Cardioplegia and cardiac function : evaluated by left ventricular pressure-volume relations
Abstract: Purpose: This thesis is focused on studies of blood cardioplegia in order to evaluate regimens of myocardial protection during cardiac surgery. To further bring down operative mortality and morbidity, in patients with acutely ischaemic hearts, an optimal peroperative myocardial management is essential. During the early 90ties several new regimens of cardioplegia emerged, but their place in clinical practice has been debated. By using an animal model, subjected to severe myocardial ischaemia, different regimens of cardioplegia were investigated. By using load independent indices of cardiac function our intention was to gain clinical relevant information about the appropriateness of these cardioplegic methods. We specifically investigated the impact of cardioplegic temperature and mode of administration on postoperative cardiac function. In study I: warm vs. cold cardioplegia; in study II: antegrade vs. simultaneous antegrade-retrograde cardioplegia; and in study III: warm intermittent vs. continuous cardioplegia. A study was also performed to further study the conductance catheter method for cardiac volume measurements (study IV), and to calibrate indices of cardiac function for changes in heart rate (study V). Methods: In study I-III pigs were put on cardiopulmonary bypass (CPB) and subjected to global ischaemia followed by cardioplegic resuscitation. Following reperfusion and weaning from CPB, cardiac function was measured using the conductance catheter technique with on-line acquisitions of left ventricular (LV) pressure-volume loops. Among measured indices were the preload recruitable stroke work relation (PRSW; global cardiac function) and the end-diastolic pressure-volume relation (EDPVR; diastolic chamber stiffness). In study IV volume measurements were used to compare (by an algorithm) the wave form of the volume curve from a mid-cardiac volume segment and the volume curve of the global LV volume. In study V the influence of heart rate on indices of cardiac function was investigated and the influence of heart rate on pressure-volume loops by atrial pacing in increments up to 160 beats per minute (bpm). Results and Implications: Blood cardioplegia given continuously showed to have a potential to resuscitate the acute severely damaged myocardium. The temperature of continuously delivered cardioplegia was of minor importance for acute post-bypass cardiac function. Warm cardioplegia caused an increasing coronary vascular resistance during the time of delivery which might have implications for the cardioplegic distribution. Since warm continuous cardioplegia is not proved better than cold cardioplegia, the cold technique is advocated due to the added safety of hypothermia. Warm intermittent blood cardioplegia showed to be detrimental for post-bypass cardiac function when used in the ischaemically insulted heart and should be avoided in these situations. Simultaneous warm antegrade-retrograde continuous blood cardioplegia did not, in comparison with antegrade perfusion, impair post-cardioplegic cardiac function despite a small, but significant, increase in myocardial water content. Simultaneous cardioplegia has thus the potential of being an optimal technique for cardioplegic delivery. The LV mid-segmental and global volume curves showed a good agreement. This suggests that the conductance catheter might be simplified with a potential for clinical use. PRSW was rate independent up to 140 bpm. At 160 bpm EDPVR increased significantly and probably resulted in a decline of PRSW at 160 bpm. ESPVR was rate dependent up to 100 bpm and should be used with caution when heart rate is different from baseline. From 140 bpm LV filling started at an increased LV pressure. LV pressure then declined during the whole filling. This might be the result of a suction mechanism facilitating LV filling during tachycardia at rest.
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