Authors: Stroe M.S., DVM, Ionașcu I. DVM PhD, Ion L., DVM

Correspondence: Stroe Marina-Stefania, DVM, Marina-Stefania.Stroe-Giurca@uliege.be

Abstract

Cataracts may occur at any age and in any location in the lens. Cataracts can block tapetal reflection and fundic examination partially or completely and are often classified by stage of maturation and cause.

Cataract surgery are facilitated by a central position of the eye ball within the palpebral fissure. A centrally positioned eye is normally achieved by using of neuromuscular blocking agents (NMBAs). NMBAs also decrease the ocular muscle tone and that is very useful because an increased tonus may cause ocular structures to become displaced and distorted and can also influence IOP. Use of these agents necessitates intermittent positive pressure ventilation (IPPV).

Objective: Offering alternatives for anesthesia to perform cataract surgery in dogs and cats without using the neuromuscular block.

The safety of anesthetic protocols consisting of midazolam, tramadol, lidocaine, propofol, fentanyl, ketamine, isoflurane without using the neuromuscular block was studied in 16 cataract surgeries in dogs and cats. The protocol’s safety was expressed by monitoring heart rate, oxygen saturation and pulse rate using pulse oximetry, respiratory rate, end-tidal carbon dioxide provided by capnography, arterial blood pressure using oscillometric method. Assessments were made for quality of induction, maintenance and recovery from anesthesia.

Animals: Sixteen animals, eleven dogs and five cats, all client-owned.

Methods: All animals were examined prior to premedication, were performed blood tests hemoleucogram and biochemistry and monitored during induction, surgery and recovery. Blood samples were analyzed for standard biochemistry panel including glucose, creatinine, ureea, hepatic transaminases and hemoleucogram. Before anesthesia, HR was measured using cardiac auscultation and MAP was measured using automated oscillometry, respectively. Protocols consisting of midazolam, tramadol, or lidocaine iv was performed. IV propofol was administered to abolish the palpebral reflex, produce jaw relaxation and facilitate ETI. Topical ocular administration of oxybuprocaine (Benoxicaine®) 0.4% drops to anesthetize cornea was performed before general anesthesia. All patients received topically sprayed laryngeal 2% lidocaine. The cough response at ETI was recorded.

After intubation, auscultation of heart and lung sounds was possible by means of an oesophageal stethoscope. Pulse oximetry was used to monitor oxygen saturation of hemoglobin in arterial blood and pulse rate. The patient was connected to the inhalational anesthesia machine. The maintenance of anesthesia was achieved using isoflurane like inhalant agent and fentanyl or mixture of fentanyl, lidocaine and ketamine. Respiratory rate and end-tidal carbon dioxide was provided by capnography. Assessments were made for quality of induction, maintenance and recovery from anesthesia by evaluation of the animal’s eye position, jaw tone, heart and respiratory rates and autonomic responses to surgical stimulation.

Results: The purpose of this work was to perform anesthesia protocols without use of the neuromuscular block for phacoemulsification in dogs and cats and make preliminary investigation into safety for patient and to record the advantages and disadvantages. Cataract surgery are facilitated by a central position of the globe within the palpebral fissure. A centrally positioned eye is normally achieved by using neuromuscular blocking agents (NMBAs). NMBAs also decrease the ocular muscle tone and that is very useful because an increased tonus may cause ocular structures to become displaced and distorted and can also influence IOP. But if there is no possibility of using NMBAs solutions must be found.

Conclusion: The aim of the project was to test several variants of anesthetic protocols to compare the various effects of molecules including lidocaine, ketamine, fentanyl, tramadol, propofol, isoflurane have on the organism.

The use of anesthetic drugs without using of neuromuscular block for cataract surgery may be challenging bringing both advantages and disadvantages. The recovery period after a classic anesthesia without neuromuscular block probably is much shorter than that achieved after a curarisation and the probability for hypotension is less likely. On the other hand, without neuromuscular blocking agents we can`t obtain the central position of the eye globe and that implicate make some compromises for the surgery.

