Anesthesia and patient monitoring during sedation

Image 1: a patient recovering from anesthesia. 

Anesthesia plays a significant role in animal imaging. Unlike in human medicine, veterinarians are unable to impart the importance of staying still during scans. For this reason, anesthetics are used to sedate patients during procedures to assure an accurate diagnosis from clear scans, as well as to protect the safety of both the patient and the veterinary team.

Anesthetic agents sedate patients by depressing various biological functions, including heart and respiratory rates. As such, approval for a procedure requiring anesthesia is only granted once a patient is determined to be stable enough to handle sedation. Pre-existing conditions can make a patient more susceptible to complications while under anesthesia, and may require additional precautions or a delay in the procedure until their condition has improved.

The first step of the anesthesia process is induction. Patients are given a combination of analgesics and short-term sedatives, usually via an intravenous catheter. Catheters are placed before induction, as they provide an option for rapid delivery of rescue agents should the patient experience adverse effects while under anesthesia. Once the patient’s level of consciousness (LOC) has dropped, they are intubated with an endotracheal tube. Most tubes have an inflatable cuff that seals the trachea, allowing for adequate inflation of the lungs and sufficient delivery of oxygen and an inhaled anesthetic. The inhaled anesthetic is used to maintain the level of sedation during the procedure, and delivery levels can easily be adjusted to compensate for patient response. In addition, while patients are usually fasted prior to anesthesia, it is still possible for vomiting to occur while sedated–thus, the endotracheal tube also prevents blockage of the trachea by any regurgitated substances.

Due to the profound effects of anesthetics on vital biological functions, patients are continuously monitored to assess their wellbeing throughout the procedure. Vitals such as heart rate (HR), blood pressure (BP), respiration rate (RR), blood oxygen levels (SPO2), and end-tidal carbon dioxide (EtCO2) levels are charted, and their trends analyzed to preemptively detect any complications. These signals inform the veterinary team whether the patient is receiving enough oxygen in tissues throughout their body, if they have enough fluids to maintain proper cardiovascular function, and their level of sedation. Adjustments in fluid and inhaled anesthetic delivery rates, mechanical breath rate, oxygen levels, and temperature are made as necessary.

Image 2: example of an anesthesia chart. Each symbol represents a different vital sign. v: systolic pressure. ^: diastolic pressure. -: mean arterial pressure (MAP). O: respiration rate. X: SPO2. C: ET CO2. •: heart rate. Note that delivery rate of the inhaled anesthetic (isoflurane in this case), oxygen, and fluids are also tracked over time. This method allows for easy visualization of patient trends throughout the procedure. Image source: https://bothellveterinarian.wordpress.com/2017/05/05/allaying-fears-about-your-dog-or-cats-anesthesia/

Once the procedure is complete, the patient is removed from the inhaled anesthetic and fluid drip. Oxygen and mechanical breath delivery are continued until the patient can breathe independently. At this point, the patient is transferred to a recovery area for continued monitoring. Certain landmarks in the recovery trajectory include the return of reflexes such as eye blinking, ear flicking, and swallowing. The ability to swallow in conjunction with an adequate RR indicates that the patient is ready to be extubated, as their muscle tone has returned enough for the epiglottis to effectively protect the airway. SPO2 is an important vital to monitor in the period directly after extubation: as the patient is still sedated, their RR and HR may remain at a depressed level. This leads to a risk of insufficient oxygen delivery to vital organs, which could cause severe complications. For this reason, these vitals are monitored to ensure that the patient’s airway is maintained without the tube, and that their respiration is of a sufficient quality. The IV catheter is left in place as a safety measure in case the patient experiences complications post-extubation. Once the patient is determined to be on a positive trajectory, the catheter is removed and the site is bandaged. 

In addition to SPO2, HR, and RR, another important measurement to monitor both during and after the procedure is temperature. While sedated, the body is unable to thermoregulate on its own. This leads to an increased risk of hypothermia during anesthesia, which can cause severe damage to the body and will drastically slow the recovery rate. Oftentimes devices such as Bair huggers are used to maintain a proper body temperature until the anesthetics have worn off.

Because the level of sedation required for non-invasive procedures like MRI scans is much lighter than that required for surgery, recovery times are generally fairly rapid for healthy patients. Patients may experience mild side effects during the immediate period after anesthesia, and monitoring at home in the days following the procedure serves to prevent any long-lasting complications. However, the risk of major issues is greatly reduced by adherence to proper anesthetic protocols during the procedure.

For a general guide to anesthesia on pets, check out this link: https://vcahospitals.com/know-your-pet/anesthesia-for-dogs

For a more technical guide to the ins and out of anesthesia protocols, here is a comprehensive review: https://www.aaha.org/globalassets/02-guidelines/2020-anesthesia/anesthesia_and_monitoring-guidelines_final.pdf