Temperature management in CPB

History-

Lewis and taufic used hypothermia and inflow occlusion for repair of ASD

Sealy used hypothermia for CPB Circuit for intracardiac repair.

Physiology of Hypothermia

Normal homeostatic mechanism with exposure to cold is – activation of thermoreceptor in brain -> hypothalamic sympathetic nerve stimulation

Hence,

1. Vasoconstriction of skin venues

2. Vasodilation of skeletal muscle venules -> increase muscular activity by tensing and shivering

3. Endocrine system is activated and O2 consumption is increased

4. HR/ CO/BP are elevated

Rationale of use of Hypothermia in CPB-

1. Organ protection and safety margin in CPB

2. By decreasing metabolic rate and O2 consumption.

3. Preserves high energy phosphate stores

4. Decrease excitatory neurotransmitter release as increase in neurotransmitter like glutaraldehyde opens up calcium channels and calcium dependent caspases activation and cell death.

4. Q10 — increase or decrease in temperature dependent enzymatic reaction per 10 degree C change in temperature.

Most reactions including O2 consumption have a Q10 of 2-3.

5. In CPB it helps in

* Lowering pump flows for better visualisation of surgical field

* Better myocardial protection- by decreasing temperature

* Less blood trauma- due to decrease pump flow and hemodilution

* Better organ protection

Acid base alteration with temperature change

* With increase in temperature – PCO2 decreases

Decrease in temperature – PCO2 increase

* Major buffering system responsible for the constant relation of blood and tissue fluid pH to neutral pH with temperature change is the imidazole moiety of amino acid histidine which is commonly found in body proteins.

* PKa of histidine is nearly 7.0

* Alpha = ratio of unprotonated histidine imidazole group to H+ ion

Alpha stat — CO2 is constant  and pH changes as per the body temperature. In otherwors CO2 is not added to the system during cooling.

PH stat— pH remains constant and CO2 is added to maintain PaCO2 and a pH of 7.4

( Alpha and pH stat are important to maintain the intracellular enzyme system as most of the enzymatic reaction are pH dependant)

* With response to hypothermia-

Cerebral blood flow decreases ut with alpha stat strategy this decrease in CBF is adequate for maintaining metabolic demand as metabolic demand is also decreased due to hypothermia. Therefore alpha stat strategy will result in decreased cerebral blood flow than the pH stat strategy ( the response of cerebral blood flow is preserved with changes in PaCO2 in a alpha stat strategy.

In alpha stat strategy, hypocarbia shifts the oxygen hemoglobin dissociation curve to the left thus decreasing O2 unloading however more O2 is dissolved in blood in hypothermia cancelling each other.

Studies have demonstrated that post op neurological and neuropsychological outcome is better with alpha stat strategy.

In pH stat — Cerebral blood flow is increased and thus exposed to a greater load of –

Microemboli

Cerebral edema

High intracranial pressure

On the other hand, it is suggested that increase in cerebral blood flow may be helpful in improving cerebral cooling before the onset of circulatory arrest. And an increased amount of blood is supplied to the deep brain structures like thalamus or midbrain or cerebellum.

Crossover strategy—PH strategy for first 10 minutes of cooling to provide maximal cerebral suppression followed by alpha stat to remove severe acidosis that accumulate during profound hypothermia.

In paediatric patients – Cerebral cooling is problematic and it appears that addition of CO2 during cooling enhances cerebral perfusion and improves cerebral metabolic recovery.

Overall, in adult the primary cause of neurological injury is probably due to embolisation hence decrease CBF (alpha stat strategy) is more beneficial where as in paediatric population hypo perfusion is the primary mechanism (hence pH stat strategy) with increase in CBF is beneficial.

Effect of Hypothermia on other organs

1. Peripheries- Decrease in blood flow to all organs-

Skeletal muscles/ Extremities—> Kidney—> Splanchnic bed —> Heart ——> Brain

( In this order)

2. Heart- With cooling HR increases but contractility remains stable/ may actually increase. Dysarrhythmia becomes more prominent in  hypothermia because of – PVC. AV block, AF, VF, Asystole- due to electrolyte imbalance, uneven cooling and autonomic imbalance

3. Lungs- Progressive decrease in ventilation with cooling. Physiological and anatomical dead space increase . Gas exchange is unaffected.

4. Kidney- Kidney shows the largest proportional decrease in blood flow of all organs. Increases renal vascular resistance. Tubular transport of Na+/Cl_/water decreased. Tubular reabsorption is decreased. Glucose absorption is decreased and glucose is found in urine.

5. Liver- Hepatic arterial blood flow is decreased. Decrease in metabolic and excretory function  of liver. With rewarming, hepatic efficiency reverts to normal.

6. Metabolic / Endocrine- Marked hyperglycaemia. Insulin production is decreased. Glycogenolysis and gluconeogenesis is increased.

Hypothermia – decreased free water clearance and causes a decrease in plasma K+ and increase in osmolality.

Increase in blood viscosity due to fluid shifts , loss of plasma volume from capillary leak, cell swelling. Hematocrit increase but RBC volume unchanged. Rouleaux formation increase. Cause thrombocytopenia.

Deep hypothermia cause massive release of cathecolamines.

Complement activation or increase bradykinin or increase neutrophil activation due to hypothermia.

Use of hypothermia

* In most cardiac surgeries hypothermia  (>25 C ) is used for its protective role

* By coronary perfusion with cold cardioplegia solution or topical application of air ice slush or cold pericardial lavage

* Classification of hypothermia-

Mild- 32-36

Moderate- 28-32

Severe- 28-18

Deep – <18

Systemic O2 consumption (VO2) is reduced by 50% for every 7 degree C reduction in core temperature below normothermia.