combine with 1 g of human hemoglobin. It is commonly given
as a constant, but it varies among species. This equation calculates the amount of oxygen carried by the hemoglobin (hemoglobin
concentration [g/dl] X hemoglobin saturation [%] X 1.3) and the amount carried as dissolved oxygen in the plasma water (0.003
Anemic animals may have high PaO2 values but little oxygen content (capacity) because their hemoglobin concentrations are reduced. Hemoglobin saturation measured
from an arterial blood sample (SaO2) is a calculated value based on the oxyhemoglobin dissociation curve. Alternatively, hemoglobin saturation can be measured
with a pulse oximeter, and oxygen content can be estimated without blood gas analysis.
Normal PaO2 values will vary with FiO2 and can be calculated by using the alveolar gas equation. However, an estimate can be made quickly by multiplying the inspired
oxygen percentage by five. For example, when breathing room air (21% oxygen), a normal PaO2 should be around 100 mm Hg. For a patient receiving 100% oxygen, the PaO2 should be closer to 500 mm Hg. Hypoxemia (low PaO2) becomes a critical concern in most anesthetized animals when it falls below 60 mm Hg because SaO2 and oxygen content fall precipitously below this PaO2 value.7