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Blood Pressure Calculations using medical-grade smart wearables


As a digital health zealot, I am often asked, especially by those in medicine, to explain how a PPG sensor can determine blood pressure. In fact, I am often told that we cannot determine blood pressure from a PPG sensor waveform! To me, statements like that simply get me out of my chair and up on a whiteboard – just like when I was a lecturer in Uni and had to explain how electrons pass through semiconductors…..you can’t see that either, but it is a fact! So let me explain …..


In this article I will cover blood pressure calculations, PPG sensors (PPG) and then how PPG sensor waveform analysis can determine blood pressure. At the close, I also discuss the inaccuracies of traditional blood pressure methods and how ambulatory blood pressure monitoring is optimal.


How is blood pressure determined?


Blood pressure is determined by two important factors: the strength of your heart pumping blood and the resistance that your blood encounters in your blood vessels.


Imagine your heart as a pump and your blood vessels as pipes. When your heart beats, it pushes blood into the pipes, creating pressure. This pressure is like the force of water flowing through a hose.


So, when a doctor or nurse checks your blood pressure, they are actually measuring how hard your heart is pumping and how easily your blood is flowing through your blood vessels. This information helps them understand the health of your heart and circulatory system.


What makes up blood pressure?


There are two components that make up your blood pressure: Systolic pressure and diastolic pressure. Systolic pressure is the top number and diastolic pressure, on the other hand, is the bottom number in a blood pressure reading.


The process of measuring blood pressure has traditionally involved a blood pressure cuff to temporarily limit the flow of blood in your arm. By doing so, it allows for the assessment of the pressure exerted against the artery walls. Additionally, the sounds produced by your heartbeat are listened to in order to determine the blood pressure reading.


A nurse in scrubs is using a blood pressure cuff to measure a patient's blood pressure.
A nurse taking a patient's blood pressure using a traditional blood pressure cuff

Here's how the systolic and diastolic points relate to the cuff:


Systolic Point: Imagine the cuff is wrapped around your arm and inflated. As the pressure in the cuff increases, it squeezes your arm and temporarily stops the blood flow. Now, when the pressure in the cuff is gradually released, the doctor or nurse listens for the first sound of your heartbeat using a stethoscope placed over an artery in your arm (usually the brachial artery). This is the point when your heart's pumping force overcomes the pressure in the cuff, and blood starts flowing again. The pressure in the cuff at this moment corresponds to your systolic blood pressure. This is the higher number in your blood pressure reading and represents the maximum pressure your heart exerts while pumping blood.


Diastolic Point: As the cuff continues to deflate, the blood flow becomes smooth and uninterrupted. The doctor or nurse listens until the sound of your heartbeat fades away. This fading sound corresponds to the point when your heart is relaxed and not exerting much force. The pressure in the cuff at this moment represents your diastolic blood pressure. This is the lower number in your blood pressure reading and reflects the pressure in your blood vessels when your heart is resting between beats.


Diagram showing Systolic and Diastolic point using traditional blood pressure cuff
Diagram showing Systolic and Diastolic point using traditional blood pressure cuff

So, the systolic point is when your heart's pumping overcomes the cuff pressure and blood starts flowing again, and the diastolic point is when the cuff pressure is low enough that blood flows without any interruption. These two points help determine your systolic and diastolic blood pressure values, which together give you your overall blood pressure reading.


What we know from historical research is that each doctor or nurse is subject to their own interpretations of these sounds and so variations or differences occur in readings between individuals performing a blood pressure reading.


A New Era of Health: Tracking Blood Pressure in Real-time with Medical-Grade Wearables


Wearable technology has revolutionised the health monitoring industry, with one notable application being the measurement of blood pressure using photoplethysmography (PPG) sensors.


What is a PPG sensor?


A PPG sensor, or Photoplethysmography sensor, is a technology that works by emitting and detecting light through the skin, and commonly used in medical and wearable technology applications. It measures changes in blood volume within your body, specifically in the small blood vessels near your skin's surface.


Blood pressure calculations using smart wearables: Here's how it works in simple terms:


Light Emitting and Detecting: The sensor emits a beam of light (usually a type of visible or infrared light) onto your skin. This light contains different wavelengths.


Interaction with Blood: When the light reaches the tiny blood vessels under your skin, it interacts with the blood. Oxygen-rich blood absorbs different amounts of light compared to oxygen-depleted blood.


Reflection and Detection: A part of the emitted light is reflected back to the sensor after interacting with the blood vessels. The sensor then detects the intensity of the reflected light. Beer Lamber Law predicts the curvature of the light and so collection can occur at various depths.


Data Analysis: The sensor analyses the changes in light intensity caused by the blood vessels expanding and contracting due to your heartbeat. When your heart pumps blood, your blood vessels expand slightly, allowing more blood to flow through. This leads to a change in the amount of light reflected back to the sensor.


Generating the PPG Signal: The sensor processes this change in light intensity over time and creates a signal called a "photoplethysmogram" or PPG signal. This signal is essentially a graphical representation of your heartbeat and the changes in blood flow.



