Intracarotid Drug Injection
Robert J. MacKay • Mary O. Smith
Intracarotid drug injection is common in horses because the jugular vein and the common carotid artery are closely apposed in the caudal third of the neck.
The condition is rare in cattle because the omohyoid muscle lies between the carotid artery and the jugular vein in the posterior part of the neck.1 Hypertonic or caustic drugs (including phenothiazine tranquilizers, chloramphenicol, chloral hydrate, barbiturate anesthetics, phenylbutazone, calcium gluconate, and sodium iodide) cause cortical necrosis when injected into the carotid artery.2-5The common carotid artery provides blood supply to the circle of Willis via the internal carotid artery and thence to the thalamus and rostral and middle cerebrum. The internal carotid artery also provides arterial supply to the eyeball via the ophthalmic artery. In contrast, the blood supply to the caudal cerebrum, midbrain, pons, medulla, cerebellum, and inner ear is via the vertebrobasilar arteries and originates in the vertebral branches of the subclavian arteries. Thus the carotid artery injectate travels principally to the ipsilateral thalamus, cerebrum, and eye but relatively spares the rest of the brain. The effect of the arrival in the forebrain of a bolus of irritant material is akin to a cerebral stroke of variable severity. Intense vasospasm occurs, as does physical endothelial damage with endothelial swelling, increased vascular permeability, mural necrosis, hemorrhage, intercellular edema, and thrombosis. As a result of ischemia, glucose, oxygen, and energy starvation occur, progressing to calcium ion influx, anaerobic metabolism, and eventually loss of function and accumulation of excitotoxic amino acids, free radicals, and eicosanoids. At necropsy, ipsilateral brain swelling and hemorrhage are grossly evident.
When the drug is injected into the carotid artery, the animal recoils backward and falls over. Some horses strike or rear violently or run wildly, often in circles toward the injected side, without regard to obstacles.
After falling, many horses convulse violently, whereas others become comatose without showing excessive motor activity. Some severely affected animals die after a variable period, but others regain their footing and recover completely. Milder and residual neurologic signs include contralateral (thalamic) or ipsilateral (retinal) blindness (or both), contralateral facial hypalgesia, circling (toward the side of the injection), and contralateral conscious proprioceptive deficit.If it can be done safely, convulsing horses should be given IV anticonvulsant medications (diazepam, 0.1 to 0.4 mg/kg; pentobarbital, 2 to 10 mg/kg; or xylazine, 0.2 to 1.1 mg/kg), which can be repeated as needed to control seizures. If possible, horses should be placed in a padded stall and treated with antiinflammatories, including dexamethasone (0.05 to 1 mg/ kg IV or IM sid), flunixin (1.1 mg/kg PO or IV bid) or equivalent, and DMSO (1 g/kg IV as a 10% solution bid). The use of hypertonic antiedema drugs in this setting is controversial because of the possibility of active bleeding in the CNS and loss of the blood-brain barrier.
Intracarotid injection of drugs is best prevented by the use of large-bore needles or catheters for intravenous injections. In the horse, venipunctures should be performed in the anterior one third of the jugular furrow because the artery and vein are separated by the omohyoid muscle in this area. The inexperienced phlebotomist may prefer to place the needle into the vessel without a syringe attached to confirm that the blood is dark and drips from the hub, which indicates proper placement into the jugular vein. Conversely, intracarotid placement of the needle results in forceful, often pulsatile ejection of bright red arterial blood from the hub.