google Loss of Pulse Detection on the Google Pixel Watch 3 (opens in new tab)
Google Research has developed a "Loss of Pulse Detection" feature for the Pixel Watch 3 to address the high mortality rates associated with unwitnessed out-of-hospital cardiac arrests (OHCA). By utilizing a multimodal algorithm that combines photoplethysmography (PPG) and accelerometer data, the device can automatically identify the transition to a pulseless state and contact emergency services. This innovation aims to transform unwitnessed medical emergencies into functionally witnessed ones, potentially increasing survival rates by ensuring timely intervention. ### The Impact of Witness Status on Survival * Unwitnessed cardiac arrests currently face a major public health challenge, with survival rates as low as 4% compared to 20% for witnessed events. * The "Chain of Survival" traditionally relies on human bystanders to activate emergency responses, leaving those alone at a significant disadvantage. * Every minute without resuscitation decreases the chance of survival by 7–10%, making rapid detection the most critical factor in prognosis. * Converting an unwitnessed event into a "functionally witnessed" one via a wearable device could equate to a number needed to treat (NNT) of only six people to save one life. ### Multimodal Detection and the Three-Gate Process * The system uses PPG sensors to measure blood pulsatility by detecting photons backscattered by tissue at green and infrared wavelengths. * To prevent false positives and errant emergency calls, the algorithm must pass three sequential "gates" before making a classification. * **Gate 1:** Detects a sudden, significant drop in the alternating current (AC) component of the green PPG signal, which suggests a transition from a pulsatile to a pulseless state, paired with physical stillness. * **Gate 2:** Employs a machine learning algorithm trained on diverse user data to quantify the probability of a true pulseless transition. * **Gate 3:** Conducts additional sensor checks using various LED and photodiode geometries, wavelengths, and gain settings to confirm the absence of even a weak pulse. ### On-Device Processing and User Verification * All data processing occurs entirely on the watch to maintain user privacy, consistent with Google’s established health data policies. * If the algorithm detects a loss of pulse, it initiates two check-in prompts involving haptic, visual, and audio notifications to assess user responsiveness. * The process can be de-escalated immediately if the user moves their arm purposefully, ensuring that emergency services are only contacted during true incapacitation. * When a user remains unresponsive, the watch automatically contacts emergency services to provide the individual's current location and medical situation. By providing a passive, opportunistic monitoring system on a mass-market wearable, this technology offers a critical safety net for individuals at risk of unwitnessed cardiac events. For the broader population, the Pixel Watch 3 serves as a life-saving tool that bridges the gap between a sudden medical emergency and the arrival of professional responders.