medical electronics III

Optoelectronic plethysmography
Optoelectronic Plethysmography is a method to evaluate ventilation through an external measurement of the chest wall surface motion.
Photoplethsymograph

A photoplethysmograph(PPG) is an optically obtained plethysmograph, a volumetric measurement of an organ. A PPG is often obtained by using a pulse oximeter which illuminates the skin and measures changes in light absorption (Shelley and Shelley, 2001). A conventional pulse oximeter monitors the perfusion of blood to the dermis and subcutaneous tissue of the skin.

In theory the photoplethysmograph is simple - it measures the variation in amount of light passing through your finger caused by the pulsatile nature of blood flow. You will place a light source on one side of your finger, and a light sensitive resistor, a Cadmium Sulfide (CdS) cell, on the other side. By monitoring variations in resistance of the CdS cell you get an indication of blood flow in your finger.

The light passes through the finger and is attenuated a certain amount depending upon how much blood is in the finger. As the amount of light striking the CdS cell varies, so does its resistance. This changing resistance must be transduced, amplified and filtered. The output of the amplifier is sent to an A/D convertor, and finally to the PIC. The relative amount of fluid in the finger should be displayed on the four LEDs. Much fluid (little light) should have all LED's on. Little fluid (much light) should correspond to all LED's off.

Labels: Photoplethsymograph