21. Nausea and Vomiting

Question 1

Define nausea and vomiting.

Answer 1

Nausea is the sensation of the need to vomit.

Vomiting is the involuntary,
forceful expulsion of gastric
contents via the mouth.

Question 2

Describe the physiology of vomiting.

Answer 2

The physiology of vomiting is complex

with multiple
afferent and efferent pathways;
an overview is helpful.

The chemoreceptor trigger zone (CTZ)
lies in the floor of the fourth ventricle
in the area postrema

and

is functionally outside of the
blood–brain barrier.

It contains dopamine (D2)
and
serotonin (5-HT3) receptors.

The CTZ provides efferent input
to the vomiting centre,
which is located in the medulla.

The vestibular system, 
peripheral pain pathways, 
intestinal chemoreceptors and
the cerebral cortex all provide 
direct afferent input to the 
vomiting centre via
cranial nerves VIII, IX and X.

Question 3

Describe the process of vomiting.

Answer 3

Vomiting is an involuntary reflex
and
may be divided into two phases,

a preejection phase
and
ejection phase.

Pre-ejection phase:

• Nausea

• Sympathetic stimulation –
tachycardia, tachypnoea, sweating

• Parasympathetic stimulation –
salivation, upper and lower oesophageal
sphincters relax,
giant retrograde contraction of the small intestine.

Ejection phase:

• Respiration temporarily ceases mid-inspiration.

• Hyoid and larynx raise to open the crico-oesphageal
sphincter.

• Glottis closes.
• Soft palate elevates to close off the nasopharynx.

• Contraction of the diaphragm and
abdominal muscles results in a rise in
intra-abdominal pressure.

• Gastro-oesophageal sphincter opens.
• Ejection of gastric contents.

Question 4

What are the potential complications of vomiting?

Answer 4

Vomiting may result in potentially
life-threatening complications:

• Aspiration (particularly if GCS is reduced)
• Wound dehiscence

• Electrolyte imbalance
(loss of hydrogen, potassium and chloride)

• Dehydration
• Elevated intraocular and intracranial pressure.

Question 5

Fig. 21.1 Overview of nausea and vomiting

Answer 5

77

Question 6

What are the main risk factors for PONV?

Answer 6

Divide the answer into patient,
anaesthetic and
surgical factors.

Patient factors

• Female gender
• Non-smoker
• Previous PONV
• History of motion sickness

Anaesthetic factors
• Use of N2O

• Use of opiates
• Use of etomidate
• Use of neostigmine
• Hypotension

Surgical factors

• Middle ear surgery

• Ophthalmic surgery
(especially squint-correction surgery)

• Gynaecology surgery