Small Animal Surgical Emergencies. Группа авторов
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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 (FiO2) to the partial pressure of oxygen in arterial blood to FiO2 (PaO2/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 PaO2/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.
Box 1.1 Formula Used for A–a Gradient Calculation
The partial pressure of alveolar–arterial oxygen (P(A–a)O2) gradient provides objective information on pulmonary function by removing the influence of ventilation on PaO2.
PAO2 = alveolar gas equation
FiO2 = percentage of inspired O2
Patm = atmospheric pressure (760 mmHg used at sea level)
PH2O = water vapor pressure (53 mmHg 39°C for dogs/cats; 47 mmHg at 37°C in humans)
R = respiratory quotient (approximately 0.8–0.9)
Example Blood Gas
PaCO2 = 24.2
PaO2 = 59.5
PAO2 = (0.21 × (760 – 53)) – (24.2/0.9)
PAO2 = 121.6
P(A–a)O2 = 121.6–59.5
P(A–a)O2 = 62.1 (indicates hypoxemia is due to pulmonary dysfunction)
Thoracic ultrasound, also known as thoracic focused assessment with sonography for trauma, triage, and tracking (TFAST), allows clinicians to assess for pleural and pericardial effusion, pneumothorax, and pulmonary parenchymal infiltrates [15–20]. It is particularly useful in patients that are not stable enough for thoracic radiographs, as well as a monitoring tool to assess for response to therapy. Thoracic ultrasound may be performed with the patient in sternal or lateral recumbency. Pleural effusion is generally visible in the cranial and/or caudoventral pleural space. Ultrasound guidance to localized fluid pockets can be helpful to guide thoracocentesis. When evaluating for the presence of pneumothorax, the caudodorsal thorax is evaluated for the lack of a “glide” sign, which is diagnostic for pneumothorax. A glide sign is created by the normal back and forth respiratory motion of the interface between the visceral and parietal pleura (Video 1.1). Free air in the thoracic cavity obliterates the glide sign [15–17]. Cellular or fluid infiltrate into the pulmonary parenchyma, as with edema, hemorrhage, and pneumonia can be assessed using ultrasound in four windows in each hemithorax (caudodorsal, cranial, middle lung lobe regions, and perihilar) for the presence of increased penetration of ultrasound, which manifest as hyperechoic lines (B‐lines) in parallel with the ultrasound beam, that can be individual or coalescing (Figure 1.4 and Video 1.2) [18–22].
Figure 1.4 TFAST ultrasonographic appearance (still image) of a B‐line, which is created by increased infiltrates in the pulmonary parenchyma allowing ultrasound penetration.
Cardiovascular Assessment
The most important part of the cardiovascular assessment during emergency patient triage is the determination whether the patient is in shock. If shock is suspected, the type of shock and need for fluid therapy must then be determined. The common feature in all shock patients is inadequate cellular energy metabolism, which is