A medical monitoring system comprising: one or more biometric monitors (20, 21, 22, 23, 24, 26) configured to collect time-stamped samples of a plurality of monitored biometric parameters; an expression evaluator configured to evaluate a user-defined expression incorporating one or more of the monitored biometric parameters to generate time-stamped samples of a user-defined biometric parameter; and a further processing component (32, 50, 52, 54, 55, 56, 57, 58, 60, 62, 66, 88) comprising: - a trending processor configured to perform trending of time-stamped samples (54, 55, 56) of a plurality of parameters selected from the monitored biometric parameters (40, 41, 42, 43, 44) and the user-defined biometric parameter (80, 82) respective to time; - a display including a plurality of display regions, each being configured to display a trend of time-stamped samples of a said parameter selected from the monitored biometric parameters and the user-defined biometric parameter by plotting the time-stamped samples of said parameter respective to time; and - a data storage for enabling retrospective trending and display of stored trend samples of a said parameter selected from the monitored biometric parameters and the user-derived biometric parameter respective to time, the data storage being configured to store the time-stamped samples of each of the monitored biometric parameters (40, 41, 42, 43, 44) and the user-defined biometric parameter (80, 82) using the same data storage format for both the monitored biometric parameters and the user-defined biometric parameter, wherein the further processing component is configured to operate in the same way regardless of whether a said parameter is selected from the monitored biometric parameters or the user-defined biometric parameter. one or more biometric monitors (20, 21, 22, 23, 24, 26) configured to collect time-stamped samples of a plurality of monitored biometric parameters; an expression evaluator configured to evaluate a user-defined expression incorporating one or more of the monitored biometric parameters to generate time-stamped samples of a user-defined biometric parameter; and a further processing component (32, 50, 52, 54, 55, 56, 57, 58, 60, 62, 66, 88) comprising: - a trending processor configured to perform trending of time-stamped samples (54, 55, 56) of a plurality of parameters selected from the monitored biometric parameters (40, 41, 42, 43, 44) and the user-defined biometric parameter (80, 82) respective to time; - a display including a plurality of display regions, each being configured to display a trend of time-stamped samples of a said parameter selected from the monitored biometric parameters and the user-defined biometric parameter by plotting the time-stamped samples of said parameter respective to time; and - a data storage for enabling retrospective trending and display of stored trend samples of a said parameter selected from the monitored biometric parameters and the user-derived biometric parameter respective to time, the data storage being configured to store the time-stamped samples of each of the monitored biometric parameters (40, 41, 42, 43, 44) and the user-defined biometric parameter (80, 82) using the same data storage format for both the monitored biometric parameters and the user-defined biometric parameter, - a trending processor configured to perform trending of time-stamped samples (54, 55, 56) of a plurality of parameters selected from the monitored biometric parameters (40, 41, 42, 43, 44) and the user-defined biometric parameter (80, 82) respective to time; - a display including a plurality of display regions, each being configured to display a trend of time-stamped samples of a said parameter selected from the monitored biometric parameters and the user-defined biometric parameter by plotting the time-stamped samples of said parameter respective to time; and - a data storage for enabling retrospective trending and display of stored trend samples of a said parameter selected from the monitored biometric parameters and the user-derived biometric parameter respective to time, the data storage being configured to store the time-stamped samples of each of the monitored biometric parameters (40, 41, 42, 43, 44) and the user-defined biometric parameter (80, 82) using the same data storage format for both the monitored biometric parameters and the user-defined biometric parameter,