What is the optimum window width when calculating the interaction between rr intervals and systolic pressure levels?

The ability to reliably measure phase between changes in heart rate and blood pressure is important when developing cardiovascular models. This study recorded 20 subjects breathing normally for periods of five minutes. RR intervals and systolic pressure (SP) levels were extracted from ECG and blood pressure signals. Phase was calculated using cross spectral analysis using four different windowing techniques. An analysis of variance showed that phase was not affected by windowing techniques. However, variance of phase reduced as window width reduced and the number of windows increased. We concluded that 5 Hanning windows of width 100 s (with 50% overlap) produce a good compromise between minimum variance and maximum frequency resolution