Keywords:  cataract, anesthesia, phacoemulsification, cat, dog,

Introduction

Patients with ophthalmic disease, such as cataract, vary from young, healthy animals with congenital cataract to geriatric patients, which may have significant diseases like diabetus mellitus. When planning anesthesia for cataract surgery is important to consider the general health status because there are many patients with concurrent disease and that may present significant challenges for the anesthetist [4]. It required investigations before anesthesia like blood tests and if there are changes ideally their condition should be stabilized before anesthesia. Also need to consider that animals that are blind are more likely to be stressed and fearful compared with patients that have vision, especially if the onset of blindness was acute [4].

A complete ophthalmic examination should be performed and should include examination of PLR and menace response, Schirmmer tear test, fluorescein stain test, intraocular pressure (IOP) and a fundic examination if possible. A complete physical examination is also pertinent, as cataracts may be related to extra-ocular disease.

Electroretinography and ocular ultrasonography are standard pre-operative screening tools to confirm an eye’s candidacy for cataract surgery. Although pre-operative preparation and postoperative management can be intensive, canine cataract surgery is often successful and rewarding. Risks, time commitment, and financial demands of phacoemulsification should be discussed with the pet owner.

Materials and methods

Eleven dogs and five cats presenting to the ophthalmology service with ophtalmological conditions that cause blindness. All patients received the cataract diagnosis after a full ophthalmic examination. Once a cataract forms, surgery is the only treatment method to restore vision. Phacoemulsification uses ultrasonic energy to fragment and extract cataractous lens material from its capsular bag.  Exclusion criteria of the patients were concurrent diseases that could not be stabilized before anesthesia. Any pre-existing medical conditions and drugs administered were recorded.

Food and water were withheld from all patients for a minimum of 12 hours prior to surgery

Animals were gently restrained in a sitting or standing position for drug administration and data collection.

Mydriasis is obtained with topical mydriatic agents (Tropicamide) applied with 2-3 hours before intraocular surgery. Also, topical ocular administration of oxybupracaine (Benoxicaine®) 0.4% drops to anesthetized cornea was performed before general anesthesia. Topical local anesthetics are effective because of a direct action on the cornea and minimizing systemic side effects but their use is limited to diagnostic procedures and intra-operatively as they delay corneal healing, are epitheliotoxic and have a short duration of action [5].

The position of the animals during surgery was in lateral position for unilateral cataract and dorsal for bilateral cataract (Fig.1). HR was measured using cardiac auscultation and MAP was measured using automated oscillometry.

Anesthesia was maintained with isoflurane in a oxygen delivered via a rebreathing anesthetic circuit with the oxygen flow rate set at 60 ml/kg/min and vaporizer setting of 2%. Oxygen saturation as measured by pulse oximetry, pulse rate and respiratory rate were recorded every 5 minutes after anesthetic induction until the end of anesthesia (vaporizer turned off). Pulse quality was established by manual palpation of the femoral artery and respiratory rate was recorded by observation of the capnogram and chest movement.

Measurement of rectal and esophageal temperature was performed by use of 2 thermistor probes. Rectal temperatures were measured at initial hospital intake and after the end of anesthesia. Once each patient had been induced esophageal temperature was measured by placement of an esophageal thermistor probe and was removed at the end of anesthesia.

The premedication has been achieved with lidocaine 2 mg/kg iv or tramadol 2 mg/kg iv (Fig. 2). All the patients received the propofol-midazolam combination for anesthetic induction. The dose utilized for midazolam was 0,4 mg/kg iv.

The use of ketamine was accomplished in combination with lidocaine and fentanyl for dogs and for one cat was used the ketamine-propofol combination. There is significant interest in this combination of propofol and ketamine because has several benefits in the terms of hemodynamic stability, absence of respiratory depression, post-operative analgesia and recovery [6]. The ketamine dose that was used was low at 0,6 mg/kg iv and was mixed in the same syringe with propofol 3 mg/kg.

The extubation was performed when the coughing and swallowing reflexes had returned.

