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Definition

Blood pressure values from calm, unsedated horses range from 120–140, 80–100, and 100–120 mmHg for systolic, diastolic, and mean arterial pressure, respectively [29]. A target mean arterial blood pressure between 70 and 90 mmHg is suggested for anesthetized horses, depending on the horse’s size and corresponding muscle mass, padding, and anticipated duration of anesthesia. In foals, blood pressure may be maintained at lower values (mean arterial blood pressure between 50 and 65 mmHg) depending on their age and size, in keeping with values considered normal for them [30, 31].

Risk factors

 Use of inhalant anesthetics, especially in the absence of inotropic support

 Patients with systemic disease or compromise (e.g. endotoxemia, hypovolemia)

Pathogenesis

Hypotension is a common, even expected, complication with use of inhaled anesthetics in the horse as they dose dependently resulting in reduction of myocardial contractility. In medically compromised horses, inhaled anesthetics may also induce vasodilation as has been reported in human beings [32]. Data from horses in which inhalation anesthetics were administered in absence of other medications shows blood pressure and cardiac output reaching about half of normal awake values described in the horse at a surgical plane of anesthesia [33, 34].

Prevention

Ability to monitor blood pressure as well as knowledge and availability of the drugs used to support blood pressure during anesthesia maintenance are important in preventing hypotension and its consequences.

Monitoring

While indirect monitoring (e.g. cuff, Doppler) may be used for short procedures in healthy horses with anesthesia times of less than 1 hour, direct arterial blood pressure monitoring is generally recommended for inhalation anesthesia. Arterial catheters are commonly placed percutaneously in the facial, transverse facial, or dorsal metatarsal arteries.

Treatment

The positive inotrope dobutamine is ideal for treatment of hypotension as it counters the decrease in cardiac contractility caused by the inhalation agents and improves both cardiac output and blood pressure. Dobutamine also increases intramuscular blood flow in both the dependent and non‐dependent limbs of anesthetized horses [35]. Dobutamine is used as a constant rate infusion due to its short duration of action.

Ephedrine also increases blood pressure, cardiac output, and muscle blood flow in horses [36]. Due to its relatively longer duration of action, it is given as an intravenous bolus for the treatment of hypotension. The anesthetist should be aware that ephedrine is a central nervous system stimulant and its provision may result in a lightened plane of anesthesia. Tachyphylaxis (progressively less drug effect with subsequent dosing) is also seen, as ephedrine’s mechanism of action involves release of stores of endogenous catecholamines that eventually become depleted [37].

In patients with volume depletion, replacement of volume (e.g. crystalloid or colloid bolus, blood transfusion) should be attempted. Foals with anesthetic‐induced hypotension will also often respond positively to a crystalloid fluid bolus (5–10 ml/kg). It may be challenging to provide the appropriate volume rapidly in an adult horse, and fluid pumps can aid in providing large volume replacement.

Vasoconstrictive drugs such as norepinephrine, phenylephrine, or vasopressin may also be used if the cause of hypotension is deemed to be related to inappropriate decreases in systemic vascular resistance (e.g. as seen with endotoxemia).

The use of concurrently administered medications such as the alpha‐2 adrenergic agonist drugs can help improve blood pressure due to their effects on vascular smooth muscle receptors. Following a single dose of intravenously administered drug in both the standing and anesthetized horse, the duration of this vasoconstrictive effect is drug dependent [38, 39], but when given by a constant rate infusion, the effect is sustained with all these drugs [40–42]. Whether used as part of an injectable [40] or inhalation [43] protocol, heart rate is likely to decrease with a corresponding decrease in cardiac output; second‐degree heart block, sinus pauses, and occasionally ventricular escape beats may also be evident following administration of alpha‐2 agonist drugs. This effect is most notable after high‐dose intravenous administration. The consequence of a significant decrease in cardiac output in the face of increased vascular resistance (and thus normal blood pressure) on organ function has not been fully elucidated for the horse.

Expected outcome

Hypotension during anesthesia can range from mild and short‐lived to prolonged and life‐threatening. Horses that experience sustained hypotension are at risk for end‐organ dysfunction as a result of poor perfusion.

Documentation of the deleterious consequences of hypotension are available in humans, where low mean arterial blood pressure (<55 mmHg) for as little as 10–20 minutes during anesthesia is associated with an increased risk of acute kidney injury, myocardial damage, and 30‐day mortality [44]. Although data of this kind is not available in horses, it is prudent to consider that the same physiological consequences are possible and thus hypotension should be treated.

Specific to horses, an early landmark experimental study showed that myopathy associated with hypotension (mean blood pressure of 55–65 mmHg for over 3 hours) during inhalation anesthesia in horses contributes negatively to recovery from anesthesia and survival [45]. In addition, a long duration of hypotension poses a greater risk for the development of myopathy in clinical cases [46]. Since this time, the widespread use of the positive inotrope dobutamine to maintain mean arterial pressure above 70 mmHg has significantly reduced the severity of post‐anesthetic myopathy cases [47].

Additionally, it has been shown that (direct) blood pressure monitoring reduces the risk of cardiac arrest death in horses [58, 49], possibly because hemodynamic problems are detected and corrected earlier in their time course.