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Incomplete relaxation was detected in three of 10 (heart rate 120 beats/min) and nine of 11 (heart rate 140 beats/min) patients with coronary artery disease and this decreased to 0 of 10 (heart rate 120 beats/min) and six of 11 (heart rate 140 beats/min) patients after coronary artery bypass. Incomplete relaxation before bypass at a heart rate of 120 beats/min averaged 0.9 0.3 mm Hg. At a heart rate of 140 beats/min, incomplete relaxation averaged 5.6 + 1.6 mm Hg before and 1.4 + 0.5 mm Hg after bypass. Intake of ,3-blockers or calcium-channel blockers, body temperature, and systolic blood pressure were not found to be related to these changes. We conclude that immediately after coronary artery bypass relaxation of left ventricular muscle is enhanced and incomplete relaxation at rapid heart rates is less likely. The most probable cause of this improvement in ventricular relaxation after coronary artery bypass grafting is relief of ischemia. Circulation 77, No. 4, 886-896, 1988. RELAXATION of left ventricular muscle during diastole is an active, energy-requiring process that is prolonged during or after ischemia. The rate of relaxation can be quantified by a time constant, T, derived from the exponential fall in left ventricular pressure after peak negative dP/dt and before mitral valve opening.'The magnitude of T reflects in part the extent to which relaxation is impaired by ischemia. 1-10 In addition, as T becomes greater, a longer diastolic time is required

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SUMMARY The accurate measurement of pediatric cardiac output by thermodilution requires that the quantity of cold indicator introduced into the central circulation be known. This study defines an important source of error in the correction factor for the amount of heat gained by small volumes of cold injectate during passage through pediatric catheter systems. This error may result in significant overestimation of cardiac output (as much as 59%) when blood at body temperature is withdrawn into the injection lumen of the pediatric catheter before the injection.THERMODILUTION catheter systems, both adult and pediatric, should account for the amount of heat gained (CT) by the cool injectate as it passes through a lumen of a catheter warmed to body temperature.The CT factor is incorporated into the formula for cardiac output by thermodilution (see appendix) and a A common method for measuring and computing CT includes the volume of the catheter dead space as part of the total injectate volume.' This method was first described and used with the adult #7F flowdirected thermodilution catheter and later with the smaller pediatric catheter systems. However, the dead space of the injection lumen of the adult #7F catheter is 0.8 ml (8% of a 10-ml injection), whereas in the smaller #5F pediatric catheter system the dead space is approximately 0.65 ml (65% of a 1-ml injection). This observation led to the investigation of the effect that temperature of the dead space fluid has on the CT value supplied by the catheter manufacturer, because many investigators recommend prefilling the injection catheter with blood at body temperature"-6 and others do not. 1 7-11 Materials and Methods Two catheter systems (Edwards Laboratories) were studied. The first was a #5F, 60-cm, flow-directed catheter with four lumens: one for measuring pressure at the catheter tip, one for measuring pressure and injecting cool solution through an opening 15 cm proximal to the catheter tip, one for balloon inflation and one that contained lead wires to a thermistor. The second catheter system consisted of a #2F thermistor catheter (designed to be inserted through the right ven- tricular wall and manipulated into the pulmonary artery during thoracotomy for cardiac surgery) and a #3.5F injection catheter, 30 cm long, separately placed into the right atrium.A model was used to measure CT ( fig. 1). A heated, stirred 10-liter water bath was maintained at a temperature of 37 ± 0.2°C. Ten centimeters of the #3.5 F catheter and 25 cm of the #5F catheter were immersed in the water bath, with the tip brought out and inserted into a small plastic bag for collection of the injectate. The #5F catheter was cut at the level of the proximal injection port and the distal 15 cm of catheter was discarded so that the injection port could be inserted into the plastic bag. The immersion lengths above represent the intravascular segment of catheter exposed to body temperature and are those specified by the manufacturer.The plastic bags were made from long, thin tubes of pol...

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