Device for the indirect, non-invasive and continuous measurement of blood pressure

In a photo-electric plethysmograph blood pressure measuring apparatus including a pressurizable cuff adapted to be wrapped about a human extremity, such as a finger, a light source and light detector both being provided at an inner side of said pressure cuff for developing an electrical signal indicative of an amount of blood in the arteries of said extremity, a control valve for controlling the pressure of fluid within the cuff. and an electronic circuit for controlling the control valve in response to the electrical signal so as to control the fluid pressure in the cuff to substantially follow the blood pressure within the extremity so that the arterial volume is maintained to a predetermined value, the fluid pressure providing an indication of the blood pressure, the improvement wherein the control valve comprises:

• a valve chamber;
• a fluid inlet nozzle for admitting fluid from a fluid source into the chamber;
• a fluid exhaust nozzle for exhausting fluid from the chamber;
• an output port for transmitting a pressure in the chamber to the pressure cuff;
• and
• a single flapper member alternately movable between a first position wherein it closes the fluid inlet nozzle while allowing the fluid exhaust nozzle to be open and a second position wherein it closes the exhaust nozzle while allowing the inlet nozzle to be open, there being a flow of fluid into the chamber through the inlet nozzle and a flow of fluid out of the chamber through the exhaust nozzle when the flapper member is at any position other than the first or second position.