起搏器组成和基础知识介绍PPT课件.ppt

The greater the pulse width, the shorter the battery longevity. An increase in pulse width leads to an increase in total energy delivered. The longer the duration of the stimulus, the lower the amplitude required to capture the heart The strength-duration curve illustrates the tradeoff of amplitude (intensity of stimulation, measured in volts) and duration (the length of time the stimulation is applied, measured in milliseconds). Capture occurs when the stimulus causes the tissue to react. On the graph, capture occurs on or above the curve. Anything below the curve will not capture. The lowest voltage on a stimulation curve which still results in capture at an infinitely long pulse duration is called rheobase. A voltage programmed below is inefficient and will lead to non-capture. The pulse duration at twice the rheobase value is defined as the chronaxie. Chronaxie time is typically considered most efficient in terms of battery consumption. Safety margins have traditionally been selected as follows, when a threshold is determined by decrementing the pulse width at a fixed voltage: At a given voltage where the pulse width value is .30 ms: Tripling the pulse width will provide a two-time voltage safety margin. Pulse widths at a given voltage value which are .30 ms are not typically selected, because they are less efficient (expend more energy), while not providing further safety. In this case, the voltage should be doubled to provide a two-time safety margin. Adequate safety margins must be selected due to daily fluctuations in threshold that can occur due to eating, sleeping, exercise, or other factors that affect thresholds. Also, a patient with an acute pacing system will typically be programmed to a setting allowing a higher safety margin, due to the lead maturation process, which can occur within the first 6-8 weeks following implant. Most lithium-iodine batteries (those used most often in pacemakers today), the battery voltage is 2.8 V at the beginning o