Anesthesia 101 in cats and dogs
HALIFAX, NS – There are numerous anesthetic drugs available for veterinary patients, and they may be combined and delivered in various ways; no one protocol is appropriate for all patients all of the time. Regardless of the drugs selected, vigilant monitoring of the patient is imperative, explained Natalie Price, RVT, VTS (Anes) and Jen Patterson, RVT, speaking at the Atlantic Provinces Veterinary Conference, as supportive interventions need to be initiated quickly when necessary.
Selecting the type of drug
Premedications are an important component of anesthetics, individualized for each patient and procedure. They are used to reduce anxiety, produce sedation and muscle relaxation, reduce the requirement for drug induction, and to provide post-operative analgesia. The drug selection is based on the type and duration of procedure, and any patient-related factors. Non-invasive procedures, like x-rays and ultrasound, usually require only sedation and/or analgesia. Minimally invasive procedures, like dental cleaning and CT scans, usually require general anesthetic and monitoring. While the procedures are usually not painful, opioids are usually indicated to provide a balanced protocol. Invasive procedures with mild-to-moderate pain, such as castrations and oviohysterectomies, require general anesthesia and monitoring, as well as a strategy for managing mild to moderate pain. Invasive procedures with moderate-to-severe pain will require a general anesthetic and monitoring, as well as a multimodal pain management plan.
Patient-related factors include the animal species, age, health status, and temperament.
Types of drugs
Chemical interventions include anticholinergics, phenothiazines, benzodiazepines, alpha 2 agonists, and opioids. Anticholinergics (atropine, glycopyrrolate) act to prevent bradycardia, reduce salivation, reduce gastrointestinal motility and secretions, and cause bronchodilation. Phenothiazines (acepromazine) are a tranquilizer used to reduce anxiety, and benzodiazepine (diazepam, midazolam) causes muscle relaxation and anticonvulsive effects without analgesic effects. Alpha 2 agonists (dexmedetomidine) cause sedation, analgesia, and muscle relaxation in patients, and significantly decrease induction and maintenance drug requirements. Opioids (morphine, hydromorphine, fentanyl, buprenorphine, butorphanol) cause sedation and respiratory depression, and can cause nausea, vomiting, and possible excitement in cats.
Dissociative agents (ketamine) are a form of general anesthesia that cause incomplete unconsciousness. They have a minimal effect on the respiratory system, and normal pharyngeal-laryngeal reflexes are maintained.
Induction drugs (thiopental, ketamine/valium, propofol, alfaxalone) are generally titrated to effect to induce anesthesia prior to an inhalant agent.
Monitoring
An electrocardiogram (ECG) provides information regarding the patient’s electrical activity. It is normally used in conjunction with pulse oximetry, which measures the amount of oxygen in the blood (normal values are between 95-100%), and blood pressure. The speakers noted that when taking a patient’s blood pressure, the cuff should be 40% of the limb or tail circumference. Finally, capnometry provides information about respiratory rate, ventilation, and inspired and end tidal carbon dioxide. All information should be recorded after each reading.
Constant rate infusions (CRIs)
CRIs deliver a steady rate of drug to prevent the “roller coaster” effect. It is imperative that patients receiving CRIs are under constant supervision, with vitals regularly monitored. Ideal administration is via a syringe pump or fluid pump. A syringe pump is the gold standard; it ensures accurate dosing and is easy to use. A fluid pump is more accurate than a fluid bag alone, but is more time-consuming and wasteful than a syringe pump. Common CRIs are propofol, fentanyl, morphine/lidocaine/ketamine.
Regional blocks
Regional blocks are used to completely block nerve fibres, and provide a significant reduction in the need for anesthetic. Indications include declaws, small mass removals, and toe amputations. Both bupivacaine and lidocaine work by blocking the radial, ulnar, or medial nerves. Epidurals are used to create a loss of sensation caudal to the umbilicus. Morphine/bupivacaine, or lidocaine/morphine, and morphine/saline are the most commonly used drug combinations.
