8/12/2019 veterynary anestehesia- anesthesia of the cat KETA.pdf

1/1

Anaesthesia of the cat Chapter

519

16administration of an 2-antagonist (see earlier for doserates).

Low to medium dose rates of ketamine can be used in

combination with other drugs for premedication andinduction of anaesthesia prior to inhalation anaesthesia.

For example, acepromazine, 0.05 mg/kg, with or withoutan opioid, plus ketamine, 45 mg/kg, IM for premedica-

tion. This is followed in 1530 minutes by induction of

anaesthesia with ketamine, 5 mg/kg, IV with or withoutdiazepam, titrated to accomplish intubation and transfer

to an inhalation agent.Ketamine is often mixed with diazepam in the same

syringe for IV administration. A commonly used calcu-lated dose rate is ketamine, 5 mg/kg, and diazepam,

0.25 mg/kg; a 1 : 1 ratio when using ketamine 100 mg/mL

and diazepam 5 mg/mL. One-half to two-thirds of thecalculated dose, depending on the premedication, is

administered rapidly IV to healthy animals and theremainder titrated in two to three increments to the

desired effect. Onset of action is slow at 4050 seconds

and is slower than the other anaesthetic agents. Theanaesthetist must be patient and allow time for full effect

before intubation, at which point jaw tone will be moder-ate, the eye in a central position in the orbit, and the

palpebral reflex will be brisk. Swallowing is a commonoccurrence during intubation and is normal for this

anaesthetic agent. Despite the appearance of swallowing,pharyngeal and laryngeal reflexes will be depressed and

cats are still at risk for pulmonary aspiration of fluid

during ketamine anaesthesia in the absence of endotra-

cheal intubation.A combination of xylazine, 1 mg/kg, with ketamine,

10 mg/kg, administered together IM has been in use formany years to provide 30 minutes of anaesthesia for

ovariohysterectomy. Occasionally, apnoea and cyanosisensue requiring administration of O2and artificial ventila-

tion. A study monitoring cardiopulmonary function after

administration of this combination measured significantdecreases in cardiac output, HR and MAP at 5 minutes

after xylazine and ketamine administration that continuedthroughout the 150-minute monitoring period, but no

changes in arterial pH and blood gases (Allen et al., 1986).

Fatal pulmonary oedema has been associated with keta-mine anaesthesia (Trim CM, recipient of personal com-

munications, Van der Linde-Sipman et al., 1992). Thepresence of fluid or froth in the endotracheal tube during

anaesthesia or laboured breathing with crackles duringthoracic auscultation developing hours after anaesthesia

are common signs. Aggressive treatment with O2and furo-

semide are usually necessary to avoid a fatal outcome.

Tiletamine

The combination of tiletamine with zolazepam

(Telazol or Zoletil) is frequently used to produceheavy sedation or anaesthesia in cats. Administration of

intubation and delivery of the inhalant anaesthetic willcontribute to need of a higher dose of etomidate.

Ketamine

Ketamine is available as the water-soluble racemic mixture

of two isomers and the standard for veterinary use is asolution containing 100 mg/mL ketamine hydrochloride

with a preservative. Ketamine is subject to abuse and mustbe kept under locked storage and, in some countries, keta-

mine is a controlled substance requiring records of indi-vidual patient administration. Ketamine is a commonly

used anaesthetic agent in cats and can be administered IM,

IV, or SC and is absorbed through oral membranes. Acci-dental introduction into the eyes during oral administra-

tion does not cause corneal damage despite the low pH ofketamine (Macy & Siwe, 1977). Intramuscular injection

appears to be painful and the cat must be restrained toavoid sudden movement that might result in needle break-

age or only partial injection of the calculated dose. Onset

of sedation or anaesthesia after IM injection of ketamineis rapid. When the cat is left in its cage during onset of

anaesthesia, the water bowl and litter box must be removedto prevent accidental airway obstruction or inhalation of

water or litter particles.Ketamine induces catalepsy so that cats receiving keta-

mine alone have high muscle tone, rigid limbs, and spon-

taneous movement. Consequently, ketamine is usuallyadministered in combination with sedatives and opioids.

Ketamine has a wide variety of uses, such as providing

mild sedation, immobilization, anaesthesia with injecta-ble agents only, induction of anaesthesia for tracheal intu-bation before using inhalation anaesthesia, or as a

continuous low dose infusion as an adjunct to inhalation

anaesthesia or postoperatively. Many combinations have

been described using low (24 mg/kg IM or IV), medium(510 mg/kg IM or IV) or high dosage (2533 mg/kg IM)

based on the degree of CNS depression desired, the routeof administration, and the pharmacological effects of the

combination drugs (see Table 16.4). The ketamine doserate is generally low to medium when ketamine is com-

bined with medetomidine or dexmedetomidine, or when

used IV for induction of anaesthesia to be maintained withan inhalation agent. High dosage in combination with

acepromazine and butorphanol or buprenorphine hasbeen used for anaesthesia for ovariohysterectomy in

healthy cats. Recovery may be prolonged after high doserates when fluid therapy is not included because urine

formation is necessary for elimination of ketamine in cats

(Heavner & Bloedow, 1979). Since no antagonist is avail-able for ketamine, some have recommended inclusion of

medetomidine or dexmedetomidine in the combinationin order to decrease the ketamine dose rate, for example,

medetomidine, 0.08 mg/kg (80 g/kg) with ketamine 5 or

7.5 mg/kg, IM for ovariectomy (Verstegen et al., 1990).Shorter recovery time can be facilitated, if necessary, by