PPG sensor showing a light emitter and a photo-diode receiver. The sensor is placed against the skin, either on the wrist or finger. LED lights in green, red, or infrared penetrate through the skin's layers. Reflective wave captured due to the Beer-Lambert Law.
A close-up view of a PPG sensor, highlighting the light emitter and photo-diode receiver.

A PPG sensor contains a light emitter and a photo-diode receiver. When placed against the skin, wrist or finger, the LED light in green, red or infrared penetrates the skin through various layers and due to the Beer-Lamber Law a reflective wave (waveforms) is captured.

PPG sensors are used in Healthisense Medical-Grade Smart Watches, Smart Wristbands and Smart Rings.
PPG sensors are used in Healthisense Medical-Grade Smart Watches, Smart Wristbands and Smart Rings.

How then is blood pressure determined using PPG?


To estimate blood pressure from a PPG sensor, we focus on the changes in the amount of light that passes through your skin as your heart beats. This changing light pattern is what we call a PPG waveform.


When your heart beats, your blood vessels expand and contract. This affects how much light can pass through your skin. We use these light changes to find two key points in the PPG waveform:


Systolic Point: This is when your heart contracts and pushes blood with the most force. At this point, the PPG waveform shows a specific change due to the increased pressure of the blood against your vessel walls.


Diastolic Point: This is when your heart relaxes between beats, and the blood flow is less forceful. The PPG waveform shows a different change at this point.



Graph of PPG Waveform when calculating blood pressure.
Graph of PPG Waveform when calculating blood pressure.

By studying these changes in the PPG waveform, and applying signal processing techniques, we can estimate your blood pressure. The point where your heart's contraction affects the PPG waveform gives us the higher number, called the systolic blood pressure. The point where your heart's relaxation is reflected in the PPG waveform gives us the lower number, called the diastolic blood pressure.


So, in simpler terms, blood pressure calculations from PPG data involve observing how your heartbeats change the light passing through your skin. These changes help us find the points that relate to the pressure when your heart contracts and relaxes, allowing us to estimate your blood pressure.


Blood Pressure Inaccuracy in clinical settings[1]


Measuring blood pressure is a common procedure that is relied upon in a variety of healthcare settings. It can be used to identify clinical deterioration, inform vasoactive drug titration, and guide goal-directed treatment. In general practice, high blood pressure values are used as a basis for the diagnosis of hypertension. Therefore, inaccurate or misleading blood pressure values can be detrimental to the quality of healthcare received by patients.


Several guidelines have been published with the aim of improving the accuracy of blood pressure measurements by standardising associated procedures. The guidelines primarily address upper-arm measurements and have commonly included recommendations about patient posture, cuff size, arm height, cuff deflation rate, and the number of repeated measurements. Studies comparing blood pressure measurements taken with strict adherence to guidelines versus usual technique have reported marked variation and differential treatment decisions between methods. However, even after training on standardised procedures, blood pressure measurement may be limited in its accuracy.


To interpret blood pressure data appropriately, healthcare providers should be knowledgeable of the potential sources of inaccuracy and variability between measurements. A single blood pressure value outside the expected range should be interpreted with caution and not taken as a definitive indicator of clinical deterioration. Where a measurement is abnormally high or low, further measurements should be taken and averaged. Wherever possible, blood pressure values should be recorded graphically within ranges. This may reduce the impact of sources of inaccuracy and reduce the scope for misinterpretations based on small, likely erroneous or misleading, changes.


Current hypertension guidelines[2]

Current hypertension guidelines recommend using the average values of several blood pressure readings obtained both in and out of the office for diagnosis. In-office blood pressure measurement using an upper-arm cuff constitutes the evidence-based reference method for current blood pressure classification and treatment targets. However, out-of-office blood pressure evaluation using 24 h ambulatory or home blood pressure monitoring is recommended by all major medical associations for further insight into your blood pressure profile. Importantly, the highly variable nature of office and out-of-office blood pressure readings has been widely acknowledged, including association with cardiovascular outcomes.


But to date, the implications of blood pressure variability on cardiovascular outcomes have largely been ignored by researchers and clinicians alike. Novel cuffless wearable technologies might provide a detailed assessment of the 24 h blood pressure profile and behaviour over weeks or months. These devices offer many advantages for researchers and patients compared with traditional blood pressure monitors, but their accuracy and utility remain uncertain.


Ambulatory Blood Pressure Monitoring (ABPM)


Ambulatory blood pressure monitoring (ABPM) utilising a photoplethysmography (PPG) sensor offers a wealth of information that surpasses the insights gained from a single clinical blood pressure reading. Unlike the isolated snapshot provided by a single measurement, ABPM with a PPG sensor records blood pressure levels continuously over a span of 24 hours or more. This extended monitoring period captures the dynamic nature of blood pressure , revealing patterns and fluctuations that may remain concealed during a brief clinic visit. These fluctuations can include nocturnal dips and morning surges, which are vital indicators of a person's cardiovascular health.