Dental blocks
Dental blocks (both bupivacaine and lidocaine) are used to block nerve endings reaching the teeth and oral tissues.
C-sections
A C-section may be necessary in cases of prolonged labour, hypoxia, and dystocia. Since time is of the essence, all equipment should be set up and ready to go before anesthesia is administered. Ms. Price and Patterson stressed that since any drug given will also affect the babies, great care must be taken in choosing agents, and vitals should be monitored throughout the procedure.
Pediatric patients
Pediatric patients, between 6-16 weeks old, have low body fat, an immature thermoregulatory system, and are susceptible to hypothermia. As they have a higher metabolic rate, they consume more oxygen. As well, their kidneys are less efficient so they cannot eliminate fluids efficiently. Their immature liver means prolonged drug duration, and their pulmonary reserve is small, increasing their risk for hypoxemia during apnea.
Prior to anesthesia, they should undergo a thorough physical exam, complete blood count, biochemistry profile, urinalysis, ECG, chest x-ray, and blood glucose check. Opioids are considered an excellent choice for premedication, whereas phenothiazines should be used with caution due to their hypotensive properties. Benzodiazepines cause little or no cardiovascular and respiratory depression, are reversible, and are considered to be a good sedative in young patients. For induction, propofol and alfaxalone are good options as they are rapidly eliminated from the body, and alfaxalone has minimal cardiovascular/respiratory side effects. Ketamine/valium are acceptable but the effects may be prolonged.
Geriatric patients
Geriatric patients are those over 8 years of age regardless of species, breed, or current health status. Geriatric patients tend to have decreased cardiac output causing hypotension, and decreased lung elasticity and chest wall compliance, meaning there is the potential for severe respiratory depression. Older patients also have decreased thermoregulatory function so close attention must be paid to temperature regulation. Decreased kidney and hepatic function impact the patient’s ability to concentrate urine, therefore it will take longer for drugs to clear the body.
Prior to anesthesia, a thorough physical exam and careful auscultation of the heart should be performed, along with a CBC, biochemistry profile, ECG, urinalysis, and chest x-rays. Any pre-existing abnormalities should be corrected.
Pre-medication options include opioids to provide pain control and benzodiazepines for sedation. Alpha 2 agonists are also used but they can have profound cardiovascular effects, so the benefits must outweigh the side effects. Induction drug options are propofol and alfaxalone. While propofol is rapidly eliminated from the body, there is potential for respiratory and cardiovascular depression, whereas alfaxalone has limited or no cardiovascular side effects. Ketamine and diazepam are reasonable choices as they may improve cardiac function, but clearance may be prolonged in patients with liver or kidney function issues. Similarly, thiopental can be used in low doses but may not be appropriate for patients with decreased liver or kidney function. CVT
Case study - Gunner
- 6-month-old male yellow lab
- 21 kg
- Presented for castration
- No abnormalities on history, physical examination or routine laboratory tests
- Bright, alert and crazy
- ASA I
Considerations
- Invasive procedure associated with mild to moderate pain
- Short-to-intermediate duration
- Young, healthy canine patient
Anesthetic plan
1. Premedication plan
- Opioid: hydromorphone 0.05 mg/kg IM
- Sedative: acepromazine 0.04 mg/kg IM
- Anticholinergic: not included in premed, will monitor HR and administer if necessary
2. Induction plan
- Wait 20 minutes for premed to take full effect
- Place an IV catheter
- Induction Drug(s): propofol 5 mg/kg titrated IV to effect
3. Maintenance plan
- Intubate with a 9.5mm cuffed endotracheal tube
- Maintenance Drug(s): isoflurane and oxygen via a circle breathing system
4. Monitoring and supportive care
- ± IV crystalloids
- Warm water circulating blanket
- Monitor HR, RR, ABP, SpO2 and ETCO2
5. Intra- and post-operative pain management plan
- Opioid: hydromorphone 0.05 mg/kg SC post-op
- NSAID: carprofen 4 mg/kg SC post-op
