I stumbled upon a 2016 article by Dr Kate Cummings and Dr Lois Wetmore on common anesthetic complications entitled Top 5 Anesthetic Complications. It’s such a simple, clear, and informative article that I had to share it. According to Cummings and Wetmore, the five complications that commonly occur during anesthesia are hypotension, hypothermia, abnormal heart rate, hypoventilation, and difficult recovery. If you’ve had any experience anesthetizing animals, I’m sure you responded to each of these the same way I did: “Yep.” “Oh yeah.” “Been there.” “Definitely.” and “Saw that yesterday.” Here’s an overview of the 5 complications.
Hypotension
They define the minimum acceptable mean arterial pressure for anesthetized small animals is 60 mm Hg. Less than that is considered hypotension or low blood pressure. A study by Dr Anne Wagner and Andrea Gordon further explains that vital organs like the brain and kidneys have the ability to adjust blood supply to meet their metabolic needs, but only if the mean arterial blood pressure is above 60 mm Hg. A survey of anesthetic records at the Colorado State University Veterinary Teaching Hospital indicated that 32% of dogs were hypotensive at some point during anesthesia. A survey of twenty veterinary practitioners in Colorado published in 2002 suggests the percentage in private practice may be higher. In that survey, the only veterinarian who considered hypotension to be a problem during anesthesia was also the only veterinarian who regularly measured blood pressure in all her patients. Coincidence?
Cummings and Wetmore describe the most common causes of low blood pressure during anesthesia, and include a very handy treatment tree labeled Hypotension Management.
Hypothermia
Hypothermia is a complication near and dear to my heart. DarvallVet focuses on developing innovations for patient warming, including the Darvall Heated Breathing Circuit that warms from within. The article attributes much of the direct causes of hypothermia to the drugs used for sedation, analgesia, induction and maintenance of general anesthesia. Additional causes include IV fluids, shaved fur, open body cavities, high oxygen flow rates and surgical prep solutions. Additional studies report that most of patients’ lost body temperature occurs before they ever reach the surgery suite, during premeds and surgical prep. Using a heated circuit from the moment of intubation prevents most of that temperature loss. Preventing hypothermia is as simple as using a better breathing circuit.
Cummings and Wetmore report that hypothermia is known to have negative consequences on coagulation and overall immune function. Dr Robertson elaborates in the Proceedings of the World Small Animal Veterinary Association World Congress of 2015 to say that as the core temperature falls there is a drop in blood pressure, changes in cardiac rhythm, and altered platelet function. Metabolism is slowed and liver function is impaired, delaying breakdown of anesthetic drugs which will prolong recovery times. In human studies, intra-operative hypothermia has been linked to increased post-operative wound infection, and humans report that waking up cold is extremely unpleasant.
As important as it is to protect the patient’s body temperature, we are warned of the dangers of using warming devices that are not specifically designated for warming veterinary patients. Pictured here is thermal injury to a cat after direct contact with a warm fluid bag.
Abnormal Heart Rate
Abnormal heart rate and rhythm during anesthesia are distressingly common. The article stresses the need to monitor the heart with a combination of ECG, pulse oximeter, and stethoscope. Cummings and Wetmore carefully explain common abnormal heart rates and rhythms, and provide a useful table of treatments for heart complications and others. The table is organized by complication, then by drug name, and then dose range. This section of the article is fact-packed, and merits extra time and attention.
Hypoventilation
Normal respiratory rates for dogs and cats vary based on size and positioning. Most dogs breathe at 6–10 breaths/minute and cats at 16–20 breaths/minute. Hypoventilation is actually difficult to understand at first, since we usually equate breathing with oxygen. Yet ventilation is defined by carbon dioxide. It takes a mental moment to shift away from thinking about oxygen, and start thinking about carbon dioxide. Capnography is discussed a lot in the practices I visit, as awareness of the value of monitoring CO2 increases, and the price of capnographs (carbon dioxide monitors) come down. Dr Hendrix presents a good overview in her article Carbon Dioxide Monitoring in Anesthetized Animals.
Difficult Recovery
The take-home message of this section is that more than half of anesthetic deaths happen during recovery. Just when you think you’re out of the woods, sometimes you’re just entering the hard part. This highlights the need to continue close monitoring during the recovery period. The article describes the two most common difficulties during recovery: the rapid, dysphoric recovery, and the prolonged recovery. Hypothermia is a common contributor to the prolonged recovery, reminding us again of the importance of protecting our patient’s body temperature during anesthesia.
As Cummings and Wetmore stated in the article, anesthetic management of small animal patients relies on thorough patient assessment, diligent anesthetic monitoring, and supportive care into recovery. Being prepared to treat anesthetic complications offers the best outcome. Their article provides a clear understanding of the 5 complications and an easy-to-navigate path toward recognizing them and treating them.
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