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Ann Thorac Surg 1996;62:337
© 1996 The Society of Thoracic Surgeons
Division of Cardiothoracic Surgery, University of North Carolina at Chapel Hill, 108 Burnett-Womack Bldg, CB 7065, Chapel Hill, NC 27599-7065
This lucid report by Van Raemdonck and colleagues documents results of (failed) attempts to come up with a method to cool lungs via the airway after circulatory arrest but before explantation. The rationale for pursuing this line of investigation is to minimize the deleterious effects of an obligatory period of warm in situ ischemia in the clinical scenario of lung retrieval for transplantation from cadavers at intervals after death. For several years, my colleagues and I have been investigating cadaver lung retrieval to provide more lungs for transplantation. Although we have focused on other issues in published reports, we, too, have been intrigued by the possibility of cooling the lung via the airway. The idea is elegant because it is so simple: just ventilate the nonperfused lung (which we have shown may be beneficial for lung viability [1, 2], maintenance of tissue adenosine triphosphate [3], and gas exchange function after transplantation [4]) and, for good measure, simply dial in the desired inflow temperature. Presto: a hypothermic lung! As Van Raemdonck and colleagues have pointed out, hypothermia has been the bastion of organ preservation principles.
After we demonstrated the benefit of oxygen ventilation of canine cadaver lungs [4], we performed pilot experiments in dogs, attempting to cool the grafts via the airway. Drawing air for a ventilator circuit through coils immersed in dry ice, we succeeded in "icing up" ventilator circuits and even had ice forming on a Harvard ventilator, but could not budge downward the measurement recorded by a temperature probe placed in the canine donor right upper lobe any quicker than with ventilation using "room air temperature" oxygen.
Sadly, great ideas are all too often thwarted by the realities of physical laws. The first law of thermodynamics becomes the thwarter in this case. This law explains why, when a new empty refrigerator is plugged in, the air within it is quickly cooled; however, as we all observed firsthand in college, if you fill that new refrigerator with large volumes of warm beer, it could take forever to chill the brew.
There are several reasons why lungs are difficult to cool via the airway, but the predominant one relates to the specific heat in the water compartment of the tissue. The specific heat of water exceeds that of air by a factor of 18. In fur-bearing animals such as rabbits, the surrounding insulation may be even more efficient than in humans. Ventilation with cold air can also induce bronchospasm, potentially limiting the ability of the cold air to come in contact with the large liquid surface area it is intended to cool. Fortunately for Van Raemdonck and others interested in the use of cadaver lungs, it may not matter that it is impractical to cool the lungs via the airway. The lung may be relatively tolerant of a period of ischemia sufficient to allow for retrieval after death. Even more critical will likely be the events affecting the pulmonary microcirculation immediately preceding circulatory arrest in the cadaver. A better understanding of the physical laws governing ischemia/reperfusion events may allow the more liberal use of lungs from cadavers for transplantation.
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