Anesthesia is often frightening and used as a scapegoat when a patient dies during surgery. Mortality is commonly used to measure anesthesia success, even though it should be the lack of complications that should be used. Unfortunately, totally safe anesthesia does not exist. Therefore, it is important to understand what factors can be life-threatening to reduce anesthesia risks.
- Mortality risk is approximately 0.05 to 0.1% for healthy patients. Complications rate is much higher.
- The majority of anesthesia deaths occur during recovery and maintenance of anesthesia, when monitoring may be lacking.
- The principal mortality risk factors are physical status (ASA), age, lack of monitoring, some breeds and endotracheal intubation in cats.
- Death is far from being the only complication of anesthesia. The main complications are hypotension, hypothermia, hypoxemia, and hypoventilation.
- Other reported complications include regurgitation, aspiration pneumonia, corneal ulcers, and blindness in cats.
- Untreated pain can also put the patient’s health in danger.
Numerous studies have reported different mortality rates in veterinary medicine. Overall, for healthy dogs and cats, the mortality rate is approximately 0.05 to 0.1% and is probably around 1-2% for sick dogs and cats. It is also important to consider that the risk of death is the same for sedation and general anesthesia. Commonly reported risk factors for death include poor health status (ASA), age, poor monitoring, endotracheal intubation, and possible breed association.
Time of death
The induction of anesthesia and the immediate recovery period are typically considered at risk for the patient. However, the vast majority of deaths occur during the post-operative recovery period (50-60% of deaths in the first 48 hours after anesthesia/surgery and most of them, within 3 hours post-anesthesia) and maintenance of anesthesia (30-40%). Only 6-8% of deaths occur during induction. It does not mean that induction and recovery are not a risk for the patient. During those periods, vigilance is usually increased, allowing the team to react before complications can be harmful.
The ASA (American Society of Anesthesiology) physical status table is a tool used to evaluate whether a patient is healthy, on a scale of 1 to 5 (see table 1). It is used to learn how the health of the patient can alter anesthesia. The complexity of the surgery in itself is not a factor in the ASA physical status; however, it is necessary to understand what the risks of the surgery are for the patients too, especially regarding blood loss and length of the procedure. Because the patient cannot protect himself when anesthetized, operation and manipulation can also put the patient’s life in danger. One known example is when cats wake up blind from anesthesia because a mouth gag left opened too wide stopped cerebral circulation ischemia.
A patient with an ASA status above 2 is 10 times more at risk and should get more attention during anesthesia, or could even be referred to a specialist. The risk is also higher for patients that are anesthetized for emergency after hours of procedures, when the staff is tired and/or in insufficient number, and the patient is usually not completely stabilized.
|1||A normal healthy patient||Healthy patient for routine surgery|
|2||A patient with mild systemic or localized disease, that does not limit normal function||Skin tumor, fracture, non-complicated umbilical hernia, and localized infection.|
|3||A patient with severe systemic disease that limits normal function||Fever, dehydration, anaemia, cachexia and mild to moderate hypovolemia|
|4||A patient with severe systemic disease that is a constant threat to life||Uraemia, toxaemia, severe dehydration and hypovolemia and congestive heart failure|
|5||A moribund patient who is not expected to survive without procedures in the next 24h||Shock, multisystemic insufficiency, sepsis, severe trauma|
|E||A patient that needs anaesthesia in emergency, without enough data to allow ASA status classification||Gastric dilatation and volvulus, respiratory distress|
Elderly patients are more at risk, independently of their physical status. Even if age in itself is not a disease, elderly patients have decreased physiological reserve; therefore, cannot compensate for the effects of anesthesia as well as a young healthy patient. Additionally, they often have comorbidities that can interfere with anesthesia.
Lack of monitoring
One study reports that patients that have pulse and pulse oximetry monitoring are 5 times less likely to die than patients without any monitoring. Human medicine studies also showed that the use of a capnograph with a pulse oximeter could prevent up to 93% of complications during anesthesia. Hypotension is also a risk factor for mortality in human medicine. Therefore, it is highly recommended to have dedicated staff to monitor anesthesia and to use at least a pulse oximeter, a capnograph, an ECG, a pressure monitor and a thermometer. An audible signal should be heard.
