Non-Invasive Blood Pressure
The control of the arterial blood pressure (BP) is influenced by a very large number of factors but the systemic pressure is ultimately determined by the relationship between cardiac output and the total peripheral resistance. The cardiac output is determined by the product of the heart rate and the stroke volume. The contraction of the ventricles (systole) causes a blood flow into the aorta. This flow is faster than the drain through the arterioles and therefore, the pressure in the aorta increases. The maximum pressure during systole (systolic pressure) is determined largely by the peak rate of the contraction and the distensibility of the aortic walls. After ventricular systole is completed the aortic valves are closed by a retrograde flow of blood shown as an dicrotic notch in the blood pressure signal. After the aortic valves are closed the arterial pressure starts to decrease (diastole) as blood flows out through peripheral vascular networks.
The continuous blood pressure in the finger was measured with a non-invasive Portapres blood pressure device (manufactured by TNO TPD Biomedical Instrumentation). The device uses the volume-clamp method where the diameter of an artery under a cuff wrapped around the finger is kept constant (clamped), in spite of the changes in arterial pressure during each heart beat. Changes in arterial diameter, detected by means of an infrared photo-plethysmograph built in the finger cuff, are opposed by a fast pressure servo controller that changes pressure in an inflatable air bladder, which is also mounted in the finger cuff.
We have calculated systolic, diastolic and mean blood pressures for each cardiac cycle. Systolic and diastolic pressures were obtained by detecting the peaks and troughs of the blood pressure signal. The detection was implemented by removing the slow baseline trend from the BP signal and then setting proper thresholds for systolic and diastolic pressures. Two different definitions for the mean blood pressure were taken i.e. functional and arithmetic means. The arithmetic mean is simply the average of the systolic and diastolic pressures. The functional mean pressure, on the other hand, is the pressure level dividing the blood pressure signal into two parts with same size. The definition of functional mean pressure is probably best understood from the figure below where the blue line marks the functional mean pressure for the specified cardiac cycle.