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 with other parameters that support hypoperfusion. Changes in the ratio with treatment, whether intravenous fluids or diuretics, can be useful for monitoring response to therapy.
The use of point of care ultrasound techniques in dogs has been used to objectively assess vascular status and may be used as an adjunct diagnostic to guide fluid and/or vasopressor therapy. In normal dogs with furosemide induced hypovolemia, the caudal vena cava to aorta ratio measured from the right intercostal space in left lateral recumbency corresponded with decreasing body weight consistent with volume loss [38]. This ratio was also decreased after blood donation in healthy dogs [39]. In greyhounds, the iliac location for this ratio did not detect volume status changes before and after blood donation [40]. While additional work is needed in canine clinical cases of hypovolemia as well as normal and critically ill cats, this technique appears to be a promising method for non‐invasive volume assessment.
Hypovolemic Shock and Fluid Therapy
Hypovolemic shock, which is the most common type of shock seen in veterinary medicine, can be due to blood loss, fluid loss, or inadequate intake, and results in decreased tissue delivery of oxygen. Clinical signs consistent with hypovolemic shock are mental depression, weakness, pale mucous membranes, prolonged CRT, tachycardia, weak peripheral pulses, cool extremities, and tachypnea.
To treat hypovolemic shock, intravascular volume replacement is essential and generally accomplished with intravenous crystalloids, colloids, blood products, or a combination of the fluid replacement options. “Shock” doses of fluid therapy are based on the blood volume for a given species, and amounts for replacement are based on the percentages of volume loss to cause cardiovascular changes secondary to hypovolemic shock. Blood volume is approximately 90 ml/kg in dogs and 45–60 ml/kg in cats. Generally, patients are given portions of their shock dose of fluids, such as 10–30 ml/kg of balanced isotonic crystalloid solutions or 5–10 ml/kg of colloid solutions as a bolus over 15–20 minutes and assessed for improvement in perfusion parameters. The bolus is repeated if indicated. Hypertonic saline (7.5%, 3–5 ml/kg IV over 15–20 minutes in dogs, 2–3 ml/kg IV over 15–20 minutes in cats) is also effective for rapid volume expansion in hypovolemic shock but should only be used in patients with normal hydration. The decision about whether crystalloids or colloids should be chosen as the initial resuscitation fluid is controversial and has yet to be determined in both human and veterinary medicine [41–47]. In veterinary patients, the decision is often based on availability, cost, and whether there are concerns about the patient's colloid osmotic pressure and the ability to maintain fluid within the intravascular space. In June 2013, a boxed warning was placed on hydroxyethyl starch (HES) solutions, such as hetastarch, due to concerns for increased mortality, severe renal injury, and risk of bleeding associated with their use in critically ill adults, including those with sepsis and admitted to the intensive care unit. VetStarch® (Abbott Laboratories, Chicago, IL), a veterinary specific HES solution, is commercially available as a synthetic colloid for plasma volume expansion. Preliminary veterinary studies have conflicting evidence on association between the use of synthetic colloids and acute kidney injury in dogs; they should be used with caution until further research is available [48, 49].
Hypovolemic resuscitation or controlled intravascular volume replacement titrated to a mean arterial blood pressure (MAP) of 60 mmHg is widely used for human and veterinary patients with hemorrhagic shock [50–54]. The goal is to preserve perfusion to the vital organs, particularly the kidneys and cerebral circulation, without supranormalizing blood pressure, to prevent disruption of any clots tempering further hemorrhage. Experimental evidence in a swine model shows that rebleeding occurs when MAP is greater than 60 mmHg, while maintaining the MAP at approximately 60 mmHg maintains renal and cerebral blood flow [54]. Recommendations for decreased volume fluid resuscitation for crystalloid boluses are between 20 and 30 ml/kg and 5 ml/kg for colloid boluses titrated to effect and target blood pressure [52].
Transfusion