ANALYZER OUTPUTS

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.

PaO ₂

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 ₂ )

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 X PaO₂).

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