DRUGS AND INTERNAL COOLING— DAMMANN Axn MULLER 423 



tares and noted an improved tolerance to prolonged cardiac nianipnlation. This 

 method of cooling has the advantage of increasing or controlling coronary blood 

 flow and systemic blood pressure. The chief disadvantage is that one is introducing 

 into the arterial system unoxygenated blood. Furthermore, if clots form in the cool- 

 ing coil, they are introduced into the arterial circulation and may damage a critical 

 area. Using the vein-to-vein method, any clot that forms in the cooling coil is 

 filtered out by the pulmonary capillary bed and the systemic circulation is protected. 



The third and perhaps most promising method involves vein-to-artery cooling 

 plus some form of oxygenator. This method has been extensively studied by Gollan, 

 using an oxygen-diffusion oxygenator. Peirce has used vein-to-artery cooling plus 

 a dog lung as the artificial oxygenator. ^^ 



All three methods are open to certain criticisms. Cannulation of veins alone or a 

 vein plus an artery adds to the total trauma. A complicated pump and cooling coil 

 that needs careful watching is added to an already complicated setup. When an 

 artificial oxygenator is used, the machine becomes even more complex. Any me- 

 chanical device is subject to breakdown and if the breakdown occurs at the wrong 

 moment, the result may be a fatality. When difficulties do occur, it is not always 

 easy to determine whether the extracorporeal circulation or hypothermia is at fault. 

 When blood is passed through a system of coils and through an artificial oxygenator, 

 damage may be done to some of the constituents of normal blood. Blood clots, 

 hemolysis, loss of platelets and alteration in electrolytes comprise a few such altera- 

 tions. These comprise the chief disadvantages of blood stream cooling. 



In the questionnaire to which Dr. ^Muller referred, rapid cooling was preferred 

 by the majority of those queried. If this view is correct, blood stream cooling would 

 seem to offer a distinct advantage over other methods. The rates of both cooling 

 and rewarming can be greatly accelerated. The total period under anesthesia is 

 decreased. The surgical procedure can be started immediately and by the time the 

 chest is entered, the patient's temperature will have reached the desired level. At 

 low temperatures where dangerous arrhythmias seem more common, heart action 

 can be observed directly and complications treated rapidly. As soon as the procedure 

 within the heart has been completed, rewarming can be started. Consequently, the 

 patient's temperature can Ije brought up above the danger level before the chest 

 is completely closed. Swan- has stressed the importance of warming to the point 

 where pulse and pressure are strong and regular before the chest is closed, for he 

 feels that complications can be recognized more rapidly and handled more effi- 

 ciently with the chest open. Brock,-^ using vein-to-vein venous warming, has found 

 that 20 to 30 minutes rewarming can be provided while the chest is being closed and 

 that during this time the temperature can be raised to 32° C. 



A common experience when external cooling of the body is used is the drift of 

 the temperature downward after external cold is removed. Such a drift is not asso- 

 ciated with blood stream cooling. The desired temperature can be selected, reached, 

 and maintained until the procedure is completed. If, for reasons of necessity, rapid 

 rewarming is desired, this can be carried out within a few minutes. 



Ross states that a fall of one degree centigrade in five minutes is ideal when using 

 the vein-to-vein method. Cooling can be faster but he does not feel that rapid cool- 

 ing is as well tolerated by the animal. One result of very rapid cooling is that the 



