D:\E-TALC\DESIGN~1\CD-ROMS\E-TALC~3\UPDATE~1\16.pmd

Update in Anaesthesia

Question 1

Nerve stimulators

A. Double burst is of particular value in assessing blockade

when there is no response to the train-of-four

B. A fading pattern to the train-of-four excludes prolonged

action of suxamethonium

C. When used to locate nerves for regional blockade the

positive electrode should be attached to the locating needle

D. The post tetanic count is particularly useful for assessing

the patient’s suitability for extubation

E. The ideal nerve for neuromuscular blockade assessment is

generally considered to be the facial nerve

Question 2

The following potentiate the action of non-depolarising

neuromuscular blockers

A. Diethyl ether

B. Quinidine

C. Enflurane

D. Lithium

E. Dantrolene

Question 3

When looking at an ECG

A. A bifid P wave may indicate that the patient has mitral

stenosis

B. Peaked P waves are associated with right atrial hypertrophy

C. A short PR interval indicates that the patient has

Wolff-Parkinson-White syndrome

D. Left bundle branch block is associated with a secondary R

wave in V6

E. U waves are associated with hypothermia

Question 4

The following ECG changes are associated with the correct

cause

A. U waves - Hypothermia

B. T wave flattening - Hypothyroidism

C. Short QT Interval - Hypercalcaemia

D. Biphasic P wave in V1 - Mitral stenosis

E. T wave inversion - Hypokalaemia

Question 5

The following ECG changes occur with hypokalaemia

A. Tall T waves

B. ST elevation

C. Loss of P waves

D. T wave inversion

E. Prominent U waves

Question 6

The CVP trace

A. The c wave follows the X descent

B. The X descent occurs in (ventricular) diastole

C. The v wave results from right atrial filling against a

closed tricuspid valve

D. Cannon waves are seen in complete heart block

E. The a wave is of variable size in atrial fibrillation

Question 7

Anaesthesia breathing circuits

A. The Lack circuit is a Mapleson D system

B. The Magill attachment is more efficient for spontaneous

breathing than for controlled ventilation

C. The recommended fresh gas flow rate for controlled

ventilation through a Bain circuit is 70 ml/kg body weight

D. Minimum fresh gas flow to avoid rebreathing using a

Mapleson A is equal to that patient’s minute ventilation

E. The Mapleson F system is a modification of the Mapleson

E system

Question 8

The following cause a rise in the end-tidal carbon dioxide level

(assuming constant ventilation)

A. Hypothermia

B. Malignant hyperpyrexia

C. Pulmonary embolus

D. Disconnection of the inner tube of a Bain circuit

E. Failure of the endotracheal tube cuff

Question 9

The concentration of vapour in the gas mixture emerging from

the outlet port of a vaporiser depends on

A. Saturated vapour pressure of the agent

B. Flow characteristics of the vaporiser

C. Duration of use

D. Surface area of the agent in contact with the gas mixture

E. Temperature

MCQ QUESTIONS

Dr Ed Hammond, Exeter

Update in Anaesthesia

Question 10

Anaesthesia at high altitude (using a plenum vaporiser)