Steroidal anti-inflammatory drug (Betametazone, Diprophos®) was administered intraconjuctival at the end of the surgery.

Results

In total sixteen animals (eleven dogs and five cats) were enrolled in the project.

All patients were in good condition of general health just 2/11 dogs were stable diabetic patients and for they measurements have been taken to monitoring the blood glucose level before, during and after surgery.

Premedication with lidocaine 2 mg/kg was performed for 6 dogs and was made observation about cough during endotracheal intubation. IV lidocaine can decrease the incidence of cough during endotracheal intubation but does not appear to have a sparing effect on the dose of propofol required for endotracheal intubation.

Two patients receive tramadol 2 mg/kg iv in premedication, one in combination with lidocaine 2 mg/kg iv and the other just the tramadol. For the patient that receive just tramadol was not observed any changes in the propofol dose.

One dog received fentanyl in premedication and after induction was observed significant respiratory depression compared with the others. Two dogs and 6 cats did not receive anything for premedication.

The diabetic protocol for phacoemulsification consist in tramadol 2 mg/kg iv for premedication, induction with midazolam 0,4 mg/kg and propofol at effect. Maintenance of anesthesia has been achieved using isoflurane like inhalant agent and mixture of fentanyl, lidocaine and ketamine. The glucose level was measured before and every hour during anesthesia.

For all patients, cats and dogs, the induction was performed with propofol and midazolam 0,4 mg/kg and topical laryngeal lidocaine was used prior to intubation. One cat received the ketamine-propofol combination for induction.

The cough response at ETI was observed for 3 dogs, the patient that receive tramadol in premedication and the others that was not premedicated and 2 cats. In propofol anaesthetized dogs iv and topical laryngeal lidocaine attenuated the pressor response to ETI where iv lidocaine reduced the cough response.

Duration of the anesthesia from intubation to extubation was 80 min ±10 min depending of the surgical procedure, unilateral/bilateral cataract.

After induction, a rotation of the eyes towards the internal angle was observed. To achieve the phacoemulsification surgery, the eye was brought to the central position by means of the traction sutures.

Cardiovascular and respiratory parameters were well maintained during induction, maintenance and recovery periods for all patients. All patients receive Ringer Lactate infusion at 5 ml/kg/h. The anesthesia was maintained with isoflurane delivered via a rebreathing anesthetic circuit with the oxygen flow rate set at 60 ml/kg/min and vaporizer setting of 2%. This was completed by analgesia offered by combination of fentanyl-lidocaine-ketamine for dogs and fentanyl CRI for cats. The doses utilized for fentanyl was 4 μg/kg/h in combination with lidocaine 2 mg/kg/h and ketamine 0,6 mg/kg/h and when fentanyl was used alone, the dose was between 5-10 μg/kg/h.

Pulse oximetry was used to monitor oxygen saturation of hemoglobin in arterial blood and was maintained at >95%. MAP was measured using automated oscillometry and was stabilized at 80-110 mmHg.

Respiratory rate, end-tidal carbon dioxide was provided by capnography. The respiratory rate was maintained at 10 ± 5 rpm and the level of CO2 was 45-60 mmHg. All patients breathed themselves spontaneously, just one cat need the controlled ventilation because of the elevated level of EtCO2, up to 65 mmHg and the low respiratory rate.

For all patients the recovery from anesthesia was fast and without any complication. The temperature at the end of anesthesia was 37,2 ± 5ºC.

Discussions

The ideal anesthetic protocol for cataract surgery should provide central position of the eye, decrease the ocular muscle tone, provide analgesia and narcosis for optimal operating conditions, be safe for the patient and comfortable for the surgeon [4] (Fig. 5).

Good communication with the surgeon before the procedure and an understanding of the surgeon’s requirements are essential when formulating an anesthesia plan. The patient position with the head lower than the heart should also be avoided and at 15 degrees head-up position during intraocular surgery has been recommended in humans.

