HandRate: Heart Rate Monitoring While Simply Holding a Smartphone

We present HandRate, the first smartphone-based system using a standard sensor (accelerometer) for opportunistically computing heart rate while a user holds their phone. Fundamentally, HandRate revisits ballistocardiography (BCG), a century-old technique for monitoring heart activity by measuring the body movement caused by the cardiac cycle. Traditionally performed using custom hardware, attached to a subject's body, revisiting BCG for the smartphone, held in hand, faces several challenges. The hand is an external organ furthest from the aorta and subject to motion artifacts, leading to a weak and noisy signal, while the position the phone is held in can impact which accelerometer axis best captures BCG. HandRate addresses these challenges by introducing a design involving two modules operating in tandem: the first aimed at transforming the accelerometer readings into a single-dimensional signal oblivious to how the phone is held, while the second module making heartbeat predictions based on this signal. Results from testing HandRate using data collected from 18 subjects show that it can estimate heart rate with accuracy similar to or better than systems requiring special sensors and/or active user participation.

I. Introduction

Essential to human life, healthcare remains costly, administered by a complex and insular apparatus, and often out of reach for many. Omnipresent and with advanced sensing and computing capabilities, smartphones can place, for the first time, advanced diagnosing and health monitoring capacities in people’s hands, significantly democratizing healthcare [1]. With cardiovascular diseases (CVD) claiming more lives every year than cancer and chronic lung diseases combined [2], heart activity monitoring attracted some of the first works in the area [3]. Initially, researchers proposed adding custom hardware to smartphones, such as photoplethysmogram (PPG) [4] or electrocardiogram (ECG) [5] sensors. While important in demonstrating the potential of smartphone-based solutions, requiring custom hardware places a significant barrier to wide adoption. Sensing an opportunity, major smartphone manufacturers have introduced special sensors for heart activity monitoring in some of their high-end models, e.g. Samsung Galaxy S7. However, such models are accessible only to a limited number of users. To relax the requirement for custom hardware or high-end phones with special sensors, [6], [7] introduced solutions that use a standard smartphone camera to measure heart rate using the PPG technique. However, users are required to place the finger on the camera, with the accuracy tightly coupled to the precise placement [7]. To remove the requirement for physical contact, [8] introduced a solution for computing heart rate variability using a video of the user’s face captured by the smartphone front camera. Nevertheless, it requires active user participation in the form of taking a video in good lighting conditions, making it unsuitable for continuous heart monitoring. Relying on seismocardiography (SCG), the technique of measuring and interpreting the acceleration in the chest wall in response to the heartbeat, [9] introduced a smartphone-based solution for estimating heart rate using the gyroscope. If an elegant and practical solution and relying on a common smartphone sensor it needs the phone placed on the chest, thus requiring active user participation.