Blood Pressure Monitor (A3051)

Contents

Description

[12-MAR-25] The Blood Pressure Monitor (A3051) is an implantable telemetry sensor for small animals that allows us to record blood pressure versus time. The A3051A1 is small enough to implant in a mouse. It provides 128 SPS (samples per second) of absolute pressure measurement with 6-Pa (0.04 mmHg) resolution, allowing us to see the pressure waveform generated by mouse's beating heart. Its measurement range is 2.6-406 kPa (19-3000 mmHg). Other versions of the A3051 provide faster sample rates, higher precision, and temperature measurement.

Figure: Implantable Blood Pressure Monitor (A3051A). A perpendicular steel tube protrudes from the epoxy and silicone encapsulation. A replaceable silicone sleeve accepts the end of a catheter.

The A3051 is equipped with an LPS28DFW absolute pressure sensor. The sensor package provides a flange into which we glue a 7-mm steel tube. Within the flange is a silicone barrier that protects the sensor from fluids, but also conveys air or fluid pressure to the sensor mechanism. We encapsulate the circuit and its battery in epoxy, coat with silicone, and leave the 5 mm of the steel tube protruding. We slide a silicone sleeve over the tube. This sleeve fits tightly over the tube, and also makes a tight fit with the catheter we will insert into one of the animal's blood vessels. We recomment a 640-μm diameter catheter tube, such as the BB31695-PE/1.

The LPS28DFW proviodes an auxiliary temperature measurement, which we enable in some versions of the A3051. In these versions, the temperature is transmitted on its own telemetry channel.

Figure: Micropower Pressure Sensor with Flange. Exctracted from LPS28DFW data sheet.

The median arterial blood pressure in a conscious mouse is roughly 103 mmHg (14 kPa) above atmospheric (PMC3115725). When under anethesia, the median pressure can drop to 64 mmHg (8.5 kPa) above atmospheric. With certain stimulants, the median pressure can reach 150 mmHg (20 kPa) above atmospheric. Variation in pressure during heartbeat is ±30 mmHg (±4.0 kPa). The absolute blood pressure at sea-level will never exceed 101 + 20 + 4 = 125 kPa. If we ascend to the top of a 4000-m mountain, the atmospheric pressure will be only 60 kPa. If we perform experiments on mice in vacuum chambers, we might see absolute blood pressure drop to 10 kPa. In a pressurized chamber, we might see 300 kPa. The A3051A pressure sensors provide a measurement range of 2.6-406 kPa with precision of better than 10 Pa rms. The A3051A1 provides a sample rate of 128 SPS. The A3051A1 is able to measure mouse blood pressure changes with precision of 0.1% and provide at least 13 samples per heartbeat, and do so in any vacuum or pressure chamber we are likely to encounter.

Expansion of the catheter, as well as compression of air bubbles within the catheter tube, will generate systematic errors in our blood pressure measurement. We are as yet uncertain of the magnitude of these errors, but we suspect they will be of order ±10% of the diastolic-systolic variation, so the errors introduce by our pressure sensor appear to be negligible compared to those that will be generated by the catheter system.

The A3051A1 provides 128 SPS of sixteen-bit resolution absolute pressure measurement, with output 0 counts representing pressure zero and output 65535 representing pressure 406 kPa. Current consumption is roughly 180 μA at 37°C. Its CR1620 battery has capacity 3300 μAdy, so the device will run continuously for 18 days. Its volume of 1.4 ml and mass of 2.9 g are small enough for implantation in an adult mouse.

The A3051B1 provides the same 128 SPS pressure measurement as does the A3051A1, but in addition it provides 128 SPS of sixteen-bit resolution temperature measurement with resolution 0.01°C. It is also equipped with a much larger battery, suitable for implantation in a rat, but not a mouse. This CR2330 battery has capacity 11000 μAdy. Current consumption at 37°C is roughly 200 μA, so the device will run continuously for 55 days. Its volume is 2.6 ml and its mass is 5.8 g.

Versions

[10-MAR-25] We define the following versions made with this circuit.

The shelf life is the time it takes to use up 10% of the battery capacity when the device is left inactive on the shelf. The operating life is how long the device can produce reliable measurements when starting with a fresh battery.

Design

[13-JAN-25] All our designs are free and open-source, with copyright protection presented in the GNU Public License, Version 3.0.

Modifications

[22-NOV-24] The A305101C PCB need a link from U6-19 to 1V8. We propose changing the 1V2 regulator to part ST1PS02DQTR so that we can power up at 1.2 V and drop down to 1.0 V once configuration is complete, thus reducing current consumption.

Operation

[23-OCT-24] The BPM turns on and off with a magnet. When turned off, it consumes less than 1 μA from its battery. When turned on, it transmits its temperature and pressure measurements continuously. See our Telemetry Manual for a description of the telemetry system. The blood pressure signal will appear in your telemetry recordings along with any other biopotential you may be receiving from other telemetry devices. You record all these signals with the Neurorecorder and play them back or export them to another file format with the Neuroplayer.

Pressure measurements are provided as unsigned sixteen-bit numbers in the range 0-65535. For sensors with pressure range 26-406 kPa, we obtain the pressure in Pascal (Pa) by multiplying the sixteen-bit sample by 6.25 Pa. Thus a measurement of 16160 is 101 kPa. When the pressure range is 26-126 kPa, we multiply the sixteen-bit sample by 3.125 Pa to obtain pressure in Pa. Thus a measurement of 32068 is 100.2125 kPa.

Temperature measurements are provided as unsigned sixteen-bit numbers in the range 0-65535. To obtain temperature in Centigrade, we first subtract 32768, then multiply the result by 0.01°C. Thus a measurement of 32768 is 0°C and 35286 is 25.18°C.

Development

[22-NOV-24] For details of the development and production of the A3051, see its Developement page.