ANESTHETIC MANAGEMENT IN SMALL ANIMALS

Dr. Satyajeet Singh

General Considerations

1. breed

2. temperament

3. physical/health status

4. purpose of anesthesia

5. familiarity with the drugs to be used

6. any concurrent medication

7. equipment and available assistance

Safe Anesthetic Practices

1. use of reversible agents whenever possible

2. endotracheal intubation should be mandatory to assure patent airway at all times

3. careful monitoring of all anesthetized patients, including the preanesthetic and recovery periods

4. preanesthetic fasting (food 8-12 hours; water 2-4 hours in small animals), except in the very small, young or debilitated

Pre-Anesthetic

A. Be sure you are familiar with the physical status of each patient 1. obtain complete history and review prior to

anesthesia 2. perform a thorough physical examination 3. an accurate weight is important particularly in small animals adjust for lean body weight 4. review laboratory data

determines need for preanesthetic stabilization directs drug selection directs further preoperative tests

Pre-Anesthetic

B. Formulate specific anesthetic plan 1. choose a protocol appropriate for the patient

2. anticipate complications that may arise based on physical examination and laboratory data, the procedure being performed, anesthetic drugs used (plan for failure)

3. calculate doses carefully

4. draw up drug, label syringes appropriately

5. prepare anesthetic record

Pre-Anesthetic

C. Place intravenous catheter 1. aseptic technique 2. place after premedication, prior to

the induction of anesthesia 3. exceptions

intractable animals that must be chemically immobilized prior to handling

restraint for catheter placement causes excessive stress and danger to the patient

short procedures in healthy patients

Pre-Anesthetic

D. Gather equipment and supplies

1. Endotracheal tube

2. Laryngoscope

3. Preparation of Anesthetic Machine

4. Assemble monitors

5. Other Equipment

Premedication

A. Selection is based on:

1. patient condition

2. patient temperament

3. procedure

4. familiarity with drugs

Premedication

B. Route of Administration

1. based on temperament

2. SC, IM, IV all possible routes

Premedication

Dogs

1. Possible Drugs/Combinations Acepromazine + Hydromorphone

Medetomidine + Hydromorphone

Midazolam + Hydromorphone

Hydromorphone (or some other opioid) alone

Premedication

Cats

1. Possible Drugs/Combinations Acepromazine + Hydromorphone (or some other

opioid)

Midazolam + Hydromorphone

Medetomidine + Hydromorphone

Medetomidine + Hydromorphone + Ketamine

Telazol

INDUCTION

A. Intravenous Agents

a. never inject rapidly

b. preanesthetics alter the required dose of inhalants, sometimes profoundly

c. most induction agents are titrated to effect

d. adjust induction dose according to the degree of sedation following premedication

Induction

thiopental

Propofol

Thiopental + propofol (1:1)

diazepam-ketamine

Telazol

Neuroleptanalgesic combinations

Induction

Specific Drugs

dose of induction drug can be reduced by administering lidocaine (1 mg/kg) or diazepam (0.2 mg/kg)

useful in depressed patients

Lidocaine helps stabilize myocardium

Induction

 Endotracheal Intubation be prepared – this should be accomplished quickly

(but don’t panic!!)

choose a range of tube sizes cats: 3.0 – 5.0 mm i.d.

dogs: 4.0 mm i.d. – 12 mm i.d.

adequate anesthesia is required prior to intubation laryngospasm can occur (especially in cats)

Induction

Procedure 2 people required animal in sternal position, lined up along its longitudinal

axis once appropriate depth of anesthesia has been reached,

pull tongue out of mouth use laryngoscope! cats

larynx is sensitive and prone to laryngospasm lidocaine spray or 0.1 ml 2% lidocaine will reduce incidence of

laryngospasm brachycephalic breeds

use a laryngoscope elongated soft palate may need to be moved dorsally to

release the epiglottis.

