From the Ward to the Wrist

September 22, 2025
By Rebecca Handler

When Sumbul Desai, MD is asked what it feels like to release a health feature that can land on millions of wrists seemingly overnight, she smiles at the premise. Overnight is the public’s view — behind the scenes, it takes years of methodical science, gritty iteration, and a lot that goes wrong before it goes right.

Desai, a Clinical Associate Professor at Stanford Medicine and Apple’s Vice President of Health, returned to her academic home for a candid, fast-paced conversation at the Department of Medicine’s Grand Rounds. She spoke about building medical products – from a new wrist-based hypertension predictor to the landmark Apple Heart Study with Stanford – at scale, and emphasized the design principles she insists on before anything ships to consumers: scientific rigor, actionability, and privacy.

Years before medicine, Desai was on a business track at Disney and ABC. Then her mother suffered a hemorrhagic stroke. Thrown into the role of advocate, she watched multidisciplinary care pull her mother from coma to rehabilitation. “That’s when I fell in love with medicine for the first time,” she said. She went back to school at 30, trained at Stanford, and never forgot the neurosurgeon’s advice: You have to advocate for your mom because no one else will. It became a throughline: patients and families must be equipped to speak up, and technology should help them do it.

A First-in-the-World Hypertension Predictor

Desai’s newest headline-making project is a wrist-based predictor for possible hypertension. Apple Watch uses its optical heart sensor to measure pulse wave signals beat by beat, then applies machine learning tuned over years of studies totaling over 100,000 participants and a 2,000 person study conducted with traditional cuffs for ground truth, to estimate whether someone’s typical blood pressure pattern sits in stage 1 or 2 hypertension.

One unique challenge was deciding how to balance accuracy with usefulness in the clinic. At consumer scale, false positives can erode trust and flood clinics. So Desai’s team dialed toward “high specificity," which roughly translates to the idea that when a user is notified, it should really mean something, and lower sensitivity be accepted as the price of credibility. After a notification, users are guided to log home blood-pressure readings for seven days before seeing a clinician, aiming to replace the usual “come back with a log” visit.

Critics will focus on sensitivity, Desai knows. But she returns to the counterfactual: most undiagnosed people currently get no medical alert or notifications. “Better than zero,” she says, is a defensible first step, “if you’re transparent about performance, publish methods, and design the experience to be clinically useful.”

Science You Can Cite 

Apple may very well be famous for secrecy; medicine, by necessity, is not. Desai knew it was important for the company to “show its work” when it came to health. That started with the Apple Heart Study in collaboration with Stanford, a massive investigation of irregular rhythm notifications that enrolled hundreds of thousands in months and widely published its results. Since then, Apple’s health group has paired regulated features with public white papers and, when appropriate, peer-reviewed publications.

None of this is academic theater. It’s a precondition, Desai argues, for clinical adoption. If physicians can’t see how a feature performs, across age, skin tones, and geographies, they won’t trust it. And if users get numbers they can’t act on, they’ll tune out. 

The Principles Behind Every Product

Desai outlined the three tests every feature must pass:

Scientific validity: The team looks at mountains of peer-reviewed publications, and months and years of testing and trials for new features. Every feature is tested in feasibility studies and then validated in large, diverse cohorts.

Actionability: Insights and data must lead to a behavior change, decision, or clinical conversation. Otherwise they’re just noise.

Privacy by default: Processing happens on-device; Apple doesn’t see health data from these features. 

Those principles are why some popular asks don’t ship: if a metric can’t be interpreted reliably in everyday life, or would provoke anxiety without a clear next step, her team says no.

Designed for Doctors & Patients Alike

In an era when startups promise to “disrupt doctors,” Desai’s design target is resolutely with, not against, clinicians. Consumer views are paired with one-page physician reports meant to be scanned in 30–60 seconds: concise trends, clear thresholds, minimal friction. “We want to enrich the clinical encounter,” she said, “not add busywork.”

That framing explains choices like the two-step approach in arrhythmia and hypertension: a wide-net screen (optical sensor) paired with a more specific confirmation (ECG on the wrist; a blood-pressure log). Each step is designed to move patients toward a higher-value clinic visit.

Access is a Design Decision

Equity starts long before the product ships. Desai’s teams over-recruit in underrepresented groups for validation studies and tune algorithms to perform consistently across diverse racial, ethnic, and regional demographics. On access, she pointed to lower-cost devices that still support core health features (like Apple Watch SE with irregular rhythm notifications and sleep apnea notifications), donation programs that supply devices for investigator-initiated studies, and pragmatic moves such as at-home hearing tests and hearing-assistance features on AirPods, an order-of-magnitude price difference compared with many traditional devices.

None of this, she emphasized, replaces clinical care. It lowers barriers. “We’re trying to make the proactive stuff easier and earlier,” she said. “If we can help people engage with their health in their 20s and 30s, we may change what shows up in their 60s.”

Designing for Impact at Scale

If the Apple Heart Study taught Stanford and Apple anything, it’s that scale can remake clinical research: a single email boosted enrollment by hundreds of thousands in days. But scale also raises the stakes. A poorly tuned threshold becomes a million nudges; a confusing chart becomes a million moments of anxiety.

Desai’s answer is to keep medicine at the center of the process: validate, publish, design for the clinic, and default to privacy. That ethos, rooted in a daughter’s advocacy, honed in residency, and stress-tested in product reviews, may be her most influential export back to the ward: insist on rigorous science, and build the bridge between consumers and clinicians.