Anesthesia and analgesia

Chapter 2


Anesthesia and analgesia




Anesthesia


A full clinical examination of the oral cavity and all oral procedures require general anesthesia (GA). In rare circumstances, e.g. a brief oral examination or taking a few radiographs, sedation may be sufficient. However, as soon as any therapy needs to be performed, the animal should be placed under GA. Oral/dental procedures range from simple procedures in young healthy patients to lengthy complicated procedures in older, systemically compromised individuals.


GA can be maintained using an inhalational or injectable technique. However, if an injectable technique is used, the airways should always be secured with an endotracheal tube to prevent aspiration of saliva, debris and irrigation fluids.


Anesthesia is an unnatural state, and the induction process always carries a risk. The degree of risk varies and this should always be explained to the owner. It is essential that the owner or, in their absence, adult agents sign a consent form for anesthesia, indicating that they are giving their consent and have understood what has been explained to them. While the anesthetic mortality rate in fit and healthy cats and dogs is 1 in 679 (0.15%), it increases to around 1 in 31 (3.2%) in animals that have a disease (Clarke and Hall 1990). In a more recent study (Dyson et al. 1998) investigating the morbidity and mortality associated with anesthesia (8087 dogs and 8702 cats), the incidences of complications were 2.1% in dogs and 0.13% in cats and the mortality rate was 0.11% in dogs and 0.1% in cats. Among other factors, continuous monitoring of anesthesia was associated with reduced mortality.


A thorough clinical examination must be performed prior to anesthesia. A full hematology and biochemistry panel is recommended for all geriatric (75–80% of the animal’s anticipated life span is completed) patients. In the elderly, there is increasing likelihood of systemic disease that may have gone unnoticed by the client. Irrespective of age, the brachycephalic breeds pose an anesthetic challenge. Anesthesia for the trauma patient also requires careful management.



General principles of anesthesia for the dental patient




Endotracheal tubes


Endotracheal tubes must be checked for defective cuffs and obstructed lumens before use. Any defective tubes should be discarded. Lightweight circuits are recommended.


To reduce apparatus dead space and the risk of endobronchial intubation, the tubing should be cut to fit the patient from midneck to the level of the incisor teeth. Excessively long tubes that protrude from the oral cavity are prone to kinking, which may lead to pulmonary oedema as the patient inspires against an obstructed airway. The use of guarded endotracheal tubes should be considered for patients at high risk of tube kinking. Moreover, excessively long tubes are difficult to secure to the jaw with gauze bandage, which increases the risk of accidental extubation. Knots should be tied around the adaptor and not around the endotracheal tube itself.


The cuff should be carefully inflated to a point where there is no air leaking around it. Be careful not to inflate the cuff excessively as this can cause tracheal injury.







Long anesthetic periods


Dental procedures are often lengthy and close attention to life support is needed:



• Oxygen should be delivered at an inspired concentration of at least 33% to compensate for the deterioration in pulmonary function that accompanies anesthesia even in healthy young patients.


• Reduced cardiac output and arterial blood pressure produced by anesthesia should be offset by intravenous fluid therapy. A catheter should be aseptically placed in an appropriate superficial vein before inducing anesthesia. Hartmann’s (lactated Ringer’s) solution should be given at a rate of 10 mL/kg per h. Catheters allow immediate venous access in an emergency and they ensure that irritant injectable agents are not given perivascularly. They should not be removed until the patient is fully recovered from anesthesia.


• Hypothermia is a complication of lengthy anesthesia and the use of cool irrigation fluids. Hypothermia results in anticholinergic-resistant bradycardia, reduced cardiac output and hemoconcentration. Cardiac fibrillation can occur at a body temperature of around 28°C. Moreover, requirements for anesthetic agents are reduced during hypothermia and care should be taken to prevent relative overdose. Body temperature should be monitored during dental procedures and the development of hypothermia should be prevented by supplying external heat by blankets and warmed intravenous and irrigation fluids. Patients should be insulated with towels or bubble pack to prevent thermal injuries due to ‘hot spots’ that may occur with electrical heating mats. Circulating warm water mats may be safer.


