Nonlinear Relationship Between Pacing and Evoked QT Intervals

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Abstract
The paced evoked response is an established biosensor which has been used in the design of a rate responsive pacemaker. The unit is capable of sensing the interval between the delivery of a pacing stimulus and the downslope of the evoked T wave. With fixed rate pacing this interval has been shown to shorten with exercise and the main cause of this effect is thought to be mediated by the increase in the plasma catecholamines which are released on exertion. The detection of a reduction in the stimulus‐T interval results in an increase in pacing rate. The rate of change of pacing rate is re/erred to as the slope setting and this must be determined for each individual patient so that optimal rate responsive pacing can be effected. The algorithm underlying the slope setting is the pacing rate‐evoked QT interval relationship. This relationship was implemented as a linear function, but this study, which was conducted to reevaluate it, has demonstrated nonlinearity between the pacing and evoked QT intervals. The degree of QT shortening is least at low heart rates. This finding has resulted in the development of a new algorithm for the pacemaker in the form of a new program for the pacing system. This should result in a more physiological rate of change of heart rate with exercise and less chance of sudden changes in rate. These postulates are the subject of current clinical trials.