GI emergencies are one of the most common emergency presentations. Patients presenting with gastrointestinal disorders or dysfunctions—gastric dilatation volvulus, haemoabdomen, intestinal/abdominal foreign body, gastrointestinal perforation, etc.—routinely require general anaesthesia to facilitate complete examination, obtain necessary diagnostic testing or imaging, or for surgical intervention. Specific considerations regarding the type of gastrointestinal dysfunction should be made for each step in the anaesthetic process from premedication and induction to the recovery and postoperative phases. Good anaesthetic monitoring during critical GI procedures can mean the difference between life and death.
All elements of the anaesthetic plan come into play when considering what monitors to use and knowing what values to expect from those monitors. Many premedicants have expected side effects and it is necessary to differentiate between what is caused by a drug and what is caused by the procedure or condition. It is also imperative to know what drugs are on board that could potentially be increased, decreased, or reversed.
When creating an anaesthetic plan the premedication combination typically consists of a sedative and an analgesic. The sedative or analgesic chosen will depend greatly on the type and severity of the gastrointestinal issue and the personality of the patient. Standard choices for sedatives include benzodiazepines, phenothiazines, and alpha-2 adrenergic agonists.
Benzodiazepines provide adequate sedation (but no analgesia) for most patients with calm personalities. They have little negative effect on cardiovascular, respiratory, or gastrointestinal systems. Benzodiazepines can be quickly reversed using flumazenil.
Phenothiazines provide good sedation (but again, no analgesia) for most animals and are a good choice for aggressive or anxious patients. They are potent antiemetics, which is of benefit when dealing with patients who should not be vomiting (i.e., GDVs, foreign bodies with a sharp element, megaoesophagus, perforation, etc.). Phenothiazines may decrease sensitivity to some ventricular arrhythmias and can decrease the seizure threshold. Phenothiazines have little effect on respiratory function, but can result in dramatic reduction in cardiovascular output. It may cause some decrease in the time of gastric emptying. There is evidence that phenothiazines can cause splenic enlargement. This can cause difficulty for surgeons working in the abdominal cavity. This category of drugs may also interfere with platelet aggregation. Phenothiazines are not reversible.
Alpha-2 adrenergic agonists provide excellent sedation for most patients and provide very minor analgesia (not enough for any surgical purpose). The level of respiratory depression can vary greatly and the response may be erratic. This class of drug can cause dark mucous membrane colour that is unrelated to oxygenation of tissue. In patients with respiratory compromise (if abdominal contents are restricting respiration) this can make it more difficult to visually assess potential hypoxaemia. It can cause profound bradycardia and vasoconstriction. It is not an appropriate drug choice for any patient with cardiac compromise. It causes emesis in some dogs and most cats, though this is seen less often when administered intravenously. It may decrease gastric motility. Alpha-2 adrenergic agonists are reversible with atipamezole.
The second part of the premedication mixture is an appropriate opioid analgesic. There are many choices available for a variety of situations. Butorphanol is an agonist-antagonist and buprenorphine is a partial agonist. Both drugs provide some sedation and are less likely to cause vomiting. They can cause decreased gastrointestinal motility, but less markedly than other opioids. Both drugs can be reversed with naloxone, but this may be difficult as they bind tightly to receptor sites. The analgesia provided is mild and in the case of butorphanol, very short acting (30–60 minutes), so these drugs are only appropriate for short, relatively painless procedures like endoscopy, diagnostic imaging or minor biopsies.
Opioid agonists such as morphine, methadone and fentanyl are better choices for painful or invasive procedures such as an abdominal exploratory laparotomy or oesophageal foreign body removal. Morphine and hydromorphone, when given intramuscularly, usually cause vomiting and ptyalism. If given IV, the incidence of vomiting is dramatically decreased (if giving morphine IV it should be done very slowly to prevent sudden histamine release). Fentanyl given IV is the best choice for any case where risk of vomiting would be detrimental.
All opioid agonists can cause depression of the cardiovascular and respiratory centres and are known to slow gastrointestinal transit time. This drug class provides sedation and excellent analgesia for varying periods of time. All can be given intraoperatively as constant rate infusions and can be quickly reversed with naloxone.
The most common choices for anaesthetic induction are propofol, ketamine/diazepam, and etomidate. Propofol is a sedative-hypnotic that is relatively safe for use in most patients. It does cause transient apnea and a dose-dependent cardiovascular depression. In cats it is known to cause Heinz body formation following repeated administration. Ketamine/diazepam is readily available and inexpensive. It is safe for use in most patients. Ketamine causes an increase in cardiac contractility, which can lead to an increase in heart rate and blood pressure. Etomidate is an imidazole derivative that has minimal cardiovascular or respiratory effect. It is expensive and tends to be used for extremely critical patients. Etomidate can cause suppression of adrenal functioning and acute haemolysis. Most importantly when considering this agent for gastrointestinally dysfunctional cases, it often causes nausea, retching and vomiting at induction. This side effect can be avoided if the patient is appropriately sedated and if an adequate amount of the drug is used.
Intubation of patients should be done quickly and the cuff of the endotracheal tube checked and sealed immediately. This group of patients is at high risk of regurgitation of stomach contents and it is important to avoid aspiration of any fluid or material. Check the seal on the endotracheal tube again after 15 or 20 minutes as further muscle relaxation usually occurs and can allow additional air/gas/fluid to pass around the endotracheal tube.
Considerations should be made if the patient has any co-morbidity that could make intubation difficult, such as an oropharyngeal mass or oesophageal foreign body displacing the trachea; in challenging cases, supplies for a tracheostomy should be ready.
