complications “Vascular puncture” earlier in the chapter.
Local and systemic effects of tourniquet ischemia
Definition
During tourniquet application, there is no blood flow to the limb, resulting in ischemia. During this time, there are local changes from the anaerobic metabolism and upon release of the tourniquet the pooled blood and metabolites from the ischemic limb are released into the systemic circulation. The extreme situation would be what is called in human medicine “rescue cardioplegia,” which consists of myocardial stunning (stopping) that can occur immediately following the release of a compressing force, harness or tourniquet [108].
Risk factors
Long tourniquet application time
Decreased cardiovascular reserve (e.g. geriatric, cardiac disease, general anesthesia)
Pre‐existing acid–base imbalances
Quick tourniquet release
Pathogenesis
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.
Prevention
The absolute safe limit of tourniquet duration has not been established and may depend on location and vary from animal to animal. The usual clinical recommendation is to limit the time of tourniquet to 2 hours, although in the author’s clinical experience this time has sometimes been exceeded with no negative consequences. In experimental rhesus monkeys, the systemic changes produced as a result of the application of a tourniquet for periods of up to 3 hours were not marked and readily reversible, and the local acid–base changes in the ischemic limb recovered in less than 40 min post‐release [111]. However, it should be noted that these were healthy animals, and shorter tourniquet times are recommended in debilitated animals.
It is recommended to use a staggered tourniquet release, which will both avoid a sudden release of local anesthetic into the systemic circulation and will reduce the cardiovascular consequences of limb reperfusion.
Diagnosis
The clinical manifestations of tourniquet release may include hypotension, brady‐ or tachy‐cardia, arrhyhtmias, tachypnea, and in extreme cases cardiovascular collapse or cardiac arrest. Close monitoring of the cardiovascular system is therefore recommended during and at least 30–40 min following tourniquet release.
Treatment
In healthy animals and when the cardiovascular changes are mild, no treatment is necessary. In cases of severe hypotension or cardiovascular collapse, supportive treatment with intravenous fluids, positive inotropic drugs (e.g. dobutamine) and/or vasoconstrictors (e.g. phenylephrine) may be necessary. Also, a venous blood sample should be obtained to check acid–base balance and serum electrolytes, and treat derangements (e.g. hyperkalemia, hypercalcemia) as necessary.
Expected outcome
The outcome is good if the cardiovascular effects are mild, but it could be fatal if cardiovascular collapse or cardiac arrest occur.
References
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