A. The concentration delivered by the vaporiser will be

higher than the dialled value

B. The concentration dialled into the vaporiser will need

to be higher for the same effect

C. The concentration dialled into the vaporiser will need

to be lower for the same effect

D. The inspired oxygen concentration may need to be

increased

E. The anaesthetic potency of 50% nitrous oxide will be

reduced

Question 11

A low resistance to gas flow is a feature of the following

vaporizers

A. Oxford minature vaporizer

B. Epstein Macintosh Oxford vaporizer

C. Copper Kettle Vaporizer

D. Goldman Vaporizer

E. Boyle’s bottle

Question 12

The following are true concerning humidity and humidification

of gases

A. Relative humidity is the ratio of absolute humidity to

saturated humidity at a specified temperature

B. Operating theatre humididty should be maintained at

no more than 30%

C. Heat and moisture exchangers can achieve 40% humidity

D. A nebuliser works on the poiseuille effect to entrain

water across a pressure drop

E. The water trap for a simple bottle humidifier must be

larger than the humidifier bottle

Question 13

During brachial plexus blockade

A. The interscalene approach commonly leads to inadequate

blockade of the ulnar nerve

B. The axillary approach may lead to Horner’s syndrome

C. The supraclavicular approach commonly leads to

inadequate blockade of the axillary nerve

D. Bilateral interscalene blocks should be used for bilateral

shoulder manipulation surgery

E. The axillary approach commonly leads to inadequate

blockade of the median nerve

Question 14

Complications of retrobulbar blockade for cataract surgery

include

A. Bradycardia

B. Retinal detachment

C. Brain stem anaesthesia

D. Vitreous haemorrhage

E. Optic nerve damage

Question 15

Effects of epidural blockade include

A. A greater degree of hypotension when adrenaline

(epinepherine) containing local anaesthetics are used

B. Sympathetic blockade before sensory blockade

C. Anterior spinal artery syndrome

D. Reduced tidal volume with a normal block to T4

E. Reduced peristalsis

Question 16

A successful stellate ganglion block may cause

A. Ipsilateral miosis

B. Contralateral nasal congestion

C. Bilateral ptosis

D. Ipsilateral exomphalos

E. Horners syndrome

Question 17

Concerning brachial plexus blockade

A. The interscalene approach provides for anaesthesia to

the ulnar border of the forearm

B. The supraclavicular approach is not reliable to produce

anaesthesia to the hand

C. The axillary approach is least likely to cause pneumothorax

D. Diaphragmatic paralysis is a complication

E. Puncture of an artery may be deliberate

Question 18

Inguinal hernia field block

A. Blocks the ilioinguinal, iliohypogastric and genitofemoral

nerves

B. May be employed for testicular surgery

C. Prilocaine 0.5% may be used

D. Depends on depositing local anaesthetic between internal

and external oblique

E. Quadriceps weakness is a complication

Question 19

Serum Na

120 mmol/l and K

6.4 mmol/l are consistent with

A. Hyperaldosteronism

B. Renal failure

C. Hypopituitarism

D. Adrenocortical insufficiency

E. Cushings disease

Question 20

The following conditions are associated with a haemoglobin

concentration of 7 g/dl and a mean corpuscular volume of 70 fl

A. Iron deficiency anaemia

Update in Anaesthesia

B. Acute blood loss

C. Folate deficiency

D. Renal failure

E. Thalassaemia

Question 21

Obesity is associated with

A. An increase in the incidence of airway complications

B. Increased functional residual capacity (FRC)

C. Increased DO

(Oxygen delivery)

D. Pulmonary hypertension

E. Quetelet index of 24.5

Question 22

Diathermy safety

A. Ohm’s law states that voltage = current x resistance

B. Diathermy works because there is a high current density at

the active electrode

C. The heat energy produced by cautery is proportional

to the current at the tip of the active electrode

D. Bipolar diathermy requires a passive electrode (‘diathermy

plate’)

E. Poor contact of the passive electrode (‘diathermy plate’)

may lead to inadvertent patient burns

Question 23

With regard to electrical safety and prevention of explosions

A. Nitrous oxide is not flammable at atmospheric pressure

B. Anaesthetic machines should be isolated from ‘earth’

to prevent completion of an electrical circuit

C. Currents of 10 microamps may initiate ventricular

fibrillation

D. The neutral connection of a circuit is not at earth

potential at the patient end of the circuit

E. In surgical diathermy the heat released depends on the

square of the potential difference between electrodes

Question 24

Ephedrine is unlikely to be effective in reversing hypotension

in patients chronically receiving the following medication

A. Reserpine

B. Alpha-methyl dopa

C. Phenoxybenzamine

D. Clonidine

E. Propranolol

Question 25

Heat loss

A. Conduction is the largest factor in patient heat loss

B. Radiation accounts for about 10% of the total heat loss

C. Convection is due to heating of the adjacent air layer

which is replaced by cooler air from the surroundings

D. Heat lost in breathing dry gases is approximately 15% of

total heat loss in the anaesthetised subject

E. Burns from faulty heating equipment are more likely

in the vasoconstricted patient

Update in Anaesthesia

Question 1

A. false B. false C. false D. false E. false

The pattern of peripheral nerve stimulation used for assessment

of neuromuscular blockade most commonly is the train-of-four.

