THERMOREGULATION

Peri-operative TeachingJune 2008

Dr Mohua JainSpecialist Anaesthetist

Definitions CORE TEMPERATURE

PERIPHERAL TEMPERATURE

NORMOTHERMIA

HYPOTHERMIA

CORE TEMPERATUREThermal compartment of body, highly perfused

tissues, uniform and higher temperature.Trunk, brain – 2/3 body heat

PERIPHERAL TEMPERATURESkin, subcutaneous – all body, inc limbsUsually 2 to 3 °C below core but can be much more

• Core and peripheral temperatures both influence comfort about equally.

• Only core influences metabolic processes

• As peripheral temp drops, heat flows from core to periphery (gradient)

NORMOTHERMIACore temp range of 36°C to 38°C

HYPOTHERMIACore temp less than 36°C

MILD HYPOTHERMIACore temp range 34°C to 36°C

Definitions (NICE)

•Preoperative - 1 hour before induction •Intraoperative - the total anaesthesia time•Postoperative - 24 hours after entry into the

recovery area in the theatre suite

•Hypothermia - a patient core temperature of

below 36.0°C.•Comfortably warm - the expected normal

temperature range of adult patients

•Temperature - used to denote core temperature

Maintenance of Heat Balance of heat production and loss

Nervous system Hormones

Vessels Behaviour

Variations during day and month

Heat Production Metabolism Exercise Shivering Non-shivering thermogenesis (fat

and muscle)

Basal metabolic rate (BMR) is energy needed to maintain constant temperature

Heat Loss Radiation (40 to 60%) Convection (25 to 30%) Evaporation (10 to 20%) Respiration (10% by heating of air

and evaporation)

Energy loss can be up to 15 x BMRSweating can be up to 1 litre per hour

for short time, taking heat with it!

Don’t forget

Hypothermia can be present regardless of temperature if patient complains of feeling cold or has the obvious signs

Body needs to maintain set temperature as all processes involving enzymes are sensitive to temp and pH

Signs Usually below 36.5°C Peripheral vasoconstriction (esp

stressed patients) Hairs standing on end (pilo-erection) Shivering Cold peripheries High diastolic blood pressure

Importance of ‘behavioural’ actions

Measurement of Core Temperature

accurate (patient, operator, instrument - variable readings)

consistent, repeatable, keeping up with rapid changes, accessible, safe

Core Temperature Measurement Sites

RELIABLE

• Pulmonary artery• Tympanic

membrane (direct and indirect)

• Nasopharynx• Distal oesophagus

ESTIMATE

• Rectal• Bladder• Oral• Skin• Axillary

Adverse Effects of Hypothermia CNS (Nervous system) RS (Respiratory system) CVS (Cardiovascular system) Renal and electrolytes Immune Blood Drug effects Others

CNS• Reduced neuronal function • Confusion• Disorientation• Stupor• Raised intracranial pressure from

shivering• Seizures• Coma

RS

• Hyperventilation then hypoventilation

• Lower respiratory rate• Lower volumes (effect on CNS)• Increased oxygen consumption

from shivering• Organ ischaemia

CVS

• More adrenaline (and other catecholamines)• Vasoconstriction• Raised blood pressure• Bradycardia• Myocardial ischaemia and infarction• ECG changes• Arrhythmias

Renal & Electrolytes

‘Cold diuresis’

• Renal tubule damage• Constriction of skin and gut vessels

• Potassium, Magnesium, Calcium and Phosphate all decrease

Immune

Infections

Wound breakdown and infections• Collagen linking less as oxygen

drops• Less subcutaneous oxygen• White blood cells function less

Blood

• Less coagulation• Less platelet function• More viscosity• More blood loss• More blood transfusions

Drug effects

• Usually prolongs actions of all drugs, (esp those needing enzymes for their metabolism)

• Muscle relaxants and opiates last longer

• Less IV and volatile agents needed for same degree of unconsciousness

Other• More patient discomfort with

shivering• More time in PACU / Recovery• Thresholds for pain and nausea• Difficulty with cannulation• More time in hospital• More time to establish diet• More costs from all above

Shivering Usually temperature related –

uncomfortable involuntary rhythmic muscle contractions to maintain core temperature

Complex , patterns of tremors Can occur post GA or during labour even

with normal temperature. Mechanism unknown ?pain and stress

Post-op 20 to 40%? Problem for monitoring Elderly rarely shiver

Drugs to treat Post-op Shivering (clinical and experimental)

• Tramadol• Pethidine• Alfentanil• MgSo4• Clonidine• Ketamine• Propofol• Ondansetron

• Doxapram• Nefopam• Meperidine• Ketanserin• Physostigmine

Effects of General and Regional Anaesthesia

Impaired thresholds for responses so they happen later

3 stage drop in temperature 1 to 3°C 1.Rapid in 1st hour (Redistribution of heat

from core to periphery - vasodilation)2.Gradual (Heat loss causes then exceed

heat production causes)3.Plateau (Production catches up)

So far...