Moreover, ABPM with PPG enables the assessment of blood pressure in real-life settings, reflecting a person's blood pressure during daily activities, exercise, rest, and sleep. This holistic perspective offers a more accurate representation of an individual's blood pressure profile, reducing the potential white coat effect—a phenomenon where blood pressure rises due to the anxiety of being in a clinical environment. By obtaining data outside the clinical setting, healthcare professionals can make more informed decisions about hypertension diagnosis and treatment adjustments.


Furthermore, the continuous monitoring provided by PPG-based ABPM allows for the early detection of masked hypertension or white coat hypertension. Masked hypertension refers to individuals who exhibit normal blood pressure in clinical settings but experience elevated readings during routine activities. Conversely, white coat hypertension occurs when a person's blood pressure is elevated solely during clinic visits due to anxiety. Identifying these conditions is crucial for preventing misdiagnosis and ensuring appropriate treatment strategies. In essence, the comprehensive and extended data offered by ambulatory blood pressure readings using a PPG sensor paint a more accurate and insightful picture of an individual's cardiovascular health, enhancing diagnostic accuracy and personalised medical interventions.


Trends Supporting Ambulatory Vitals Monitoring:


Advancements in Wearable Technology: Wearable devices, such as smartwatches, fitness trackers, and patches, have been rapidly evolving. These devices are capable of continuously monitoring various vital signs, such as heart rate, blood pressure, and even ECG. As technology improves, these wearables could become more accurate and capable of tracking a wider range of health metrics.


Preventive Healthcare: There's a growing emphasis on preventive healthcare. Continuous monitoring of vitals can help individuals and healthcare providers detect anomalies and potential health issues at an early stage, allowing for timely interventions and improved health outcomes.


Chronic Disease Management: Many chronic conditions, such as hypertension, diabetes, and cardiovascular diseases, require regular monitoring of vital signs. Ambulatory monitoring can provide healthcare professionals with a more comprehensive and accurate view of a patient's health status, leading to better disease management.


Remote Patient Monitoring: The COVID-19 pandemic highlighted the importance of remote healthcare solutions. Ambulatory vitals monitoring allows healthcare providers to remotely monitor patients' health without requiring them to visit healthcare facilities frequently. This is especially beneficial for those who are elderly, have limited mobility, or live in remote areas.


Data-Driven Healthcare: The healthcare industry is becoming more data-driven. Continuous monitoring generates a wealth of data that can be analysed to identify trends and patterns, leading to more personalised treatment plans and better understanding of individual health behaviors.


Integration with Telemedicine: Telemedicine and virtual healthcare visits have gained popularity. Ambulatory vitals monitoring can provide real-time data during virtual consultations, enabling healthcare providers to make more informed decisions about a patient's health.


Consumer Interest and Demand: As people become more health-conscious, there's a growing demand for tools and technologies that empower individuals to take control of their health. Ambulatory vitals monitoring aligns with this trend, offering individuals the ability to track their health metrics and make informed lifestyle choices.


It's important to note that while ambulatory vitals monitoring holds significant potential, there are also challenges to consider, such as data privacy concerns, data accuracy, and the need for healthcare professionals to interpret and act on the collected data effectively.


Conclusion


In conclusion, there have been many developments in blood pressure techniques since Hales first measured the blood pressure of a horse in 1733.


Blood Pressure Through History
Blood Pressure Through History

From the manometer, through the sphygmograph and to the oscillometric devices of the 70’s and 80’s, there have continued to be improvements. The introduction of digital health technologies, such as PPG sensors and the associated signal processing and AI tools we now see becoming more prevalent, is merely a continuation of that history.


At Healthisense, we believe that the use of sensors, data and signal processing and AI tools are the future of not only blood pressure measurements but measurement of all health vitals – especially in an ambulatory mode – or at-home. The monitoring of ambulatory vitals is crucial to the early detection and early prevention of issues that will eventually result in chronic illnesses.


In the 2017-2018 National Health Survey conducted by ABS, it was reported that on average, people aged 65-74 had 2.7 chronic conditions. On average, people aged 75 and over had 3.4 chronic conditions. These averages take into account a range of chronic conditions, such as cardiovascular diseases, arthritis, diabetes, respiratory conditions, and more. As at 2023 estimates show that Australians over the age of 70 have 4-4.25 chronic conditions on average.


By failing to adopt a preventive approach to health now, the number of health issues will continue to escalate. This places an enormous burden on individuals, as well as hampers our taxes and healthcare system. To address this problem, we need to immediately embrace ambulatory vitals monitoring.


[1] Kallioinen, N., Hill, A., Horswill, M. S., Ward, H. E. & Watson, M. O. Sources of inaccuracy in the measurement of adult patients’ resting blood pressure in clinical settings: a systematic review. J. Hypertens. 35, 421–441 (2017).


[2] Schutte, A.E., Kollias, A. & Stergiou, G.S. Blood pressure and its variability: classic and novel measurement techniques. Nat Rev Cardiol 19, 643–654 (2022). https://doi.org/10.1038/s41569-022-00690-0


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