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Careful monitoring is the key to avoiding possible perioperative complications

Although every anaesthetic and surgical procedures carries certain risks, most can be prevented by performing proper (pre-anaesthetic) examination, using appropriate equipment and techniques and providing careful monitoring.
This article addresses the most common possible perioperative complications in veterinary dentistry patients and what to do to prevent them.

Complications related to surgical procedure

We discussed the most common possible intra and post-operative surgical complications during dental extractions (e.g., root fracture, haemorrhage, soft tissue trauma, jaw fracture, air embolism, emphysema) in our previous blog.
Especially in cats, central neurological deficits, including temporary or permanent blindness, may be a sequel of keeping the mouth of a cat open widely with a mouth gag for extended periods of time, which reduces the maxillary artery blood flow. It is therefore recommended to avoid especially spring-loaded mouth gags and use smaller (up to 30 mm) plastic mouth gags, or try to avoid using mouth gags at all. Post-anaesthetic blindness may also be related to anaesthesia procedure (e.g., hypotension, hypoxemia).

Complications related to local anaesthesia

Nerve blocks are commonly used in animals undergoing oral/dental procedures as a part of multimodal approach to analgesia/anaesthesia. Administration of nerve blocks has been shown to help reduce the amount of general anaesthetic used and the postoperative pain.

Careful technique and appropriate maximum dosage/volume of the local anaesthetic used are of the utmost importance to avoid complications.

Nerve blocks may be associated with systemic toxicity of the local anaesthetic, including life-threatening neurotoxicity and cardiotoxicity. Accidental direct intravascular injection and/or excessive dose of local anaesthetic is the most common mechanism for production of excess plasma concentrations of local anaesthetics and their adverse effects. Hence maximum total dose (in milligrams) must be calculated and aspiration must be performed prior to injection. Local anaesthetics can also be locally toxic causing nerve damage if injected intravascicularly or deposited within the nerve as the needle is withdrawn. As nerve damage may possibly be related to mechanical trauma of the needle, gentle technique and use of fine needles (27G) with short bevel, which is oriented in the same direction as the nerve fibres, is recommended. Local anaesthetic should not be injected if resistance to injection is encountered, which likely indicates nerve penetration - in such case, gentle repositioning of the needle, aspiration and application should be performed. If hematoma occurs at the site of injection, it usually resolves without complications.

Infections associated with nerve blocks appear to be extremely rare, however, aseptic technique is recommended especially when using extraoral approaches, and disinfection of the oral cavity (with e.g., 0.12 % chlorhexidine) is recommended prior to any oral/dental procedure.

With inferior alveolar nerve block one needs to be aware of the possibility to block the lingual and mylohyoid nerves, if the local anaesthetic is deposited too far away from the mandibular foramen, which may result in (temporary) desensitization of the tongue and related tongue chewing post-operatively.

Maxillary nerve block was associated with a globe penetration and subsequent need for eye enucleation in cats, hence the knowledge of the anatomy of the maxilla careful nerve block technique (infraorbital nerve block is considered preferable) are of an utmost importance.

Complications related to general anaesthesia

General anaesthesia is required for all dental procedures. It has been reported, that, overall, cats have a higher risk of complications from anaesthesia compared to dogs. Reported risk factors included higher ASA grade, old age, extremes of body weight, urgency of procedure, endotracheal intubation and intravenous fluid therapy.

The risks of general anaesthesia can be significantly reduced with meticulous pre-anaesthetic examination and preparation of the patient, good anaesthesia planning, monitoring of the life functions during anaesthesia.

Monitoring is ideally done by an experienced anaesthetist and involving monitoring of at least body temperature, blood pressure, capnography, pulse oximetry), and close observation of the patient during the recovery period.

Hypothermia is a common problem during anaesthesia (mostly in cats, small dogs and other small mammals), especially with long dental procedures. It is expected even more so in geriatric patients with underlying diseases. Hypothermia can affect function of several body systems and hence impair anaesthesia and general recovery, or even lead to peri- and post-operative complications. Body heat is dissipated with evaporation, conduction, convection and radiation – most important ways are through the anaesthetic system (especially non-rebreathing systems) and cold anaesthetic gases, by large amounts of water used in the oral cavity and cool surgical tables/rooms. Other than shivering, the most common complications associated with hypothermia are a threefold increase in morbid myocardial events, a threefold increase in the risk of surgical wound infection, and an increase in blood loss. Adverse cardiovascular events can follow perianesthetic depression of cardiac output and heart rate, which typically do not respond to parasympatholytic administration, when consequence of hypothermia. Hypothermia during the postoperative period markedly impairs thermal comfort, and physiologic stress leads to increases in heart rate, blood pressure, and oxygen consumption. Hypothermia most likely contributes to wound infection through impairment of immune function and through thermoregulatory vasoconstriction, which, in turn, diminishes oxygen delivery to surgical sites. Even mild hypothermia impairs blood clotting. Drug metabolism can be markedly decreased by hypothermia. During intravenous infusion of propofol, plasma concentration is increased by 30% in patients who are 3°C hypothermic. The pharmacodynamics and pharmacokinetics of volatile anesthetics are likewise altered. Minimum alveolar concentration (MAC) is reduced by 5% for each °C below normal.

Therefore, it is very important to prevent hypothermia from occurring as much as possible by close monitoring of the patient’s body temperature throughout the procedure (every 5-10 minutes) and after the procedure until the patient is normothermic, and provide adequate thermoregulatory support.

To avoid any long-term detrimental effects, body temperature should be maintained above 35.5°C. Special attention should also be paid on not to overheat a patient and carefully using certain heating devices not to cause burns. Wet animals in particular, are at risk of burns as wet surfaces transfer heat more readily. Direct contact with heating pad must be avoided. As an anaesthetised animal is unable to change position in response to excessive heat, anaesthesia staff must monitor the animal regularly, inspecting the surface of the animal for any signs of increased heat.

Aspiration of liquids from the oral cavity is possible, especially if animals are placed in dorsal recumbency. Hence, airway protection is needed and the animals should be endotracheally intubated, pharynx gently packed with absorbent pack (e.g., gauzes) that is changed during the procedure (and removed at the end of the procedure!) when saturated with fluid, and aspiration used at all times. Note that sublingual edema may result from pharyngeal pack packed too tightly.

Tracheal rupture has been reported to occur in cats during anaesthesia, commonly related to dental procedures, although the cause has remained undetermined. To prevent tracheal damage, head and neck should be carefully manipulated during dental procedure and endotracheal tube disconnected from the breathing system any time when changing patient’s or tube’s position. If using the stylet during intubation, special care should be employed. Endotracheal tube cuff must be carefully inflated, as its’ overinflation has been considered the most likely cause of tracheal rupture with ruptures being of a greater length when high-volume low-pressure cuff was used. The cuff should be carefully inflated also to avoid pressure on the mucosal blood flow in the trachea. After initial light inflation of the cuff, a test can be performed to evaluate the effectiveness of the seal by manually delivering a positive pressure breath up to 15-20 cm of water and listening for escaping gas around the cuff. If a leak is detected, the cuff can be inflated further until a seal is maintained. The cuff must be deflated before extubation.

Corneal damage can be a result of physical trauma or decreased production of tears during anaesthesia (especially if using medetomidine/ketamine and volatile agents). Eyes should be lubricated and protected from the physical trauma (cover surgical field) in order to avoid corneal damage and ulceration. Lubricant should be applied every 30 minutes throughout anaesthesia and recovery period.

If you have noted any problems with your animal, please consult your veterinarian.