1. A method for computing an estimate of a physiological variable in a medical device, the method comprising: enabling the medical device to sense a physiological signal; measuring an event of the physiological signal; initializing a value of 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; updating in a memory of the medical device the estimate of the long-term metric using a previous long-term metric and a current measurement of the event; detecting a need for computing the physiological variable; and enabling a processor of the medical device to compute an estimate of the physiological variable using the updated long-term metric.