Post-operative nausea and vomiting

Post-operative nausea and vomiting (PONV) is one of the most common side effects associated with surgical procedures.^[1]  It can be very distressing for patients, can lead to medical complications and imposes an economic burden.  The medical complications of PONV include possible wound disruption, oesophageal tears, gastric herniation, muscular fatigue, dehydration and electrolyte imbalance.  There is also an increased risk of pulmonary aspiration of vomitus.  Aside from the medical complications, PONV can have psychological effects that may result in patients experiencing anxiety about undergoing further surgery.  The cost implications of PONV can be major because of delayed recovery and discharge, increased medical care and occasionally re-operation.

The aetiology and risk factors for PONV

A large study reported rates of 37% for nausea and 20% for vomiting in patients undergoing general anaesthesia^[1].  There are, however, considerable variations in the reported prevalence of PONV^[2],[3],[4], which can be attributed to a number of factors, as discussed below.

In the UK, it has been estimated that PONV affects between one and two million patients every year^[5],[6].  However with the growth in the number and complexity of procedures since this analysis was reported, and the growth in day cases this may be an underestimate. 

Risk factors for PONV can be divided into patient risk factors, procedural risk factors, anaesthetic risk factors and post-operative risk factors.

Patient risk factors

Certain patient groups are at a higher risk of PONV than others.  The following are some of the particular risk factors ^{[2], [4], [7], [8]}.

• Gender.  The prevalence of PONV is three times higher in women than in men.  This gender difference is not evident in pre-pubertal children or in the elderly, which indicates that there may be hormonal involvement.
• Age.  Children are two times more likely to develop PONV than adults.  PONV is low in very young children, increases up to the age of 5 and is highest in children between the ages of 6 and 16 years.
• Obesity.  Fat-soluble anaesthetics may accumulate in adipose tissue and continue to be released for an extended period resulting in prolonged side effects, including PONV.
• Migraine.  Patients with a history of migraine are more likely to experience PONV.
• Pre-operative eating patterns.  Adequate pre-operative fasting reduces the risk of PONV, whereas excessive starvation appears to increase the risk.  In emergency surgery where there has not been an adequate fast the risk is increased.
• History of PONV or motion sickness.  Such patients may have a lower threshold to nausea and vomiting than the rest of the population.  Anxiety, due to a previous experience of PONV may add to the risk.
• Gastroparesis.  Patients with delayed gastric emptying secondary to an underlying disease may be at increased risk of PONV.

Procedural risk factors

The type and duration of surgery is a major factor in PONV.  Extended surgical procedures are more likely to lead to PONV than shorter operations, and the following surgical procedures predispose to a higher incidence of PONV.

• Gynaecological
• Abdominal, especially gastrointestinal
• Laparoscopic
• Ear Nose and Throat
• Ophthalmic

Anaesthetic risk factors

Certain anaesthetic agents have been associated with a higher incidence of PONV than others.  The following can increase the risk of PONV.

• Choice of premedication
• Use of opioid analgesics
• Use of nitrous oxide
• Use of some inhalation agents
• Longer procedures and greater depth of anaesthesia

Post-operative risk factors

A number of post-operative factors can influence the risk of PONV.

• Pain - relief of pain is often associated with the relief of nausea, though the use of opioid analgesics may exacerbate the risk because of their known emetic potential.  However some patients may be willing to tolerate a degree of pain provided they are free of nausea and vomiting.
• Dizziness - PONV is increased in patients who experience dizziness.
• Early ambulation - Early or sudden movement can increase the risk of PONV, especially if the patients have received opioids
• Use of opioids - the use of opioids may exacerbate the risk of PONV because of their known emetic potential
• Hypotension - post-operative hypotension is common and can trigger PONV
• Premature oral intake - It is generally considered wise to restrict oral intake, and then to recommend small sips of water to minimise the risk of PONV.

