Measurement of heat exchanger performance

_{Performance
factor =} ^{Blood outlet temperature —
Blood inlet temperature}

^{Water inlet temperature — Blood inlet temperature}

[Also known as Heat Transfer coefficient]

Arrest times related to temperature

· Q_{10 [brain]}: 37—27°C 2 (ie a 10°C reduction in temp reduces metabolic rate by half)

27—14°C 4.5

· See precipitous drop in cerebral metabolic rate with profound hypothermia

18°C—20°C

> 45 minutes associated with increased stroke

> 60 minutes associated with increased mortality

Svensson et. al. , J Cardiovasc Surg 1993;106:19

Probability of “safe’ circulatory arrest

Duration arrest (min)	37°C	28°C	18°C
1	1	1	1
2	0.99	1	1
3	0.98	1	1
4	0.9	1	1
5	0.8	1	1
10	0.02	0.999	1
20	0	0.6	1
30	0	0.058	1
40	0	0	0.96
50	0	0	0.66
60	0	0	0.3
70	0	0	0.09
80	0	0	0.03
90	0	0	0.001

Duration arrest (min)

rate of cooling & warming and the effect on monitored temperatures

Temperature management entails 3 principles:

1) During both cooling & rewarming a maximum of 10°C is maintained between the water temperature (of the heater-cooler) and the blood temperature

a) Too rapid cooling may be associated with cerebral injury [eg cerebral vasoconstriction may be associated with reduced uniform cooling of brain)

b)Too rapid rewarming may be associated with the danger of gas bubble formation as gases become less soluble in blood as temperature increases + critical post ischaemic period characterised by high cerebrovascular resistance -impaired autoregulation & high cerebral metabolic demand for oxygen

2)Secondly, once the desired patient temperature has been reached, a further 10 minutes at least is required at this temperature to allow for uniform cooling of brain when cooling & uniform warming of body when rewarming

3) Once on HCA, must reduce rewarming-maintain hypothermia: use cooling blanket

· Is important to ensure cooling is not done too rapidly and once have reached target temperature a further interval of cooling is achieved prior to circulatory arrest to allow for uniform cooling of the brain and other vascular organs

value of surface cooling & bypass cooling

1. Once on HCA, must reduce rewarming-maintain hypothermia: use cooling blanket

2. Minimising rewarming of the brain is essential, therefore external heat sources (eg overhead lights, ambient room temperature) should be minimised

3. Application of external ice packs to the head have been shown to delay brain rewarming and increase ischaemic tolerance

4. Methods of inducing & reversing hypothermia (esp paediatrics)

a) Total extracorporeal circulation (core cooling)

b) Surface cooling

c) Surface cooling + supplementary partial CPB

d) Deep hypothermic total circulatory arrest

e) Low flow, profoundly hypothermic arrest

As a means of inducing hypothermia (as opposed to maintaining hypothermia) surface cooling is used only for very small infants

5. Infants can be cooled rapidly by surface cooling + vasodilation 2° anaesthesia

1. Cooling of heart

a) Core cooling of the heart occurs via bypass cooling (eg rapid cooling of blood prior to X-clamping) and via cold cardioplegia

b) Rapid cooling of heart via bypass cooling may result in VF

c) Gradual cooling via cardioplegia may provide more uniform cooling of the myocardium

d) External cooling of myocardium (ice slush, cooling jacket)

i) help maintain myocardial hypothermia between cardioplegia dosages

ii) augment cardioplegia effect with severe coronary artery disease & hypertrophic heart where not all the myocardium may be perfused with the cardioplegia

iii) Help minimise rewarming 2°

a) Warmer venous blood

b) Conduction from warmer adjacent tissues

c) Noncoronary collateral circulation

d) Environment: lights, room temperature

iv) Disadvantages:

a) Ice slush may be associated with phrenic nerve injury

b) Cooling jackets are cumbersome