.heat transfer during cpb

1. Heat transfer during CPB

2. Introduction
 Heat exchangers are integral components of
all oxygenators.
 Heat exchangers can be external in
oxygenators used for ECLS/ECMO.
 Heat exchanger function in combination with
an external heater cooler unit.

3. Function of heat exchanger
 To regulate the temperature of the blood
perfusing the patient.

4. Design charecteristics
 Heat exchangers must be made of
biologically inert materials.
 Should not cause excessive activation of
cellular and noncellular elements of blood.
 The heat exchanger is seperated from the
blood phase of the oxygenator by a highly
conductive material.

5. Design contd.,
 To maximize heat efficiency of heat transfer,
blood and water pathways flow in a counter
current direction.
 This reduces outgassing of solutions due to
rapid change in temperature.
 Increase in heat exchanger surface area
results in greater heat transfer but priming
volume also increases.

6. How it works ?
 Transfer of energy occurs by circulation of
water from heater-cooler unit.
 While cooling the water temperature is
reduced by activating a chilling unit of HCU.
 Warming of water is a function of high-
wattage resistance unit.
 Precise control of temperature is achieved by
digital thermostat.

7. Contd.,
 Other technique of controlling patient
temperature with the use of warming or
cooling blankets placed below the patients.
 By controlling room temprature.
 To maximize heat efficiency of heat transfer,

8. Priinciples of heat exchanger
 Conduction ( solid to solid)
 Convection ( solid to liquids)

9. Materials used
 The heat transfer surface is usually made of
stainless steel, aluminum, or polypropylene.
 The material used should have good thermal
conductivity.
 They are coated with polymers or other
surface-modifying agents to minimize blood
activation.

10. Constraints on rate of heat transfer
 The temperature difference b/w circulating
water and blood determines heat transfer.
 A thermal boundry layer exists in the blood
flowing just beside the wall of the heat
exchanger.
 Here the temperature varies from wall to free
stream temperature.

11. Constraints contd.,
 Changing temperatures alters the solubility of
gases.
 Higher partial pressures of gas increase the rate of
outgassing of the solution.
 Due to the potential for gas emboli from outgassing
the temperature gradient should not be more than
10 degrees.
 Reduced temp gradient (<6) and longer rewarming
recommended for improved neurocognitive
outcomes

12. Contd.,
 Blood damage in the form of protein
denaturation limits the maximum temperature
to 42 degrees.