How can I produce a envelope curve (p_ampl vs. p_trend) when using the blood pressure sensor?

The blood pressure sensor BT17i (or 0377i) is a relatively complicated sensor for which the generation of a relevant graph is difficult to automate. This is because the procedure requires manual selection of relevant datapoints for each measurement. When using this sensor, please take note of the following:

• The procedure for creating a envelope curve is extensively described in the sensors’s User Guide. Read the guide carefully and follow the described steps.
• An example result in which the necessary processing steps have been carried out can be downloaded here. With this example, you know how the data is supposed to look.
• You can also access the instructions in the lesson materials provided here in our library of Teaching Resources.

There are a few tips that make the selection of relevant datapoints easier. In general, more datapoints means that the analysis can be done more accurately. You may, however, already achieve acceptable results using as little as 30 datapoints. Please take note of the following:

• Make sure the measurement takes no longer than 40 to 60 seconds. This can be achieved by tweaking the pressure release valve to allow the cuff to deflate faster.
• First select the interval in which you want to select the datapoints. This can be done by choosing “Range” instead of “Point-to-point” in the “Select/Remove Data” window. This considerably reduces the amount of points, making it easier to select relevant datapoints.
• Maximize the selection screen by setting the window to full size using the icon in the top right corner of the window. The selection of points is easier when using a large window.

If you succeeded in selecting relevant datapoints and performing other processing steps, but did not get a envelope curve, please take note of the following:

• It is possible that the range for p_trend was too small. The best results are acquired with a 40 – 140 mmHg range. When a smaller interval is used, part of the curve may be lost. Select datapoints over the entire interval. Especially the datapoints at 40 and 140 mmHg are important for the envelope curve.
• The smoothing for p_pulse or p_trend may be wrong. In this case, the envelope curve is plotted over the entire range, but the actual envelope shape is not visible. This may be caused by insufficient/incomplete datapoint in one of the two measurements (either p_pulse or p_trend), for example when the selected points are in a smaller range or not distributed over the entire range. It is good practice to always select the accompanying local minimum (p_trend) for every local maximum (p_pulse).