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AN ULTRARAPID HEAT EXCHANGER FOR PROFOUND HYPOTHERMIA
A N ULTRARAPID HEAT FOR P R O F O U N D
EXCHANGER
HYPOTHERMIA
A.
DeGasperis, M.D., A. Demetz, M.D., and
R.
Donatelli,
M.D., Milano,
Italy
S
Gollan10 first demonstrated that the body temperature could be lowered to profound hypothermic levels without untoward effects for the organism, several other investigators have used extracorporeal blood stream cooling to induce deep hypothermia, experimentally 3 ' "• 1 0 ' 1 4 ' 1 7 ' 1 S | 2 0 > 2 1 - 2 3 - 2 S and clinically. 2 ' 7 INCE
8, 9, l.rj, 19, •!•!, 20, '11, 30
Various types of heat exchangers were developed 1 ' 6 ' 1 0 , 2 4 > 2 n ' 2 9 using ice watei' as a cooling medium and hot water for rewarming, but we believe that, in order to prevent blood element damage, blood should not be exposed to critical temperatures either during cooling or rewarming, such as, temperatures below 5° C. and above 38° C. The cooling of the patient should be achieved rapidly, within the shortest possible time, to reduce blood damage during extracorporeal circulation for the induction of profound hypothermia. The use of an efficient heat exchanger is necessary to achieve such a rapid caloric exchange, and must be built in such a way that moderately low blood flows and elevated water flows are permitted. The high water flows accelerate the caloric exchange and, at the same time, make it possible to reduce the temperature gradient between inflowing blood and inflowing water. Low blood flows reduce blood injury. The function of the heat exchanger is that of rapid heat transfer between water and blood. Therefore, the apparatus must meet thermodynamic as well as clinical prerequisites; it should afford rapid cooling and rewarming, and the temperature control should be precise. Simple design and compact construction should facilitate its cleaning, assembly, and sterilization, reduce the priming volume, exclude air trapping or blood contamination, and cause no impedance of blood flow or injury to blood elements. "With these objects in mind we designed and developed a new heat exchanger and combined it with a thermic unit capable of withdrawing the calories required to achieve rapid cooling of a patient, regardless of his weight. THEORETICAL BACKGROUND
To achieve a desired temperature level by means of extracorporeal cooling of From the Department of Caruio-Thoracic Surgery, Ospedale Maggiore, Milano, Italy. Received for publication April 5, 1962. 343
344
DE GASPEEIS, DEMETZ, DONATELLI
J. Thoracic and
Cardiovas. Surg.
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5.55.04,4 3,83.3 2,89.0
13.S
18,0
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27.2
31.7
Patient
36.3
40,8
Weight
(Kg)
45.4
49,9
54,4
59.0
63.5
Pig-. 1.—The relationship between water temperature and patient weight is plotted allowing1 esophageal temperature to be reduced to 12° C. within 15 minutes with a water flow of 180 L. per minute and a mean blood flow of approximately 45 ml. per kilogram per minute.
the blood, it is essential to know the amount of heat which must be transferred to reach the desired temperature level and to use an efficient heat exchanger capable of transferring this heat within a desired time period. The quantity of heat which must be dispersed from a uniform and homogeneous body to lower its temperature to the desired level can be calculated from the formula: Q = m • (t t - t 2 ) ■ c in which Q = kilogram calories (kcal), m = mass in kilograms, t t = initial temperature, t2 = final temperature, and c = specific heat. This formula, however, cannot be directly applied to the human organism because of the differential cooling of the various areas of the human body (see Pig. 6). In our experience we have observed that to reach an esophageal temperature between 10° C. and 15° C. in 15 to 20 minutes it is necessary to use an apparatus capable of removing, during the same period, an amount of calories equal to approximately 70 per cent of the kcal calculated with the above formula, which applies to a homogeneous body with uniform cooling. The heat transfer, calculated by multiplying the arteriovenous temperature difference by the blood flow in liters, is still lower because of the heat loss which occurs in the apparatus and environment. The amount of heat which can be transferred by a heat exchanger may be calculated from the formula: Q = S -a- Atm in which Q = quantity of heat the heat exchanger is capable of transferring in
Vol. 45, No. 3 March, 1963
ULTRARAPID HEAT EXCHANGER
345
the time unit; S = heat exchange surface; a = coefficient of heat transfer, which depends on the thermic wall resistance, the thermodynamic characteristics of the fluids, and their flow rates. A tm = mean logarithmic difference between the inflow and outflow temperatures of blood and water. A careful analysis of the second formula shows that the rate of caloric transfer is directly proportional to: (1) the heat exchange surface, (2) the flow rates of the fluids, and (3) the temperature gradient between the fluids. The rate of caloric exchange rises with increasing flow rates, but, since the variations of blood flow are small, the heat transfer will primarily depend on the water flow rates. Also, an increase in the water flow rate permits reduction of the temperature gradient between the heat exchanging fluids. If the amount of calories which must be transferred are known and the flow rates of blood and water are maintained constant at a preset level, that is, 45 ml. per kilogram per minute and 180 L. per minute, then the duration of cooling will depend on the temperature of the cooling water (Fig. 1). DESCRIPTION OF EQUIPMENT
The apparatus employed for direct blood stream cooling consists of two parts: (1) the heat exchanger, and (2) a thermostatically controlled thermic unit including hot and cold water reservoirs and a circulating pump.
Fig. 2.—Blood heat exchanger assembled.
