Short-term variability of pulse rate and blood pressure in cardiac surgery patients

The magnitude and mathematical nature of short-term variations of pulse rate and mean arterial blood pressure has been studied in 27 post-operative cardiac surgical patients over a continuous period of 200 min in each case. Similar variability was observed in all patients. Short-term variations were composed predominantly of a series of rhythmic changes ranging from one synchronous with respiration to others between 2 and 5 min in cycle length. There was consistent variance of the beat-by-beat values for both variables about a continuously updating 5 min mean. The average standard deviation was 3.75 beats/min for pulse rate and 0.48 kPa (3.64 mm Hg) for mean arterial blood pressure. For both variables the distribution about the 5-min mean did not differ significantly from a normal distribution in 50 out of the 54 records. These findings have implications for the reproducibility of current methods of estimating mean pulse rate and blood pressure, and the change necessary before two estimates may be regarded as significantly different. The results are applicable both to the interpretation of Ward Charts by medical staff and to automated monitoring systems.