Blood Pressure Variability (BPV) Analysis
Beat-to-beat values of arterial blood pressure along with the heart period duration are the most easily accessible variables providing information about the autonomic nervous systems cardiovascular control mechanisms. The analysis methods used for blood pressure variability (BPV) are usually the same as used for heart rate variability (HRV). Whereas the HRV time-series is constructed from RR interval durations as a function of R-peak occurrence times the blood pressure series is formed from either systolic, mean, or diastolic pressure values as a function of the R-peak times. To form the BP series a continuous blood pressure measurement device is needed. We have used a non-invasive Portapres measuring device (manufactured by TNO TPD Biomedical Instrumentation). The figure on the right presents typical BP measurement and the detected systolic, mean, and diastolic pressures values.
The Portapres device uses a volume-clamp method where the diameter of the finger artery under the cuff is kept constant, in spite of the changes in arterial pressure during each heart beat. However the unloaded diameter is usually not constant during measurement and therefore it must be adjusted at intervals. These adjustments are seen in the figure on the right as short constant pressure steps. The missed BP values due to these adjustments are recovered using a cubic interpolation.
Usually biomedical signals such as EEG or ECG are measured in AC-mode in order to avoid disturbing baseline drifts. However, BP must be measured in DC-mode so that the true amplitude information is not lost. The problem arises when both e.g. EEG/ECG and BP signals need to be measured synchronously. We have solved the problem by implementing correlation tools, enabling the synchronous linking of a DC-mode BP measurement with the AC-mode measurements, to the software.