346
D E GASPERIS, DEMETZ, D O N A T E L L I
J. Thoracic and Cardiovas. Surg.
The heat exchanger is mounted vertically on a stand (Fig. 2) and is placed on the table of the pump cart. It is made of a stainless steel, inverted " U " tube, the branches of which are placed in water jackets formed by two concentric cylinders (Fig. 3). The outer surface of the " U " tube and the internal surface of the inner cylinder delimit a narrow space, "blood chamber," containing the circulating blood. Both these surfaces are highly polished. A Plexiglas headpiece at the upper end of the water jackets covers the curvature of the " U "
Fig-. 3.—Cross section of blood heat exchanger. Blood and water flow in opposite directions. a, Mercury thermometer, b, Plexiglas headpiece, c, Air vent.
tube, completing the blood chamber in that portion. The cold or warm water circulates inside the " U " tube along the internal surface and inside the water jackets along the outer surface of the blood chamber with the exception of the portion covered by the Plexiglas headpiece, which allows priming of the apparatus and elimination of residual air bubbles through an air vent. The " U " tube is 150 cm. long with an inner diameter of 0.5 cm.; the water jackets are 70 cm. long and have an inner diameter of 0.5 cm. The width of the blood chamber is 1.5 mm., and the heat transfer surface measures 3,542 sq. cm. The priming
Vol. 45, No. 3
ULTRARAPID H E A T EXCHANGER
347
March, 1963
°* '
volume amounts to only 300 ml. The water flows in opposite direction to the blood stream, establishing a maximal temperature gradient at all points along the exchange surface. Largo water inlets and outlets permit flows of more than 180 L. per minute. A plastic tubing of 4.5 cm. inside diameter connects them to the thermic unit. This unit consists of a 3,000 calories per hour refrigerator, a 1,000 watt heater, and two 150 liter water reservoirs. The temperature of the water in the reservoirs (5° to 38° C.) is thermostatically controlled. A circulating pump maintains a constant water flow of 180 L. per minute. Two mercury thermometers are connected with the water inlet and outlet for continuous reading of the water temperatures. Thermistor leads from the esophagus, skin, muscles, rectum, arterial inflow and venous outflow blood are connected to a multiple channel telethermometer. The heat exchanger is autoclaved after assembly. DISCUSSION
An efficient heat exchanger has been achieved. It affords rapid cooling and rewarming of any patient independently of his weight, requires a low priming blood volume, is easy to clean and sterilize, and causes little or no blood damage. The heat exchanger has an exchange surface which is considerably larger than that of other types previously tested and used (Table I ) . Furthermore, adopting the principle of a single, narrow "blood chamber" delimited by two concentric cylinders, the ratio between exchange surface and priming volume is higher and the chances of central streaming, which may occur in wider tubes, are eliminated. In the heat exchangers with multiple wider tubes the central blood stream is thermically insulated from the exchange surface. The small cross-sectional area (180 mm.) increases the velocity of the circulating blood, thus raising the coefficient of caloric exchange. TABLE I.
1 2 8 24 2
H E A T EXCHANGE SURFACE OF DIFFERENT T Y P E S OF H E A T EXCHANGERS
Coil, % 6 in. I.D. and 7 ft. lengthz* Coils, % 6 in. I.D. and 17 ft. lengths* Tubes, 6 mm. I.D. and 160 cm. lengths Tubes, 4.6 mm. I.D. and 40 cm. length* Concentric cylinders, 4.0 cm. and 4.3 cm. I.D. and 150 cm. length
SURFACE (SQ. CM.)
VOLUME (ML.)
318 1,549 2,411 1,868
38 184 362 175
s/v 8.3 8.4 6.6 10.2
3,542
300
11.8
The resistance to blood flow, caused by the insertion of the heat exchanger in the arterial line, is shown by the pressure gradient obtained by measuring the line pressure immediately proximally and distally to the heat exchanger. The line pressures registered with different blood flows at blood temperatures of 38° C. and 10° C. are illustrated by Pig. 4. At 38° C , with flow rates of 1,500, 2,300, and 3,000 ml. per minute, the pressure gradient amounts to 90, 130, and 180 mm. Hg, respectively. These values were seen to be equivalent to those measured when placing a 25 cm. length of tubing, 14 in. in inner diameter, in the % inch arterial line in place of the heat exchanger. At 10° C , with the same flow rates,
DB GASPEBIS, DEMETZ, DONATELLI
Pig. 4.—Effect of blood flow rates on line pressure proximal (upper curves) curves) to the heat exchanger.
