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Considerations for anesthesia in the ophthalmic patient (Proceedings)
There are four main goals for anesthesia in the ophthalmic patient.
The goals for anesthesia in the ophthalmic patient include providing appropriate analgesia for the particular procedure or condition, maintaining or lowering intraocular pressure (IOP), preventing activation of the oculo-cardiac reflex (OCR), and providing a level of anesthesia that will allow surgical manipulation of eye and surrounding structures while maintaining normal cardiovascular function
Intra-ocular pressure (IOP)
Normal IOP in the dog ranges from 10-26 mmHg. In the cat it ranges from 12-32 mmHg. Significant increases in intraocular pressure can lead to globe prolapse or rupture, damage to the optic nerve, or retinal detachment. There are many factors that can affect IOP in the peri-anesthetic period. Restraint, neck leads, jugular compression for blood draws, coughing, gagging or vomiting can result in increased IOP. Endotracheal intubation, as well as many anesthetic drugs can also cause increased pressure. Even physiologic factors such as sudden increases in blood pressure and hypercapnia from hypoventilation can increase IOP and potentially lead to damage or blindness.
The oculo-cardiac reflex (OCR)
The oculo-cardiac reflex or OCR refers to a sudden drop in heart rate associated with traction on the eye and/or surrounding structures. Traction on the eye puts pressure on the trigeminal / vagus nerves of the parasympathetic nervous system leading to bradycardia and brady-arrhythmias such as atrioventicular block or sinus arrest. Certain patients are more likely to have this drop in heart rate than others. Pediatric/neonatal patients and brachycephalic patients tend to have high vagal tone and are pre-disposed to activation of the OCR. Anti-cholinergic drugs, such as glycopyrrolate, can be administered as needed or as part of the premedication to prevent or treat this physiologic response.
The goals of premedication in the ophthalmic patient are to minimize stress, to decrease or maintain IOP and to prevent stimulation of the OCR. Sedation can be provided by a tranquilizer such as acepromazine in many patients. This drug provides excellent anxiolysis and can lower blood pressure. This drug should be used with care in pediatric or geriatric patients and avoided in patients with hepatic disease. In some patients a benzodiazepine tranquilizer, such as midazolam or diazepam, is preferable. Sedation is generally adequate in patients that are debilitated in some way. Analgesia is often provided by the use of opioid drugs as they provide excellent pain control as well as sedation. One drawback to opioid use is that full agonist opioid drugs (morphine, hydromorphone) can cause vomiting after administration and thus can increase IOP. The pre-emptive administration an anti-cholinergic drug should be considered based on the patient and the preferences of the clinician and/or anesthetist. An anticholinergic drug should always at least be available and can be given as needed.
Induction of patients with intraocular disease or those that are undergoing intraocular surgery can be achieved with thiopental, propofol or etomidate depending on the needs of the individual. Ketamine and ketamine combinations should be avoided since this drug increases IOP. Mask and box inductions should also be avoided since excess struggling often occurs. A patient undergoing extraocular surgery where IOP is of less concern may receive any standard induction drug or drug combo and ketamine may actually be helpful as it provides additional analgesia for painful surgeries.
Maintenance of anesthesia is usually with volatile inhalant anesthetics such as isoflurane or sevoflurane. Nitrous oxide can also be used but is not recommended for intraocular procedures as nitrous oxide diffuses into closed, air-filled spaces and can increase IOP if air bubbles are present. Injectable agents may also be used for certain procedures. Propofol can be given as a constant rate infusion or intermittently as needed. For short extra-ocular procedures patients can be sedated with alpha-2 combinations. Dexmedetomidine/butorphanol (dogs) or ketamine/ dexmedetomidine/ butorphanol (cats) can be helpful combinations.
Neuromuscular blocking drugs
Neuromuscular blocking drugs are important for anesthesia of patients undergoing intraocular surgeries and any ophthalmic surgery that requires the eye to be in a central position. Normally, at a medium anesthetic plane the eye rotates ventrally and would be out of the view of the surgeon. Administration of a neuromuscular blocker (NMB) will cause the eye to move centrally without the need of an excessively deep anesthetic plane. NMB's are also known as "paralytic" drugs or peripheral muscle relaxants. These drugs provide skeletal muscle relaxation which helps to prevent movement of the eye and keep it in an accessible position. NMB's do not provide analgesia or sedation and adequate analgesia and anesthesia are required. Reflexes that are normally used to indicate anesthetic depth (palpebral reflex, jaw tone) will not be present so signs of inadequate anesthetic plane may include tachycardia, hypertension, lacrimation, and salivation.