This shows characteristic fade when recovery from non-

depolarising neuromuscular blockade is occurring. However in

prolonged blockade with suxamethonium there is a conversion

to the non-depolarising pattern with fade appearing. This is called

dual block and occurs if the prolonged block is due to either

repeated doses of suxamethonium or impaired metabolism

(suxamethonium apnoea). The train-of-four is of no value when

there is no response to the first twitch, and in this case post-

tetanic count is used. During recovery it is difficult to assess the

ratio between the first (t1) and fourth twitch (t4) and this should

be 100:70 for successful extubation. This comparison is easier

using the double burst pattern. Stimulators used for regional

blockade should be used with the locating needle attached to the

negative electrode as this ensures depolarisation of the nerve at a

lower applied current.

Question 2

A. true B. true C. true D. true E. false

All the volatile agents enhance the action of neuromuscular

blockers by reducing the tone of skeletal muscle, an action

mediated by an effect at the post junctional membrane. Quinidine

is a membrane stabiliser and an isomer of quinine. Dantrolene

disrupts excitation-contraction coupling and so does not directly

potentiate the action of NMBs.

Question 3

A. true B. false C. false D. true E. false

Bifid P waves are associated with left atrial hypertrophy and may

indicate the presence of mitral stenosis. Tall peaked P waves

(>3mm) are associated with raised right atrial pressure and

hypertrophy. The presence of a short PR interval suggests pre-

excitation. If there is a delta wave present then this is known as

the Wolff-Parkinson-White syndrome. The absence of a delta

wave suggests that it is the Lown-Ganong-Levine syndrome. In

left bundle branch block the left ventricle depolarises late and so

there is a slurred or secondary R wave in V6 and S wave in V1

(M shaped complex in V6). U waves are associated with

hypokalaemia and occur after the T wave. J waves are associated

with hypothermia and are a characteristic deflection at the end of

the QRS complex.

Ref: Yentis,Hirsch and Smith. Anaesthesia A to Z. Butterworth

Question 4

A. false B. true C. true D. true E. true

Hypothermia is associated with J waves which are positive

deflections at the end of the QRS complex. Hypokalaemia is

associated with U waves as well as T wave flattening, inversion,

and prolongation of the PR and QT intervals. U waves may also

be a normal finding and be associated with both hypercalcaemia

and hyperthyroidism. A short QT interval is a recognised sign of

hypercalcaemia. Biphasic P waves is V1 represent left atrial

enlargment and are associated with P mitrale where the P wave

is bifid in lead II. Hypothyroidism is associated with low voltage

complexes, bradycardia and T wave flattening or inversion.

Question 5

A. false B. false C. false D. true E. true

Hypokalaemia causes P-R prolongation, ST depression, T wave

inversion and prominent U waves.

Ref: Ganong WF. Review of Medical Physiology. Lange.

Question 6

A. false B. false C. true D. true E. false

The CVP trace consists of three main waves and two descents,

(in chronological order):

The a wave - right atrial (RA) contraction. Absent in atrial

fibrillation; enlarged with tricuspid stenosis, RV hypertrophy,

pulmonary stenosis or pulmonary hypertension; cannon waves

(giant A waves) occur if the RA contracts against a closed tricuspid

valve (e.g. in complete heart block)

The c wave - bulging of the tricuspid valve at the onset of

ventricular systole

The X descent - atrial relaxation during ventricular systole

The v wave - RA filling with a closed tricuspid valve

The Y descent - opening of the tricuspid valve with blood flow

into the right ventricle.

Question 7

A. false B. true C. true D. false E. true

The Mapleson classification of breathing systems has six

classifications A-F. The Lack circuit and the Magill attachment

are examples of Mapleson A systems. They are very efficient for

spontaneous breathing (requiring a fresh gas flow (FGF) equal

to alveolar ventilation - less than minute ventilation (MV)). They

are much less efficient for controlled ventilation (FGF = 3 x MV).

The Mapleson D (including the coaxial Bain circuit) is most

efficient for controlled ventilation. The Bain circuit requires a

FGF of 70 ml/kg to maintain normocarbia. The Mapleson F

(Jackson-Rees) is a modification of the Ayre’s T-piece (Mapleson

E) designed to minimise resistance to gas flow and ideal for

paediatric use.

Ref: See Update No 7.

Question 8

A. false B. true C. false D. true E. false

A rise in end tidal carbon dioxide may be due to:

Inspired carbon dioxide

Rebreathing in the circuit (e.g. disconnection of the inner tube in

a Bain circuit or increased dead space)

Increased production of carbon dioxide (e.g. malignant

hyperpyrexia)

Inadequate ventilation

Leaks, reduced metabolic rate (due to hypothermia) and impaired

Update in Anaesthesia

cardiac output (pulmonary embolus) all cause reduced end tidal

carbon dioxide.

