eLFH - Heat Transfer

Question 1

Factors which impact heat loss of patients in theatre

Answer 1

Environmental
Anaesthetic
Surgical
Patient

Question 2

Environmental factors affecting heat loss

Answer 2

Theatre environment set to lower temperature for staff and scrubbed surgeon comfort - increases radiation heat loss

Question 3

Surgical factors affecting heat loss

Answer 3

Orthopaedic laminar flow - increases heat loss by convection

Exposed body cavities - increased evaporative losses

Cold irrigation fluid

Question 4

Anaesthetic factors affecting heat loss

Answer 4

Increase heat loss and decrease heat production

Also infusion of cold fluids / blood

Question 5

How is heat loss increased with anaesthesia

Answer 5

Most anaesthetic agent (inc regional) cause vasodilation
Increased radiation and convection heat loss

Also moves blood from central compartment to peripheral compartment causing rapid decrease in central temperature

Question 6

How is heat production reduced with anaesthesia

Answer 6

Basal metabolic rate

Spontaneous respiration

Resting muscle tone

Physiological response to hypothermia

Behavioural responses

Question 7

Effect of general anaesthesia on basal metabolic rate

Answer 7

GA reduces basal metabolic rate

Therefore reduces heat generation by metabolically active tissues such as liver

Question 8

Effect of GA on spontaneous respiration

Answer 8

Neuromuscular blocking agents and some opiates prevent spont breathing

Therefore reduced heat production by respiratory muscle contraction

Question 9

Effect of GA on resting muscle tone

Answer 9

All resting muscle tone is reduced which reduces heat production

Question 10

Effect of GA on physiological response to hypothermia

Answer 10

Unable to generate heat by moving / shivering

Impaired vasoconstriction and piloerection

Question 11

Effect of GA on behavioural responses

Answer 11

Behavioural responses are removed

Eg. finding warmer environment, wearing warmer clothes

Question 12

Patient factors affecting heat loss

Answer 12

Extremes of age lose heat more readily

Question 13

Paediatric patients and heat loss

Answer 13

Increased surface are to weight ratio (esp neonates) so lose heat more quickly

Disproportionate losses through head as larger in relation to body

Question 14

Elderly patients and heat loss

Answer 14

Reduced basal metabolic rate
Lower muscle mass

Reduced shivering response
Reduced insulating adipose tissue layer

Also may have pathological conditions (eg hypothyroid)

Question 15

Characteristic pattern of temperature fall in anaesthetised patient

Answer 15

Initial sharp drop in temperature due to blood redistribution on induction following vasodilation

The gradual linear heat loss until reach equilibrium with environment - heat loss = heat generated

Question 16

Methods of managing heat loss in theatre

Answer 16

Methods to minimise heat losses

Methods to actively warm the patient

Question 17

Methods of reducing heat loss of patient in theatre

Answer 17

Avoid unnecessary exposure
Head wraps
Reflective space blankets
Avoiding use of muscle relaxants
Spont breathing techniques
HME filter
Warm infusion fluids

Question 18

Methods of active patient warming in theatre

Answer 18

Hot air blanket
Warmer ambient temperatures in theatre
Warm mattress
Warmed IV infusions

Question 19

Theatre environment humidity and why

Answer 19

Maintain humidity at least 50% to reduce evaporative losses