Hypothermia

Definition?

Mild 35-32

Moderate 32-28

Severe 28-20

Profound <20

Classification:

Primary: Otherwise healthy individual exposed to adverse weather or cold water immersion

Secondary: Core temp drop from a disease process, eg alcohol intoxication, trauma, MI.

Elderly are more prone

How do we measure temperature?

Adults:

Commonly we use tympanic thermometers – works well for pyrexia, can be used as a core temp initially but if really low will need further invasive monitoring

Whats the problem with this in hypothermic patients? – well we are more interested in their core temps

-          Oesphageal

-          Rectal

Paeds:

Small babies wont fit a TM thermometer so they use an under arm temp

Pathophysiology:

Core temp is maintained between 36.5 and 37.5

Heat is generated by cellular metabolism – can be increased by striated skeletal muscle contraction (shivering can generate 2-5x the basal amount of heat)

In normal circumstances 60% of heat is lost through radiation – conduction and convection 15% and respiration and evaporation the remaining 15%

Controlled from the hypothalamus via heat conservation (peripheral vasoconstriction and behaviour responses) and heat production (shivering , increased thyroxine and adrenaline production)

When the mechanism for maintaining a normal temp are overwhelmed (cold stress or fatigue/glycogen depletion) then core temp drops

Physiological Impacts:

Cardiovascular:

-          Initial response reflects the increased metabolic rate (tachycardia) and vasocontriction which leads to hypertension

-          With gradual decompensation and lower temps bradycardia and then VF and finally asystole occurs

-          The Myocardial instability with hypothermia leads to them being very prone to VF with even slight movements so need very careful handling

o   IF they tip into VF then it is often refractory and given how peripherially vasoconstricted they are we need to change our ALS algorhythm

Coagulation:

-          Low temp inhibits the enzymes of the coag cascade – wont always be reflected in PT/APTT

-          Thrombocytopenia can be caused by hypothermia and may develop physiological hypercoagulability similar to DIC

CNS:

-          Progression from impaired judgment and memory, slurred speech, ataxia and reduced conciousness

-          Hypothermic patients may foot stomp and paradoxically undress

Renal:

-          Intial Diuresis is common (increased BP and CO from vasoconstriction)

-          Severe hypothermia can cause renal failure

Respiratory:

-          Initial response is an increased in RR and a respiratory alkalosis

-          With progressive low temps they develop hypoventilation, respiratory acidosis

Afterdrop/rebound hypothermia:

Commonly after initial rewarming there will be a second drop in core temp

-          Theory being that vasodilation leads to cold blood from peripheries reaching the core

-          Increases arrhythmia risk

This is why we temp and cardiac monitor anyone with a temp<32

You can also see a physiological worsening similar to a mild reperfusion injury as they rewarm and  vasodilate – all  that static, peripheral blood is physiologically abnormal. (raised K, raised lactate, acidotic) and this can be reflected in tachycardia/hypotension

How do we approach it in real life?

                “Hey Doc, this patient’s temp is 32”

What Do we need to consider?

Causes: primary vs secondary

“serious and most common” – sepsis, environmental

Initial Management:

                Investigations:

                                Sepsis work up

                                TFTs

                                ECG

                Warming Options:

                                Passive – dry, warm environment, insulation, movement

                                Active – easy options: bairhuggers, warm fluid, heating packs

                                                More complex: Warm, humidified gases (if intubated), Cavity lavage (eg peritoneal, gastric but bladder is easier)

                                                Really complex: ECMO/bypass, Renal replacement treatment

Rewarming rates:

-          Shivering 1.5° C/hr

-          Warming Blanket 2° C/hr

-          Warm O2 1 °C/hr with mask; 1.5° C/hr ET tube

-          IV Fluids do not add, but do not take away either

-          Peritoneal Lavage 3° C/hr

-          Thoracic Lavage with Chest Tubes 3-6° C/hr

-          Cardiac Bypass 9-18° C/hr

-          Think about ways that we will cool them unnecessarily and prevent them

o   Exposing

o   Cold IVs

o   Leaving them wet

So how are we going to choose which options and when?

                So like all medicine it’s a judgment call:

1.       The teenager that has fallen through Ice on a lake – every option we can organize

2.       The pensioner in a cold house whose a bit confused at 32 degrees – bairhugger and warm fluids