Checking for proper tube placement

1. check for respiratory gas condensation on the inside of the tube during inspiration

2. a correctly placed tube will often stimulate a cough reflex immediately following placement

3. watch for movement of the rebreathing bag (keeping in mind that a patient may become apneic immediately following induction)

Checking for proper tube placement

4. palpate the neck for two tubes – if you feel two tubes, you are feeling the trachea and the endotracheal tube in the esophagus.

5. auscultate the chest during assisted ventilation.

6. hemoglobin saturation (Pulse oximetry)

7. Capnography

Maintenance

General Considerations once the patient is at an appropriate

level of anesthesia, turn down vaporizer and oxygen flow rate to maintenance levels.

immediately following induction, check pulse, respiratory rate, pulse quality, capillary refill time.

start anesthetic record attach monitors start fluid administration at an

appropriate rate (5-10 ml/kg/hr)

Recovery

Continue oxygen administration as long as possible after vaporizer has been turned off

Deflate cuff and remove tube only when swallowing is observed

Watch carefully during recovery.

Recovery

Post-anesthetic monitoring should continue until the animal can maintain sternal recumbency or lift its head and until vital signs are stable

External heat source should be applied to raise body temperature to within 1 or 2 degrees of normal body temperature

Stimulating the animal will speed recovery but keep in mind that once the stimulation is stopped that the animal will likely go back to sleep

Post-operative analgesics should be administered as required.

Recovery

Sedation may be required for the animal that is experiencing a rough recovery a. acepromazine b. medetomidine

Fluid therapy should be continued in the recovery period until the animal is completely recovered from anesthesia. Some disease states will require continued fluid administration.

MAINTENANCE OF ANESTHESIA WITH INJECTABLE AGENTS used for short duration procedures or when

inhalant anesthesia is unavailable or contraindicated

achieved by a single dose for short procedures or small intermittent boluses doses of a drug or a constant rate infusion in longer duration procedures

MAINTENANCE OF ANESTHESIA WITH INJECTABLE AGENTS Specific drugs or drug combinations:

1. propofol

2. ketamine combinations

3. Ketamine/Alpha 2 Agonist Combinations

4. Telazol

5. GXK (Triple drip) in horses/cattle

References

Slatter, D. Textbook of Small Animal Surgery 3rd Edition, Volume II, 2003, Section 17, p. 2503-2623. Pain Management for the Surgical

Patient Patient Monitoring Operating Room Emergencies Anesthesia and Analgesia for the

Trauma or Shock Patient Anesthesia for the Pediatric and

Geriatric Animal

Common Anesthetic Problems

Bradycardia Tachycardia Hypotension Hypertension Blood loss Apnea Tachypnea Too Light Too Deep

Bradycardia

Remember that a heart rate defined as bradycardic is specific to a particular animal/breed. Smaller breeds are much less tolerant of lower heart rates while larger breeds don’t have a problem with a heart rate of 60.

The target heart rate for an animal should be its pre-anesthetic heart rate, keeping in mind that the pre-anesthetic rate will often be elevated because of stress/anxiety.

Bradycardia

Possible causes: opioid administration

traction on viscera

alpha 2 agonist administration

hypothermia

dobutamine/dopamine administration

hypertension

hypoxemia

Bradycardia

Treatments:

remove cause (if possible – as in hypothermia)

anticholinergic

glycopyrolate 0.005 – 0.02 mg/kg IV

atropine 0.01 – 0.02 mg/kg IV

if administering dopamine or dobutamine – discontinue and wait approximately 5 minutes before administering anticholinergic

Bradycardia

Atropine – faster onset than glycopyrolate

don’t be afraid to administer > 1 dose of an anticholinergic. If the first dose doesn’t work, try a second or a third. After 3 doses of one anticholinergic, I will usually administer a dose of the other to see if that works.

Bradycardia

bradycardia induced by alpha 2 agonists such as Domitor can be approached this way:

if the heart rate is just low, blood pressure is fine and there are no arrhythmias, then you don’t really need to treat.

if arrhythmias do start to appear (second degree AV block, escape complexes) then it may be best to administer a small amount of the reversal (atipamezole) to bring the heart rate up rather than giving an anticholinergic (debatable).

