Brandon Ballinger explores the potential of the Apple Watch to measure blood pressure, a feature already seen in Samsung watches internationally and anticipated to enter the US market pending FDA approval. In 2018, Ballinger was part of a pioneering study demonstrating that health sensor data from wearables, when combined with a deep neural network, can detect signs of high blood pressure, sleep apnea, atrial fibrillation, and more. While sensor technology has advanced over the past seven years, the foundational science remains unchanged, offering insights into future developments. This article delves into the science behind wrist-based blood pressure measurement, the historical medical literature on using deep neural networks to extract signals from consumer wearables, the limitations of wrist-based blood pressure, and how doctors and patients can integrate this technology into medical practice.
Pulse Wave Analysis for Blood Pressure
When the heart beats, it sends a pressure wave—your pulse—throughout the body. Historically, doctors could roughly gauge blood pressure by feeling the pulse with their fingers. The first machine for measuring blood pressure, the sphygmograph, was invented in 1854, and the modern blood pressure cuff, or sphygmomanometer, is its descendant. These devices require active pressure on the artery to measure blood pressure. But what if you can't apply force to the artery? On a watch, you only have optical sensors, which are nearly touchless. However, there are still clues that allow you to infer blood pressure from the speed and shape of each pulse wave.
Image: Apple Watch with Health Monitoring Features
Pulse Wave Velocity (PWV) is the speed at which the pulse travels through the circulatory system. Similar to a string being pulled taut, when blood pressure is higher, the wave travels faster. If the Watch measures the precise times when (a) the heart contracts and (b) the pulse wave arrives at the wrist, the difference can indicate how blood pressure changes throughout the day. The exact time of the heartbeat can be measured through an ECG (which Apple Watches, Samsungs, Fitbits, etc., already have) or through the body's mechanical response to the blood ejected from the heart during each heartbeat (known as a ballistocardiogram). Additionally, the shape of the wave provides indirect clues to blood pressure. The pulse rise time and various wave amplitudes (pulse wave amplitude, pulse wave systolic peak, etc.) correlate with blood pressure. Generally, a higher amplitude and a faster rise time correlate with more arterial stiffness and higher blood pressure.
Calibration and Health Implications
Wrist-based blood pressure needs to be calibrated initially by comparing it to a cuff and periodically re-calibrated (e.g., every 30 days) because blood pressure can drift over time, and each person's parameters are unique. This future blood pressure sensor on the Apple Watch holds significant potential for health. While blood pressure is typically measured during doctor visits, 120 million people in the US are diagnosed with high blood pressure, but only 23% have it under control. This means 77% of patients do not have their blood pressure under control. These figures are from HHS, the American College of Cardiology, and the American Heart Association. Blood pressure control often requires multiple medications and dietary changes, which can be challenging to achieve.
High blood pressure is a major risk factor for heart attacks, alongside cholesterol or apoB. Every 20 mmHg increase in systolic blood pressure or 10 mmHg increase in diastolic blood pressure doubles mortality. For example, a 40-year-old man with good cholesterol numbers (160 total cholesterol, 60 HDL) has a 5% lifetime risk of a heart attack if his blood pressure is 115/75 (good). If his blood pressure is 150/110 (high), his lifetime risk is 10 times higher—50%.
Image: Blood Pressure Monitoring
Blood pressure is sensitive to daily, counterintuitive nutritional choices. People with high blood pressure are often recommended the DASH diet, which involves consuming 3,500 to 5,000 mg of potassium per day and under 1,500 mg of sodium. Controlling dietary potassium and sodium is surprisingly difficult because the set of foods is counterintuitive. For instance, bananas don't even make the top 10 of high-potassium foods. High-potassium foods are diverse—spinach, avocado, sweet potato, and white beans are all among the top 10. Sodium is even more insidious, as much of our consumption comes from foods like bread, which don't necessarily taste salty. Continuous glucose monitors have revolutionized diabetes treatment. Could a continuous blood pressure monitor provide similar real-time feedback on dietary choices? With AI, everyone could have a virtual nutritionist.
What's Next?
As of September 2024, blood pressure measurement on the wrist remains speculative. Updates will be provided as the situation evolves. If you own an external blood pressure cuff and want to manage your hypertension, try out Empirical Health on the App Store today. Join over 2,000 people already on their way to better health.
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Innovative Health Solutions: The potential for wearables like the Apple Watch to measure blood pressure opens new avenues for startups in the health tech sector. By leveraging AI and deep neural networks, startups can develop solutions that provide real-time health monitoring and personalized feedback, revolutionizing how we manage health. For more insights on integrating AI, check out this comprehensive guide to AI integration.
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AI Integration: The integration of AI in health monitoring devices can provide users with personalized insights and recommendations. Startups can harness AI to create virtual nutritionists or health coaches, offering tailored advice based on real-time data, thus enhancing user engagement and satisfaction. Discover how to automate tasks and workflows to improve efficiency.
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