Maintenance and monitoring
Anesthesia is typically maintained using inhalant anesthetics, although maintenance can also be achieved with continuous infusions or intermittent doses of injectable agents, or a combination of injectable and inhalant drugs. An O2-enriched gas mixture is necessary for the safe and effective administration of inhalant anesthesia.23,29
O2 flow rates depend on the breathing circuit used. For a circle rebreathing system, use a relatively high flow rate when rapid changes in anesthetic depth are needed, such as during the transition from injectables to inhalants (induction), or when turning the vaporizer off at the end of the procedure. During the maintenance phase, total O2 flow rate should typically be between 200 and 500 mL. The system must be leak-free for these flow rates to be effective. These are, perhaps, lower O2 flow rates than many are accustomed to. The benefits of lower flow rates include decreased environmental contamination and the economy of decreased consumption of O2 and volatile anesthetic gases. Lower flow rates also conserve moisture and heat. Disadvantages of lower flow rates include increased time to change anesthetic depth. Administer an O2 flow of approximately 200 mL/kg/min to patients connected to a non-rebreathing circuit.22
Guidelines for anesthesia monitoring are available from the American College of Veterinary Anesthesiologists (ACVA).35
Continue the cardiovascular monitoring and physiologic support measures that began in the patient preparation and/or induction periods. Monitoring includes evaluation of oxygenation, ventilation, cardiac rate and rhythm, adequacy of anesthetic depth, muscle relaxation, body temperature, and analgesia. Blood pressure, heart rate and rhythm, mucous membrane color, and pulse oximetry provide the best indexes of cardiovascular function. Multiparameter electronic monitors are available and serve as tools to assess physiologic parameters during the perianesthetic period (Table 3). One must always evaluate the data the monitor is conveying in light of all other parameters and make treatment decisions based on the whole picture. Vigilant monitoring, interpretation, and responding to patient physiologic status by well-trained and attentive staff are critical.
Provide thermal support and monitor body temperature throughout the perianesthetic period. Supplemental heat may include warm IV fluids, use of a fluid line warmer, insulation on the patient’s feet (e.g., bubble wrap), circulating warm-water blankets, and/or warm air circulation systems. Do not use supplemental heat sources that are not designed specifically for anesthetized patients, as they can cause severe thermal injury.36
Troubleshooting anesthetic complications
Recognize and then quickly and effectively respond to complications as they develop. Anesthesia-related complications are responsible for a significant number of AVMA PLIT insurance claimsj.
Hypoventilation is an expected effect of general anesthesia and can be estimated by observing respiratory rate and depth, but can be quantified using capnometry. Observation of respiratory tidal volume is subjective, and it can be difficult to distinguish a normal from an abnormal tidal volume. Normal end-tidal CO2 is approximately 35–40 mm Hg in awake patients and approximately 40–50 mm Hg in patients in a light surgical plane of anesthesia. With increasing CO2, identify causes such as excessive anesthetic depth, provide initial patient support by positive pressure ventilation, and adjust anesthetic management as indicated. Hypotension is a common complication during anesthesia. Diagnose hypotension through blood pressure monitoring and evaluation of other physiologic parameters. Therapies for hypotension include decreasing the depth of anesthesia, administering crystalloid and/or colloid boluses, and/or administering vasopressors and inotropes.
Monitor for arrhythmias via auscultation, electrocardiography, or observing pulse–heart rate discongruity when using Doppler ultrasound. Common perioperative arrhythmias include bradycardia and ventricular arrhythmias. The decision of whether to treat a given arrhythmia should be based on the severity, the effect on other hemodynamic parameters (e.g., blood pressure), and the likelihood of deterioration to a more significant arrhythmia.
There are limited data to provide insight into the causes of anesthetic and perianesthetic deaths in dogs and cats.37 Many complications and deaths occur during recovery. Most anesthetic deaths are unexplained because of insufficient information regarding the event. Increased monitoring and early diagnosis of physiologic changes and earlier intervention may reduce the risk of anesthetic death.
After an anesthetic death, offer clients the option of having a necropsy performed. Necropsy may detect preexisting disease that contributed to anesthetic death that was not detectable with preoperative evaluation. Empathetic communication may help clients deal with loss, anger, and the grief process.