Pressure monitoring method and apparatus

• US 4,393,878 A
• Filed: 05/11/1981
• Issued: 07/19/1983
• Est. Priority Date: 11/28/1980
• Status: Expired due to Term

• Current Assignee: Meadox Instruments Incorporated (Boston Scientific Corporation), Kenneth Clark
• Original Assignee: Meadox Instruments Incorporated (Boston Scientific Corporation)
• Inventors: Kahn, Alan R.
• Primary Examiner(s): Cohen, Lee S.
• Application Number:
  US06/262,381
• Time in Patent Office: 799 Days
• Field of Search: 128/748, 128/672, 128/673, 128/675, 128/687-690, 73/715, 73/729
• US Class Current: 600/561
• CPC Class Codes: A61B 5/031 Intracranial pressure
• CPC Class Codes: G01L 11/006 hydraulic or pneumatic counterbalancing forces

Abstract

An apparatus (10) which measures and monitors the pressure within a human body includes a pressure sensor (11) having a housing (30) closed by a pressure sensitive diaphragm (34) to define a plenum (35), an exhaust tube (36) mounted within the plenum (35) which terminates adjacent the inner surface of the diaphragm (34), and tubes (13, 15) which transmit a substantially constant flow of gas into the plenum and which withdraw gas from the exhaust tube (36) to a location remote from the body. Changes in pressure within the body cause the diaphragm (34) to alternately close and open the end of the exhaust tube (36), thereby resulting in an increase or decrease in the pressure within the plenum until an equilibrium pressure is reached. A source (12) of gas pressure is provided, as well as a means for providing a substantially uniform flow rate (R) and means for measuring the pressure (14) within the tube (13) that transmits the gas flow to the sensor (11). The flow rates to and from the sensor (11) are compared, and the flow of gas to the sensor (11) is cut off if the flow rates are unequal, a condition indicative of a leak in the sensor (11) or in the tubing (13, 15, 17) that transmits the gas to and from the sensor.

Claims

1. Pressure monitoring apparatus comprising:

(a) a fluid pressure pump supplying gas under pressure;

(b) a pressure sensor including;

(1) a cup shaped housing having an open mouth and having an inlet and an outlet formed therein;

(2) a flexible diaphragm sealed over the open mouth of the housing to define a plenum between the walls of the mouth in the housing and the diaphragm, and wherein the inlet formed in the housing is in communication with the plenum;

(3) an exhaust tube centrally mounted in the housing within the plenum with an end thereof terminating adjacent the inner side of the diaphragm and connected to be in communication with the outlet from the housing;

(c) tubing connecting the pump to the inlet in the sensor housing to supply gas under pressure to the inlet and also connecting the outlet in the sensor housing to the pump to form a closed system;

(d) a flow restriction connected in the tubing between the pump and the inlet to the sensor housing so that the flow to the sensor is substantially uniform; and

(e) means for measuring pressure connected to measure the pressure in the tubing between the flow restriction and the inlet to the sensor housing, whereby changes in ambient pressure at the pressure sensor will result in movement of the diaphragm to alternately open and close the exhaust tube to automatically maintain the pressure within the plenum approximately equal to the ambient pressure, and whereby the means for measuring pressure in the tubing will measure a pressure which is approximately equal to the pressure within the plenum.

2. The pressure monitoring apparatus of claim 1 in which the pressure pump supplies gas at a pressure at least twice the maximum pressure to be measured by the means for measuring pressure.

3. The pressure monitoring apparatus of claim 1 including means connected to the tubing returning gas from the sensor to the pump for providing a stable negative pressure reference in the tubing which enables the measurement of negative pressure at the pressure sensor.

4. The pressure monitoring apparatus of claim 1 including a sealed vessel connected in the tubing between the outlet of the sensor housing and the pump and means for maintaining a negative pressure in the sealed vessel, whereby negative ambient pressures can be sensed by the pressure sensor and whereby, if a leak develops in the pressure sensor, the gas supplied to the sensor will tend to be withdrawn into the vessel rather than leaking out of the sensor.

5. The pressure monitoring apparatus of claim 4 wherein the means for maintaining a negative pressure in the sealed vessel includes a piston slideably engaging the walls of the vessel in substantially pressure tight relationship and a weight operatively connected to the piston to provide a constant force pulling on the piston to tend to draw the piston in a direction to enlarge the internal volume of the sealed vessel.

