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ANAESTHETIC MANAGEMENT IN SMALL ANIMALS
 

ANAESTHETIC MANAGEMENT IN SMALL ANIMALS

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ANAESTHETIC MANAGEMENT IN SMALL ANIMALS

ANAESTHETIC MANAGEMENT IN SMALL ANIMALS

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    ANAESTHETIC MANAGEMENT IN SMALL ANIMALS ANAESTHETIC MANAGEMENT IN SMALL ANIMALS Presentation Transcript

    • 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 FORM Date: Time: Patient Name: Requested By: History: PROCEDURE/REASON FOR ANESTHESIA: = _________________________________ Glenn Pettifer, DVM, DVSc, DACVA