We address both technical and clinical issues of wearable biosensors (WBS). First, design concepts of a WBS are presented, with emphasis on the ring sensor developed by the author's group at MIT. The ring sensor is an ambulatory, telemetric, continuous health-monitoring device. This WBS combines miniaturized data acquisition features with advanced photoplethysmographic (PPG) techniques to acquire data related to the patient's cardiovascular state using a method that is far superior to existing fingertip PPG sensors. In particular, the ring sensor is capable of reliably monitoring a patient's heart rate, oxygen saturation, and heart rate variability. Technical issues, including motion artifact, interference with blood circulation, and battery power issues, are addressed, and effective engineering solutions to alleviate these problems are presented. Second, based on the ring sensor technology the clinical potentials of WBS monitoring are addressed.

Mobile monitoring with wearable photoplethysmographic biosensors

—To evaluate the clinical application of the second derivative of the fingertip photoplethysmogram waveform, we performed drug administration studies (study 1) and epidemiological studies (study 2). In study 1, ascending aortic pressure was recorded simultaneously with the fingertip photoplethysmogram and its second derivative in 39 patients with a mean±SD age of 54±11 years. The augmentation index was defined as the ratio of the height of the late systolic peak to that of the early systolic peak in the pulse. The second derivative consists of an a, b, c, and d wave in systole and an e wave in diastole. Ascending aortic pressure increased after injection of 2.5 μg angiotensin from 126/74 to 160/91 mm Hg and decreased after 0.3 mg sublingual nitroglycerin to 111/73 mm Hg. The d/a, the ratio of the height of the d wave to that of the a wave, decreased after angiotensin from −0.40±0.13 to −0.62±0.19 and increased after nitroglycerin to −0.25±0.12 ( P P r =0.79, P r =0.80, P y ) increased with age ( x ) ( r =0.80, P y =0.023 x −1.515). The second derivative aging index was higher in 126 subjects with any history of diabetes mellitus, hypertension, hypercholesterolemia, and ischemic heart disease than in age-matched subjects without such a history (−0.06±0.36 versus −0.22±0.41, P P

Assessment of vasoactive agents and vascular aging by the second derivative of photoplethysmogram waveform.

A monitor for determining a patient's physiological parameter includes a calibration device that provides a calibration signal indicative of an accurate measurement of the patient's physiological parameter. The monitor also includes a processor, which receives a noninvasive signal from a noninvasive sensor positioned over a blood vessel. The processor uses the calibration signal to calibrate a relationship between the noninvasive signal and a property of the physiological parameter. The processor also determines when to recalibrate the relationship.

System and method of determining whether to recalibrate a blood pressure monitor

A measurement device for generating an arterial volume-indicative signal includes an exciter and a detector. The exciter is adapted to receive an oscillating signal and generate a pressure wave based at least in part on the oscillating signal on the artery at a measurement site on a patient. The pressure wave includes a frequency. The detector is placed sufficiently near the measurement site to detect a volumetric signal indicative of arterial volume of the patient.

Rapid non-invasive blood pressure measuring device

An exciter-detector unit is disclosed which includes an exciter (104) and a detector (105) mounted on a common support (101) for inducing perturbations into the body and detecting the perturbations after they travel a distance through the body.

Exciter-detector unit for measuring physiological parameters

A new plethysmograph, the electric impedance cuff, was designed for the indirect measurement of blood pressure, volume elastic modulus Ev and compliance Ca in human limb arteries. This comprises a compression chamber filled with electrolyte solution and a tetrapolar electric impedance plethysmograph whose electrodes are placed inside the chamber; the former for controlling transmural arterial pressure Pt, and the latter for detecting total limb volume Vo, mean arterial volume
 
$$\bar V_a $$

 and its variation ΔVa. Systolic and mean arterial pressure in the upper arms, forearms and fingers were measured by detecting pulsatile impedance variation during the gradual (3–5 mm Hg per heart beat) increase (or decrease) in chamber pressure by the volume oscillometric technique. Diastolic and pulse pressure ΔP were calculated from these pressure values. Compliance Ca=ΔV/ΔP and volume elastic modulus
 
$$E_v = \Delta P/(\Delta V_a /\bar V_a )$$

 were recorded at various Pt levels, controlled by the compression pressure. Although this is a kind of impedance plethysmograph, the volume change in a limb segment can be detected by this method without passing electric current through the limb.

Electric impedance cuff for the indirect measurement of blood pressure and volume elastic modulus in human limb and finger arteries.

