A Deep Dive Into Small Animal Anaesthesia and Treatment

Common complications during anesthesia and in the perioperative period in small animals include hypotension, hypoxemia, arrhythmias, tachycardia, hypoventilation, hypothermia, and reflux. The following are some brief descriptions of the

recognition and management of these common complications.

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01 Hypotension

Hypotension is one of the common complications during anesthesia and even when inhalational anesthetics are used without injurious stimuli, cats can develop significant myocardial depression and decreased cardiac output, resulting in

hypotension (SAP < 90 mmHg, MAP < 60 mmHg).

When an animal develops hypotension, the first thing to do is to assess the depth of anesthesia, e.g. check jaw tension, heart rate, blood pressure, etc. If this is caused by too deep anesthesia, the concentration of inhaled anesthetic can be

reduced to improve blood pressure. During the preparation prior to surgical stimulation, this means that the isoflurane or sevoflurane volatile tanks can be reduced to 0.5% or 1% respectively. These concentrations are usually not sufficient for

surgical procedures and it should be anticipated that an increased concentration will be required at the start of the procedure.

This is followed by an assessment of blood volume, such as checking capillary refill times and peripheral pulse palpation, to help diagnose hypotension due to inadequate blood flow. Push balanced crystalloids (3-10ml/kg) intravenously, but if

the amount needed may be as high as 15ml/kg, the push should last longer than 5-15 minutes. In cats with known cardiomyopathy or anuric renal failure, try not to give fluids by push injection.

When neither of these methods improves hypotension, positive inotropic drugs should be given. An intravenous constant rate infusion (CRI) of dobutamine at a starting dose of 5 μg/kg/min may increase cardiac output and blood pressure. If

the initial rate does not work, the dose should be increased over a period of 5 minutes. Usually, the dose is increased by 2.5 μg/kg/min, but smaller or larger changes may be required depending on the cat's response. Dobutamine or ephedrine

has also been used to treat hypotension. Dobutamine should only be administered as a CRI at a rate of 1-5μg/kg/min. Ephedrine (0.03-0.2mg/kg IV) is diluted in 5.0 ml of balanced electrolyte solution and then administered as a small

intravenous dose. However, dobutamine and ephedrine may not be effective in raising blood pressure. Ephedrine, like sympathomimetics, can promote arrhythmias.

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02 Hypoxia

When animals are intubated and breathing 100% oxygen, hypoxemia (SpO295%, severe SpO290%) is uncommon. Observation of mucous membrane color is not a sensitive indicator of hypoxemia as cyanosis may not occur until severe

hypoxemia is present. Continuous assessment of oxygenation is best done with a pulse oximeter. For low SpO2, the anesthetist may attempt to troubleshoot the pulse oximeter by repositioning the probe, wetting the mucosa, or trying to use a

different monitor. If the problem does lie with the probe, these measures may work, but before troubleshooting the probe, make sure the animal is properly intubated and connected to an oxygen source and that the oxygen supply is adequate.

Hypoxemia can occur secondary to pulmonary atelectasis, bloating, or dorsal recumbency in obese animals, or primary lung disease (e.g. pneumonia) or pleural disease (e.g. pleural effusion). If this occurs, manual or mechanical ventilation

should be initiated and a positive end-expiratory pressure (PEEP) valve (2.5-5cmH2O) may be added to the expiratory branch of the circuit to open the collapsed airway.

Decreased tissue oxygen delivery due to perfusion problems (rather than respiratory problems) can also lead to decreased SpO2 readings. Indications of poor perfusion include slow capillary refill times, bradycardia or tachycardia, hypotension,

and a weak pulse.

If these treatments do not improve blood oxygenation, the animal should be placed in the sternal recumbent position as soon as possible and awakened from anesthesia with continuous oxygen support.

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03 Hypothermia

Hypothermia, with a core body temperature <36.6°C, can lead to a myriad of adverse effects, including delayed drug metabolism, cardiovascular dysfunction, impaired perfusion, impaired respiratory function, brain depression, and increased

incidence of wound infection. Cats are prone to hypothermia due to their large body surface area. Heat is lost mainly through radiation, evaporation, and conduction. The most effective way to prevent heat loss is therefore to raise the room

temperature or wrap the animal in a warm blanket. It is recommended that warming measures are initiated before administration and are maintained until the animal awakens. Using active warming methods to maintain the core body

temperatures, such as forced air conditioning and medical electric blankets and circulating warm water blankets to warm up, will be more effective than passive insulation methods such as blankets, towels, and bubble covers. Within limits, it is

also important to use warm solutions to flush and keep dry. Fluid therapy and dry cold gas in the breathing circuit have little effect on heat loss, but providing warm fluids and hot humidified gas can reduce heat loss.

Postoperative temperature monitoring should be continued to prevent hypothermia or hyperthermia.

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04 High body temperature

Reactive hyperthermia has been reported in cats following general anesthesia or sedation, with body temperatures as high as 41.1-42.2°C. The first reported hyperthermia was associated with hydromorphone administration and several other

opioid drugs and ketamine can also cause elevated body temperature. The degree of hypothermia may be related to the degree of hypothermia during anesthesia, and Posner and colleagues showed that sick animals with a lower core

the temperature at the end of anesthesia produced significantly more heat in the recovery period.

Treatment of hypothermia is usually with supportive therapy, including the use of acetyl Prozac (vasodilator), removal of heat sources, moistening the animal with warm water, and the use of air conditioning.

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05 Cardiac arrhythmias

Common arrhythmias in the perioperative period include sinus tachycardia, sinus bradycardia, atrioventricular block, and ventricular arrhythmias. Monitoring is performed using auscultation or ECG and/or by observing the inconsistency of the

pulse heart rate with the Doppler ultrasound SpO2 waveform. The decision to treat the arrhythmia should be based on the severity, the effect on other hemodynamic parameters (e.g. blood pressure), and the potential for deterioration to a

more severe arrhythmia.

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06 Tachycardia

Tachycardia is defined as a heart rate (HR) >180 bpm in cats and >150-190 bpm in large and small dogs during anesthesia. Tachycardia cannot be simply attributed to the inadequate depth of anesthesia but may be secondary to injurious

stimuli, hypoxemia, hypercapnia, and hypovolemia, or to the use of drugs such as alfaxalone, ketamine, atropine, and dopamine.

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07 Regurgitation

Regurgitation can cause oesophagitis and aspiration pneumonia, and in extreme cases can lead to oesophageal stricture. When reflux occurs, oesophageal suctioning followed by saline or tap water irrigation is recommended, along with

tracheal intubation to protect the airway. Dilute bicarbonate drops can be introduced into the esophagus to increase pH. maropitant prevents vomiting and promotes a faster return to normal feeding, but the incidence of reflux is less affected.

Metoclopramide, ranitidine, and omeprazole also appear to have a minimal effect on reflux. When cisapride 1mg/kg was combined with omeprazole 1mg/kg, the incidence of reflux was significantly reduced.

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Reference:

【1】AAFP Feline Anesthesia Guidelines

【2】AAHA Troubleshooting Anesthetic Complications