Current methods
The current methods which are used to monitor heart rate and blood oxygen levels are Phonocardiography, Oscillometry, Photoplethysmography and Electrocardiography. The sounds generated due to heart beat and murmurs can be recorded using through a microphone. This acoustic property which determines heart rate can be studied using phonocradiographs. As a result of repeated relaxations ad contractions variations in blood pressure is observed. The frequent pulsation of the vessels is detected by automatic blood pressure monitors via peizo sensors. The rhythmic properties of blood can be sensed through alight ray reflected from the finger or an ear lobe through Photoplethysmography. The relaxation and contractions of cardiac muscles are involved in pumping blood. The rhythmic relaxation and contraction of the pace maker which generated impulse can be recorded by electrocardiogram.
The non contact methods which are used for heart monitoring are Electromagnetic based monitoring systems, Laser based monitoring systems, Image based monitoring systems. Wide band pulsed and continuous wave band are the variants which are based on radar.
Vibriocardiography is used to measure the heart beat along with respiration activity, artery stiffness and heart mechanics can be studied. Image based application methods generally involve the detection due to heart beats via laser. Cardiocam is one of the image based technique which is advanced and of low cost. The limitations of the pulse oximetry are due to motion artifact, ambient light, skin pigmentation, low peripheral perfusion states, dyshaemoglobinemia, low oxygen saturation, nail polish, irregular heart rhythm and temparature.
Principal Operation
It uses a pair of LEDs which operates at two wavelengths that is one infrared at 910 nm and the other at 660 nm which is infrared. LEDs are positioned opposite to the photodiode which is capable of detection light. The deoxygenated heamoglobin and oxyhaemoglobin differ significantly on their absorption rates at significant wavelengths. Through the variability of absorbance among infrared and red light the calculations will reflect the ratio of deoxy/oxyheamoglobin. The amount of the pressure will change drastically circling heart and pulse cycle. This is how the heart rate is measured. The principle operation of a pulse oximeter of heamoglobin in IR and red region can be represented as below.
A fingertip will be inserted in the path of LEDs which is considered as source and the photodiode which is the receiver and this acts as translucent site with a better blood flow. After detecting the absorption levels the ratio of absorption between varying wave lengths will be determined.
It relies on the measurement of extinction and absorptivity coefficient of interested sample at certain wave lengths. A spectrum will be generated through which the substance can be identified by spectrophotometer but in case of pulse oximetry, two wave lengths are utilized to detect the concentration of the two solutes under study. The saturation of arterial blood can be measured. The reading obtained is oxygen concentration of functional artery and can be represented as Sp O2.
The conventional type of pulse oximetry is characterized by light emission diodes with varying wave lengths in red and infra red region. The emission is through cutaneous vascular bed. The oxygenated heamoglobin absorbs light at 940 nm and RHb absorbs at 660 nm.
Design of a probe
The photo detector is placed around the IR and red LED. The photo detector outputs the current which has been processed by an intact amplifier, A/D convertor, noise filter and MCU which is used in digital signal processing. The current will be systematically analyzed in order to extract saturation of the pulse.
The maximum separation should be 3.5 mm id current is low that is greater than or equal to 2 mA in between middle of the detector and the center of the source. The photodiode should be capable of detecting most of the backscattered light which is the AC variation. These photo diodes should be will be placed around the dual LED in order to increase signal to noise ratio.
The chip will be mounted on a printed circuit board along with electronic components with the help of the contacting pads. The another side faces the tissue and the light will bombard at this side so that the light at the opposite side will be compared to the back side i.e., junction location.
The solution which is present in the sensor must be compatible with the thermistor. In order to protect the photodiode from the stray light, metal covering is done.
The chip with the photodiode and the sensor for the temparature is flip chip which is mounted on the flex print and the electronics are mounted on the other side. The light will be able to pass along the hole via a patch at the bottom.
References
Goldman, J.M, Petterson, M.T., 2000.Signal extraction pulse oximetry, Journal of clinical monitoring and computing, 16,475-483
Polk,T., Walker,W., Hande.,2006.wireless telemetry for oxygen saturation measurements. In Biocas: Biomedical circuits and systems conference.IEEE,174-177