When NMB's are used, it is not just the muscles of the eye that are affected. The intercostal muscles and those of the diaphragm will be paralyzed as well so the patient will not have the ability to ventilate themselves. This means that endotracheal intubation and intermittent positive pressure ventilation (IPPV) must be provided.
The length of blockade provided by NMB drugs can be dependent on a variety of factors. First, the specific drug used is important. Atracurium, a commonly used NMB in veterinary medicine, has a relatively short duration as is lasts only 15-30 minutes. Pancuronium, on the other hand, can last up to 60 minutes. Temperature, acid/base balance, fluid and electrolyte balance, and co-administration of other drugs can affect the length of blockade as well. Patients that are hypothermic and/or acidotic can have a prolonged duration of blockade. Hyperthermic patients may require re-dosing more frequently.
Before administering a paralytic drug, several steps should be taken by the anesthetist. First, an appropriate level of anesthesia should be confirmed since paralyzation of an inadequately anesthetized animal can result in extreme anxiety. Analgesia should be provided if the animal is to undergo a potentially painful procedure. It is important to confirm that some form of IPPV is available whether it is provided manually or by a mechanical ventilator. This means that the patient must be endotracheally intubated with a cuffed ET tube in order to provide efficient ventilation. If an antagonist is available for the NMB drug chosen, it should be nearby. Proper anesthetic monitoring should be instituted and should include the use of a respirometer or capnograph to aid in the monitoring of ventilation. A peripheral nerve stimulator can also be used to help the anesthetist judge the level of blockade and help them determine when adequate reversal has occurred.
A peripheral nerve stimulator is a very useful monitor when using NMB drugs as part of the anesthetic protocol. The ulnar, tibial, facial, or peroneal nerves can be used but the peroneal nerve is the easiest to use in small animals. The nerve is stimulated on the lateral aspect of the stifle and a muscle twitch of the hind foot is seen. The pre-blockade twitch is then compared to the twitch after administration of the NMB drug. This can give the anesthetist some information about the level of neuromuscular blockade and helps to indicate when adequate reversal of the drug has occurred.
Antagonists, or reversal, drugs are only available for non-depolarizing NMB's due to the way these drugs provide neuromuscular blockade. These reversal drugs are also known as anti-cholinesterase drugs. These antagonist drugs work by inhibiting acetylcholinesterase at the neuromuscular junction allowing acetylcholine to build up. This leads to a return to normal muscle function. Commonly used antagonists include neostigmine (0.02mg/kg) and edrophonium (0.5-1.0mg/kg). Before the antagonist is administered there should be some signs of recovery present to prevent the occurrence of re-paralyzation. This can be assessed best with the use of the peripheral nerve stimulator. It is recommended that an anti-cholinergic drug (such as glycopyrrolate, 0.01mg/kg) be given before the anti-cholinesterase drug to prevent the muscarinic side effects of the drug, mainly bradycardia. The antagonist should then be given over 5-10 minutes. Once reversal is achieved, the anesthetic can be discontinued and oxygen administered for about 5 minutes. A capnograph or respirometer should be used to help judge the adequacy of the patient's spontaneous ventilation. The motion of the re-breathing bag and the patient's chest excursions can also be helpful in determining if the patient is breathing adequately. Ventilation should be assisted if needed and additional reversal agent can be given if necessary.
Any patient undergoing anesthesia for ophthalmic surgeries should be monitored closely in the recovery period. Anxiety and pain can increase systemic blood pressure which can lead to increased intra-ocular pressure as well as hemorrhage. Analgesics and sedatives should be readily available and administered preemptively if a rough recovery is anticipated. Self-trauma can be avoided by placement of an Elizabethan collar, providing padding and blankets in the patient's cage, and by providing a dark, stimulus-free environment. Though uncommon, after neuromuscular blockade, it is possible for re-paralyzation to occur. This can occur if an antagonist drug is given too soon (before signs of recovery from NM blockade have occurred). In recovery, respiratory rate and effort as well as mucous membrane color should be monitored for several hours post-operatively. If a problem is suspected, pulse oximetry and arterial blood gases can be useful. A decreased Spo2 and/or increased PaCO2 can indicate problems with ventilation.
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