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Endotracheal intubation in cats
Endotracheal intubation of cats has been reported as being a risk factor for mortality. Their airways are smaller and sensitive to trauma, spasms, and edema, compared to other species. These results should not be used as an excuse not to intubate cats, as it is still fundamental to secure airways. However, it is important to be careful and delicate when intubating cats and to use suitable endotracheal tubes. The trachea of cats is also fragile. Cuff insufflation and manipulation of the endotracheal tube should be done carefully, as tracheal ruptures have been reported. Laryngeal masks could be an alternative for short duration procedures.
Breed and size
Increased mortality risk is reported for small dogs and cats. They are more prone to hypothermia, easily overdosed (inaccurate weight estimation, errors when preparing small volume injections), and perioperative difficulties are harder to manage (catheter, intubation). Obesity is also a risk factor for cats. Obese animals are prone to hypoventilation, and management of perioperative complications are even harder to manage. Brachycephalic and terrier dogs are frequently represented amongst the fatalities, but part of the risk is caused by their small size. Nonetheless, caution with the anesthesia of these breeds may be advisable.
Other risk factors
Some drugs are risk factors in many studies, but nowadays, those drugs are not used in everyday practice (especially xylazine for dogs and cats, and halothane). However, even if modern drugs have not been reported to be risk factors, caution is advisable with the use of any of them.
Mask induction is one example of a practice that can put patients at risk. Mask induction is considered more stressful, longer, and less controlled than induction with injectable agents. Additionally, if it is done without monitoring (e.g., the technician is busy doing something else such as preparing the surgery site to go “faster”), the patient is at risk to be overdosed, which can in itself cause complications (hypotension, hypoxemia, hypoventilation, even death). Finally, mask induction participates in the pollution of room air by anesthetic agents. Thus, it is not advised to practice mask induction.
Risk of other complications
Mortality is not the only anesthetic risk. Generally speaking, all anesthesia can cause hypothermia, hypotension, hypoventilation and/or hypoxemia. Other complications could happen, such as aspiration pneumonia, regurgitations, and corneal ulcers. However, only a limited number of studies report those risks; therefore, factors affecting those risks and their frequency might vary depending on the study population (different breeds, surgeries or habits).
In one study, the frequency of hypothermia was 92% in dogs and 98% in cats. Risk factors were major surgeries (orthopedic and abdominal), ASA physical status, diagnostic procedures, duration of anesthesia, and patient positioning (dorsal and lateral more at risk).
In another study, the frequency of hypotension during anesthesia of healthy patients was 63%. Risk factors included underlying cardiovascular diseases, obesity, drugs choice, and surgery. In human medicine, good perioperative management of hypotension decreased complications rate and improve the outcome.
The frequency of some other complications associated with anesthesia has also been reported. The incidence of postoperative pneumonia is between 0.1 to 4.5%. Risk factors include the type of procedures, previous neurologic deficits, duration of anesthesia, and per-operative regurgitation. The frequency of regurgitation during anesthesia is between 0.4 to 5%, and the frequency of gastro-oesophageal reflux is around 16.3 to 55%. Risk factors for gastro-oesophageal reflux include the duration of anesthesia, abdominal surgeries, and some drugs. Risk factors for regurgitation include orthopedic surgeries and obesity. Knowing that the risk of regurgitation and reflux is high, it is important to secure airway for all anesthetized patients.
Anesthetized patients can develop corneal ulcers as eyelids cannot protect the eye and stretch the lachrymal film over the eye. Reported frequency of corneal ulcers is 1.9%. Risk factors include the duration of anesthesia, the shape of the skull (small skull more at risk), neurological surgeries, and having a fentanyl patch.
Pain is probably also a risk factor for mortality and other complications but no study has been published on the impact of pain on anesthesia risks. In theory, adequate analgesia allows lighter plans of anesthesia; therefore, more stable vital functions, better recovery, and a faster hospital discharge. Additionally, pain in itself can cause physiological alterations that can be harmful to the patient.
Mortality during anesthesia is rare, but complications associated with anesthesia are not. So, it is fundamental to closely monitor any patient during anesthesia and to treat any complications as soon as possible before they become a long-term issue for the patient.