We studied the effects of removing brief variations in the monitoring data on the quality of limit alarms during the postoperative haemodynamic monitoring of cardiac patients. The variations were removed by median filtering. The false alarm frequency was reduced by more than two-thirds compared with a typical patient monitor. We designed and clinically tested a novel dual-limit alarm system with two median filters. The proportion of true alarms increased from 12% to 49% as compared to a typical patient monitor. The average false alarm frequency was 4.5 alarms per monitored hour. No correct alarms were missed.