Methods and systems are presented for triggering physiological measurements in a physiological monitor. Metrics are computed for a received physiological signal (e.g., a PPG signal), or a determined physiological parameter associated with the physiological signal (e.g., blood pressure). A change parameter is determined based on one or more of the metrics, and a variable change threshold is determined. The variable change threshold may be determined over time based on a time measure, a frequency measure, or both. The change parameter is compared to the variable change threshold, and a physiological measurement is triggered based on the comparison. The variable change threshold technique may allow measurements to be taken frequently enough to catch clinically significant changes in a physiological parameter of a subject but not so often as to interfere with the subjects comfort or the function of other medical monitors.
