How to Reduce Anesthesia-Related Morbidity and Mortality: Evidence and Experience
World Small Animal Veterinary Association Congress Proceedings, 2018
Anderson Favaro da Cunha, DVM, MS, DACVAA
Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA


Review anesthesia-related mortality and morbidity facts with a special focus on how to lower the odds of a patient dying under general anesthesia.

Many studies have elucidated the anesthetic-related mortality and morbidity. The most important studies ever published for dogs and cats describe the following mortality rate:


  • Overall risk (ASA 1,2,3,4,5, combined)=0.17% or 1 in 601
  • Healthy dogs (ASA 1 and 2)=0.05% or 1 in 1849
  • Sick dogs (ASA 4 and 5)=1.33% or 1 in 7


  • Overall risk (ASA 1,2,3,4,5, combined)=0.24% or 1 in 419
  • Healthy cats (ASA 1 and 2)=0.11% or 1 in 895
  • Sick cats (ASA 4 and 5)=1.4% or 1 in 7


  • Overall risk=0.001 to 0.0025%=1 every 100.000–250.000. The explanation for the major difference between human and veterinary anesthesia-related complication rates is simple. Pre-op evaluation and vital signs monitoring in human medicine are significantly better. The more you know about your patient before the beginning of the anesthetic procedure, the more prepared you will be for a possible complication. With modern monitoring, you can identify life-threatening complications earlier and start target treatment sooner.

The next paragraphs will shine some light on factors that have been recognized as important factors that may increase the odds of anesthetic-related death. In other words, these are the factors that should be avoided:

  • Increased physical status increases the chances of having a complication under general anesthesia.
  • The more sick the patient is, the more complicated the anesthesia management will be, and obviously, the mortality rate of those sick patients is expected to be higher.
  • The American Society of Anesthesiologists (ASA) published a physical status classification that helps us to guide ourselves:


Physical status

Possible examples


Normal healthy patient

Elective procedures, ovariohysterectomy, castration, allergy test


Patients with mild systemic disease

Skin tumor removal, repair of fractures or hernias, cryptorchidectomy, localized infections, compensated cardiac disease, obesity, mild dehydration


Patient with severe systemic disease

Fever, anemia, dehydration, cachexia, moderate hypovolemia, kidney disease, C-section


Patient with severe systemic disease that is a constant threat to life

Uremia, toxemia, severe dehydration and hypoxemia, anemia, cardiac instability, emaciation, high fever, GDV, azotemia, caval syndrome


Moribund patient not expected to survive 1 day with or without operation

Extreme shock and dehydration, terminal malignancy or infection, or severe trauma, sepsis



Any category can receive an emergency status

This classification system is considered subjective. Different anesthesiologists may classify the same patient differently. But that is all right. The ASA classification is not designed to be precise but is a tool used to classify a patient based on its physical status. It is used to help the anesthesiologist to see if there is a possibility to reduce the risk of anesthesia by improving the ASA status of the patient prior to be beginning of the anesthesia. There is evidence that survival rates deteriorate, as the ASA status gets worse.

Factors to Consider When Assigning ASA Status

  • Cardiac reserve – Is it compromised? Will anesthesia drugs make it worse? Can it be better with drugs or fluids?
  • Pulmonary – What is the drug effect on the pulmonary system? What type of ventilation/oxygenation is required prior to anesthesia induction? Will patient position on the table influence the ventilation status?
  • Renal – Is dehydration, azotemia or uremia present? Will drugs be eliminated by renal system?
  • Neurologic – Any signs of CNS depression, behavior change, seizures, elevated ICP, anisocoria, nystagmus?
  • Hepatic – Is the hepatic function compromised? Liver enzymes, blood glucose, albumin, coagulation disorders?
  • Endocrine – Any clinical signs of diabetes, thyroid disease, Cushing’s, Addison’s diseases?
  • Hematologic – Is anemia present?
  • Physical status is independent of the surgical procedure.

