What's new in small animal veterinary anesthesia? (Proceedings)

The purpose of this presentation is to review several articles published in veterinary journals over the last three years and to present them in a way that is relevant to the practicing veterinarian.

Mortality and CPCR: Obviously, the veterinarian wants to avoid anesthetic-related mxortality. The only way to effectively do that is to understand the factors associated with anesthetic mishaps. The three articles summarized below provide some valuable information of the practitioner:

Brodbelt et al. The risk of death: the confidential enquiry into perioperative small animal fatalities. Vet Anaes Analg 2008.

What they did: The authors performed a prospective case-control stud of anesthetic-related mortality in 117 veterinary practices and evaluated nearly 100,000 dogs, 80,000 cats, and 8,200 rabbits. They evaluated deaths that occurred within 48 hours of anesthesia or sedation.

What they found:

     o The overall death rate for dogs, cats, and rabbits was 0.17%, 0.24%, and 1.39%, respectively.

     o The risk was lower in healthy animals (0.05, 0.11, and 0.73%)

     o Postoperative deaths accounted for 7, 61, and 64% of deaths in dogs, cats, and rabbits, respectively. The first three hours after anesthesia was the most critical.

     o Risk of death associated with sedation was less than for anesthesia. (Maybe???)

     o Most deaths in dogs and cats were related to cardiovascular or respiratory causes. Most deaths in rabbits were of unknown cause.

What you need to know: This comprehensive study proved a few things that we already suspected (sicker animals are at higher risk for anesthetic death), and perhaps revealed a few things that we didn't know. For example, the first three hours after the end of anesthesia were associated with a significant percentage of anesthesia related deaths. This implies that we need to assess our patients more aggressively in the immediate postoperative period. The risk of anesthetic-related death is higher in cats than in dogs, and it appears that the risk of death from sedation alone MAY NOT be different from the risk associated with general anesthesia.

Hofmeister et al. Prognostic indicators for dogs and cats with cardiopulmonary arrest (CPA) treated by cardiopulmonary cerebral resuscitation (CPCR) at a university teaching hospital. JAVMA 2009.

What they did:

The authors reviewed records from all animals that suffered CPA during a 60 month period .

What they found:

     o 6% of the animals that had in-hospital CPA survived to discharge. (35% of dogs and 44% of cats had a return of spontaneous circulation—successful CPCR).

     o Successful resuscitation was more likely if:

     o The dog was anesthetized at the time of CPA

     o The duration of CPA was short

     o Multiple diseases/conditions were not present

     o The following drugs were used: mannitol, lidocaine, fluids, dopamine, corticosteroids, vasopressin

     o Cardiac massage was done in lateral recumbence

     o None of the animals that arrested outside of the hospital survived to discharge.

     o Higher levels of CO2 as determined by capnography were associated with successful resuscitation (dogs and catswith a peak CO2 reading less than 15 or 20 mm Hg, respectively, were unlikely to have a return of spontaneous circulation).

     o In cats, more people involved in the resuscitation was a positive predictor of outcome. Shock prior to arrest was a negative predictor of outcome.

     o 50% of the dogs arrested in asystole, 23% had bradycardia

What you need to know:

     o CPCR is not a practice builder!!! It has a low success rate

     o Your capnograph is a good tool to help monitor your patient during CPCR

Love, et al. Anesthesia case of the month. JAVMA 2010

What they found:

This was not a research project, but a case report of a cat with primary pulmonary disease and cardiac arrhythmias. It provides a good review of anesthesia for pulmonary disease and for treating arrhythmias in cats.

What they did:

An 11 year old spayed female Himalayan cat was evaluated for a 6 day history of lethargy, anorexia, and dyspnea. Problems identified included dehydration, abnormal thoracic auscultation, and right-sided atelectasis with pneumothorax. A thoracentesis of the atelectic lung revealed a broad-based budding yeast consistent with blastomyces dermatidis.

The cat was anesthetized for exploratory thoracotomy and right cranial lung lobectomy. The procedure was being performed to debulk compromised lung tissue and decrease compression of remaining lung tissue. Thoracostomy and esophageal tubes were to be placed.

