A method that is correlated to cardiac output is ballistocardiography (BCG), which is a measure of the body’s reaction force to the blood expelled by the heart. However, most BCG measurement systems involve a chair, bed, or a scale which do not allow continuous and wearable monitoring. To measure blood oxygenation, a photoplethysmography (PPG) is typically obtained through an optical transmission measurement at the finger or the ear lobe. Both locations are non-rigid and yield significant motion artifacts. Reflectance PPG has been shown to be feasible on the forehead due to its reflective bony structure. Unfortunately, the forehead is an impractical location for a long-term wearable device due to low patient acceptance. To address these issues, we propose the site behind the ear as a location for an integrated wearable vital sign monitor. This site is superior for both physiological and mechanical reasons. Physiologically, the bony structure behind the ear and above Reid’s base line is optimal for reflectance PPG. The same location also contains BCG signal, which yields heart rate and respiratory rate, and correlates to cardiac output and blood pressure. Mechanically, the ear remains in the same orientation relative to the heart when upright, which reduces the need for complex calibrations when calculating PTT. The ear location is discreet because a small device can be concealed by the ear and hair.

In order to verify the reflectance PPG measurement, transmission PPG is taken with a Criticare Systems 504-US from the left index finger and a chest ECG is taken in lead II configuration. The delay between the PPG peak and the R peak is shorter with the right ear reflectance measurement (243ms±8.0ms) when compared to the finger transmission measurement (318ms±4.6ms) because the ear is physically closer to the heart than the finger.