• Hyperthermia can occasionally occur in large heavy-coated dogs connected to rebreathing circuits for long periods. Active cooling must then be initiated before damage occurs to vital organs.



Hemorrhage


The conditions covered in this book rarely result in extensive hemorrhage, unless the patient has an underlying disorder, e.g. coagulopathy, septicemia. A full hematologic examination and clotting profile should be performed prior to any potentially hemorrhagic procedure. The patient should also be cross-matched with a healthy donor prior to any such procedure. An alternative to cross-matching is autologous transfusion, where a week before surgery 10% of the patient’s blood volume is removed and replaced with intravenous fluids. The blood is stored at 4°C in acid citrate–dextrose or citrate–phosphate–dextrose transfusion packs until required.


During the procedure, blood loss should be estimated either by weighing blood-soaked swabs or by measuring the amount of blood collected in a suction jar. As a rough guide, a saturated 3 × 3 inch swab contains 7 mL of blood and a saturated 4 × 4 inch swab contains 10 mL of blood.


The normal patient can compensate for a blood loss of up to 20% of circulating volume. A dog’s blood volume is 80–90 mL/kg and a cat’s blood volume is 60–70 mL/kg. To compensate for hypotension, intravenous isotonic crystalloid fluid infusion should be increased to 30–40 mL/kg per h. Colloids can be used (up to 20 mL/kg) to maintain tissue perfusion but they are not a replacement for red blood cells. As the blood loss approaches 20% of circulating volume, fluid replacement therapy with blood should begin. Donor blood should be given at the same rate as patient blood is lost.




Patient monitoring


All patients should be monitored continuously. Careful monitoring should enable the detection of problems before they become severe, so that they can be treated appropriately and crises can be avoided. Continuous anesthetic monitoring is associated with reduced mortality (Dyson et al. 1998).


Routine anesthetic monitoring includes inspection of respiratory function and the color of the mucous membranes, capillary refill time, listening to the sound of breathing and palpation of the peripheral pulse. This basic monitoring can be augmented with mechanical aids which give additional information and allow a more precise picture of the patient’s status. This allows closer control over the course of the anesthetic. The disadvantage of mechanical monitoring devices is that they in turn must be monitored to ensure that the information they are giving is accurate. Unexpected readings should be verified by examination of the patient before they are acted on, i.e. monitor the patient, not the equipment (Box 2.1)!



Box 2.1   Anesthesia and monitoring checklist




• Endotracheal tube is correctly positioned and the cuff is not overinflated


• Endotracheal tube is securely fastened and not kinked


• Accidental extubation or circuit disconnection has not occurred (apnea alarms and capnograms are useful for detecting accidental disconnection)


• Monitor the central nervous system (ocular signs and muscle tone will indicate the depth of anesthesia)


• Monitor the cardiovascular system (pulse quality, auscultation of heart sounds, mucous membrane color and capillary refill). Monitoring devices that aid clinical assessment of cardiovascular function include esophageal stethoscopes, blood pressure monitors and ECG


• Monitor the respiratory system (tidal volume assessment by observing the rebreathing bag and chest wall excursions, respiratory rate, and mucous membrane color). Monitoring devices include apnea alarms and pulse oximeters


• Monitor and record body temperature (rectal or esophageal)


• Monitor renal function (a urinary catheter connected to an empty intravenous fluid bag via an administration set can measure urine output and thus give an indication of organ perfusion)


• Estimate blood loss and take appropriate measures


• Replace saturated pharyngeal packs


• Release mouth gags at regular intervals


• Reapply eye ointment as required.



Geriatric patients


Many of the patients that require dental procedures are geriatric. It must be remembered that even clinically healthy geriatric patients have physiologic changes in the cardiopulmonary system that can influence the course of anesthesia. Important age-related changes include:



A noticeable decrease in circulation time is seen during induction, and further increments of injectable anesthetic agents should not be given too soon.


In addition to the age-related physiologic changes, elderly patients also have psychologic requirements in that they are easily distressed and confused by changes in routine and require gentle handling and constant reassurance.