Bradycardia

bradycardia in response to hypertension is a good thing. Leave it alone. Correct the hypertension and the bradycardia will usually go away.

Tachycardia

anticholinergic administration

hypovolemia

too light/pain during surgery

too much dobutamine/dopamine

hyperthermia

high ETCO2

Tachycardia

Treatment: remove cause in animals with high heart rates and

normal blood pressures that are at an appropriate depth of anesthesia, it is often the case that they have lower than normal circulating volume so that the heart has to work harder/faster to maintain good cardiac output and blood pressure. Often, the first response to tachycardia in an animal that is assessed to be appropriately managed in terms of its pain, is to administer a fluid bolus (10 ml/kg crystalloids over 15 min) to see if an increase in volume will bring the heart rate down.

Tachycardia

high ETCO2 will produce sympathetic stimulation that can cause an increase in heart rate. Solution: ventilate

when an animal is responding to surgical stimulation, then you should reevaluate your approach to pain management in this animal. Often, a supplemental dose of an opioid (hydromorphone, fentanyl) will provide you with the additional analgesia that is required.

Hypotension

Possible Causes:

hypovolemia/blood loss

excessive anesthetic dose (usually inhalant but can occur with injectables such as propofol)

poor myocardial contractility

bradycardia (see above)

Hypotension

Treatments: generally, the first approach to dealing

with hypotension in an animal under anesthesia is to a) reduce the dose of inhalant anesthesia and b) administer a bolus of either crystalloids (10 ml/kg over 15 min) or dextrans (5 ml/kg initially – can go up to 20 ml/kg in more emergent cases) (if non-responsive to a bolus of crystalloids or you don’t want to give any more crystalloids.

Hypotension

in cases where the inhalant concentration cannot be lowered but you REALLY need to reduce it because of cardiovascular depression, you can consider giving a supplemental dose of narcotic (say hydromorphone = 0.05 – 0.1 mg/kg IV). This will allow you to reduce the dose of inhalant required for anesthesia.

Hypotension

poor contractility can lead to hypotension and can be the result of cardiovascular depression due to the inhaled anesthetic or preexisting cardiovascular disease (i.e. dilated cardiomyopathy). In these cases administering either dobutamine (3 – 10 ug/kg/min IV) or dopamine (3-10 ug/kg/min) may be helpful.

Hypertension

Possible Causes:

too light/inadequate anesthesia

excessive dobutamine or dopamine administration

alpha 2 agonist administration

excessive fluid administration

elevated ETCO2

Hypertension

Treatments:

increase depth of anesthesia (either by increasing inhalant concentration or administering an additional dose of narcotic)

reduce dobutamine or dopamine administration

ventilate if ETCO2 is elevated

diuresis if volume overload

Blood Loss

Most blood loss during anesthesia occurs because of a surgical or medical procedure. Acute blood loss can lead to hypovolemia, hypotension and reduced oxygen delivery. Anesthetized animals have greater difficulty compensating for blood loss than do conscious animals. Ongoing efforts to quantify blood loss must be made so that adequate volume replacement can be made. When quantifying blood loss, keep in mind that for every 1 ml of blood lost, you need to give 3 ml of crystalloids back to restore the volume. This should be provided in addition to the calculated maintenance solutions. Animals that lose 20% of their blood volume should have that volume replaced with whole blood rather than crystalloids.

Apnea

can occur at 2 different, distinct time points in the anesthetic process. 1

. following induction (particularly with an injectable anesthetic)

occurs frequently and will generally resolve on its own

proceed with intubation, denitrogenating the anesthetic circuit and lungs (by giving 2-3 good breaths of oxygen once the animal is connected to the anesthetic machine)

wait for the animal to redistribute the induction drug to the point where it begins to respond to carbon dioxide again.

Apnea

In animals where you are particularly concerned about induction apnea (i.e. animals that already have some respiratory embarrassment), preoxygenating prior to the induction of anesthesia will help to reduce the impact of induction apnea on oxygen levels.

