tailieunhanh - Programmability and specialized circuits

Programmability often allows for noninvasive correction of pacemaker malfunction, optimizing battery life and adjustment of the pacemaker to the patient s physiologic needs, which may change over time. Before external programming is performed, any problems must be appropriately diagnosed and the clinical status of the patient, especially the degree of the patient s pacemaker dependency and the safety margin left after programming, must be considered. | preset by the manufacturer at approximately 60 to 70 bpm. Many studies have attempted to determine the pacemaker rate that optimizes cardiac output and, in general, it has been found that cardiac output varies little over a fairly wide range of paced rates, even when an atrial pacemaker is used. The increase in cardiac output that occurs normally with exercise involves not only an increase in heart rate but also peripheral vasodilatation in areas of increased metabolic activity and an increase in cardiac contractility. Simply increasing the rate of the pacemaker does not duplicate these complex physiologic events. In some individual patients and in some clinical situations, however, an increase in pacemaker rate does increase cardiac output. For example, a pacemaker-dependent patient who is metabolically unstable after surgery may benefit from a faster-paced rate. If the clinical situation is critical, this can even be determined by use of thermodilution cardiac outputs. Rate programmability allows for flexibility in these instances. Another reason for increasing rate is to overdrive cardiac arrhythmias.