Читать книгу Small Animal Surgical Emergencies - Группа авторов - Страница 19

Hypoxemic shock occurs secondary to decreased arterial blood oxygen content. Common causes of hypoxemic shock include pulmonary parenchymal disease, such as pneumonia, severe anemia, and hypoventilation (Figure 1.2). Many veterinary patients in hypoxemic shock are at the limits of their physiologic reserves, and are intolerant of excessive handling, restraint, and manipulation; they should be handled carefully. Clinical signs include weakness, mental depression, pale mucous membranes, dyspnea, crackles or increased bronchovesicular lung sounds, or decreased lung sounds ventrally (pleural effusion) or dorsally (pneumothorax), and cyanosis. Patients with diaphragmatic hernia may have decreased lung sounds dorsally or ventrally. Cyanosis is only seen with severe hypoxemia (at least 5 g/dl of deoxygenated hemoglobin), and thus the absence of cyanosis absolutely does not rule out hypoxemia. In anemic animals, cyanosis is unlikely to be detected due to decreased hemoglobin concentration, and therefore should not be relied upon to diagnose hypoxemia [8].

Figure 1.2 Lateral thoracic radiograph showing cranioventral pulmonary infiltrates creating an alveolar pattern consistent with aspiration pneumonia.

In any patient with suspected hypoxemic shock, supplemental oxygen should be provided until the ability to adequately oxygenate is confirmed. Diagnostics that can be helpful for the patient in hypoxemic shock include pulse oximetry (peripheral capillary oxygen saturation, SpO₂), arterial blood gas analysis, thoracic radiographs, thoracic/trauma computed tomography (CT), and thoracic ultrasound. SpO₂ may be less effective with bright lighting, poor perfusion, high motion, and pigmentation of the skin. It is convenient since it can noninvasively determine percentage of oxygenated hemoglobin, and, for sedentary patients, can be left in place for continuous monitoring (Figure 1.3). Many patients in respiratory distress will not tolerate the restraint necessary for arterial blood gas collection and thoracic radiographs, especially on presentation. If obtaining an arterial blood gas is feasible, findings may include decreased SpO₂, decreased partial pressure of carbon dioxide (PaCO₂) consistent with hyperventilation, increased PaCO₂ consistent with hypoventilation, decreased partial pressure of oxygen (PaO₂) consistent with hypoxemia, and an increased partial pressure of alveolar–arterial oxygen gradient P(A–a)O₂). Calculation of the P(A–a)O₂ gradient provides objective information on pulmonary function by removing the influence of ventilation on PaO₂. When a patient is breathing 21% oxygen, the P(A–a)O₂ should be less than 10–15 mmHg. When a patient is breathing 100% oxygen, the P(A–a)O₂ should be less than 150 mmHg. If the P(A–a)O₂ gradient is greater than 15 mmHg while breathing 21% oxygen, it is consistent with pulmonary dysfunction. For A–a gradient calculation, see the formula in Box 1.1.

Figure 1.3 Continuous pulse oximetry assessment in a laterally recumbent dog receiving oxygen supplementation via nasal prongs.

Preliminary evaluation of the ratio of SpO₂ to fraction of inspired oxygen (F_iO₂) to the partial pressure of oxygen in arterial blood to FiO₂ (P_aO₂/FiO₂) showed good correlation between the two values in dogs. It is possible that with further investigation, the SpO₂/FiO₂ may become a reliable, less invasive alternative to determining P_aO₂/FiO₂ [9].

Thoracic radiographs may show pulmonary parenchymal infiltrates ventrally consistent with pneumonia, caudodorsally consistent with non‐cardiogenic pulmonary edema, and in the perihilar region consistent with congestive heart failure. In the trauma patient, pulmonary contusions, which can be present in any lung field(s), may not become radiographically apparent for up to 48 hours, although peak opacification has been shown to occur at 6 hours in human trauma patients [10]. Additionally, up to 30% of human trauma patients do not have radiographic evidence of contusions on initial thoracic radiographs, which is why CT is often proposed as the preferred method of thoracic imaging [10–12]. In a study of dogs that had succumb to vehicular trauma, thoracic radiographs underestimated the presence of contusions, while also overestimating their severity. The same study also noted that thoracic radiographs were less sensitive than CT for detecting rib fractures [13]. Initial investigation in the use of thoracic ultrasound for detection of pulmonary contusions in dogs with vehicular trauma showed a high sensitivity for diagnosing contusions compared with CT, and even noted improved sensitivity compared with thoracic radiographs [14]. Therefore, cautious respiratory monitoring and repeat thoracic imaging may be indicated in any patient with a history of known or suspected trauma.