Definitions Heat balance – how and why needed Measurement of core temperature Bad effects of Hypothermia Shivering (normally and post-op) AnaesthesiaSo, how can we prevent hypothermia?

Evidence - Research and Clinical Recommendations and guidelines (esp 2000

onwards) WHO - ambient temperature American Society of Anesthesiologists (ASA) American Society of PeriAnesthesia Nurses

(ASPAN) National Institute of Clinical Excellence (NICE)

Common Sense Guidelines

• Minimising heat loss from the body

• Giving heat to the body

Common Sense Guidelines

Pre-operative Intra-operative Post-operative

• ASSESSMENT (identify, measure, observe & ask)

• INTERVENTION (preventative, passive and active)

Identification of Risks

• Very young• Very old• Female• GA / RA• Large surface area

/ gut exposed• Ambient temp

(circulating air)

• Poor nutritional status

• Length of surgery• Fluid shifts• Irrigation fluids• Trauma/burns• Cold transfers

Patients at higher risk of perioperative hypothermia

(NICE) Some patients are at higher risk of inadvertent perioperative hypothermia; they should be managed accordingly if any two of the following apply:

•ASA grade II to V•preoperative temperature below 36.0°C•undergoing combined general and regional

anaesthesia•undergoing major or intermediate surgery •at risk of cardiovascular complications.

Expectations

• Core temperature never to drop below 36°C at any stage

• To avoid symptoms and signs• If GA will last 30 mins or more, must

measure temp through operation• More strict if high risk group• Start actions BEFORE theatre

Preoperative warming

If the patient’s temperature is below 36.0°C in the hour before they leave the ward or emergency department:

• forced air warming should be started preoperatively on the ward or in the emergency department (unless there is a need to expedite surgery because of clinical urgency)

• forced air warming should be maintained throughout the intraoperative phase.

Intraoperative phase

The patient’s temperature should be measured and documented before induction of anaesthesia and then every 30 minutes until the end of surgery.

Induction of anaesthesia should not begin unless the patient’s temperature is 36.0°C or above.

Passive – to minimise heat lossFor hypothermic AND normothermic patients

• Ambient temp at least 20°C (upto 30° if burns or neonates!)

• Passive insulation (layer of air)• Warmed cotton aircell blankets• Space blanket?• Circulating water mattress?• Hats (esp Paeds) Socks etc• (Special cases – pre veins, post flaps)• (Before – preop vasodilation)

Active – add to heat gainFor hypothermic patients• Skin – Forced air warming / convective (Bair Hugger) – upto 50 W heat given (no

infection evidence)• Internal – IV, irrigation (1 litre fluid at

room temp will lower core temp by 0.25°C)

• Airway - humidification (HMEF)

• Cardiopulmonary bypass• Dialysis• (Protein infusion to increase

metabolism)

• Watch out for over-heating of skin and fluids (keep below 45°C)

Warming intravenous fluids

Intravenous fluids (500 ml or more) and blood products should be warmed to 37°C using a fluid warming device.

Postoperative phase

The patient’s temperature should be measured and documented on admission to the recovery room and then every 15 minutes

• Ward transfer should not be arranged unless the patient’s temperature is 36.0°C or above.

• If the patient’s temperature is below 36.0°C, they should be actively warmed using forced air warming until they are discharged from the recovery room or until they are comfortably warm

Any questions???

Costs and savings per 100,000 population

Recommendations with significant costsCosts

(£ per year)

Increased use of forced air warming blankets 43,000

Increased warming of IV fluids and blood products 23,000

Estimated cost of implementation 66,000

Recommendations with significant savingsSavings

(£ per year)

Expected reduction in surgical site infections –43,000

Estimated annual net cost of implementation 23,000

Discussion

• Which key areas of local practice differ from the guideline?

• To ensure effective implementation:- what equipment is needed?- what are staff training needs?

• What will the impact be on the average length of patient stay if the guideline is implemented fully?

• How should Risk and Safety Managers be involved in the implementation of the guideline?

SUMMARY

• Understanding of heat balance• Understanding why this is important • Why to prevent temp below 36°C• How to measure temperature• Recommendations of how to assess• Passive and active ways of helping

the patient from pre- to post-op