Anesthesia Tenants

Regardless of species, I tend to approach animals that have a human bond similarly. Small mammals that are pets typically fall in this category. Very small patients, regardless of species, have high metabolic rates, and thus shorter periods of tolerance for sub-optimal physiological parameters (oxygenation). Uptake, distribution, and elimination of parenteral or inhaled drugs will also be more rapid. Rate of cooling (or rewarming) is very rapid due to high body surface area.

1.  Assess and manage stress, anxiety, and pain.

2.  Premedicate in concert with item #1, with judicious attention to timing of procedure.

3.  Start anesthetic support, as indicated, concomitant with, or prior to, induction.

4.  The pharmacology of anesthetic induction and maintenance.

5.  Appropriate withdrawal of support during recovery.

6.  Ongoing analgesia.

Prior to Interventions

Risk of hypoglycemia, and in many cases, resistance to vomiting indicates that fasting should not take place. A large gastric volume can be problematic during anesthesia, so pelleted food and water alone should be given for the 2–4 hours prior to anesthesia. Rabbits, guinea pigs, and ferrets may benefit from actual fasting for a couple of hours, at most.

Fear/Anxiety

Non-pharma is integral-medications can be added, but in small mammals this is uncommon.

1.  Environment (warm, food and water still present, noise).

2.  Lavender lotion for handling (odor).

Premedicate (or Restraint)

1.  Small mammals in general: Injectable medications, smooth anesthetic induction, even if inhalants are chosen for induction as well as maintenance. Also, consider intubation doses of inhalants (3 MAC) relative to toxic doses (4 MAC), and the advantage of increasing this therapeutic gap. Use caution extrapolating doses from research studies, that may not have been designed for recovery (use text such as Quesenberry/Carpenter).

a.  Opioids.

b.  Benzodiazepines.

c.  Alfaxalone.

d.  Ketamine (combined).

e.  Alpha-two agonists (deprioritized in domesticated pets).

2.  Ferrets: IV catheterization after sedation is very possible (cephalic, saphenous, jugular - thick skin so consider pilot hole and/or topical lidocaine) - consider IV induction.

3.  Rabbits: IV catheterization after sedation is very possible (cephalic, saphenous). Thin skin - use care with shaving.

a.  Consider terbutaline with premedications - clinical impression of less respiratory arrest.

b.  Atropinase - consider glycopyrrolate with premedication if an anti-cholinergic is needed.

4.  Guinea pigs:

5.  Chinchillas:

6.  Hedgehogs: Premedication facilitates handling for induction.

7.  Sugar gliders:

Support and Monitoring

1.  Heat support short start at the time of premedication (prewarm the area where prep will occur). Heat support continues throughout the procedure and recovery. Consider perforated forced air devices that surround the patient with warmed air.

2.  IV fluid support - rabbits require high fluid rates (10–20 ml/kg/hour). Others should have typical maintenance (5–10 ml/kg/hour). If there is not an IV catheter, this can be given pre-warmed SQ at the time of induction.

3.  Transfusion parameters are very similar to other mammals (PCV <20; Blood loss >30% blood volume). Blood volume approximately 10 m mL/kg.

4.  Very small patients should have non-heparinized saline used for flushes

5.  Monitoring intensity needs to balance with level of complexity of case, fragility of patient, and NFS. See notes from monitoring lecture.

Induction and Intubation

1.  Small mammals in general: Length of procedure and ability to support and monitor help dictate the priority tor speed (NFS) versus increased control and complexity.

a.  Very short procedures: mask induction and low maintenance monitoring.

i.  Pulse oximeter.

ii.  Hands, eyes, and ears (RR rate and TV, HR, and rhythm).

iii.  +/- doppler crystal over artery, heart, or eye.

iv.  Note - capnography not useful in this setting.

a)  Longer procedures:

(1)  Intubate or LMA and support ventilation (see species notes).

(a)  Always evaluate depth of ET tube (avoid endobronchial intubation).

(b)  Keep tubes as short as possible to reduce dead space.

(c)  Very small airways are narrowed by even the smallest tube - consider Cole tubes that are wider before the airway.

(i)  ECG.

(ii)  Doppler sound or cuff when possible.

(iii) Arterial line in complex/specialist situations.

2.  Rodents: Often not IV catheter amenable, so mask inductions (preferably with appropriate premedications) are typical. Tight fitting mask - consider use of plastic wrap.
Intubation is difficult but can be attempted (IV catheter tubes). An otoscope cone modified with a gap may be useful. Larger rodents (prairie dogs, squirrels) can be intubated with blind technique is often possible. Rats require direct visualization.

3.  Rabbits: Intubation requires hyperextension of head and neck. Nasal breathers - so must displace soft palate with either intubation or supraglottic airways. Verify placement with breath sounds or capnography wave. (Can also use modified otoscope as with rodents).
Prefer IV induction (alfaxalone, propofol, ketamine/benzo, etomidate).

4.  Ferrets: Intubation is much like cats - consider gauze for jaw (high tone remains), drop of lidocaine for arytenoids, and cotton-tipped swab to retract tongue.

a.  (Alfaxalone, propofol, ketamine/benzo, etomidate.)

b.  Similar intubation to cats - consider lidocaine on arytenoids (watch total dose <4 mg/kg).

5.  Guinea pigs and chinchillas: Mask induction required due to palatal ostium and frequent regurgitation with laryngeal stimulation.

6.  Hedgehogs: Direct visualization using gauze/lidocaine/cotton-tipped swab.

7.  Sugar gliders: Direct visualization is possible, but usually a mask approach is used.

Maintenance

1.  Inhalant (either isoflurane or sevoflurane: they are basically equivalent). Run semi-closed systems at ∼30 ml/kg/min of oxygen or non-rebreathing at 200 mg/kg/min.

a.  Semi-closed - increase work of breathing in inspiration, better heat control, difficult to see breath size, able to use side-stream capnograph without adapter.

b.  Non-rebreathing - increased work of breathing in exhalation, cools patient, difficult to impossible to see breath size, capnograph non-effective.

2.  Local anesthetics in every surgery, wound, etc.
(Total dose <2 mg/kg bupivacaine and <4 mg/kg lidocaine or mepivacaine.)

3.  When needed to augment inhalant to improve hemodynamics, analgesia, and anesthetic plane (limiting ups and downs) the following can be used as intermittent bolus or CRI:

a.  Alfaxalone.

b.  Ketamine.

c.  Fentanyl.

d.  Dexmedetomidine.

4.  Metabolic: blood glucose. Blood loss- PCV/TP.

Recovery

1.  Anti-inflammatory if not contra-indicated once patient is recovering consciousness and body temperature has been maintained or corrected.

2.  Repeat any narcotic doses when nearing the end of their duration of action.

3.  Repeat any local blocks that are nearing their duration of action (lidocaine, mepivacaine 2 hours. Bupivacaine 4 hours).

a.  Consider long-acting bupivacaine – Nocita - 3 days duration.

4.  Utilize non-drug options - at a minimum: ice.

a.  Acupuncture.

b.  Laser.