and the wrist band 20 the pulse at the patch 10 may be determined from the ecg data for example the r peak in the ecg the pulse at the wrist band may be measured by any convenient pulse detection technique the simple algorithm used in the embodiment is as follows as illustrated in fig4 firstly the accelerometer is used step 50 to detect posture change and in particular to detect when the patient is stationary in a standing position step 52 then as the patient stands still for a complete minute the system measures a reference pulse transit time pat 0 the time between a predetermined feature of the ecg pulse and the detection of the pulse at the pulse sensor step 54 then the system continues to monitor the pulse transit time in the same way step 56 the ratio pat pat 0 of the measured pulse transit time pat over the reference pulse transit time is calculated when this ratio exceeds a predetermined limit step 58 the context data is checked step 60 to see if the patient is in a position or state for which the alarm is not to be sounded in particular the accelerometer data provides important context data if.