Fifty to sixty years ago heart-lung machine was used only for cooling and rewarming.
Sarns heater-cooler "Old Betsy"
For millions of years human organism have been homoiothermic.
Studies on hypothermia had begun in 1941 in Dachau
Anesthetized patients are poikilothermic.
Hypothermia exacerbates dysrhythmias and coagulopathy, potentiates the effects of anesthetic drugs and neuromuscular blockers, increases vascular resistance, decreases the availability of oxygen, and contributes to postoperative shivering.
pH/Pco2 MANAGEMENT: There are two strategies for managing pH/Pco2 during hypothermia: pH stat and alpha stat. During deep hypothermia and circulatory arrest (see below) there is increasing evidence that pH-stat management may produce better neurologic outcomes during pediatric cardiac surgery. Alpha stat may be better in adults. pH stat maintains temperature-corrected pH 7.40 at all temperatures and requires the addition of CO2 as the patient is cooled. Alpha stat allows the pH to increase during cooling so that blood becomes alkalotic. Cerebral blood flow is higher, and pressure is passive and uncoupled from cerebral oxygen demand with pH stat. With alpha stat, cerebral blood flow is lower, autoregulated, and coupled to cerebral oxygen demand.
Alpha-stat method is responsible for the steal of blood from the cerebral to the pulmonary circulation because of the opposing effects of alkalosis on cerebral and pulmonary resistance.
S-shaped curve #1. Oxyhemoglobin dissociation curve
Oxyhemoglobin dissociation curve shift to the left reproduces fetal type of oxyhemoglobin dissociation.
S-shaped curve #2.
Arrhenius equation states that the rate of chemical reaction is inversely proportional to the absolute temperature. The nomogram describing this relationship is S-shaped such that at high temperatures, the reaction rate ceases increasing with temperature.
Hypothermia decreases action potential conduction velocity.
In the clinical setting, electrocerebral silence (as assessed by electroencephalography) is obtained at a mean nasopharyngeal temperature of 17.5°C.
Alpha-stat management (the mechanism prevailing in reptiles) aims at maintaining normal acidemia and blood gases (a pH of 7.40 and a PaCO2 of 40 mm Hg) in the rewarmed (to 37°C) blood. In vivo, the hypothermic blood is alkalemic and hypocapnic.
pH-stat management (the mechanism prevailing in hibernating animals) aims at maintaining normal values in vivo, in the hypothermic blood. When rewarmed to 37°C, the blood becomes acidemic and hypercapnic.
Alpha-stat management preserves autoregulation of brain perfusion and optimizes cellular enzyme activity. Because of blood alkalemia, the curve of oxyhemoglobin dissociation is shifted toward the right, corresponding to an increased affinity of oxygen for hemoglobin. With the further shift of oxyhemoglobin to the right due to hypothermia, the availability of oxygen carried by the hemoglobin molecule becomes tremendously reduced. At deep temperature reductions, oxygen diluted in blood represents the major source of oxygen to tissues.
The pH--stat strategy results in a powerful and sustained dilatation of the cerebral vessels because of the high level of carbon dioxide. Autoregulation of brain perfusion is lost and cerebral blood flow greatly increased. The time for temperature equilibration between blood and brain is shortened, resulting in a quick and homogenous cooling of the brain. Hypercapnia shifts the oxyhemoglobin dissociation curve toward the left and results in an increased availability of oxygen to tissues.
The superiority of the pH strategy has not been confirmed in adults, however. Prospective studies found no differences or even worse neuropsychologic outcome. Because it maintains a physiological coupling between cerebral blood flow and metabolism, the alpha-stat strategy appears advantageous in adults, in whom the risk of under- or overperfusion within the brain is substantial.
Hypothermia reduces oxygen consumption by a factor of 0.5 for every 10°C decrease in temperature. However, at both normothermia and hypothermia maximal oxygen consumption falls with decreasing flow as described in the following equation:
Kirklin and Barratt-Boyes recommend that flows be reduced only to levels which permit at least 85% of maximal an oxygen consumption. At 30°C this flow rate is approximately 1.8 L/min/m2; at 25°C, 1.6 L/min/m2; and at 18°C, 1.0 L/min/m2.
Hypothermia reduces cerebral metabolic rate, but also causes cerebral vasoconstriction. Hypothermia shifts the oxygen dissociation curve to the left, and there is increased CO2 solubility and alkalosis, resulting in a further shift of the oxygen dissociation curve to the left.
pH-stat: As CO2 solubility increases with hypothermia, the partial pressure decreases and CO2 is added to maintain normal pH and pCO2.
Arguments against pH-stat centered on the potential for impaired cellular function following reperfusion and associated increased cerebral blood flow.
alpha-stat: This method results in relative alkalosis, but it maintains electrochemical neutrality. The intracellular pH remains constant, and this may have a beneficial effect on protein structure and function, enzyme activity, and substrate ionization.
Viscosity increases approximate 2% for each °C decrease in temperature.
Oxygen availability is reduced during hypothermia because of the shift to the right of the oxyhemoglobin curve.
During rapid cooling, however, the affinity of oxygen to hemoglobin rises sharply during the period that the tissue temperature is not equilibrated with that of blood. This effect, combined with the dilution of blood by the priming volume of cardiopulmonary bypass, may temporarily create a state of insufficient oxygen availability.
Hypothermia increases the solubility of volatile anesthetics in blood. Rewarming returns anesthetic requirements to baseline levels and predisposes to inadequate anesthesia if therapeutic drug concentrations are not maintained.
Alpha-stat management (pH increases during cooling) decreases cerebral perfusion during hypothermia; pH stat (pH 7.40 is maintained by adding CO2) improves organ perfusion but may increase embolic injury.
СО2 flow on pH-stat is maintained at 50 to 100 ml per minute.
Hypothermia is considered to be mild when patient's core temperature is 28 to35C, moderate when it is 21 to 27C, and deep when 15 to 20C.
Safe periods of low flow bypass
1.6 l/m/m2@ 30C for max 120 min
0.5 l/m/m2 @ 28C for max 20 min
0.5 l/m/m2 @ 26C for max 30 min
0.5 l/m/m2 @ 22C for max 45 min
0.5 l/m/m2 @ 17C for max 90 min
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