8
| BT17i
Blood Pressure Sensor User’s Guide
more points. These are the pulse pressure data, you can rename this run into
p_pulse
.
III. Determining the envelope curve
10.
To be able to calculate the pressure difference first copy the pressure data from Run
3
p_trend
into Run 5
p_pulse
.
•
Select a data series (at the right side of the table) or select a cell in a column of
the data series of Run 3.
•
When you start to drag a variable heading
p
the cursor changes into a graph icon.
Continue dragging until you place the cursor over run number 5, at the left side
of the table (to this run you are copying the data).
•
When the run number is selected (a blue frame appears) release the icon. A new
variable column with the heading
Copied of p(mmHg)
will be added to Run 5.
•
The data points from the column
p (mmHg)
of Run 3 are now copied into a new
column
Copy of p (mmHg)
of Run 5. You can see it by opening Run 5. You can
rename the variables to reflect the above:
p
to
p_pulse
and
Copy of p
into
p_trend
.
11.
Calculate a new variable
p_difference
.
•
Right-click the Data Table and select
Add a new variable > Into Data Series >
Formula
.
•
Give this variable the name
p_difference
and unit
mmHg
and enter the formula:
p_pulse – p_trend
. This calculates the pressure difference.
12.
Create a new graph
Envelope curve
, which displays
p-difference
against
p-trend
. The
resulting graph has a bell-shape form.
13.
Use the function fit option to approximate the graph with a function.
•
Right-click the
p-difference
vs.
p-trend
graph and select
Analyze/Process >
Function fit.
•
Select under Run the option
Run 5
(where the calculations were made).
•
Select the function
f(x)=a*exp(-(bx+c)
2
)+d
.
•
Press
Estimate
.
If necessary manually adjust the individual parameter values and
press
Refine
.
Press
OK
when you are satisfied with the result. The fit will be
added as a new variable
Fit of p_difference5
in the graph.
IV. Determining the blood pressure
The blood pressure values can be determined directly from the envelope curve or via its
derivative graph.
Method 1 - directly from the Envelope curve (see figure 5):
14.
Use the Scan option to read the highest point in the envelope curve. This point
corresponds to the Mean Arterial Pressure (MAP).
15.
Determine the systolic and diastolic pressures.
-
The systolic blood pressure is found as the pressure value at the point along this
bell-shaped envelope for pressure above MAP, for which the pressure difference
value is 50% of the pressure difference corresponding to MAP.
-
The diastolic blood pressure is found as the pressure value at the point along this
bell-shaped envelope for pressure below MAP, for which the pressure difference
value is 75% of the pressure difference corresponding to MAP.