Question 9

A. true B. true C. true D. true E. true

All of these factors will determine the concentration of vapour in

the gas mixture.

The more volatile the agent, the higher the SVP and hence the

higher the concentration emerging from the vaporiser assuming

the same splitting ratio. The temperature determines the SVP of

the agent. Duration of use may alter the SVP via its effect on

temperature. The surface area of the vaporiser must be sufficient

to ensure full saturation of the mixture passing through the

vaporising chamber. The flow characteristics are important to

ensure complete mixing and hence full saturation once again.

The splitting ration is also important as if the gas exiting the

vaporising chamber is fully saturated it will determine the total

amout of agent added to the final gas mixture.

Modern vaporisers are temperature compensated. The splitting

ratio amount of gas passing through the bypass chamber is altered

to reflect the change in SVP with temperature and thus ensure

constant output of vapour at the required level.

Question 10

A. true B. false C. false D. true E. true

The amount of oxygen, nitrous oxide and anaesthetic agent

required by the body for oxygenation and anaesthesia is related

to the partial pressure of these in the trachea. For convenience

the more easily measured concentration or fraction is used, and

the barometric pressure is assumed to be constant. At high altitude

the barometric pressure is reduced. The partial pressure of

anaesthetic agent that is delivered by the vaporiser is constant at

differing barometric pressures, so the concentration dialled into

the apparatus will have the same. clinical effect as the SVP is

unchanged if the temperature is the same. However the fraction

(%) actually delivered will be higher than the dialled value as

the absolute pressure in the enviroment is lower. Furthermore to

get the same partial pressure of oxygen at lower barometric

pressure requires a higher inspired fraction, and the same inspired

fraction of nitrous oxide will provide a lower partial pressure

and so a reduced effect.

This can be confusing and it is advisable to read the reference.

Question 11

A. true B. true C. false D. true E. false

Low resistance is a feature of the OMV,EMO and Goldman

vaporizers.

The OMV is a small non-temperature compensated vaporizer

used with portable anaesthetic equipment. It has a antifreeze filled

sealed compartment that acts as a heat sink and so minimise

temperature changes. The scale can be change for the use of the

vaporizer with different volatile agents.

The EMO is used for ether. It is still used throughout the world

and has a large water filled heat sink.

The Goldman vaporizer is a small cheap and simple device used

for halothane. It is not temperature compensated and output

depends somewhat on the gas flow through the vaporizer.

Both the Copper Kettle and Boyles bottle vaporizers are examples

of plenum vaporizer with a high internal resistance to gas flow.

Ref: See Update No 14

Question 12

A. true B. false C. true D. false E. true

Absolute humidity is defined as the mass of water in a volume of

air. Relative humidity is defined as in the question is usually

presented as a %. Humidification devices can be defined as active

or passive; vapour or droplet producing; hot or cold and finally

functioning in a breathing system or in the atmosphere. Theatre

humidity should be around 60% as a compromise between

discomfort (if too high) and the increased risk of explosion due

to static electricity (if too low). Heat and moisture exchangers

can achieve 70% humidification. A nebuliser works on the venturi

or bernouille effect. For a bottle humidifier the water trap should

be at least the same size as the humidifier bottle.

Question 13

A. true B. false C. false D. false E. false

The brachial plexus can be approached from above or below

during regional anaesthesia for surgery on the upper arm. The

interscalene approach gives excellent anaesthesia to the shoulder

and upper arm, but commonly leads to inadequate blockade of

the ulnar nerve. Potential side-effects of this block are Horner’s

syndrome, phrenic nerve block, recurrent laryngeal nerve block

and inadvertent extradural or intrathecal injection. Bilateral

interscalene blocks should not be performed. The supraclavicular

approach commonly leads to inadequate blockade of the median

nerve. Horner’s syndrome and pneumothorax are amongst the

adverse events that can occur with this approach.The axillary

approach commonly leads to inadequate blockade of the axillary

nerve, and supplemental sub-cutaneous local anaesthetic will be

required if the upper/outer aspect of the arm is involved in surgery,

or if a tourniquet is to be used. Horner’s syndrome does not occur

with the axillary approach.