Urgency of the Procedure

  • Emergency procedures are associated with higher risks of anesthesia-related mortality rate.


  • Age is not a disease but is considered a cofactor associated with anesthesia-related mortality rate. Both neonatal and geriatric patients are considered high-risk patients for anesthesia.

Intended Duration of the Procedure

  • The longer the procedure is, the higher the odds of anesthesia-related complications are.

Injectable versus Inhalant

  • Studies are showing that when inhalant anesthetics are used in cats, the chances of having a complication increase. However, the authors explain that other cofactors are associated with this observation. For instance, the use of inhalant anesthetics without any comorbidity is not associated with higher mortally rates. Usually inhalants are used for more complicated and longer procedures and, at the same time, maintenance of anesthesia with injectable anesthetics is often used during short, uncomplicated procedures. Therefore, the length and complexity of the procedure are more important than the anesthetic choice itself.

Endotracheal Intubation in Cats

  • Endotracheal intubation in cats is associated with higher mortality rates. Even though cats are more difficult to intubate than dogs and there are higher chances of tracheal tear in any very small animal, this is not the explanation. Actually, it is explained similarly as the previous factor. In the majority of the available studies in cats, endotracheal intubation was associated with longer and more complicated procedures. For example, for a simple castration, inhalants are not necessary usually used.

Fluid Administration in Cats

  • Fluid administration in cats is associated with higher mortality rates. Same as both previous items. Cats receive fluids only when they are undergoing longer and/or more complicated procedures, which are associated with higher mortality rate.


  • Obese patients have higher odds of suffering a complication under anesthesia than slim patients. While under anesthesia, obese patients are more likely to suffer from respiratory depression, overdoses, thermoregulatory issues and prolonged recovery.


  • Brachycephalic patients have higher chances of dying after anesthesia. Usually the brachycephalic patients have stenotic nares, prolonged soft palate, inverted saccules and stenotic trachea, that leads to possible respiratory obstruction and arrest.

Therefore avoiding those previous mentioned factors will help you to decrease the anesthetic-related mortality rates in your clinic.

The next paragraphs will then review the factors that have been recognized as important factors to decrease the odds of anesthetic-related death. In other words, those are the factors that should be promoted:

Equipment Check with Protocol and Checklist

  • Many anesthesia-related complications are related to equipment failure or failures to properly test the equipment. Checklists are available to remind the personnel of all important steps for the safety of the patient being anesthetized.
  • Documentation of equipment check also helps to reduce the equipment-related anesthetic complication. Knowing exactly when and what happened to the equipment helps to identify reoccurring problems that need to be corrected.

Directly Available Anesthesiologist + Full-Time Nurse

  • The availability of well-trained personnel helps decrease mortality rates when the team is able to treat complications earlier, even before they become dangerous to the patient.

No Change of Anesthetics During the Procedure

  • When changing personnel during a procedure, make sure the information regarding the patient is transferred or possible mistakes can happen. Common examples are administration of incompatible drugs, implementation of treatments that did not work previously, overdose of fluids and other drugs. Well-trained teams use standard transfer sheets to help team members remember the important factors about the patient.

Two Persons Available During Emergency

  • The evidence shows that multiple team members should manage CPR events in order to improve survival rates.

Postoperative Pain Management

  • The large majority of anesthesia-related complications occur during the recovery of anesthesia. Providing postoperative pain management helps reduce the complications rate.

Pulse Oximeter

  • Early recognition of complications is the key to improve survival outcomes. The pulse oximeter is usually the first parameter that displays errors associated with cardiorespiratory complications.


  • Most of the anesthesia-related mortality and morbidity studies show that monitoring saves lives. Again early recognition and early treatment of anesthesia-related complication is the key to decrease the odds of having a complication under general anesthesia.



References are available upon request.


Speaker Information
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Anderson Favaro da Cunha, DVM, MS, DACVAA
Department of Veterinary Clinical Sciences
School of Veterinary Medicine
Louisiana State University
Baton Rouge, LA, USA

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