and distal (.lower
the pressure gradient increases by approximately 100 per cent, reaching 170, 260, and 320 mm. Hg. This is due to the increased viscosity of blood at low temperature. The heat transfer characteristics of the heat exchanger at varying blood flow rates are illustrated by Fig. 5. This diagram outlines the importance of the high water flow rates. With the same blood flow rates and with a heat exchange surface only twice as large, the caloric transfer in our heat exchanger is three times as high as in the Brown-type 1 heat exchanger (Fig. 5). The efficiency of the heat exchanger during extracorporeal circulation is illustrated by the temperature data obtained from a patient during open-heart surgery for correction of a sub valvular aortic stenosis (Fig. 6). The elevated
Vol. 45, No. 3
ULTEAEAPID H E A T E X C H A N G E E
March, 1963
349 °*V
flow rates of the water, which flows in opposite direction to the blood stream, reduces the temperature difference between inflowing and outflowing water to negligible values (less than 0.5° C.) (Fig. 6), thus maintaining a maximal caloric transfer coefficient at all points. Cooling of the vital organs to temperature levels around 10° C. occurred in 16 minutes and rewarming was equally rapid. The differential cooling of the different structures (varying from 0.33° C. per minute for the skin to 0.87° C. per minute for the rectum and 1.5° C. per minute for the esophagus) is primarily due to the difference in blood flow to the various areas. The relationship between cooling rate and blood flow has been outlined by the work of Pierce and associates.24 We found that the combination of an efficient heat exchanger with an adequate thermic unit was necessary to insure a drop of the esophageal temperature to around 10° C. to 15° C. within 15 to 20 minutes in any patient, independently of his weight and without lowering the temperature of the cooling water below 5° C. In our earlier experience, with the use of an inadequate thermic unit (1,200 calories per hour, 18 liter reservoirs, and a flow rate of 5 to 6 L. per minute), the temperature of the inflowing water tended to rise rapidly while cooling and caloric exchange was slowed considerably. This inconvenience could only be overcome by lowering the temperature of the cooling mixture (water + alcohol) to temperatures between -3° C. and -5° C. During rewarming, the temperature of the warming water does not exceed 38° C. The decrease of the blood-water temperature gradient reduces the po-
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Flow- IBOLit./min.
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7L W) ci ^
0.9 0.8
c E
0.7
J
0.6 OS 0.4 03 02 0.1
Blood
Flow - m l . / r n i n .
Fig. 5.—Heat transfer characteristics of heat exchanger at varying1 blood flow rates. parison of caloric exchange efficiency with the Brown-type heat exchanger.
Com-
D E GASPERIS, DEMETZ, D O N A T E L L I
350
0
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I
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6
8
10
12
14 16 16min.
circulation pump on extracorporeal circulation
) |
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|
2
4
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8
J. Thoracic and Cardiovas. Surg.
10
12 16
2 0 24 25 mm
H2O circulation pump on 1 extracorforeal circulation 1
Fig. 6.—Differential cooling and caloric data during extracorporeal circulation in a patient undergoing open-heart surgery for correction of a subvalvular aortic stenosis. The arteriovenous temperature difference times the blood flow permits calculation of caloric transfer during cooling and rewarming. Age 14, male, weight, 45.9 Kg. Initial temperature, 37° C. Final temperature, 10.2° C. Blood flow (mean), 2,530 c.c./ min. (55 c.c./Kg.). Duration of cooling, 18 min. Kcal calculated to cool to 10° C, 1,196 kcal. Average A-V temperature difference, 8° C. Caloric transfer, 364 kcal. Cooling rate (esophagus), 1.5° C./min. Cooling rate (rectum), 0.87° C./min. Cooling rate (skin), 0.33° C./min. Lowest temperature of cooling water, 8° C. Highest temperature of rewarming water, 38° C. Temperature difference between in- and outflowing water, less than 0.5° C.
tential danger of air bubbles forming in oxygen saturated plasma during the rewarming, 1 ' 4 since the solubility of the oxygen in the plasma decreases with rising temperatures. CONCLUSIONS
A thorough examination of the thermodynamic principles which influence caloric exchange between two fluids outlines the importance of the exchange surface and flow rates. These two factors regulate the efficiency of a heat exchanger. Adopting the principle of a single, narrow "blood chamber" delimited by two concentric cylinders, a large exchange surface is obtained with a favorable ratio exchange surface/priming volume. The elevated (180 L. per minute) water flow rates have several advantages: marked increase of the coefficient of caloric transfer; reduction of the temperature gradient between in- and outflowing water and between inflowing blood and water; elimination of critical temperatures of the
Vol. 45, No. 3
ULTRARAPID HEAT EXCHANGER
March, 1963
351
0 0 ±
water, that is, below 5° C. while cooling and above 38° C. while rewarming. The heat exchanger in combination with a thermic unit, as illustrated, has a caloric exchange capacity of 1.3 kcal per minute per degrees centigrade A t inlet fluids with a blood flow rate of 1,500 ml. per minute, which is by far superior to that of other types of heat exchangers (Fig. 5). SUMMARY
