Poster H12 - Cuffless Blood Pressure Monitoring via the Oscillometric Finger Pressing Method: Optimizing PPG Sensor Placement for Accurate Measurements

Thursday, October 24, 2024

10:00 AM - 11:00 AM EST

Location: Exhibit Hall E, F & G

Presenting Author(s)

Jesse Z. Zhang

Undergraduate Researcher
University of Pittsburgh
Devon, Pennsylvania, United States

Co-Author(s)

MF

Mark Freithaler

Graduate Student/Researcher
University of Pittsburgh, United States
SW

Shipeng Wang

Graduate Student/Researcher
University of Pittsburgh, United States
Louis Sauter

Undergraduate Researcher
University of Pittsburgh, United States

Primary Investigator(s)

RM

Ramakrishna Mukkamala

Professor
University of Pittsburgh, United States

Introduction: High blood pressure (HBP) is a leading cause of many life-threatening conditions, including cardiovascular and kidney diseases. Despite its significant impact, global awareness and control rates of HBP remain low. Early detection and management are crucial for mitigating these risks. Traditional blood pressure monitoring methods, which rely on inflatable cuffs, are inconvenient and limit the frequency of checks. To address these challenges, smartphone-based, cuffless blood pressure measurement systems have been proposed and evaluated.

Oscillometric finger pressing is a promising cuffless and calibration-free approach that uses photoplethysmography (PPG) to measure blood pressure by detecting changes in blood volume in response to applied pressure. By integrating a PPG-force sensor into a smartphone, users can accurately monitor their blood pressure at home. However, precise sensor placement over a finger artery is crucial for reliable measurements. This study aims to systematically locate the digital distal transverse arch artery and understand its orientation to optimize sensor positioning for accurate blood pressure readings.

Materials and

Methods: Our custom-built benchtop system includes a multi-wavelength (green, red, and infrared) PPG sensor operating in reflectance mode, placed atop a calibrated load cell to measure finger blood volume oscillations and applied pressure [Figure 1A]. To analyze blood volume changes, we applied a band-pass filter to the PPG signal to obtain the AC PPG, which measures blood volume changes. Lead-I ECG data were used to identify the signal's peak-to-peak envelope, and its amplitude was plotted against the pressure to produce the oscillogram. A standard blood pressure cuff served as the reference for our measurements [Figure 1C].

To locate the area with the greatest blood volume change, we measured the PPG oscillations of the participant at -3mm, 0mm, and +3mm from the base of the nail in two trials each, as shown in Figure 2(C). We refined the search by adjusting ±1mm from the location of the largest oscillation in two additional trials. This process was repeated until we identified a local maximum in oscillations over the finger's positions. Standardized procedures and markers ensured consistent finger positioning across trials.

To evaluate whether we were measuring the target artery or a collection of arterioles, participants rotate their fingers from 0 to 180 degrees in 45-degree increments using a positioning template to pivot the finger, spinning the area of interest atop the PPG sensor, repeating the process twice.

Results, Conclusions, and Discussions: From Figure 3, we made several observations. The maximum oscillation peak varies between individuals: among the twenty participants, seven exhibited the peak at -3mm, while five showed it at -2mm relative to the base of the nail, as shown in Figure 2(A). There is a significant difference in the magnitude across positions for a specific person, highlighting the importance of finding the best location for each individual, as seen in Figure 2(B). Despite these differences, using the base of the nail as the starting point is effective due to the small range of movement required—only a few millimeters in each direction.

To evaluate the vessel's orientation and see if it changes the magnitude of the oscillations, we over at the angle study. If our PPG sensor is directly over the arch artery, assuming the anatomy described in the literature, we would expect an increase in oscillations at 90 degrees compared to 0 degrees. This occurs because more of the PPG light paths intersect the arterial volume, capturing a larger AC PPG and yielding higher oscillations, as shown in Figure 3.

Using this logic, when examining Figure 4(A), we see an equally distributed histogram from 0 to 180 degrees for our angle study. This indicates that each participant had unique pharyngeal vessel anatomy, contrary to what literature suggests. However, angle placement is important for obtaining high oscillations, which is confirmed in Figures 4(B) and 4(C), where both have significantly different-sized oscillations at different angles. For example, in Figure 4C, we see the oscillogram for 90 degrees being almost 25% larger than at 0 degrees.

In conclusion, using the base of the nail as a natural landmark for blood pressure monitoring is an effective and convenient method for obtaining high-quality oscillations. While our results conclude that each participant had a unique pharyngeal vessel anatomy, the angle of the finger relative to the PPG sensor is important and should be carefully considered. Moving forward, further efforts should focus on visually mapping the vessels of the finger to better understand optimal angle placement and enhance measurement accuracy.

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