Читать книгу Complications in Equine Surgery - Группа авторов - Страница 281

Tourniquet ischemia results in anaerobic metabolism, decrease in pH and accumulation of extracellular lactic acid, CO₂, adenosine, potassium and ionized calcium. When the tourniquet is removed, these metabolites are released into the systemic circulation causing systemic mixed metabolic‐respiratory acidosis, hyperkalemia and hypercalcemia. The longer the ischemic period the greater the accumulation of these metabolites and the systemic acid‐base imbalances upon reperfusion. The clinical consequences of these alterations are minimal in healthy patients with normal cardiovascular status; however, in patients with limited cardiovascular reserve capacity or patients with pre‐existing acid–base imbalances, the sudden release of these metabolites may lead to clinically relevant cardiovascular effects including arrhythmias, decreased myocardial contractility, vasodilation with resultant hypotension, or even cardiovascular collapse.

When the tourniquet is released quickly, the cold pooled blood under pressure in the congested limb is rapidly released into the systemic circulation, leading to a quick transient increase in preload to the right heart [108]. This results in sudden atrial stretch, which could potentially stun the myocardium into asystole or initiate atrial fibrillation [109]. Limb reperfusion also leads to a sudden reduction of systemic vascular resistance and venous pooling, also called post‐ischemic reactive hyperemia, which results in decreased venous return and cardiac output (~18% in humans) [110].

Staggered tourniquet release (deflating the tourniquet for 30 sec and subsequently re‐inflating it to 300 mmHg for 3 min, and repeating this sequence 3 times), led to lower serum lactate concentration and CO₂ and less hypotension and bradycardia following the release compared to a standard quick removal of the tourniquet [108].

There are no published reports of fatalities as a consequence of IVRA or intravenous regional limb perfusion in horses, which indicates that it is probably a very safe technique. A mild and transient decrease in blood pressure is usually observed in the experience of the author after the release of a tourniquet in horses under general anesthesia.