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 most commonly due to poor perfusion. However, metabolic and hypoxic shock can occur with normal perfusion, so one must be careful to not rule out shock on the basis of normal perfusion parameters alone [23, 24].
For cardiovascular triage, mucous membrane color, temperature, and capillary refill time (CRT) can be used to assess perfusion. Signs of poor perfusion during mucous membrane assessment include pale pink to white mucous membranes, cool temperature, and prolonged to absent CRT (> 2 seconds). Bright pink or red mucous membranes, injected capillaries, and rapid CRT can be seen with distributive shock. However, depending on the patient's stage of cardiovascular compromise and the degree of compensation, even patients with shock can have normal mucous membrane appearance. Heart rate and rhythm should be assessed simultaneously with pulse palpation to determine pulse pressure quality and for deficits. Both femoral and dorsal metatarsal artery palpation is preferred to appreciate discrepancies in proximal and distal perfusion. Extremity temperature on limb palpation and rectal temperature should be noted to complete the patient's perfusion clinical picture [23–26].
Following physical examination of the cardiovascular system, emergency diagnostic tools that can aid cardiovascular triage include indirect blood pressure, electrocardiogram (ECG), venous or arterial blood gas, packed cell volume/total solids (PCV/TS), lactate, and left atrial to aortic root ratio on ultrasound. Indirect blood pressure methods, such as Doppler or oscillometric technologies, provide rapid noninvasive determination of arterial blood pressure. Doppler is particularly useful in small patients, cats, and those with cardiac arrhythmias. Oscillometric methods are convenient as they can be programmed to cycle at predetermined intervals, such that repeated measurements can be obtained automatically. For both methods, cuff size selection in relation to limb diameter is essential for accurate results. Cuff diameter should be approximately 40% of the limb circumference in dogs and 30% in cats. Cuffs that are too large will generate falsely low blood pressure results, and falsely high results will be obtained from a cuff that is too small [27]. In hypotensive patients, noninvasive methods have been shown to have the greatest variability compared with direct measurements [28]. Direct arterial blood pressure is considered the gold standard for blood pressure determination, and offers the additional benefits of continuous, real‐time results that are accurate with arrhythmias and decreased perfusion. However, placement of an arterial catheter is technically challenging, especially in distressed or hypotensive patients and cats, uncomfortable for the patient during placement, and requires constant monitoring to ensure the catheter is not inadvertently dislodged. ECG is helpful to evaluate for the presence of cardiac arrhythmias, which can be the primary cause of shock (cardiogenic), or secondary complications of hypovolemic, metabolic, hypoxic, or distributive shock. Venous blood gas monitoring, particularly for pH, partial pressure of carbon dioxide in venous blood (PvCO2), partial pressure of oxygen in venous blood (PvO2), electrolytes, and base excess/deficit is important to help determine the underlying cause of cardiovascular compromise, assess cellular oxygen delivery and metabolism, and response to therapy. Similarly, PCV/TS are essential to evaluate for blood and/or protein loss, dehydration, and appropriate hemodilution response if fluid therapy is used. Lactate can be a marker of anaerobic metabolism and is often increased in shock patients (type A lactic acidosis), although less reliably in cats. It can support clinical assessment of poor perfusion and trended over time with treatment of the primary cardiovascular disturbance. It has been associated with outcome in gastric dilatation and volvulus, pyometra, and immune‐mediated hemolytic anemia [29–35]. It is important to remember that type B lactic acidosis, which is hyperlactatemia in the face of normal perfusion, does not resolve with fluid therapy. Causes of type B lactic acidosis include liver failure, neoplasia (especially hematopoietic), diabetes mellitus, sepsis/systemic inflammatory response syndrome, and various drugs and toxins [30].
Comparing the left atrium diameter with the root of the aorta (LA : Ao), when using a short axis view from the right parasternum, is helpful to assess left atrial volume. The LA : Ao ratio was originally developed to support a diagnosis of congestive heart failure, but it can also be used to evaluate for left atrial volume underload, which can occur with hypovolemic shock. In dogs, the normal LA : Ao ratio is 1.3, whereas the ratio is 1.5 in cats [36, 37]. Baseline LA:Ao ratio on triage can help support a diagnosis of cardiogenic shock secondary to congestive heart failure if the LA : Ao ratio is increased. When the ratio is decreased, hypovolemic shock may be present, especially if found in conjunction