Drugs used in the anesthesia protocol: Premedication: fentanyl; Induction: ketamine/propofol; maintenance: isoflurane, ketamine.

After induction, the cat was breathing at 28-35/min with an end-tidal CO2 of 42 mm Hg, HR of 165, SpO2 of 99% and MAP of 172 mm Hg. The cat was positioned in left lateral recumbence for surgical preparation. 15 minutes later, HR increased to 300 bpm (with significant multiform ventricular premature contractions/tachycardia), SPO2 decreased to 90%, end-tidal CO2 was 50mm Hg, and respiratory rate was 25/min. Thoracentesis was performed and the signs resolved over the course of several minutes.

What you need to know:

     o Cardiac rhythm can be disturbed by primary causes or secondary causes. In this case, the re-accumulation of air in the thoracic cavity caused hypoxemia and desaturation that led to myocardial hypoxia. In this case, resolving the respiratory problem was the treatment for the cardiac rhythm disturbance!

     o The presence of a pneumothorax and the history of a percutaneous needle aspirate of the lung should decrease enthusiasm for positive pressure ventilation!

     o The authors also of a balanced anesthetic technique and the multimodal approach to pain control in this cat (opioids and ketamine). Local or regional anesthesia might also be applied.

Mosing et al. Clinical evaluation of the anesthetic sparing effect of brachial plexus blocks in cats. VAA 2010.

What they did:

The authors evaluated the anesthetic-sparing effect and postoperative pain control after a brachial plexus block in cats undergoing thoracic surgery.

What they found:

A brachial plexus block was able to decrease intra-operative isoflurane requirement and pain during the postoperative period.

What you need to know:

• Brachial plexus block may be used to provide pre-emptive and postoperative analgesia in dogs and cats undergoing surgery of the thoracic limb (radius/ulna and distal)

• A mixture of lidocaine and bupivacaine was used

     o Allows for rapid onset and long duration analgesia

• The anesthesia protocol included a fentanyl infusion

     o Decreased requirement for inhalant anesthesia

     o Multimodal analgesia

• Rescue analgesia (methadone + carprofen; opioid + NSAID) was required in both groups!!!

     o Even an aggressive, multimodal approach to pain management requires assessment of the patient and may require the use of additional analgesic drugs.

Wagner et al. Clinical Evaluation of perioperative administration of gabapentin as an adjunct for postoperative analgesia in dogs undergoing amputation of a forelimb. JAVMA 2010

What they did:

Assessed effectiveness of gabapentin as an adjunct for postoperative analgesia in dogs.

What they found:

Authors were unable to detect a significant benefit associated with gabapentin (10 mg/kg once preoperatively, then 5 mg/kg BID q 12 h) after amputation.

What you need to know:

     o Dogs were given pre-emptive and multimodal analgesia (methadone, fentanyl, local anesthesia)

     o The authors used 5 different assessments of pain!

     o Although gabapentin was not shown to be effective in this study, several studies in human beings provide evidence for its effectiveness.

     o Different doses? Different timing?

Boscan et al. Fluid balance, glomerular filtration rate, and urine output for dogs anesthetized for an orthopedic surgical procedure. JAVMA 2010

What they did:

The authors evaluated urine output, glomerular filtration rate, and water balance during orthopedic surgery.

What they found:

Median urine output were 0.26 ml/kg/h. Body weight increased, PCV, total plasma protein, and body temperature decreased.

What you need to know:

     o Expected urine output in awake animals is 1-2 ml/kg/h. Anesthetized dogs in this study produced ~ 0.5 ml/kg/h

     o Hemodynamic parameters were well maintained (MAP 70-100 mm Hg)

     o The overall balance of crystalloid fluid administration and urine production resulted in a gain of 1-2 L of fluid during the 4 h anesthesia (~50 ml/kg)

     o All dogs became hypothermic

     o Calls into question the current strategy of crystalloid fluid administration during anesthesia.