A modified Trigg's Tracking Variable as an 'advisory' alarm during anaesthesia.

We wished to assess the accuracy of a modified form of Trigg's Tracking Variable (TTV) at detecting the onset of changes in systolic blood pressure. A computer model generated systolic blood pressures which changed to a new value after period of stability. A separate algorithm based on TTV indicated when a 'significant' change had been detected. This signal occurred when TTV had exceeded a set limit (0.8-0.99) a predetermined number of times (1-10). Five anaesthetists were shown 40 sets of data generated by same the computer model and asked to indicate the onset of changes. The greatest number of changes (94.1%) were correctly identified when TTV exceeded 0.92 on 4 consecutive determinations. The onset of the trend was detected after an average delay of 140 s. The anaesthetists correctly detected 85% of the changes after an average delay of 162 s. There was no statistically significant difference between the anaesthetists and the algorithm, although only one anaesthetist performed better than the modified TTV. The modified TTV detected changes in simulated invasive systolic blood pressures faster and more accurately than four of a group of five anaesthetists. Such simple trend detection systems may be useful as 'advisory' alarms.