The qualitative detection of decreases in cardiac output

BACKGROUND Cardiac output is a major factor in the maintenance of physiological homeostasis and is difficult to measure with accuracy. This study describes an evidence-based technique, based on physiological changes, which may indicate small changes in cardiac output that cannot be measured by current techniques. METHOD Synchronous changes in blood pressure, heart rate, pulse amplitude and end-tidal carbon dioxide are analysed using runs analysis and a normalisation technique. An evidence-based algorithm was used to detect possible changes in cardiac output and data extracts from 31 consenting patients are presented as examples. RESULTS The decrease in end-tidal carbon dioxide, during steady state ventilation, was greater in those events notified as hypovolaemia associated with a fall in cardiac output than those events notified as hypovolaemia alone. The difference in end-tidal carbon dioxide between the two groups was -0.25 kPa (CI -0.42 to -0.09) p<0.003. DISCUSSION Runs analysis can detect trends in EtCO2 that during steady state ventilation may indicate a decrease in cardiac output. It is a safe technique; no additional hardware is required and the generated alerts only notify the clinician of the possibility of an adverse change. Determination of the rate of clinically significant false positives and negatives requires further work.