1. A highly efficient heat exchanger for direct blood stream cooling, consisting of a single, narrow "blood chamber," delimited by two concentric cylinders, has been presented. 2. The heat exchanger is used in combination with a thermic unit yielding a water flow of 180 L. per minute. 3. The body temperature can be lowered to 10° C. in 15 to 20 minutes independently of the patient's weight. 4. The heat exchanger described has worked well for the production of profound hypothermia in 50 clinical cases of open-heart surgery. Clinical experience will be reported in a subsequent paper. 5. Blood trauma and the danger of air embolism have been lessened by reducing the blood-water temperature gradient through elevated water flow rates. REFERENCES
1. Brown, I . W., Smith, W. W., and Emmons, W. O.: An Efficient Blood Heat Exchanger for Use With Extracorporeal Circulation, Surgery 44: 372, 1958. 2. DeGasperis, A., Donatelli, R., and Rovelli, F . : Sulla correzione chirurgica a cuore aperto della malformazione cardiaca, Acta tertii Europae de Cordis Scentia conventus Romae, 179, 1960. 3. Delorme, E . J . : Experimental Cooling of t h e Blood Stream, Lancet 2 : 914, 1952. 4. Donald, D. E., and Fellows, J . L . : Relation of Temperature, Gas Tension and Hydrostatic Pressure to the Formation of Gas Bubbles in Extracorporeally Oxygenated Blood, S. Forum 10: 589, 1959. 5. Donatelli, R., a n d Pellegrini, A.: Modificazioni fisiopato-logiche da circolazione extracorporea in normotermia ed in ipotermia profonda, Chir. P a t . Sper. 9: 1131, 1960. 6. Drew, C , Keen, G., and Benazon, D. B . : Profound Hypothermia, Lancet 1: 745, 1959. 7. Drew, C. E., and Anderson, I . M.: Profound Hypothermia in Cardiac Surgery: Report of Three Cases, Lancet 1: 748, 1959. 8. Dubost, Ch., and Blondeau, P h . : The Association of the Artificial H e a r t Lung With Deep Hypothermia in Open H e a r t Surgery, J . Cardiov. Surg. 1: 85, 1960. 9. Dubost, Ch., Blondeau, Ph., Passalecq, J., and Guary, J . : L'association du coeurpoumon artificiel et de l'hypothermie profonde dans l a chirurgie a coeur ouvert, Acta cardiol. internat. 8: 95, 1959. 10. Gollan, F., Bios, P . , and Shuman, H . : Studies on Hypothermia b y Means of a PumpOxygenator, Am. J . Physiol. 171: 331, 1952. 11. Gollan, F . , Hamilton, E . C , and Meneely, G. R.: Consecutive Survival of Open Chest Hypothermic Dogs After Prolonged By-Pass of H e a r t and Lungs by Means of a Pump-Oxygenator, Surgery 35: 88, 1954. 12. Gollan, F., Hoffman, J . E., and .Tones, R. M.: Maintenance of Life Below 10° C. Without Hemoglobin, Am. J . Physiol. 179: 640, 1954. 13. Gollan, F., Grace, J . T., Shell, W. W., Tysinger, D. S., and Feaster, L. B . : Left H e a r t Surgery in Dogs During Respiratory and Cardiac Arrest a t Body Temperatures Below 10° C , Surgery 38: 363, 1955. 14. Gollan, F . : Physiology of Cardiac Surgery, Hypothermia, Extracorporeal Circulation and Extracorporeal Cooling, Springfield, 111., 1959, Charles C Thomas, Publisher. 15. Gordon, A. S., Meyer, B . W., and Jones, J . C : Open H e a r t Surgery Using Deep H y p o t h e r m i a W i t h o u t Oxygenator, J . THORACIC SURG. 40: 787, 1960.
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J. Thoracic and
Cardiovas. Surg.
16. Juvenelle, A. A., Lind, J., and Wegelius, C : A New Method of Extracorporeal Circulation, Am. H e a r t J . 47: 692, 1954. 17. Kenyon, J. R., and Ludbrook, J . : Hypothermia Below 10° C. in Dogs With Cardiac Recovery on Rewarming, Lancet 2: 171, 1957. 18. Kenyon, J . R., Ludbrook, J., Downs, D. R., Tait, I. B., Brooks, D. K., and Pryczkowski, J . : Experimental Deep Hypothermia, Lancet 2: 41, 1959. 19. Kirklin, J . W., and Devloo, R. A.: Hypothermic Perf usion and Circulatory Arrest for Surgical Correction of Fallot With Previously Constructed P o t t s ' Anastomosis, Dis. Chest 39: 87, 1961. 20. Lesage, A. M., Sealy, W. C , Brown, I. W., and Young, W. G.: Hypothermia and Extracorporeal Circulation. Experimental Studies on Profound Hvpothermia of 10° C. to 20° C , A. M. A. Arch. Surg. 79: 607, 1959. 21. Niazi, S. A., and Lewis, F . J.: Profound Hypothermia in the Dog, Surg. Gynec. & Obst. 102: 98, 1956. 22. Niazi, S. A., and Lewis, F . J . : Profound Hypothermia in M a n : Report of a Case, Ann. Surg. 147: 254, 1958. 23. Niazi, S. A., and Lewis, F . J . : Profound Hypothermia in the Monkey With Recovery After Prolonged Periods of Cardiac Standstill, J. Appl. Physiol. 10: 137, 1957. 24. Pierce, E. C , Dabbs, C. H., Rogers, W. K., Rawson, F . L., and Tompkins, R.: Reduced Metabolism by Means of Hypothermia and the Low Flow Pump-Oxygenator, Surg. Gynec. & Obst. 107: 339, 1958. 25. Piwniea, A., Weiss, M., Lenfant, C , and Dubost, Ch.: Circulatory Arrest and Deep Hypothermia Induced W i t h a Pump-Oxygenator System and H e a t Exchanger, J. Cardiov. Surg. 1: 74, 1960. 26. Sealy, W. C , Brown, I. W., Jr., Young, W. G., Jr., Stephen, C. R., and Harris, J. S.: Hypothermia, Low Flow Extracorporeal Circulation and Controlled Cardiac Arrest for Open H e a r t Surgery, Surg. Gynec. & Obst. 104: 441, 1957. 27. Sealy, W. C , Brown, I. W., Jr., and Young, W. G., J r . : A Report on the Use of Both Extracorporeal Circulation and Hypothermia for the Open H e a r t Surgery, Ann. Surg. 147: 603, 1958. 28. Senning, A.: Extracorporeal Circulation Combined W i t h Hypothermia, Acta chir. scandinav. 107: 516, 1954. 29. Shields, T. W., and Lewis, F . J . : Rapid Cooling and Surgery at Temperatures Below 20° C , Surgery 46: 164, 1959. 30. Young, W. G., Jr., Sealy, W. C , Brown, I. W., Smith, W. W., Callaway, H. A., and Harris, I. S.: Metabolic and Physiologic Observations on P a t i e n t s Undergoing Extracorporeal Circulation in Conjunction W i t h Hypothermia, Surgery 46: 175, 1959.
