Post-operative nausea and vomiting (PONV) can be defined as any nausea, retching or vomiting that occurs in the first 24 to 48 hours following general anaesthesia (GA). In humans, PONV is considered one of the most common causes of patient dissatisfaction when recovering from a GA and occurs in 30 percent of patients. It has been reported that patients would rather have a reduction in the sensation of nausea than the sensation of pain post-operatively (Pierre and Whelan, 2013).
In small animals we should also consider regurgitation when thinking about PONV. Regurgitation is the passive discharge of gastric or oesophageal fluid from the mouth or nose and can cause similar, if not the same, side effects as PONV (Lamata et al., 2012).
Although often well managed, PONV is an unpleasant experience for patients and can cause further complications including aspiration pneumonia, oesophagitis and suture dehiscence, especially for those who have had abdominal surgery. This can lead to possible prolonged hospitalisation periods and increased costs for clients (Rahman and Beattie, 2008).
Vomiting is a protective reflex coordinated by the vomiting centre, located in the medulla oblongata. The vomiting centre has input from four different locations, all of which cause nausea or vomiting for particular reasons. The cerebral cortex induces vomiting related to behaviour, anxiety and pain; the vestibular system due to vestibulitis and motion sickness; the chemoreceptor trigger zone (CTZ) receives input from blood-borne substances; and the abdominal visceral receptors are sensitive to vomiting caused by the gastrointestinal (GI) tract (Schoeman, 2008). Table 1 shows the main neurotransmitters involved in nausea and vomiting in dogs.
Vomiting is the active expulsion of gastrointestinal material, with the act of vomiting itself involving various stages and the central coordination of the reflex. The patient takes a deep breath in, the larynx is raised and the upper oesophageal sphincter opens, the glottis closes and the soft palate lifts. Following this the diaphragm and abdominal muscles contract, causing positive pressure in the abdomen. The stomach is squeezed and GI content is ejected along the oesophagus and out of the mouth. In contrast regurgitation is a passive act and is generally the return of undigested food (Lane and Cooper, 1999).
There are procedural factors that will influence a patient’s likelihood of suffering from PONV. Abdominal, ophthalmic and middle ear surgery are examples of procedures thought to be associated with a higher incidence of PONV. Abdominal surgery will often involve ischaemic events, which cause release of 5-HT3, or frequent handling of the bowel, causing ileus (Rahman and Beattie, 2008). An emergency that needs a GA is at increased risk of PONV. These cases are not pre-planned or expected, so often the patients haven’t been starved following current guidelines. Whilst under GA, patients do not have control of their reflexes, so having a full stomach is likely to cause regurgitation. Patients suffering from vestibular syndrome should be considered as they are often already vomiting. Giving them a GA will likely increase this on recovery. Reducing their kennel size and covering their eyes (Figure 1) may help.
In dogs, a common cause of PONV is the use of preanaesthetic and perioperative medications, including sevoflurane and opioids such as methadone and buprenorphine. These are all medications which are used in veterinary practice daily for patients having a procedure under GA. The incidence of vomiting can be increased by high doses being given intravenously (Hay Kraus and Cazlan, 2019).
The breed of the patient being anaesthetised is another risk factor that needs to be considered. Due to their increased effort to ventilate, brachycephalic breeds have an increase in negative pressure which draws stomach content into the chest, therefore increasing their risk of PONV when they have a GA (University of Cambridge, 2021). Dachshunds have been seen to have PONV more than other breeds. This is likely due to them commonly having spinal surgery, which has already been established as having an increased rate of PONV. Entire females are thought to be at a higher risk compared to males due to their hormones. Patients that have a history of PONV are more likely to suffer from the same problem with future GAs. Being aware of a patient’s history will enable an appropriate anaesthetic plan to be made in order to minimise PONV on recovery.
A lot of the patients we see in practice will have multiple risk factors. If each patient is assessed on an individual basis, appropriate anaesthetic plans can be made with appropriate medications to minimise these confounding factors.
What can we do?
There are anti-emetic medications, such as maropitant, that can be given prior to a GA. Maropitant is an NK1 receptor antagonist given to reduce nausea; it also makes patients twice as likely to eat once they have recovered. Maropitant has analgesic properties, so as well as helping to reduce PONV, it will help with visceral pain following abdominal surgery and surgery entering the chest cavity. Another medication that can be given as part of a premedication is omeprazole. Giving omeprazole will reduce the pH of the stomach acid making it less damaging to the oesophagus if the patient was to suffer from PONV (Panti et al., 2009).
Reducing the dose of opioids given can help reduce PONV, as long as it isn’t going to compromise the level of analgesia needed for the patient. The use of paracetamol or non-steroidal anti-inflammatory medications could be considered alongside a lower opioid dose. Local and regional blocks can be used for most surgeries; using one of these will reduce the amount of opioids and inhalational agent needed. Metoclopramide can be used to help reduce PONV. It can be used as a prokinetic to reduce acid reflux and it blocks messages between the CTZ and the vomiting centre.
There are practical considerations which may help PONV. Inflating the endotracheal tube (ETT) as soon as possible will protect the airway and prevent any complications if regurgitation occurs. Having a head-up induction and intubation (Figure 2) will lower the risk of gastric reflux; this is also helped by elevating the patient’s head. It is a good idea to check the patient’s mouth before extubation as it allows you to visualise whether there has been an occurrence of regurgitation. If there has, suction can be used to eliminate the material and minimise the risk of aspiration pneumonia.
Withholding food from our patients will prevent the risk of POVN; however, if food is withheld for too long it will reduce the pH of gastric reflux, making it more damaging to the oesophagus. There is no set answer on how long a patient should have food withheld prior to anaesthesia, but current guidelines have been produced by the American Animal Hospital Association (Grubb et al., 2020).
Assessing each patient as an individual and considering PONV when making a GA plan will help to improve patient well-being and potentially give them a more comfortable recovery. Although they cannot speak, they will thank you for it!