Question 14

A. true B. true C. true D. true E. true

Complications of retrobubar block may be systemic or local.

Retrobulbar blockade requires the injection of local anaesthetic

into the muscle cone behind the eye. The most common

complication is retrobulbar haemorrhage due to puncturing the

vessels within the retrobulbar space. The increased pressure in

the globe can cause central artery occlusion. Other complications

include bradycardia secondary to the oculocardiac reflex,

posterior globe puncture with resultant retinal detachment and

vitreous haemorrhage, penetration of the optic nerve, brain stem

anaesthesia from local entering breeched dura around the optic

nerve and subarachnoid blockade or inadvertant intraocular or

intravascular injection; hence the preference of most anaesthetists

for peribulbar blocks.

Update in Anaesthesia

Question 15

A. true B. false C. true D. false E. false

More hypotension occurs when adrenaline containing local

anaesthetics are used for epidural blockade. This may be due to

the beta 2 effects of the absorbed adrenaline causing vasodilation

in peripheral beds. It is countered by the chronotropic and

inotropic effects on beta 1 receptors. However the more prolonged

hypotension seen is probably due to the achievement of a more

profound degree of sympathetic blockade. Sympathetic blockade

occurs after sensory blockade. Small unmyelinated sensory fibres

with no barrier to local anaesthetic diffusion are blocked before

the larger autonomic B fibres. Anterior spinal artery syndrome is

due to severe hypotension secondary to epidural blockade and

not due to the technique itself. This leads to infarction of the

spinal cord and results in a lower motor neurone paralysis at the

level of the lesion and spastic paraplegia with decreased pain

and temperature sensation below the level. Epidural blockade

may cause lower intercostal muscle and abdominal muscle

weakness resulting in impaired coughing and exhalation.

However with a T4 block diaphragmatic innervation (C3-C5) is

maintained and tidal volume and inspiratory pressure are

maintained. Bowel contraction results from blockade of the

sympathetic outflow and unopposed parasympathetic activity.

Sphincters relax and peristalsis increases.

Question 16

A. true B. false C. false D. false E. true

Horners syndome is the triad of enopthalmos, ptosis and miosis.

Nasal congestion and anhydrosis are common but ipsilateral.

Remember that exomphalos is a neonatal condition!

Question 17

A. false B. false C. true D. true E. true

There are 3 common approaches to blocking the brachial plexus.

The interscalene approach is ideal for shoulder and upper arm

operations. It however frequently spares the C8 and T1 fibres

which innervate the ulnar border of the forearm. Injection of local

anaesthetic by this approach may produce cervical plexus block

which may cause diaphragmatic paralysis. The phrenic nerve may

also be blocked because of diffusion or inappropriate injection

to the anterior side of the anterior scalene. The supraclavicular

approach attempts to block the plexus at the first rib and is most

reliable at producing anaesthesia of all four terminal nerves of

the forearm and hand. It does however carry the greatest risk of

pneumothorax. The axillary approach is simplest and has the least

chance of pneumothorax. If paraesthesia cannot be elicited during

this approach then one alternative is to delibrately pucture the

axillary artery and advance the needle through the opposite wall

where half the anaesthetic solution is deposited. The remainder

is injected once the needle has been pulled back through the

“anterior” wall of the artery.

Question 18

A. true B. false C. true D. true E. true

Local anaesthetic is deposited between the internal and external

oblique muscle layers. If the internal oblique is penetrated, local

anaesthetic may track back to the lumbar plexus and affect the

femoral nerve, producing quadriceps weakness. Prilocaine may

be used because of the larger amount of solution which may safely

be injected, but most would use bupivacaine. The testicle is

innervated by T10 and so this block is quite ineffective for

testicular surgery.

Question 19

A. false B. true C. false D. true E. false

Aldosterone causes sodium retention and potassium loss. In

chronic renal failure hyponatraemia or hypernatraemia can occur.

In acute renal failure fluid retention can lead to hyponatraemia.

Hyperkalaemia can also occur and is an indication for dialysis.

Hypopituitarism leads to a reduced secretion from the anterior

pituitary gland and hence ACTH insufficiency and reduced

cortisol. Mineralocorticoid production remains largely intact as

this is predominantly stimulated by angiotensin II. Destruction

of the entire adrenal cortex reduces glucocorticoids,

mineralocorticoids and sex steroids. As such hyponatraemia,

hyperkalaemia and a raised urea result. Cushings results in excess

cortisol which has some mineralocoticoid activity. This can lead

to loss of potassium.