EXCHANGER
HYPOTHERMIA
A.
DeGasperis, M.D., A. Demetz, M.D., and
R.
Donatelli,
M.D., Milano,
Italy
S
Gollan10 first demonstrated that the body temperature could be lowered to profound hypothermic levels without untoward effects for the organism, several other investigators have used extracorporeal blood stream cooling to induce deep hypothermia, experimentally 3 ' "• 1 0 ' 1 4 ' 1 7 ' 1 S | 2 0 > 2 1 - 2 3 - 2 S and clinically. 2 ' 7 INCE
8, 9, l.rj, 19, •!•!, 20, '11, 30
Various types of heat exchangers were developed 1 ' 6 ' 1 0 , 2 4 > 2 n ' 2 9 using ice watei' as a cooling medium and hot water for rewarming, but we believe that, in order to prevent blood element damage, blood should not be exposed to critical temperatures either during cooling or rewarming, such as, temperatures below 5° C. and above 38° C. The cooling of the patient should be achieved rapidly, within the shortest possible time, to reduce blood damage during extracorporeal circulation for the induction of profound hypothermia. The use of an efficient heat exchanger is necessary to achieve such a rapid caloric exchange, and must be built in such a way that moderately low blood flows and elevated water flows are permitted. The high water flows accelerate the caloric exchange and, at the same time, make it possible to reduce the temperature gradient between inflowing blood and inflowing water. Low blood flows reduce blood injury. The function of the heat exchanger is that of rapid heat transfer between water and blood. Therefore, the apparatus must meet thermodynamic as well as clinical prerequisites; it should afford rapid cooling and rewarming, and the temperature control should be precise. Simple design and compact construction should facilitate its cleaning, assembly, and sterilization, reduce the priming volume, exclude air trapping or blood contamination, and cause no impedance of blood flow or injury to blood elements. "With these objects in mind we designed and developed a new heat exchanger and combined it with a thermic unit capable of withdrawing the calories required to achieve rapid cooling of a patient, regardless of his weight. THEORETICAL BACKGROUND
To achieve a desired temperature level by means of extracorporeal cooling of From the Department of Caruio-Thoracic Surgery, Ospedale Maggiore, Milano, Italy. Received for publication April 5, 1962. 343
344
DE GASPEEIS, DEMETZ, DONATELLI
J. Thoracic and
Cardiovas. Surg.
10.7 10,09.5 8.8 8,4' ^
7.7£
7.3
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6,6
E
6.1
4.
(S c a, ^
5.55.04,4 3,83.3 2,89.0
13.S
18,0
22,7
27.2
31.7
Patient
36.3
40,8
Weight
(Kg)
45.4
49,9
54,4
59.0
63.5
Pig-. 1.—The relationship between water temperature and patient weight is plotted allowing1 esophageal temperature to be reduced to 12° C. within 15 minutes with a water flow of 180 L. per minute and a mean blood flow of approximately 45 ml. per kilogram per minute.
the blood, it is essential to know the amount of heat which must be transferred to reach the desired temperature level and to use an efficient heat exchanger capable of transferring this heat within a desired time period. The quantity of heat which must be dispersed from a uniform and homogeneous body to lower its temperature to the desired level can be calculated from the formula: Q = m • (t t - t 2 ) ■ c in which Q = kilogram calories (kcal), m = mass in kilograms, t t = initial temperature, t2 = final temperature, and c = specific heat. This formula, however, cannot be directly applied to the human organism because of the differential cooling of the various areas of the human body (see Pig. 6). In our experience we have observed that to reach an esophageal temperature between 10° C. and 15° C. in 15 to 20 minutes it is necessary to use an apparatus capable of removing, during the same period, an amount of calories equal to approximately 70 per cent of the kcal calculated with the above formula, which applies to a homogeneous body with uniform cooling. The heat transfer, calculated by multiplying the arteriovenous temperature difference by the blood flow in liters, is still lower because of the heat loss which occurs in the apparatus and environment. The amount of heat which can be transferred by a heat exchanger may be calculated from the formula: Q = S -a- Atm in which Q = quantity of heat the heat exchanger is capable of transferring in
Vol. 45, No. 3 March, 1963
ULTRARAPID HEAT EXCHANGER
345
the time unit; S = heat exchange surface; a = coefficient of heat transfer, which depends on the thermic wall resistance, the thermodynamic characteristics of the fluids, and their flow rates. A tm = mean logarithmic difference between the inflow and outflow temperatures of blood and water. A careful analysis of the second formula shows that the rate of caloric transfer is directly proportional to: (1) the heat exchange surface, (2) the flow rates of the fluids, and (3) the temperature gradient between the fluids. The rate of caloric exchange rises with increasing flow rates, but, since the variations of blood flow are small, the heat transfer will primarily depend on the water flow rates. Also, an increase in the water flow rate permits reduction of the temperature gradient between the heat exchanging fluids. If the amount of calories which must be transferred are known and the flow rates of blood and water are maintained constant at a preset level, that is, 45 ml. per kilogram per minute and 180 L. per minute, then the duration of cooling will depend on the temperature of the cooling water (Fig. 1). DESCRIPTION OF EQUIPMENT
The apparatus employed for direct blood stream cooling consists of two parts: (1) the heat exchanger, and (2) a thermostatically controlled thermic unit including hot and cold water reservoirs and a circulating pump.
Fig. 2.—Blood heat exchanger assembled.