The causes of PONV

As discussed in more detail elsewhere numerous neuronal pathways converge on the Vomiting Centre in the medulla in the hind brain where the vomiting reflex is initiated.  These include vagal sensory pathways from the gastro-intestinal tract and neuronal pathways from the labyrinths, higher centres of the cortex, intracranial pressure receptors and the Chemoreceptor Trigger Zone (CTZ).  The exact involvement of each of these pathways in PONV is not known and it is likely to vary with the surgical procedure and drugs employed. Activation of the CTZ (which monitors agents in the bloodstream and cerebrospinal fluid) by anaesthetics, opioids and humeral factors released during surgery is thought to be important, as is activation of the labyrinths and gastrointestinal tract resulting from surgical manipulation.

The factors implicated in PONV are illustrated schematically in Figure 5.

The consequences of PONV

PONV can have practical consequences for patients and carers, can lead to medical complications and can impose an economic burden on the hospital.

Practical consequences

PONV is very distressing to patients at a time when they are already feeling very uncomfortable and anxious.  They may be embarrassed by vomiting in the presence of staff and other patients, and by the need for hospital staff to clear up after them.  Patients who have experienced PONV after previous surgery can be anxious, adding to the risk of further occurrences.  Patients are reported to be willing to suffer a degree of pain in preference to experiencing PONV ^[9].

Medical complications

Medically there is a risk that the powerful muscular contractions associated with nausea and vomiting could lead to damage to the stitches of wounds and to an increased risk of bleeding, so affecting the outcome of the operation.  There is also a possibility of the regurgitation of stomach contents, leading to risks of respiratory obstruction, pulmonary inflammation and aspiration pneumonia.  Electrolyte imbalance and dehydration can occur if PONV is severe, which can be a particular issue with young children.  Finally the delayed ability to take oral therapy and nutrition may be a concern.

Economic burden

There have been many studies that have tried to quantify the costs associated with PONV (see for example ^{[2], [13], [10]}). The factors generally considered to be important are personnel time in clearing up and material costs of disposable products, laundry, caring for patients, delayed discharge, unplanned admission leading to bed blocking, delayed surgical throughput and potential re-operation costs.  An Audit Commission Report in 1997 reported that the most common cause for unplanned overnight hospitalisation after surgery is PONV ^[11].

The management of PONV

No single drug or class of drug is fully effective in controlling PONV, presumably because none block all pathways to the Vomiting Centre.  However, because of the multi-receptor origin of PONV, combination therapy is being more widely employed.  A recent British Journal of Anaesthesia editorial suggests this approach may be appropriate ^[12].  Figure 6 summarises the sites of actions of the drugs that influence PONV.

Using a decision-analysis treatment model, it has been suggested that prophylactic anti-emetic therapy can be more cost-effective compared with treatment of established symptoms where operations are associated with a high risk of emesis^[13].

In view of the complex nature of PONV and the many factors that predispose to the risk, many hospitals have introduced protocols or guidelines to standardise the assessment and management of PONV (see for example ^{[8], [9]}).  Some examples are reproduced in this resource with permission of the hospitals.  Adequate pain relief, hydration and maintenance of blood pressure will contribute to the control of PONV, as will prior patient education and information. In many cases protocols will involve either prophylactic drug treatment for all patients or pre-operative risk assessment with appropriate treatment for at risk groups.

Summary

• Post-operative nausea and vomiting (PONV) is one of the most common side effects associated with surgical procedures and is distressing to patients, can lead to medical complications and has an economic cost
• In the UK, it is estimated that PONV affects between one and two million patients every year
• Several factors govern the susceptibility of an individual to PONV.  These include age, gender, the anaesthetics and analgesics used, obesity, a history of motion sickness and the type and duration of surgery.
• The exact involvement of the multiple emetic pathways in PONV is not known and it is likely to vary with the surgical procedure and drugs employed.
• Pre-screening patients for potential risk of PONV and then the prophylactic use of appropriate drugs is becoming common practice.  For high risk patients combination therapy with drugs with different mechanisms of action is becoming more widespread.

References