The OCR is a physiologic phenomenon whereby manipulations of the eye and associated tissues or an increase in intraocular pressure results in sudden bradycardia.1 The ophthalmic branch of the trigeminal nerve (cranial nerve V) is the afferent branch of this reflex arc. It carries sensory signals from the orbit to the reticular formation within the CNS. The vagus nerve (cranial nerve X) mediates the efferent parasympathetic signaling of the OCR. The OCR has been described in humans,1–3 dogs,4,5 and other species.5,6 Inherent protective mechanisms limit the severity of vagal stimulation such as vagal escape and vagal fatigue; however, such mechanisms may fail in anesthetized dogs given that anesthetics are known to blunt at least part of the sympathetic responses.7
Heart rate and blood pressure are important determinants of cardiac output. Maintenance of heart rate and blood pressure within reference limits is fundamental during anesthesia to ensure adequate tissue perfusion and prevent organ damage secondary to tissue hypoxia. If unrecognized and left untreated, an OCR can lead to severe hypotension secondary to bradycardia and result in hypoperfusion and tissue hypoxia. An OCR may also result in asystole and death,8 which emphasizes the importance of vigilant patient monitoring during general anesthesia.
Evidence from the human and veterinary medical literature suggests that administration of anticholinergic drugs (atropine or glycopyrrolate)9,10 or RNBs with local anesthetics6 may prevent an OCR from occurring.11 In veterinary species, RNBs provide adequate perioperative analgesia and reduce the need for postoperative analgesic administration.12 The infratemporal approach to RNB has been described as one of the easiest methods to provide adequate retrobulbar cone anesthesia and reduce the need for systemic administration of neuromuscular blocking agents.13 However, RNBs have been associated with potential complications, including ineffective blockade, retrobulbar hematoma, intrathecal injection, inadvertent IV injection, respiratory depression, and, rarely, sudden death.
Although anticholinergic drugs can be administered to treat intraoperative bradycardia,14 whether these drugs should be routinely administered prior to ophthalmic surgery remains controversial in both human and veterinary medicine because such drugs have a wide range of effects with the potential to cause important adverse effects.2,15 Anticholinergic drugs can cause severe tachycardia in dogs,16 resulting in an increase in myocardial oxygen demand and decrease in coronary perfusion, potentially leading to myocardial ischemia and hypoxia. These drugs can also cause gastrointestinal ileus and decrease lower esophageal sphincter tone, thereby increasing the risk of regurgitation and aspiration pneumonia.17
To the authors’ knowledge, the prevalence of an OCR occurring or the effectiveness of anticholinergic drugs or RNBs in preventing an OCR from occurring in dogs during enucleation has not been reported. The aim of the retrospective study reported here was to determine the prevalence of and covariates associated with an OCR occurring in a heterogeneous population of dogs during enucleation. The hypothesis was that preoperative administration (vs no administration) of RNBs or anticholinergic drugs would be associated with a lower prevalence of OCR in dogs undergoing enucleation.
No third-party funding or support was received in connection with this study or the writing or publication of the manuscript. The authors declare that there were no conflicts of interest.
Retrobulbar nerve block
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