Question 20

A. true B. false C. false D. false E. true

These indices indicate a microcytic anaemia (normal MCV = 85

fl). The most common cause is iron deficiency. The red cells will

also be hypochromic (MCH less than 27 pg). In thalassaemia

there is a deficiency in the synthesis of the globin chains of

haemoglobin. In addition the accumulation of abnormal chains

within the red cell leads to its early destruction. This causes an

anaemia with reduced MCV and MCH. The reticulocyte count is

also raised. The anaemia of renal failure is normocytic and

normochromic, in common with anaemias of chronic disease. In

renal failure it is due to reduced erythropoietin production and in

severe uraemia, > 30 mmol/l, a shortened red cell life and marrow

toxicity. Folate and vitamin B12 deficiency cause macrocytic

(high MCV) megaloblastic anaemia. Acute blood loss will cause

an anaemia with a normal MCV and normal shaped existing red

cells.

Question 21

A. true B. false C. true D. true E. false

The Quetelet index (or body mass index) is weight (kg) divided

by the square of height (in m2). The upper limit of normal is

24.9. Overweight is 25-29.9, obese is 30-34.9, and morbid obesity

over 35. Hypertension, ischaemic heart disease and increased

oxygen consumption are characteristics, as are airway obstruction,

right heart strain and, eventually, pulmonary hypertension and

right-sided failure. The FRC is reduced with a tendency to V/Q

mismatch and hypoxaemia. Hiatus hernia with reflux, and

diabetes are other considerations.

Ref:Shenkman et al. Perioperative management of the obese

patient. British Journal of Anaesthesia 1993;70:349.

Update in Anaesthesia

Question 22

A. true B. true C. false D. false E. true

Diathermy relies on generation of heat energy from electrical

energy. Ohm’s law states that voltage = current x resistance

(V=IR). The heat energy produced = (current)2 x resistance

(E=I2R). At the active electrode there is a high current density

due to the small area of the electrode. This leads to the heat energy

and cautery. The passive electrode (‘diathermy plate’) used with

unipolar diathermy has a large area and so the current density

(and hence the heat produced) is small. However if the contact is

poor then either there is a reduced area of contact (with a higher

current density) or increased resistance. As E=I2R both these

faults lead to an increased risk of burning. Bipolar diathermy

does not have a passive electrode, and the current passes from an

active electrode to a return electrode; these are the two blades of

the diathermy forceps.

Question 23

A. true B. false C. false D. true E. false

Nitrous oxide is not flammable. An anaesthetic machine should

be connected to a conducting floor of an operating theatre to

prevent the production of static electrical sparks. A current of

approximately 100 microamps is needed to produce a microshock

to the heart, capable of producing ventricular fibrillation. Since

all connections have some finite resistance the neutral connection

is not exactly at earth potential at the patient end of the circuit.

The heat released in surgical diathermy depends on the resistance

and to the square of the current flowing.

Question 24

A. true B. true C. true D. false E. true

Indirectly acting sympathomimetics like ephedrine are unlikely

to increase blood pressure in patients taking drugs which alter

neuronal storage, uptake, metabolism or release of

neurotransmitters. Reserpine depletes neuronal granules of

noradrenaline. Alpha-methyl dopa acts as a false transmitter.

Phenoxybenzamine and propranolol block peripheral receptors

and industrial doses of directly acting sympathomometics may

be required to overcome their blockade. Clonidine works on

central adrenergic receptors and the peripheral effect of indirectly

acting sympathomimetics is not decreased, in fact smaller doses

may be required due to receptor up regulation.

Question 25

A. false B. false C. true D. true E. true

Patient heat loss is mainly due to radiation (40-50%) and, to a

lesser extent, convection (defined as in the question) and

evaporation. About 15% of heat loss in the anaesthetised patient

is via the respiratory tract. The main part of this is due to the

latent heat needed to vaporise water to humidify the gas within

the trachea. This can be minimised by humidification. Warming

equipment such as electric blankets and water-filled mattresses

have caused thermal burns in anaesthetised patients. The risk of

burns is greater if there is poor blood supply as blood can help

conduct the heat away from the site of potential injury.