There are various patterns of nerve stimulation that may be used. A single twitch is the simplest. When activated, the device will show one twitch of the selected muscle group. Once the NMB has been administered, the muscle twitch should diminish or be completely abolished. A more common nerve stimulation pattern is called the “Train of Four” (TOF). This pattern is more useful than a single twitch because it gives the anesthetist some qualitative information about the level of the block. Four electrical stimuli are given over two seconds, and four corresponding muscle twitches should be observed. As the degree of muscle relaxation increases, the muscle twitches will be abolished, starting with the fourth. If all four muscle twitches are abolished, it means that at least 95% of the acetylcholine receptors have been blocked (Karas and McCobb 2005). Once the neuromuscular blockade begins to wear off, the twitches should start to reappear, starting with the first. The level of muscle relaxation can be determined by comparing the height of the fourth to the first twitch.

Reversal of neuromuscular blockade

Nondepolarizing NMBs, such as atracurium, are reversible by anticholinesterase drugs. These reversal agents work by inhibiting acetylcholinesterase. This allows acetylcholine to build up in the neuromuscular junction for restoration of muscle movement. Before giving an anticholinesterase drug, it is important to wait for signs of recovery from neuromuscular blockade. Because of this, monitoring with a nerve stimulator is very helpful. At least two twitches from a TOF nerve stimulation pattern should be present before the reversal agent is given. This is important because adequate reversal may not occur if signs of recovery are not seen first. Commonly used reversal agents include neostigmine (0.02 mg/kg) and edrophonium (0.5–1 mg/kg). An anticholinesterase drug should be preceded by an anticholinergic to prevent side effects, bradycardia in particular. Glycopyrrolate (0.01 mg/kg IV) should be given about 5 minutes before reversal. After the anticholinergic drug has taken effect, the reversal agent may be given slowly over 5–10 minutes while watching for bradycardia. If an anticholinergic is contraindicated (severe heart disease), the reversal agent may be administered slowly and, if excessive bradycardia is observed, the anticholinergic may then be given. Recovery from neuromuscular blockade has occurred when the TOF has returned to pre-blockade levels and the patient is able to maintain an adequate tidal volume. A respirometer or capnograph can be used to establish whether ventilation is adequate. Only after reversal has occurred should anesthesia be discontinued. If the patient were to wake up before adequate reversal of the block, extreme anxiety could result.

Anesthesia for Electroretinogram (ERG)

For ERG, generally only light anesthesia or deep tranquilization is necessary. Propofol may be a good choice for this procedure because it can easily be titrated to effect. It is helpful that a light plane of anesthesia be maintained so that the patient will remain still but the eye may stay in an adequate position for examination. Some clinicians may want the eyes covered with a towel or drape for about 10 minutes once the patient is sedated or asleep so that the eyes have time to adjust to the dark.

Anesthesia for Extraocular Procedures

Generally, for extraocular procedures, no special considerations are needed. Analgesia should be provided as needed and a combination of opioids and NSAID pain relievers may be helpful. Even with extraocular procedures, premedication with an anticholinergic should be considered because pressure on the muscles surrounding the globe can occasionally activate the OCR. This is particularly important in pediatric and brachycephalic patients.

Anesthesia for Ocular and Intraocular Procedures

Enucleation

Enucleation is generally considered a very painful procedure. It may be difficult to maintain an adequate level of anesthesia. The increase in blood pressure caused by insufficient analgesia may also cause excessive bleeding during surgery. A pure agonist opioid, such as hydromorphone, should be included in the preanesthetic protocol. A sedative and an anticholinergic should be included as well. Normal anesthetic induction and maintenance regimes may be used. Additional analgesia may be provided in the form of a constant rate infusion (CRI) during the procedure. Fentanyl, lidocaine/ketamine, or morphine/lidocaine/ketamine (MLK) are some possible choices. The addition of an NSAID can help potentiate the analgesia provided by these drugs. Local anesthetic infusion into the eyelid before it is cut and anesthetic splashed into the orbit before closure can be helpful. The retrobulbar block is also a possible choice, though it is associated with more complications and may be difficult to perform in smaller patients. Applying an ice pack to the surgical site before the patient is awake is also helpful in reducing pain from inflammation. These techniques together can help maintain a smoother anesthesia and provide a quiet recovery with a reduced risk of bleeding and self-injury.

Conjunctival flap

The main anesthetic concerns for this procedure are reducing pain, maintaining proper eye position for surgery, maintaining or reducing intraocular pressure, and providing a smooth recovery. This procedure is performed as treatment for corneal ulceration, which is generally quite painful. The drugs chosen should provide analgesia and sedation to prevent struggling in the preanesthetic period. Drugs that increase IOP should be avoided. Ketamine and succinylcholine are the anesthetic drugs most commonly associated with increases in IOP. Neuromuscular blockade is usually necessary to maintain the eye in a central position, so ventilation support must be provided and proper monitoring of neuromuscular blockade should be available. Additional sedation in the recovery period may be necessary to prevent self-injury.

Cataract removal

Patients that present with cataracts frequently have systemic disease that may increase their anesthetic risk. Often, these patients are geriatric or diabetic and additional precautions may need to be taken. Diabetic patients should have their blood glucose (BG) checked frequently in the perianesthetic period. Generally, a BG should be taken at the beginning of surgery and then every hour during the procedure. These patients should be given drugs that will not prolong recovery so that they will be awake and able to get back on their usual eating schedule. Insulin and dextrose can be given as needed. Geriatric patients may have heart or renal disease, and the anesthetic protocol will have to be adapted to whatever condition is present. Patients presenting for cataract removal also will require the use of NMB drugs to maintain the eye in a central position, so ventilation and proper monitoring must be provided. In addition, intraocular procedures, such as cataract removal, require dilation of the pupil. Opioid pure agonists cause miosis (constriction of the pupil) in dogs and may be contraindicated preoperatively. It is important to discuss with the surgeon his or her preference about the preoperative administration of opioids.

Positioning

Many ocular and intraocular procedures require positioning in dorsal recumbency for surgery. This can be a problem because, while the patient is on its back, the head and nose must be positioned toward the chest so that the surgeon has access to the eye. Figure 19.2 shows a patient in position for a conjunctival flap. This head position can cause normal endotracheal tubes to become kinked and cause an occlusion of the airway. Signs of airway occlusion include increased respiratory effort or apnea, difficulty in providing positive pressure ventilation, lack of chest wall motion, lack of breath sounds, and decreased or absent end-tidal carbon dioxide values. The incidence of this positional complication can be greatly reduced by using a wire-reinforced (guarded or armored) endotracheal tube. These tubes have a wire coil embedded in the wall, which helps to prevent the tube from kinking and occluding. It is highly recommended that a wire-reinforced ET tube be used for ophthalmic procedures that require the neck to be pulled forward into this position.

Figure 19.2. Patient in position for conjunctival flap surgery. Note the position of the head and the potential for kinking of an unguarded endotracheal tube.

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