346
D E GASPERIS, DEMETZ, D O N A T E L L I
J. Thoracic and Cardiovas. Surg.
The heat exchanger is mounted vertically on a stand (Fig. 2) and is placed on the table of the pump cart. It is made of a stainless steel, inverted " U " tube, the branches of which are placed in water jackets formed by two concentric cylinders (Fig. 3). The outer surface of the " U " tube and the internal surface of the inner cylinder delimit a narrow space, "blood chamber," containing the circulating blood. Both these surfaces are highly polished. A Plexiglas headpiece at the upper end of the water jackets covers the curvature of the " U "
Fig-. 3.—Cross section of blood heat exchanger. Blood and water flow in opposite directions. a, Mercury thermometer, b, Plexiglas headpiece, c, Air vent.
tube, completing the blood chamber in that portion. The cold or warm water circulates inside the " U " tube along the internal surface and inside the water jackets along the outer surface of the blood chamber with the exception of the portion covered by the Plexiglas headpiece, which allows priming of the apparatus and elimination of residual air bubbles through an air vent. The " U " tube is 150 cm. long with an inner diameter of 0.5 cm.; the water jackets are 70 cm. long and have an inner diameter of 0.5 cm. The width of the blood chamber is 1.5 mm., and the heat transfer surface measures 3,542 sq. cm. The priming
Vol. 45, No. 3
ULTRARAPID H E A T EXCHANGER
347
March, 1963
°* '
volume amounts to only 300 ml. The water flows in opposite direction to the blood stream, establishing a maximal temperature gradient at all points along the exchange surface. Largo water inlets and outlets permit flows of more than 180 L. per minute. A plastic tubing of 4.5 cm. inside diameter connects them to the thermic unit. This unit consists of a 3,000 calories per hour refrigerator, a 1,000 watt heater, and two 150 liter water reservoirs. The temperature of the water in the reservoirs (5° to 38° C.) is thermostatically controlled. A circulating pump maintains a constant water flow of 180 L. per minute. Two mercury thermometers are connected with the water inlet and outlet for continuous reading of the water temperatures. Thermistor leads from the esophagus, skin, muscles, rectum, arterial inflow and venous outflow blood are connected to a multiple channel telethermometer. The heat exchanger is autoclaved after assembly. DISCUSSION
An efficient heat exchanger has been achieved. It affords rapid cooling and rewarming of any patient independently of his weight, requires a low priming blood volume, is easy to clean and sterilize, and causes little or no blood damage. The heat exchanger has an exchange surface which is considerably larger than that of other types previously tested and used (Table I ) . Furthermore, adopting the principle of a single, narrow "blood chamber" delimited by two concentric cylinders, the ratio between exchange surface and priming volume is higher and the chances of central streaming, which may occur in wider tubes, are eliminated. In the heat exchangers with multiple wider tubes the central blood stream is thermically insulated from the exchange surface. The small cross-sectional area (180 mm.) increases the velocity of the circulating blood, thus raising the coefficient of caloric exchange. TABLE I.
1 2 8 24 2
H E A T EXCHANGE SURFACE OF DIFFERENT T Y P E S OF H E A T EXCHANGERS
Coil, % 6 in. I.D. and 7 ft. lengthz* Coils, % 6 in. I.D. and 17 ft. lengths* Tubes, 6 mm. I.D. and 160 cm. lengths Tubes, 4.6 mm. I.D. and 40 cm. length* Concentric cylinders, 4.0 cm. and 4.3 cm. I.D. and 150 cm. length
SURFACE (SQ. CM.)
VOLUME (ML.)
318 1,549 2,411 1,868
38 184 362 175
s/v 8.3 8.4 6.6 10.2
3,542
300
11.8
The resistance to blood flow, caused by the insertion of the heat exchanger in the arterial line, is shown by the pressure gradient obtained by measuring the line pressure immediately proximally and distally to the heat exchanger. The line pressures registered with different blood flows at blood temperatures of 38° C. and 10° C. are illustrated by Pig. 4. At 38° C , with flow rates of 1,500, 2,300, and 3,000 ml. per minute, the pressure gradient amounts to 90, 130, and 180 mm. Hg, respectively. These values were seen to be equivalent to those measured when placing a 25 cm. length of tubing, 14 in. in inner diameter, in the % inch arterial line in place of the heat exchanger. At 10° C , with the same flow rates,
DB GASPEBIS, DEMETZ, DONATELLI
Pig. 4.—Effect of blood flow rates on line pressure proximal (upper curves) curves) to the heat exchanger.
