PHYSIOLOGIC RATIONALE FOR ABBREVIATED LAPAROTOMY

Trauma patients who succumb to their injuries do so by one of several mechanisms. The most common include head injury, exsanguination, sepsis, and multiple organ failure. Another cause of death, previously unrecognized, evolved in the late 1970s and early 1980s, when an epidemic of penetrating injuries was encountered in many urban trauma centers. Blood banks in these institutions developed the capability of delivering massive quantities of red blood cells and other components to keep pace with the transfusion requirements of all but the most severely injured patients. As a result, trauma surgeons were able to continue operating upon such patients until a constellation of metabolic derangements developed. These were characterized by the triad of a clinically obvious coagulopathy, profound hypothermia, and metabolic acidosis. 5,7 Hypothermia from evaporative and conductive heat loss and diminished heat production occurred in spite of warming blankets and blood warmers. The metabolic acidosis of shock was exacerbated by aortic clamping, vasopressors, massive transfusions, and impaired myocardial performance. Coagulopathy due to dilution and hypothermia completed the lethal triad. Each of these factors reinforced both of the other derangements, resulting in an extremely fragile patient who was moments away from a fatal arrhythmia. Kashuk et al 7 referred to this downward spiral as the "bloody vicious cycle." Stone et al 11 were the first to advocate abdominal packing and rapid abdominal closure under tension to treat the coagulopathy. They emphasized the importance of elevated intra-abdominal pressure to retard hemorrhage while the coagulopathy was being corrected. 9 Although increased intra-abdominal pressure does slow the rate of blood loss, it also creates a potentially deleterious condition that has come to be known as the abdominal compartment syndrome. 1,10 Today general agreement exists that the concept of "abbreviated laparotomy" with planned reoperation is valid and that the rapid conclusion of the initial laparotomy is essential for success. 8 However, we now believe that the primary benefit of rapid abdominal closure is the prevention of further heat loss rather than the tamponade effect of increased abdominal pressure. The adverse consequences of hypothermia have been appreciated for many years. Most early scientific observations were made in studies of humans immersed in cold water. These experiments were performed to determine the tolerance of and rescue logistics for downed pilots in the North Atlantic during World War II. From a practical standpoint for the surgeon, impaired myocardial contractility, cardiac irritability, and impaired coagulation capability are the most important complications. The reason that correction of hypothermia is crucial for survival after abbreviated laparotomy is that neither coagulopathy nor acidosis can be corrected until the patient's core temperature approaches normal. Furthermore, experience has shown that it is virtually impossible to achieve normothermia in these patients as long as the abdominal (and/or thoracic) cavity is open. Several major and minor sources of heat loss are present in injured patients undergoing operations. Conductive heat loss occurs by the direct transfer of thermal energy. One common source is the infusion of unwarmed fluids such as crystalloid solutions and blood products. Another source of conductive loss occurs when the patient lies on blood-saturated sheets that are in contact with the cold operating table. Shed blood and urine are additional sources of conductive heat loss. Less obvious but more important is the heat that is lost as moisture evaporates from the exposed visceral and serosal surfaces. In addition, evaporative heat loss also occurs from the blood-saturated drapes, thereby augmenting conductive heat loss to the operating table. Another mechanism of loss of thermal energy is convection: Heat radiates directly from the surface of the body to the environment. Compared with other mechanisms, however, convective heat loss is trivial. A final mechanism of heat loss occurs when cool dry air is inhaled and warm moist air is exhaled. Given these various mechanisms by which the body loses heat and the impaired production of energy associated with anesthesia and shock, it is possible to gain further insight into the dynamic process of intraoperative heat loss by developing a mathematical model that describes the thermal fluxes of injured patients. For this model we make several assumptions based on figures documented in patients treated by abbreviated laparotomy. 2 We assume that the patient weights 70 kg, has a skin surface area of 1.6 m 2 and a peritoneal surface of 1.5 m 2 , receives 22 units of red blood cells and 11.5 L of saline, loses 10 L of blood, and has an average body temperature during the operation of 34°C. Although it is difficult, if not impossible, to measure heat losses by all the various mechanisms, the overall heat loss from the skin and peritoneum of anesthetized pigs has been measured directly using sophisticated heat flux transducers. 4 These devices measure total heat loss from a surface caused by all the mechanisms. We also assume that, for the purpose of the model, heat flux in pigs is similar to that in humans. Overall heat balance in our model (QT) can be described by the equation: where Q S is the heat flux from the skin, Q p is the heat flux from the peritoneum, Q F is the heat flux due to intravenous infusions and blood loss, and Q A is the heat flux from pulmonary ventilation. Q s equals the product of the heat flux transducer measurement (74 W/ m 2 ), the percent of exposed surface area (80%), and the body surface area (1.56 m 2 ). 4 This product equals−92 w. Q P uses the same calculation except that the surface area is 1.5 m 2 , the percent of exposed surface in an eviscerated patient is estimated to be 15%, and the heat flux transducer measurement for the peritoneal cavity is 350 w/m 2 . 3,4 This results in a total heat exchange from the peritoneal cavity of −396 w. The net heat exchange due to infused and shed fluids is equal to: We further assume that 50% of the fluid is infused at 39°C with blood warmers and that the remainder is infused at 21°C, representing room temperature. The heat exchange for each of the above terms is calculated using the formula: where p=density, Cp=heat capacity, F=flow rate, and ΔT = the change in temperature. Using these figures, Q F is −85 w. Q A , or the heat exchange via airway, is the product of the mass flow rate (pF), the heat capacity (Cp), and the change in temperature of the gas. We assume that 100% O 2 is administered and that the gas is 100% saturated with H 2 O. pF equals the volumetric flow rate in liters per hour times the density. The result for Q A is + 2 w. When all the above values are added, the total heat exchange for our model is (−92) + (−396) + (−85) + 2, or −571 w. This figure converts to 491 Kcal per hour. Because a negative heat exchange of 58.1 Kcal yields a temperature loss of 1°C, our mathematical model predicts heat loss at a rate as high as 8.5°C per hour. This is somewhat offset by the body's own generation of heat, which is usually in the range of 81 w. 6 However, in a shocked patient, the value for endogenous heat production must be much lower. Nevertheless, using the optimistic figures, our model still predicts heat loss at a rate of 7.2°C per hour. Finally, if all intravenous fluids are administered at 39°C rather than just half, heat loss drops to 4.6°C per hour, which is still an alarming rate. Although this model has considerable room for error, it is important because it puts any discussion of hypothermia during damage control surgery into a quantitative context. It clearly shows that evaporative heat loss from an open peritoneal cavity with eviscerated bowel is a major contributor to hypothermia in a critically injured trauma patient undergoing a damage control laparotomy. This source of heat loss can be immediately terminated by closure of the abdominal cavity and therefore is the physiologic rationale for abbreviated laparotomy.