Arterial Blood Gas Analysis
Arterial samples are obtained by direct puncture of the dorsal metatarsal artery in dogs and the femoral artery in cats and small dogs. In anesthetized animals, the lingual artery can yield a sufficient sample for analysis. It is easiest to use a self‐filling, pre‐heparinized syringe to obtain an arterial blood gas. The artery is palpated and stabilized with two fingers of one hand while the syringe is firmly placed through the wall of the artery. Typically, the syringe will fill quite rapidly when the artery is entered (versus the vein), although low pulse pressures in sick animals can make this difficult to interpret. When there is uncertainty about whether or not an arterial sample has been obtained, a venous sample can be submitted for comparison. Approximately 0.5 ml blood is needed for analysis and the sample must be tightly stoppered to prevent entrance of air into the sample and stored on ice until evaluated. After withdrawal of the needle from the artery, firm pressure is applied to the vessel for 3–5 minutes. An arterial blood gas analysis measures PaO2, PaCO2, pH, total carbon dioxide (CO2), and hemoglobin saturation with oxygen, and allows calculation of bicarbonate, base excess and deficit, and oxygen content (Table 2.1).
Table 2.1 Normal blood gas values for dogs and cats.
| Dog | Cat | |
| PaO2 (mmHg) | 90 (80–105) | 100 (95–105) |
| PaCO2 (mmHg) | 37 (32–43) | 31 (26–36) |
| pH | 7.35–7.45 | 7.35–7.45 |
| HCO3 (mmol/l) | 22 (18–26) | 18 (14–22) |
HCO3, bicarbonate; PaCO2, partial pressure of carbon dioxide; PaO2, partial pressure of oxygen.
Alveolar–Arterial Oxygen Gradient and PF Ratio
The alveolar–arterial (A–a) oxygen gradient estimates the difference between the calculated alveolar oxygen level expected for the animal and the measured arterial oxygen level. Thus, the A–a gradient corrects for the level of ventilation performed by the animal and allows comparison of blood gas data through the course of disease that is not impacted by the effect of an increase or a decrease in PaCO2 on PaO2. The A–a oxygen gradient is calculated as:
where FiO2 is the fraction of inspired oxygen (0.21 on room air), PB is the barometric pressure (in mmHg), PH2O is the water vapor pressure (47 mmHg at 37 °C), and R is the respiratory quotient (ratio of CO2 production to O2 consumption, usually assigned a value between 0.8 and 1.0). PaO2 and PaCO2 are obtained from blood gas analysis. Normal value for the A–a oxygen gradient is <15.
The PaO2/FiO2 ratio (PF or oxygenation ratio) provides a measure of the ability of the lung to oxygenate as the fraction of inspired oxygen changes from room air to 100% oxygen. This is calculated by dividing arterial oxygen by FiO2 (ranging from 0.21 to 1.0). Normal animals have a PF ratio of >500 at sea level. Values between 300 and 500 indicate mild impairment of oxygenation, while values <200 indicate serious decrements in oxygenation. A PF ratio <200–300 is one of the criteria for a diagnosis of acute respiratory distress syndrome.
Causes of Hypoxemia
Obtaining an arterial blood gas, calculating the A–a gradient, and assessing response of hemoglobin saturation or PaO2 to exogenous oxygen supplementation allow assumptions to be made about the most likely mechanism causing hypoxemia (Table 2.2). This can help determine the most likely cause of hypoxemia, although ventilation/perfusion mismatch underlies the pathophysiology of hypoxemia in almost all lung diseases, and many clinical disorders have multiple contributors to hypoxemia.
Diagnostic Imaging
Radiography
Radiography is often the key to creating an appropriate list of differential diagnoses for the respiratory case and for determining the type of sampling method that is most likely to achieve a final diagnosis, such as endoscopy, fine‐needle aspiration (FNA), or a tracheal wash (Table 2.3). It will also help determine the need for advanced imaging, including fluoroscopy, ultrasound, nuclear scintigraphy, or computed tomography (CT). The widespread use of digital radiography has enhanced the evaluation of pulmonary patterns, although overlying structures can still confuse interpretation. Specific features of these tests are presented in the relevant disease sections.
Table 2.2 Respiratory causes of hypoxemia.
| Mechanism | Clinical attributes | Causes |
| Hypoventilation | High PaCO2 Normal A–a gradient Improved by oxygen supplementation Improved by increasing alveolar ventilation | Anesthesia Upper airway obstruction Neuromuscular weakness CNS disease |
| V/Q mismatch | Increased A–a gradient Mildly increased PaCO2 Markedly improved by oxygen supplementation | Virtually any lung disease |
| Shunt | Increased A–a gradient Not improved by oxygen supplementation Not improved by increasing alveolar ventilation | Congenital right to left cardiac shunts Acute respiratory distress syndrome |
| Diffusion impairment | Increased A–a gradient Seldom a major cause of hypoxemia at rest Causes hypoxemia during exercise or with low inspired oxygen Improved by oxygen supplementation | Interstitial lung disease Pulmonary edema |
| Reduced inspired oxygen | Improved by oxygen supplementation Causes hypoxemia during exercise or when diffusion is impaired | High altitude |
A–a, alveolar–arterial; CNS, central nervous system; PaO2, partial pressure of oxygen.