Apnea

May also occur because of being either too deep under anesthesia (or too light)

Tachypnea

Possible causes:

too light for the procedure

excessive ETCO2

hyperthermia

hypoxemia

The most common cause of tachypnea during anesthesia is an inadequate depth of anesthesia – too light (see below).

Tachypnea

Treatments:

correct the cause

TOO LIGHT

Possible causes:

mismatch between the depth of anesthesia (inadequate) and the level of surgical stimulation (excessive).

Usually occurs at the beginning of surgery.

during the transfer of an animal to inhalant anesthesia following induction with propofol.

TOO LIGHT

Treatments: if the animal is moving, then you need

to increase the depth of anesthesia - giving a supplemental dose of the induction drug is indicated.

if you just need to increase the depth slowly because you have noticed that the animal is responding a little to surgical stimulation (i.e. mild increase in blood pressure, heart rate or respiratory rate), then you can produce an increase in depth of anesthesia by a) turning up the vaporizer and b) increasing the fresh gas flow rate.

TOO DEEP

Possible Causes:

mismatch between the depth of anesthesia (excessive) and the level of surgical stimulation (inadequate.)

TOO DEEP

too much induction drug vaporizer set too high. Preventing an animal from becoming

too deep under anesthesia requires close attention to the anesthetic needs of the animal.

Before administering an injectable induction drug, carefully evaluate how sedate the animal is following the premedication. If it is very sedate, then you will not have to give much induction drug.

In the same vein, if an animal is very deep after you induce anesthesia, then you will not have to give much inhalant initially.

TOO DEEP

Treatment:

if the depth of anesthesia is too deep because of injectable drugs you can either

a) reverse part or all of any reversible drugs

b) support the animal as it metabolizes any non-reversible drugs.

TOO DEEP

inhaled anesthesia:

decreasing the vaporizer setting

increasing the fresh gas flow (this increases the rate at which the gas containing the reduced concentration replaces the gas with the “old”,

higher concentration of anesthetic.

gpettifer@sympatico.ca

5. The components of an anesthetic machine are listed below. Choose the order that best describes the flow oxygen through the machine:

a) cylinder, pressure gauge, pressure regulator, precision vaporizer, flow meter, fresh gas outlet/port, breathing circuit, overflow (pop-off) valve.

b) pressure regulator, cylinder, pressure gauge, precision vaporizer, flow meter, fresh gas outlet/port, overflow (pop-off) valve, breathing circuit

c) cylinder, pressure gauge, pressure regulator, flow meter, precision vaporizer, fresh gas outlet/port, overflow (pop-off) valve, breathing circuit

d) cylinder, pressure gauge, pressure regulator, flow meter, precision vaporizer, fresh gas outlet/port, breathing circuit, overflow (pop-off) valve.

e) pressure regulator, cylinder, pressure gauge, precision vaporizer, flow meter, overflow (pop-off) valve, fresh gas port, breathing circuit

6. All of the following statements concerning the rebreathing bag are correct except:

a) it acts as a reservoir bag from which the animal may breath oxygen and anesthetic gas

b) it can be used to manually support ventilation

c) it allows visual assessment of the respiratory rate

d) it traps carbon dioxide expired by the animal

e) it allows visual assessment of the tidal volume

Physical Exam/Data:Chest:Rhythm:Other:Labs/Rads/ECG/Other:

Assessment:

ASA Status:I II III

IV V

PLAN/RECOMMENDATIONS:FURTHER DIAGNOSTICS:PRE-ANESTHETIC THERAPY: PREMEDICATION:INDUCTION:MAINTENANCE:ANALGESIA:FLUIDS: MONITORING:

PRE-ANESTHETIC EVALUATION FORMDate: Time:Patient Name:Requested By:History:

PROCEDURE/REASON FOR ANESTHESIA:=

_________________________________Glenn Pettifer, DVM, DVSc, DACVA