The LabVIEW VI can be broken down into four parts: the inflation; the timer; the deflation during which systolic pressure is sensed; and the isolation of pulse oscillations during which MAP is determined.
One important calibration incorporated into the VI is that the pressure in the sensor is zeroed at the beginning of each run. This is done using a case structure that subtracts the baseline value from itself during the second sampling; it is important then, that the cuff is in place before the program is run to prevent any pressure changes while the calibration is completed. For further calibration, testing will again be performed using either a market blood pressure monitor or an analog gauge to determine the error from the program. Using Microsoft Excel, the ratio between experimental and expected results will be used to obtain a best-fit calibration plot; once enough tests are run, the slope and intercept from this plot will be incorporated into the LabVIEW VI as conversion constants.
The countdown timer VI was adapted from a VI found on a LabVIEW forum. It was created by Andy F. of National Instruments. It begins to run when the desired pressure is reached. Shown here, it is set to run for 40 seconds. This time may need to be adjusted to allow more time for removal of the bike pump and application of the release valve.
Systolic Blood Pressure
The voltage obtained by the DAQ device is converted to a pressure value using constants that were determined through subject testing. Once filtered, the peak detection tool is used and the array of peaks is subtracted from itself to find the greatest difference in pressure values. The pressure value after this largest drop in pressure was found, through testing, to be a reliable measure of systolic blood pressure. A second filter is implemented to find the pulse oscillations in order to find the MAP value. A peak detection too is also used on the filtered signal here. To find the oscillations that correspond with blood flow from the waveform, a range tool is used and only considers peaks during which oscillations are within the 60-100 Hz range, which is the normal pulse range for humans.
Mean Arterial Pressure
Additionally, a loop is used to isolate only the section of the waveform for which the oscillations within pulse range are continuous. The range tool outputs an array of 0s and 1s to show whether a peak occurs in the 60-100 Hz range (0 if not, 1 if so). The loop requires that three peaks in a row must be within the 60-100 Hz range, which indicates the beginning of the patient's pulse. Outside of the loop, the array position at which these steady oscillations begin is used to create a new array of peak values for a certain length. In the VI shown, this is a 300-value array, which will surely be truncated in the final design, as the MAP value, found using an array maximum tool, occurs early on in the pulse oscillations. Once this value is obtained, it is converted to a pressure value based on testing. Using the known relationship of MAP = (2/3)*Diastolic + (1/3)*Systolic, Diastolic is found to equal [MAP - (1/3)*Systolic]*(3/2).
Once the pressure detection part of the VI is complete (after 40 seconds in this case), the systolic and diastolic values will appear next to their respective labels on the front panel.