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Noninvasive Blood Pressure Measurement Techniques
 

Noninvasive Blood Pressure Measurement Techniques

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Blood pressure measurements are often used to monitor cardiovascular health. Blood pressure is the lateral pressure produced by circulating blood on vessel walls. Close monitoring is important to detect and treat hypotension, low blood pressure, and hypertension, high blood pressure, both of which can impair the function of vital organs, such as the heart, kidneys, and brain.

Blood pressure is measured manually by auscultation, where the clinician occludes the artery using an inflatable cuff and then listens for the sound of blood flow through the brachial artery as it deflates. Blood pressure can also be measured automatically by oscillometry, which allows patients to measure their blood pressure at home. The inflatable cuff contains a pressure transducer that converts the arterial vibrations to systolic and diastolic pressures based on hemodynamic principles.

This video will illustrate the principles of blood pressure, demonstrate the current methods to measure the systolic and diastolic blood pressures of humans and small animals, and discuss some applications.

Blood pressure measurements consist of systolic and diastolic components, both of which are important indicators of cardiovascular health.

The systolic blood pressure is a measure of the maximum intraluminal pressure exerted against the artery during heart contraction when your heart beats, and the diastolic pressure is a measure of the minimum intraluminal pressure, or the pressure in between beats when the heart relaxes and the aortic valve is closed. To measure blood pressure, a cuff is placed on the patient's upper arm and then inflated to the point where blood flow is completely occluded. As the cuff deflates, the clinician auscultates while observing the manometer readings.

The first audible Korotkoff sound corresponds to the systolic pressure, the pressure at which blood begins to flow again through the open vessel. As the cuff continues to deflate, blood flows more freely as the vessel remains open. Diastolic pressure is recorded at the point when the clinician no longer hears the beating sound or the self-monitoring device detects a leveling-off of oscillations.

A similar method can be used to monitor rodent blood pressure. A cuff is inflated and deflated around the tail artery, while a pressure transducer detects systolic and diastolic pressure. Now that you understand the difference between systolic and diastolic pressure, let's demonstrate how to measure blood pressure using a wireless cuff for humans and a noninvasive technique for rodents.

Turn on the wireless cuff and the mobile device and open the cuff application. Then, pair the devices via Bluetooth and open the cuff application on the mobile device. Then, strap the cuff onto your upper arm. Make sure that the blood pressure monitor is placed against your inner bicep. The strap should be secured tightly on your arm. Sit in a position so that the cuff is at the same level as your heart. Make sure you are calm and relaxed before starting.

Now, begin the blood pressure measurement by pressing start. Each measurement takes about one to three minutes. Repeat the measurement several times to obtain an average value in the results. The result will be displayed on the screen of the mobile device. After saving the results, unstrap the cuff from your arm and shut off the device.

To measure the blood pressure of a rodent, first turn on the desktop computer, blood pressure system, and heated stage. Then, attach the VPR cuff and occlusion cuff to the blood pressure system. Use software to confirm that the cuffs are properly calibrated and working as expected. Choose the appropriate parameters for data acquisition. Parameters include deflation time, the number of sets per cycle, occlusion pressure, and the delay between sets. Occlusion pressure is typically set to 250 millimeters of mercury, and there are usually 20 sets per cycle. Other parameters can be specified based on user needs.

Calm the animal to prevent it from experiencing unnecessary stress. The environment should be quiet, with minimal noise. Then, place the animal in an appropriately sized holder. Make sure the animal's paws do not get stuck in the attachments of the holder and that the tail is outside of the restrainer. Now, place the holder on the heated stage. Slide the occlusion cuff, followed by the VPR cuff onto the base of the tail, approximately one to two millimeters apart. Once the cuffs are in place, tape them to the stage. This will restrict tail movement.

Then, heat the tail to 32 to 34 degrees Celsius to promote vasodilation and blood flow through the tail. As the stage is warming, choose the type of animal on the computer whose blood pressure you will be acquiring and then enter its animal ID into the system so you can access its data later. Once the stage is sufficiently heated, start acquiring blood pressure data by pressing start. Try to maintain a calm environment. Keep the position of the cuff constant and make a note if the animal moves substantially. If the cuffs slide down the tail, or if the animal moves inside the restrainer, press the pause button, then make adjustments after the current measurement is completed, and resume testing.

Check to make sure the blood pressure data are being acquired as expected. Collect 20 to 40 measurements. Once all data has been collected, remove the animal from the holder and place it back into a cage. Then turn off the equipment and clean the surfaces that were in contact with the animal.

The results using the blood pressure monitor for humans are outputted on a screen as shown. Using this technology, systolic and diastolic blood pressure measurements are displayed and multiple readings are averaged.

Healthy blood pressures in humans are typically below 120 millimeters of mercury for the systolic pressure and below 80 millimeters of mercury for the diastolic pressure. Blood pressures above these values could indicate that a patient is starting to develop high blood pressure, which can lead to further health complications.

Here is a typical graph that is obtained when taking rodent blood pressure. The two curves on the graph represent data collected from the VPR sensor, the blue line, and the occlusion cuff, the red line. The inflection points of the blue curve are used to calculate the systolic and diastolic pressure. The measurements are listed at the bottom of the graph along with the OK status, which indicates that this measurement is deemed acceptable by the system.

Typical blood pressure measurements for mice are around 120 over 70 millimeters of mercury, whereas rats have slightly higher blood pressures at around 130 over 90 millimeters of mercury.

Now, let's take a look at how blood pressure is noninvasively measured in the public and scientific community.

Portable noninvasive blood pressure cuffs allow hypertensive patients to monitor their health between visits to the clinic. Pregnant women at risk for developing pre-eclampsia can monitor their risk for this condition in the critical months before the baby is born.

Research on the development of new therapeutic agents requires frequent blood pressure assessment. While invasive techniques such as subcutaneous telemetry and carotid artery cannulation in mice provide long-term blood pressure data, noninvasive tail-cuff systems are an attractive method to monitor the blood pressure in animal models of cardiovascular disease.

You've just watched JoVE's introduction to blood pressure measurement techniques. You should now understand how noninvasive measurement methods are used for both humans and rodents and how the technology is applied to improve health monitoring for the community. Thanks for watching!

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