Also, the position of the animal during surgery may influence the choice of breathing system and endotracheal tube (ETT). Related to intubation should be remembered that the mouth during tracheal intubation can increase IOP as the choroid process of the mandible moves into the orbit. Care must be taken when positioning patients for tracheal intubation, as pressure may be exerted on the globe while the maxilla is held; this is especially the case for brachycephalic breeds. An armored ETT is recommended to use[4].

The ability to influence IOP is very important part of anesthesia management. Is necessary to avoid increased IOP because in these circumstances may result in a globe rupture, risk for intraocular bleeding or retinal detachment.

The use of ketamine, a dissociative anesthetic, for ophthalmologic procedures is controversial. Ketamine used alone is likely to significantly increase IOP because it causes an increase in extraocular muscle tone [4]. The good benefits of ketamine administration consist in increased of the amount of circulating norepinephrine, increase in peripheral arteriolar resistance and muscle activity and decrease the extent of redistribution hypothermia [3]. The use of ketamine has beneficial effects on the blood pressure, cardiac output, corporal temperature and contributes to realization of a balanced anesthesia based on a multimodal analgesia. On the other hand, ketamine can increase IOP but considering that in the protocols used in this study was never used alone and the fact that the surgical procedure involves making a break through the incision of the cornea and penetrating the eye globe this pressure can be adjusted naturally without becoming hazardous for the structures of the eye.

Is mandatory to avoid coughing, sneezing, vomits when there is a risk of globe rupture because this can result in an increased central venous pressure [2]. Therefore, drugs like morphine that causes vomiting should be avoided. On the other hand, the use of alpha 2 adrenergic agonist is not prohibited; although may induce vomiting especially in cats the alpha 2 adrenergic agonist can be very useful when we are dealing with uncooperative patients and the risk of globe rupture is bigger because of the stress and manipulation. In this study, for avoiding the coughing response was used lidocaine. Both iv and topical laryngeal lidocaine attenuated the pressor response to ETI and iv lidocaine 2 mg/kg reduced the cough response to ETI in propofol anaesthetized dogs [1] [2].

Intraocular blood volume is influenced by intraocular vascular tone (vasodilatation or vasoconstriction), arterial blood pressure (ABP) and outflow of the blood from the globe [4]. Is well known that exist an inverse proportional relationship between arterial carbon dioxide tension (PaCO2) and vascular tone. Increased carbon dioxide tension causes choroidal vessel vasodilatation and an increase in IOP. Hypoxaemia can be detect using pulse oximetry and should be avoided by oxygen supplementation and ventilation. PaCO2 can be monitored by capnography or arterial blood gas analysis and controlled using IPPV. However, inappropriate use of IPPV can increase CVP by increasing intrathoracic pressure during inspiration, resulting in an increase in IOP.

Cataract phacoemulsification is not a very painful procedure except during the incision and suturing of the corneal limbus. Traditionally, most anesthetic molecules mildly decrease IOP by increasing the outflow of aqueous humor. The use of anesthetic induction agents such as propofol, alfaxalone, ketamine and etomidate may all increase IOP. All are ameliorated by co-induction agents like opioids,  midazolam or diazepam [4].

One limitation to the present study was the small number of the patients (sixteen animals – eleven dogs and five cats) used.

In conclusion, if the realization of the neuromuscular block for phacoemulsification is not possible, we can perform anesthesia for this procedure using just the standard molecules like lidocaine, propofol, midazolam, fentanyl, ketamine and tramadol. The recovery period after a classic anesthesia without neuromuscular block is much shorter than that achieved after a curarisation and the probability for hypotension is less likely. On the other hand, after induction, a basculation of the eyes towards the internal angle was observed for all studied cases. In order to achieve the phacoemulsification surgery, the eye was brought to the central position by means of the traction sutures.

The great disadvantage is the fact that without neuromuscular blocking agents we can`t obtain the central position of the eye globe and that implicate make some compromises from the surgeon.

Acknowledgments

The project was provided by Di-Vet Medical – pet emergency and critical care clinic, Bucharest, Romania.