A Wearable Vital Signs Monitor at the Ear for Continuous Heart Rate and Pulse Transit Time Measurements
Continuously monitoring vital signs such as heart rate (HR) and heart intervals can provide the data necessary for early diagnosis of CVD. Several groups have designed measurement systems to monitor many of these vital signs for personalized health care. Guo et al. designed a wearable vital signs monitor with a chest belt for electrocardiography (ECG) and an ear-worn probe for photoplethysmography (PPG) . They measured heart rate, peripheral oxygen saturation (SpO2) and systolic blood pressure using the pulse-arrival time (PAT) method. Pinheiro et al. measured the PPG at the finger, the ECG at the chest and the ballistocardiogram (BCG) from a chair for heart rate variability (HRV), HR, PAT and SpO2 measurements. Here, we demonstrate a continuous, wearable and wireless vital-sign monitor worn at the ear in the form factor of a hearing aid. The ear location is chosen because it is a natural anchoring point, and it is discreet since the device can be partially hidden by hair and the ear.
Several heart intervals can be determined via the three physiological signals measured with the vital signs monitor; Pre-ejection period (PEP), defined as the electro-mechanical delay between the onset of ECG’s Q wave and the time when the aortic valve opens ; pulse arrival time (PAT), defined as the time between the Q wave and the arrival of the pulse wave to a location on the body; pulse transit time (PTT), defined as the time it takes for a pulse wave to travel from one location to another location in the vasculature.
Compared to measuring PTT from a finger PPG, calibration for the vital signs monitor should be greatly reduced since the head is almost always upright or level compared to the heart. The ability to subtract PEP accurately from this equation removes one of the large error sources associated with transit-derived ABP.
Conclusion: A wearable and wireless vital-signs monitor at the ear has been demonstrated. The portable device measures the ECG, BCG and PPG, all from a single area behind the ear. R-J interval and ±ln(PTT) have been shown to correlate to pre-ejection period and arterial blood pressure, respectively.
(Eric S. Winokur, 2012)