and distal (.lower
the pressure gradient increases by approximately 100 per cent, reaching 170, 260, and 320 mm. Hg. This is due to the increased viscosity of blood at low temperature. The heat transfer characteristics of the heat exchanger at varying blood flow rates are illustrated by Fig. 5. This diagram outlines the importance of the high water flow rates. With the same blood flow rates and with a heat exchange surface only twice as large, the caloric transfer in our heat exchanger is three times as high as in the Brown-type 1 heat exchanger (Fig. 5). The efficiency of the heat exchanger during extracorporeal circulation is illustrated by the temperature data obtained from a patient during open-heart surgery for correction of a sub valvular aortic stenosis (Fig. 6). The elevated
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ULTEAEAPID H E A T E X C H A N G E E
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349 °*V
flow rates of the water, which flows in opposite direction to the blood stream, reduces the temperature difference between inflowing and outflowing water to negligible values (less than 0.5° C.) (Fig. 6), thus maintaining a maximal caloric transfer coefficient at all points. Cooling of the vital organs to temperature levels around 10° C. occurred in 16 minutes and rewarming was equally rapid. The differential cooling of the different structures (varying from 0.33° C. per minute for the skin to 0.87° C. per minute for the rectum and 1.5° C. per minute for the esophagus) is primarily due to the difference in blood flow to the various areas. The relationship between cooling rate and blood flow has been outlined by the work of Pierce and associates.24 We found that the combination of an efficient heat exchanger with an adequate thermic unit was necessary to insure a drop of the esophageal temperature to around 10° C. to 15° C. within 15 to 20 minutes in any patient, independently of his weight and without lowering the temperature of the cooling water below 5° C. In our earlier experience, with the use of an inadequate thermic unit (1,200 calories per hour, 18 liter reservoirs, and a flow rate of 5 to 6 L. per minute), the temperature of the inflowing water tended to rise rapidly while cooling and caloric exchange was slowed considerably. This inconvenience could only be overcome by lowering the temperature of the cooling mixture (water + alcohol) to temperatures between -3° C. and -5° C. During rewarming, the temperature of the warming water does not exceed 38° C. The decrease of the blood-water temperature gradient reduces the po-
1.8
1.7 16
Water
Flow- IBOLit./min.
15 1.4 1.3 -S 3 ~-
12 1.1
7L W) ci ^
0.9 0.8
c E
0.7
J
0.6 OS 0.4 03 02 0.1
Blood
Flow - m l . / r n i n .
Fig. 5.—Heat transfer characteristics of heat exchanger at varying1 blood flow rates. parison of caloric exchange efficiency with the Brown-type heat exchanger.
Com-
D E GASPERIS, DEMETZ, D O N A T E L L I
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I
2
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ffiO
6
8
10
12
14 16 16min.
circulation pump on extracorporeal circulation
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J. Thoracic and Cardiovas. Surg.
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H2O circulation pump on 1 extracorforeal circulation 1
Fig. 6.—Differential cooling and caloric data during extracorporeal circulation in a patient undergoing open-heart surgery for correction of a subvalvular aortic stenosis. The arteriovenous temperature difference times the blood flow permits calculation of caloric transfer during cooling and rewarming. Age 14, male, weight, 45.9 Kg. Initial temperature, 37° C. Final temperature, 10.2° C. Blood flow (mean), 2,530 c.c./ min. (55 c.c./Kg.). Duration of cooling, 18 min. Kcal calculated to cool to 10° C, 1,196 kcal. Average A-V temperature difference, 8° C. Caloric transfer, 364 kcal. Cooling rate (esophagus), 1.5° C./min. Cooling rate (rectum), 0.87° C./min. Cooling rate (skin), 0.33° C./min. Lowest temperature of cooling water, 8° C. Highest temperature of rewarming water, 38° C. Temperature difference between in- and outflowing water, less than 0.5° C.
tential danger of air bubbles forming in oxygen saturated plasma during the rewarming, 1 ' 4 since the solubility of the oxygen in the plasma decreases with rising temperatures. CONCLUSIONS
A thorough examination of the thermodynamic principles which influence caloric exchange between two fluids outlines the importance of the exchange surface and flow rates. These two factors regulate the efficiency of a heat exchanger. Adopting the principle of a single, narrow "blood chamber" delimited by two concentric cylinders, a large exchange surface is obtained with a favorable ratio exchange surface/priming volume. The elevated (180 L. per minute) water flow rates have several advantages: marked increase of the coefficient of caloric transfer; reduction of the temperature gradient between in- and outflowing water and between inflowing blood and water; elimination of critical temperatures of the
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water, that is, below 5° C. while cooling and above 38° C. while rewarming. The heat exchanger in combination with a thermic unit, as illustrated, has a caloric exchange capacity of 1.3 kcal per minute per degrees centigrade A t inlet fluids with a blood flow rate of 1,500 ml. per minute, which is by far superior to that of other types of heat exchangers (Fig. 5). SUMMARY
