1. A medical device comprising: a sensing module configured to receive a physiological signal and measure an event of the physiological signal; a memory configured to store a long-term metric of the event measurement, wherein the long-term metric corresponds to a time interval correlated to a response time of the physiological variable to changes in the event; and a control unit configured to: initialize a value of the long-term metric of the event measurement; update the long term metric in the memory using a previous long-term metric of the event measurement and a current measurement of the event; detect a need for computing the physiological variable; compute an estimate of the physiological variable using the updated long-term metric.