Gastrointestinal Disease and Anaesthesia

Introduction

The major role of the gastrointestinal tract is the uptake of important electrolytes, nutrients and water and the excretion of some breakdown products. In cases of gastrointestinal disease, patients can often become dehydrated and in cases of malabsorptive disease, such things as hypoproteinaemia may develop. This has important implications for those patients requiring general anaesthesia as the patient is not only in a deteriorated state but things such as hypoproteinaemia and dehydration can alter the action of the drug due to these affecting protein binding and distribution for example. Weight loss is often a clincial sign in these patients, and so problems with temperature regulation may also be seen during anaesthestic procedures.

The liver is also plays an important role as it is a major site of breakdown and generation of compounds. It is a major site of drug breakdown also, including many anaesthetic agents. This means that if there is any form of hepatic disease, it has implications on anaesthetic procedures in these patients.

Oesophagus

There are many disorders affecting the oesophagus. These include foreign bodies, megaoesophagus and oesophagitis. In these patients, it is important to select agents that will not induce vomiting as this may exacerbate the condition and impact on the condition of the patient. These patients can also often be dehydrated due to reduced intakes, and in severe cases, loss of condition. This means that the patients may often need to be stabilised with fluids before the procedure if possible, and during the procedure, fluids should be warmed to help with temperature regulation in poor condition patients. Intubation should be performed carefully in these patients as the airway may already be restricted depending on the condition affecting the oesophagus, and it is important to avoid intubating the oesophagus as it may push an foreign body further down, making retrieval more difficult, or cause further damage to the oesophagus. There is also a higher risk of aspiration in these patients so they should be placed in sternal recumbancy with their head's slightly elevated. These patients should also be assessed and given appropriate analgesia.

Oesophagitis

Oesophagitis can be caused by aspiration of gastric material during general anaesthesia. Clincal signs which should make the clinican suspicious of reflux having occured include profuse salivation and repeated and painful attempts to swallow. In these cases, the oesophagus should be lavaged with fluids to try and remove the material to prevent further inflammation.

Stomach and Intestine

General anaesthesia may be required for a number of procedures in patients, including diagnostics and surgery. Most commonly, general anaesthesia is required for foreign body removal, biopsies and correction of gastric dilatation-volvulus (GDV). As mentioned before, these patients can often be severely dehydrated and so many require stabilisation before surgery, if possible. Analgesia is an important consideration in these patients and often included in the premedication.

Gastric Dilatation-Volvulus

Gastric Dilatation-Volvulus (GDV) is a life threatening condition seen in deep chested breeds of dogs, requiring surgical correction. Due to the nature of the condition, hypovolemic shock occurs due to occlusion of venous return through the caudal vena cava because of the dilated stomach. It also decreases flow through the portal vein reducing venous return. This reduction in venous return leads to a poor cardiac output resulting in pale mucous membrane colour, reduced capillary refill time, poor pulse quality and tachycardia. This means that the patient needs to be cardiovascularly stablised before undergoing general anaesthesia. A stomach tube should be passed to deflate the stomach, and intravenous fluids should be administered to help correct the poor cardiac output. Baseline bloods should be obtained to check for any underlying electrolyte and acid-base disturbances as these may need to be corrected before surgery.

In some severe cases, premedication may not be necessary, otherwise agents which have minimal effects on the cardiovascular system should be selected and at the lowest dose to give the desired effect. If possible, monitoring equipment should be set up before induction and the patient preoxygenated. Induction agents should be selected again, based on their effect on the cardiovascular system, so those agents with minimal effect should be chosen and given to effect. Maintenance can be done using gaseous agents, such as isoflurane in oxygen but nitrous oxide should be avoided in these patients as it may exacerbate the dilated stomach. Monitoring throughout the anaesthetic should include the normal parameters and include blood pressure and an ECG. If possible, acid base and blood gases should also be performed throughout the procedure. The ECG is important as cardiac arrhythmias are common in correction of GDVs.

Liver

The liver plays an important role in many biological functions. It is the site of albumin and coagulation factor production as well as the site of drug metabolism. If there is an alteration in albumin concentrations then the degree of protein binding of anaesthetic agents is affected, similarly if the liver is damaged then breakdown of some anaesthetic agents will be altered also. It is therefore important that agents are carefully selected such that they are minimally metabolised by the liver and protein binding has little effect on the pharmacokinetics of that agent. Glucose concentration should be closely monitored throughout any anaesthetic in these patients as the liver is a site of gluconeogenesis and so hypoglycemia may be encountered in these patients. Blood pressure should be monitored closely in patients undergoing portosystemic shunt ligation as this alteration in blood flow to the liver can affect systemic blood pressures.