Abdominal distention, electrolyte imbalance, and haemorrhage are the biggest complications associated with GI emergencies that most directly impact anaesthetic monitoring. Each obstacle has a string of further difficulties tied to it. Abdominal distention affects cardiac and respiratory function, which in turn affect blood pressure, tissue perfusion, and oxygenation. Electrolyte imbalances affect organ function and cardiac rhythm. Haemorrhage affects cardiac function, blood pressure, and tissue perfusion.
The gold standard for anaesthetic monitoring of GI emergencies would include: pulse oximetry, capnometry, invasive blood pressure, blood gas sampling, and electrocardiography.
Pulse oximetry measures oxygen saturation of haemoglobin. It can give some degree of information about ventilation status and tissue perfusion. The pulse wave can alert an attentive anaesthetist to possible arrhythmias. A pulse-ox monitor can be placed in a number of areas—tongue, toes, ears, vulva, prepuce, or rectally. Pulse oximetry is a very popular monitoring choice for several reasons—it is simple to use, noninvasive, provides quick and constant information, and is relatively inexpensive. However, much of the information given by the pulse-ox must be taken with a grain of salt. There can be discrepancies in the readings given if the tissue where the probe is placed is too dry, dark, or thick. The readings are not always precise in terms of actual arterial oxygenation. The pulse-ox may still read high even though the arterial oxygenation level is falling. It isn’t until the arterial oxygenation level has fallen far enough that the pulse-ox will begin to show lower readings.
Abdominal distention can impair cardiac function and impede appropriate ventilation. Mechanical ventilators are often needed for adequate ventilation. Capnometry is a vital monitor in these cases and will give information on the inspired and expired amounts of carbon dioxide. A capnograph can give information on cardiac function, respiratory function, and machine malfunction. The capnometry wave can highlight hypercapnia, hypocapnia, decreased cardiac output, poor pulmonary perfusion, airway obstruction, and anaesthetic system malfunctions. Capnometry is a simple, noninvasive tool that can give the anaesthetist vital information on ventilatory status.
Impaired cardiac function equals hypotension and decreased perfusion. Direct arterial pressure monitoring is the best way to obtain constant, accurate, real-time information about blood pressure. Arterial catheters can be placed in the dorsal pedal, lingual, auricular, or coccygeal arteries. Invasive blood pressure monitoring requires a higher level of skill and slightly more equipment than oscillometric monitoring, but provides a wealth of information. In addition to systolic and diastolic numbers, arterial monitors can highlight volume issues, ventilator induced cardiac depression, and cardiac dysrhythmias. Invasive blood pressure monitoring can often be the first to pick up haemorrhage or hypovolaemia.
Many patients that suffer dehydration from excessive vomiting/diarrhoea or those with a gastric dilation-volvulus may have electrolyte imbalances and/or acid base disturbances. These abnormalities should be corrected before anaesthesia if at all possible. If stabilization is not possible pre-anaesthesia, aggressive fluid therapy with appropriately balanced solutions should begin immediately. Electrolyte panels and arterial blood gas parameters should be checked regularly during the intraoperative period. If an arterial line is in place, blood sampling is much more convenient.
Respiratory disturbances causing acidosis or alkalosis occur when a patient is hyperventilating (in response to pain, fever, overcompensation with manual/mechanical ventilation) or hypoventilating (as a result of drug administration, CNS trauma, or restricted thoracic movement). Metabolic disturbances causing acidosis or alkalosis occur in response to electrolyte imbalances. Acidosis is common in conjunction with shock, sepsis, pancreatitis, hypoxaemia, and some toxins. Alkalosis commonly occurs in conjunction with vomiting, hypokalaemia, steroid therapy or bicarbonate therapy. Serial blood gas analysis can highlight metabolic abnormalities, assess adequacy of ventilation, and monitor response to treatment.
Cardiac dysrhythmias are extremely common in volvulus cases, abdominal exploratory laparotomy with splenic involvement, and may be seen in patients with severe electrolyte imbalances so ECG monitoring is strongly recommended.
ECGs show electrical activity of the heart and can alert the anaesthetist to arrhythmias and dysfunctions, but in no way tells the level of cardiac output or tissue perfusion. ECGs are easy to use, noninvasive, and can be easily placed before anaesthesia if warranted. ECGs show common dysfunctions that can be linked to electrolyte imbalances, levels of anaesthesia, ventilation issues. Bradycardia is commonly associated with hyperkalaemia, anaesthetic drugs, extreme hypoxia, hypoglycaemia, and vagal stimulation. Tachycardia can be associated with pain, light anaesthetic plane, hypercapnia, hyperthermia, hypoxia, and haemorrhagic/hypovolaemic shock. VPCs are commonly associated with GDVs and splenic manipulation. Arrhythmias are associated with electrolyte imbalances and ECGs can be the first line to alert the anaesthetist to conductive issues.
Serious cases should be recovered and maintained in a critical care unit for continued fluid support and postoperative monitoring. Cardiac dysrhythmias are common postoperatively in many surgical cases and ECG monitoring may be necessary. It is also essential to monitor for both electrolytes and acid-base disturbances in the postoperative period. Shock, hypo/hyperkalaemia, acidosis/alkalosis, respiratory impairment, and hypovolaemia can all be consequences associated with gastrointestinal dysfunction. Depending on the severity of the case additional analgesia may be needed. CRIs of fentanyl or morphine-lidocaine-ketamine are often used, as are fentanyl patches and IV or IM doses of opioid analgesics.