I'm not suggesting that absolute blood pressure is being measured by a phone PPG, only relative changes. Far from being just a doodle, the PPG readout ("ECG-li

I'm not suggesting that absolute blood pressure is being measured by a phone PPG, only relative changes. Far from being just a doodle, the PPG readout ("ECG-like trace") comes from changes in skin reflectivity that indicate relative changes in blood pressure. [1] Obviously this isn't as good as an actual ECG (and can be disrupted by movement), but it's far more information than is needed to accurately track heart rate. These devices can be as accurate as a pulse oximeter [2], though they don't account for the movement of the user. [1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4822420/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4822420/ [2] https://www.ncbi.nlm.nih.gov/pubmed/28464700 https://www.ncbi.nlm.nih.gov/pubmed/28464700