Brachycephalic patients


In brachycephalic patients, upper airway obstruction should be anticipated. The degree of obstruction, assessed from clinical history and physical examination, needs to be determined prior to anesthesia and surgery. Chronic severe upper airway obstruction eventually results in cor pulmonale, and evidence for this should be checked.


Brachycephalic patients pose a challenge at both induction and recovery. Induction of anesthesia causes relaxation of pharyngeal musculature, and the degree of upper airway obstruction is increased until endotracheal intubation is performed. The ideal is rapid induction and rapid expert endotracheal intubation to shorten the period of increased upper airway obstruction. Mild sedation with low doses of acepromazine and an opioid, e.g. buprenorphine, is adequate in dogs. Boxers are prone to vasovagal syncope with acepromazine and should receive an anticholinergic if acepromazine is used. Alternatively, it should be avoided. Preoxygenation by mask for 5 min, if the animal will allow it, helps prevent hypoxia during induction, but mask induction using an inhalational agent should be avoided.


Airway obstruction during recovery can be dealt with in two ways. First, using an induction agent with a short plasma half-life, e.g. propofol, will ensure a rapid recovery and return of the patient’s ability to maintain its own airway. Isoflurane or sevoflurane provides more rapid recoveries than halothane. Second, the use of an opioid with potent antitussive action, e.g. butorphanol, morphine or oxymorphone, can be used to allow tolerance of the endotracheal tube for a prolonged period. The endotracheal tube should be left in place for as long as possible. Ideally, the animal should be able to sit up or even stand before the endotracheal tube is removed.


Once the endotracheal tube is removed, there is still a risk of obstruction until the patient is fully awake. It is wise to have a small dose of an induction agent available so that reintubation can be performed rapidly if required. Continued oxygenation via a nasal catheter to prevent hypoxia following removal of the endotracheal tube is prudent. The patient’s tongue should be pulled forwards to alleviate obstruction and the mouth kept open to encourage mouth breathing. Recovery in sternal recumbency is ideal as it allows more uniform expansion of the lungs and may promote a more rapid return to consciousness.




Analgesia


Humans can express and describe the sensations of discomfort and/or pain that they experience, and these descriptions are well accepted. Assessment of pain in animals is much more difficult. One must rely on overt signs and the correct interpretation of these signs. Animals probably have no psychologic expectation of pain, so the confounding influence of anticipation is removed. Changed responsiveness to human contact is often a first indicator that the animal is in discomfort. Aggression or avoidance of human contact may occur, but some animals seek excessive human reassurance. Disturbance in the sleep pattern, with an animal sleeping less, is also an indicator of discomfort. Reduced grooming and changes in eating behaviour are often manifestations of chronic pain.


In the presence of oral/dental disease it is rare for the animal to stop eating, instead they change their food preferences (e.g. an animal will selectively only eat soft food) or change the way they chew (e.g. chew selectively on one side). A common feedback from clients after their pet has undergone a remedial dental procedure is that the animal is brighter in general, often showing more interest in exercise and games than prior to treatment. One can speculate that this commonly reported change in general behaviour is attributable to the removal of chronic discomfort and pain.


In human dentistry, there is a good understanding of which disease processes cause discomfort and pain. We also know which procedures are associated with postoperative pain. It seems reasonable to assume that dogs and cats experience discomfort and pain when afflicted by the same diseases and after receiving similar treatment. In following this line of reasoning, overtreatment with analgesics may occur, but the adverse consequences of this are minimal compared with the distress of withholding pain relief.


Common conditions that we know are likely to cause discomfort and/or pain in people, and are thus likely to cause similar sensations to an affected animal, include:


Related posts:

  1. Preventive dentistry
  2. Anatomy of the teeth and periodontium
  3. Dental diseases in lagomorphs and rodents
  4. Tooth extraction

Stay updated, free articles. Join our Telegram channel
Tags:
Oct 9, 2016 | Posted by in GENERAL | Comments Off on Anesthesia and analgesia

Full access? Get Clinical Tree
Get Clinical Tree app for offline access