Diastolic pressure Pd was indirectly measured by vibrating a finger artery with a 10 Hz sinusoidal pressure variation during a gradual increase (or decrease) in occlusive cuff pressure Pc. Pulsatile arterial volume changes on which sinusoidal variations are superimposed were detected by a transmitted infra-red photoelectric plethysmograph (TIPP). It is known that volume change in an artery shows a maximum amplitude at the transmural pressure Pt level equal to 0 mm Hg due to the nonlinear viscoelastic properties of the arterial wall. For the same reason, the amplitude of the sinusoidal volume variation reached its maximum at the end-diastolic phase, when Pc was controlled to be exactly equal to Pd. The indirect Pd values determined from Pc were compared with those simultaneously measured by a direct method in rabbit forelegs and by the volume-compensation method in human fingers. Using the principle of the volume oscillometric method systolic and mean pressures were also determined by this system.

Vibration technique for indirect measurement of diastolic arterial pressure in human fingers

Using a volume-compensation technique, a portable device has been designed for the indirect measurement of beat-to-beat arterial pressure and its waveforms in the basal phalanx of fingers of ambulatory subjects. The device consists of (1) a transmission infra-red photoelectric plethysmograph (TIPP) to detect the variation of arterial volume, (2) a pneumatic cuff with an actuator, (3) a servosystem to control the cuff pressure, and (4) a stereo cassette tape recorder. Arterial pressure was determined from the cuff pressure which was controlled by the servosystem so as to maintain the arterial volume constant at the ‘vascular unloading’ state. This device is equipped with a compensator for any hydrostatic pressure difference between the heart and finger. Thus, the blood pressure at heart level can be obtained for any finger height. The total weight of the device was 1·6kg. Blood pressure changes during walking, jogging, jumping, and exercises such as side-stepping, Master's two-step test and car driving, have been successfully recorded.

Ambulatory monitoring of indirect beat-to-beat arterial pressure in human fingers by a volume-compensation method.

Arterial elasticity expressed by such indices as volume elastic modulus Ev and compliance Ca were noninvasively measured in various human limb segments; the upper arms, forearms, fingers, thighs, calves and toes These indices are defined, respectively, as$E_v = \Delta P/(\Delta V/\bar V_a )$ and Ca=ΔV/ΔP, where ΔP is pulse pressure,$\bar V_a $ mean arterial volume and ΔV its pulsatile variation ΔP was calculated from systolic Pas and mean Pam arterial pressures determined by volume oscillometric sphygmomanometry using the following equation:$\Delta P = 3(P_{as} - P_{am} )/2\bar V_a $ and the ΔV were detected by electrical admittance plethysmography at various transmural pressure Pt levels controlled by a compression cuff The values obtained in these limb segments were compared with each other at Pt levels 0,30 and 60 mm Hg and the differences between them were discussed

Noninvasive measurement of arterial elasticity in various human limbs

A new Plethysmograph, the electric impedance-cuff, was designed for the indirect measurement of blood pressure and flow, volume elastic modulus (Ev) and compliance (C) in human finger arteries. The device comprises a compression chamber filled with electrolyte solution and a tetrapolar electric impedance Plethysmograph whose electrodes are embedded inside the chamber; the compression chamber for controlling transmural arterial pressure (Pt) and the impedance Plethysmograph for detecting total finger volume (Vo), mean arterial volume (Va) and its variations (Av). Systolic (Pas) and mean (Pam) arterial pressures in fingers were determined by detecting pulsatile impedance variations during the gradual increase (or decrease) in Pc using volume oscillometric technique. Diastolic pressure (Pad) and pulse pressure (AP) were calculated using the equations: Pad = (3Pam -_Pas)/2 and ΔP = Pas — Pad. Ev and C defined as Ev = ΔP/(ΔV/Va) and C = ΔVa/ΔP were measured at various Pt levels controlled by Pc. Arterial pressures, blood flow and Ev were measured in 171 healthy subjects. In some subjects, the change in pressure, blood flow and Ev of finger arteries were detected during acupuncture.

A. Kawarada

Papers

Electric impedance cuff for the indirect measurement of blood pressure and volume elastic modulus in human limb and finger arteries.

Vibration technique for indirect measurement of diastolic arterial pressure in human fingers

Ambulatory monitoring of indirect beat-to-beat arterial pressure in human fingers by a volume-compensation method.

Noninvasive measurement of arterial elasticity in various human limbs

Indirect Measurement of Arterial Pressure, Blood Flow and Volume Elastic Modulus in Human Fingers Using Electric Impedance-Cuff