1. A highly efficient heat exchanger for direct blood stream cooling, consisting of a single, narrow "blood chamber," delimited by two concentric cylinders, has been presented. 2. The heat exchanger is used in combination with a thermic unit yielding a water flow of 180 L. per minute. 3. The body temperature can be lowered to 10° C. in 15 to 20 minutes independently of the patient's weight. 4. The heat exchanger described has worked well for the production of profound hypothermia in 50 clinical cases of open-heart surgery. Clinical experience will be reported in a subsequent paper. 5. Blood trauma and the danger of air embolism have been lessened by reducing the blood-water temperature gradient through elevated water flow rates. REFERENCES
1. Brown, I . W., Smith, W. W., and Emmons, W. O.: An Efficient Blood Heat Exchanger for Use With Extracorporeal Circulation, Surgery 44: 372, 1958. 2. DeGasperis, A., Donatelli, R., and Rovelli, F . : Sulla correzione chirurgica a cuore aperto della malformazione cardiaca, Acta tertii Europae de Cordis Scentia conventus Romae, 179, 1960. 3. Delorme, E . J . : Experimental Cooling of t h e Blood Stream, Lancet 2 : 914, 1952. 4. Donald, D. E., and Fellows, J . L . : Relation of Temperature, Gas Tension and Hydrostatic Pressure to the Formation of Gas Bubbles in Extracorporeally Oxygenated Blood, S. Forum 10: 589, 1959. 5. Donatelli, R., a n d Pellegrini, A.: Modificazioni fisiopato-logiche da circolazione extracorporea in normotermia ed in ipotermia profonda, Chir. P a t . Sper. 9: 1131, 1960. 6. Drew, C , Keen, G., and Benazon, D. B . : Profound Hypothermia, Lancet 1: 745, 1959. 7. Drew, C. E., and Anderson, I . M.: Profound Hypothermia in Cardiac Surgery: Report of Three Cases, Lancet 1: 748, 1959. 8. Dubost, Ch., and Blondeau, P h . : The Association of the Artificial H e a r t Lung With Deep Hypothermia in Open H e a r t Surgery, J . Cardiov. Surg. 1: 85, 1960. 9. Dubost, Ch., Blondeau, Ph., Passalecq, J., and Guary, J . : L'association du coeurpoumon artificiel et de l'hypothermie profonde dans l a chirurgie a coeur ouvert, Acta cardiol. internat. 8: 95, 1959. 10. Gollan, F., Bios, P . , and Shuman, H . : Studies on Hypothermia b y Means of a PumpOxygenator, Am. J . Physiol. 171: 331, 1952. 11. Gollan, F . , Hamilton, E . C , and Meneely, G. R.: Consecutive Survival of Open Chest Hypothermic Dogs After Prolonged By-Pass of H e a r t and Lungs by Means of a Pump-Oxygenator, Surgery 35: 88, 1954. 12. Gollan, F., Hoffman, J . E., and .Tones, R. M.: Maintenance of Life Below 10° C. Without Hemoglobin, Am. J . Physiol. 179: 640, 1954. 13. Gollan, F., Grace, J . T., Shell, W. W., Tysinger, D. S., and Feaster, L. B . : Left H e a r t Surgery in Dogs During Respiratory and Cardiac Arrest a t Body Temperatures Below 10° C , Surgery 38: 363, 1955. 14. Gollan, F . : Physiology of Cardiac Surgery, Hypothermia, Extracorporeal Circulation and Extracorporeal Cooling, Springfield, 111., 1959, Charles C Thomas, Publisher. 15. Gordon, A. S., Meyer, B . W., and Jones, J . C : Open H e a r t Surgery Using Deep H y p o t h e r m i a W i t h o u t Oxygenator, J . THORACIC SURG. 40: 787, 1960.
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16. Juvenelle, A. A., Lind, J., and Wegelius, C : A New Method of Extracorporeal Circulation, Am. H e a r t J . 47: 692, 1954. 17. Kenyon, J. R., and Ludbrook, J . : Hypothermia Below 10° C. in Dogs With Cardiac Recovery on Rewarming, Lancet 2: 171, 1957. 18. Kenyon, J . R., Ludbrook, J., Downs, D. R., Tait, I. B., Brooks, D. K., and Pryczkowski, J . : Experimental Deep Hypothermia, Lancet 2: 41, 1959. 19. Kirklin, J . W., and Devloo, R. A.: Hypothermic Perf usion and Circulatory Arrest for Surgical Correction of Fallot With Previously Constructed P o t t s ' Anastomosis, Dis. Chest 39: 87, 1961. 20. Lesage, A. M., Sealy, W. C , Brown, I. W., and Young, W. G.: Hypothermia and Extracorporeal Circulation. Experimental Studies on Profound Hvpothermia of 10° C. to 20° C , A. M. A. Arch. Surg. 79: 607, 1959. 21. Niazi, S. A., and Lewis, F . J.: Profound Hypothermia in the Dog, Surg. Gynec. & Obst. 102: 98, 1956. 22. Niazi, S. A., and Lewis, F . J . : Profound Hypothermia in M a n : Report of a Case, Ann. Surg. 147: 254, 1958. 23. Niazi, S. A., and Lewis, F . J . : Profound Hypothermia in the Monkey With Recovery After Prolonged Periods of Cardiac Standstill, J. Appl. Physiol. 10: 137, 1957. 24. Pierce, E. C , Dabbs, C. H., Rogers, W. K., Rawson, F . L., and Tompkins, R.: Reduced Metabolism by Means of Hypothermia and the Low Flow Pump-Oxygenator, Surg. Gynec. & Obst. 107: 339, 1958. 25. Piwniea, A., Weiss, M., Lenfant, C , and Dubost, Ch.: Circulatory Arrest and Deep Hypothermia Induced W i t h a Pump-Oxygenator System and H e a t Exchanger, J. Cardiov. Surg. 1: 74, 1960. 26. Sealy, W. C , Brown, I. W., Jr., Young, W. G., Jr., Stephen, C. R., and Harris, J. S.: Hypothermia, Low Flow Extracorporeal Circulation and Controlled Cardiac Arrest for Open H e a r t Surgery, Surg. Gynec. & Obst. 104: 441, 1957. 27. Sealy, W. C , Brown, I. W., Jr., and Young, W. G., J r . : A Report on the Use of Both Extracorporeal Circulation and Hypothermia for the Open H e a r t Surgery, Ann. Surg. 147: 603, 1958. 28. Senning, A.: Extracorporeal Circulation Combined W i t h Hypothermia, Acta chir. scandinav. 107: 516, 1954. 29. Shields, T. W., and Lewis, F . J . : Rapid Cooling and Surgery at Temperatures Below 20° C , Surgery 46: 164, 1959. 30. Young, W. G., Jr., Sealy, W. C , Brown, I. W., Smith, W. W., Callaway, H. A., and Harris, I. S.: Metabolic and Physiologic Observations on P a t i e n t s Undergoing Extracorporeal Circulation in Conjunction W i t h Hypothermia, Surgery 46: 175, 1959.