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.