Safety factors of heat exchanger design including: a)materials; b)construction; b)temperature monitoring; c)temperature gradients; e)pressure gradients; f)operational checks

a) Materials

• The heat transfer surface is usually made of stainless steel or aluminium and rarely plastic

• Both aluminium & stainless steel have good thermal conductivity and are readily coated with polymers to minimise blood interactions

b) Construction

• The heat transfer surface may be organised into: spiral thin tubes, tubes, pleated sheets

• Consideration of design to minimise “hot spots” which may damage blood components

• Heat exchange may be enhanced by:

i) flowing the blood and water in opposite directions (countercurrent)

ii) flowing the blood and water perpendicularly (crosscurrent) to create secondary currents and minimise boundary layers

• To maximise heat efficiency of heat transfer, the surface for heat exchange is maximised by:

i) larger heat exchanger (associated with increased priming volume)

ii) Use of fins extending into blood (advantage of reduced number of water channels required with associated decrease number of sites for fluid leakage into blood)

· Heat exchanger before oxygenator

c) Temperature monitoring

• Rewarming water bath temperature should never exceed 42°C

• Thermal injury and destruction of blood formed elements (in form of protein denaturation) occurs above 43°C

• Heat exchangers therefore have a temperature limiter at 42°C

d) Temperature gradients

Cooling

• Rapid cooling may be associated with GME production when the cooled arterial blood mixes with the warmer blood in the aorta

• The rate of rapid cooling (circulating water approaching 0°C) is limited by thermal boundary layer and the temperature difference between water & the blood

• Slow cooling probably results in more uniform cooling of the brain

• Rate of cooling should be at less than 1°C/minute to minimise intravascular aggregation

Warming

• Rapid rewarming may be associated with GME production when the rapidly warmed blood in the heat exchanger has a reduced gas solubility

• Manufacturers often position heat exchanger before oxygenator where lower oxygen tensions are present

• The maximum difference in temperature between water and the venous blood is limited in order to prevent GME secondary to rapidly decreasing gas solubility.

• GME formation can be avoided by limiting the blood/water temperature difference (and tissue/blood temperature difference) to 10°C • THe temperature gradient between the water bath temperature and the patient should never exceed 12°C in the adult or 8°C in the child

• Rate of rewarming should be ≤ 1°C increase in 3 - 5 minutes

• As the patient is rewarmed and the gradient between the blood and the water bath temperatures becomes smaller, the rate of rewarming becomes slower

e) Pressure gradients

• ?An excessively high pressure in water circuit versus blood component in heat exchanger may predispose to leakage?

• Water leakage into the blood path will manifest as haemolysis and subsequently sepsis

f) Operational checks

• Temperature stress test (by initially cooling then rapidly warming heat exchanger whilst priming) to evaluate integrity of heat exchanger (if integrity impaired, blood circuit will fill with water from heat exchanger)

• Observation of appropriate changes in arterial temperature whilst performing temperature stress test during priming (to test for operational functionality of heat exchanger)

• Temperature limiter set at 42°C