Practical problems of determining the dimensions of heart rate data.

The practical problems are explored of determining the dimension of the phase space set generated from real, experimental and simulated data of the times between consecutive heartbeats in normal and diseased rabbits. It is determined how different measures of dimension have depended on the procedures used to construct the phase space set and on such properties of the data as the amount of data, noise, long-term trends and stationarity. Reproducible estimates of the dimensions of different physiological states are found to require considerable amounts of data recorded under stationary conditions.