Effects of moderate hypothermia on O2 consumption at various O2 deliveries in a sheep model.

The effects of modest hypothermia on oxygen consumption (VO2) were studied at various levels of oxygen delivery (DO2) in six sheep. Each animal was placed on cardiopulmonary bypass by extrathoracic cannulations. DO2 was varied by changing blood flow through an extracorporeal circuit. VO2 was measured spirometrically across a membrane lung. VO2 was initially measured at various levels of DO2 at normothermic temperatures (39 degrees C). The animals were then cooled to 33 degrees C. DO2 was varied, and the corresponding VO2's were determined. The data at both temperatures demonstrated the biphasic relationship of VO2 to various levels of DO2. A critical level of DO2 (DO2 crit) was defined to reflect the transition area between the dependent and independent portions of the consumption-delivery curve. The average baseline VO2's on the delivery independent portion of the curve were calculated to be 5.33 and 3.17 ml O2.kg-1.min-1 at 39 and 33 degrees C, respectively (P less than 0.001). The corresponding DO2 crit's were 6.17 and 4.57 ml O2.kg-1.min-1 (P less than 0.05). The oxygen extraction ratios at DO2 crit for each of these temperatures did not differ significantly. We conclude that hypothermia, by lowering baseline VO2, reduces DO2 crit. Hypothermia may therefore reduce or eliminate the anaerobic metabolism and subsequent acidosis that would otherwise occur during normothermia at low levels of DO2.

Early experience with adult extracorporeal membrane oxygenation in the modern era.

In 1980 we stopped using extracorporeal membrane oxygenation for adults because only 1 of 20 patients treated between 1973 and 1979 survived. In October 1988 we returned to adult extracorporeal life support (ECLS) with a modified protocol including venovenous access when possible, large oxygenators for CO2 clearance, activated clotting time of 180 to 200 seconds, and case selection based on 90% mortality (30% transpulmonary shunt). Of 19 patients referred, 14 met criteria for ECLS. Three of these 14 patients with isolated respiratory failure died before ECLS could be started, and 1 patient refused ECLS and died. Ten were placed on ECLS for 2 to 24 days. Indications were pneumonia (3), post-cardiac operation (2), and adult respiratory distress syndrome (5). Five recovered and 5 died. The cause of early death was progressive pulmonary injury (3), hemorrhage (1), and ventricular arrhythmia (1). One late death occurred at 3 months secondary to intraabdominal complications related to liver transplantation. In conclusion, 10 adult patients with severe respiratory failure were treated with extracorporeal life support; 5 patients recovered lung function and 4 of these patients survived and were discharged to home. Surviving patients were typically younger and were placed on ECLS early in their disease process, emphasizing that early intervention is one key factor to a successful outcome.

Evaluation of extracorporeal perfusion catheters using a standardized measurement technique--the M-number.

Cannulas used for extracorporeal circulating devices are made in many shapes and sizes. Side hole placement, length, and wall thickness can make the pressure-flow characteristics of catheters with similar French sizes quite different in their actual performance. A single value, the M-number, has been developed that describes the pressure-flow characteristics of an individual cannula. This number can be used to compare catheters and assess their potential utility for given bypass requirements. The M-numbers for commonly used catheters that are placed by extrathoracic dissections are determined. Clinical situations are presented that demonstrate applications of the M-number.