6. The pressure monitoring apparatus of claim 5 including a pair of electrical switches connected in an electrical circuit supplying electrical power to drive the pump, means connected to the piston and responsive to movement of the piston beyond preselected limits of either increased or decreased volume of the sealed vessel for opening one or the other of the switches to cut-off electrical power to the pump and thereby shut off the flow of gas to the pressure sensor, whereby the pressure monitoring apparatus will automatically shut down if a leak of gas supplied to or withdrawn from the sensor occurs.

7. The pressure monitoring apparatus of claim 1 wherein the housing of the pressure sensor is formed of plastic having a circular open mouth formed therein to define a cup-shape, the exhaust tube is formed integrally with the housing and extends upwardly from the bottom surface of the mouth of the housing at the center thereof and has a surface area adjacent the diaphragm which is not more than 30% of the area of the mouth of the housing covered by the diaphragm, and wherein the outlet in the housing includes a channel formed in the housing extending from the end of the exhaust tube adjacent the diaphragm and to the surface of the housing for connection to the tubing connected to the pump, and wherein the inlet is formed as a hole in the wall of the housing which terminates in the plenum and which is connected to the tubing leading from the pump, and wherein the points of connection of the tubing to the inlet and the outlet are adjacent to one another.

8. The pressure monitoring apparatus of claim 1 wherein the pressure sensor housing is formed as a circular plastic disc having a circular open mouth formed in one surface of the housing which is surrounded by a peripheral annular face and leaving a top wall at the other surface of the disc, and wherein the diaphragm is sealed to the surface of the peripheral annular face to cover the mouth of the housing and define the plenum between the diaphragm and the interior walls of the mouth of the housing, wherein the exhaust tube is formed integrally with the housing, extends from the center of the top wall of the housing to terminate at a position adjacent the inner side of the diaphragm, and has a circular bore therein, and wherein an outlet pipe formed of metal extends through the wall of the exhaust tube into communication with the bore within the exhaust tube and also extends through the walls of the housing to the outside thereof to define the outlet from the housing and is connected to the tubing leading to the pressure pump, and wherein the inlet includes an inlet pipe formed of metal extending through the wall of the housing from a position outside of the housing into the plenum within the housing, the piston of the inlet pipe outside of the housing being connected to the tubing leading from the pressure pump, the inlet and outlet pipes being mounted closely adjacent to one another to minimize the space taken up by the pipes and by the tubing connected to them, and the inlet and outlet pipes being located at a position in the wall of the sensor being between the diaphragm and the top wall of the housing such that the thickness of the top wall of the housing can be minimized.

9. The pressure monitoring apparatus of claim 7 or 8 wherein the pressure sensor housing is formed of a material selected from the group consisting of silicone and polyurethane.

10. The pressure sensor of claim 8 wherein the height of the sensor from the surface of the diaphragm to the top surface of the housing is approximately 1.5 mm.

11. The pressure sensor of claim 1 wherein the exhaust tube terminates just short of the diaphragm whereby gas can flow from the plenum into the exhaust tube when the pressure on either side of the diaphragm is approximately equal.

12. A pressure sensor adapted for implantation in a human body comprising:

(a) a cup-shaped housing formed as a circular plastic disc having a circular open mouth formed in one surface which is surrounded by a peripheral annular face and leaving a top wall at the other surface of the disc;

(b) a pressure sensitive diaphragm sealed to the surfaces of the peripheral annular face to close the open mouth of the housing to define a plenum between the diaphragm and the interior walls of the mouth in the housing;

(c) an exhaust tube formed integrally with the housing and extending from the top wall of the housing within the center of the plenum to terminate at a position adjacent to the inner side of the diaphragm, the exhaust tube having a circular bore therein;

(d) an outlet from the housing including an outlet pipe formed of metal extending through the walls of the exhaust tube into communication with the bore within the exhaust tube and also extending through the walls of the housing to the outside thereof where it is adapted to be connected to external tubing;

(e) an inlet for the housing including an inlet pipe formed of metal extending through the wall of the housing from a position outside the housing into the plenum within the housing, the portion of the inlet pipe outside of the housing being adapted to be connected to external tubing, the inlet and outlet pipes being mounted closely adjacent to one another to minimize the space taken up by the pipes and by the tubing to be connected to them, and the inlet and outlet pipes being located at a position in the wall of the sensor housing between the diaphragm and the top wall of the housing such that the thickness of the top wall of the housing can be minimized.

13. The sensor of claim 12 wherein the pressure sensor housing is formed of a material selected from the group consisting of silicone and polyurethane.

14. A method of measuring the pressure within a human body comprising the steps of:

(a) implanting within a human body a sensor which includes a housing closed by a diaphragm to define a plenum therein and an exhaust tube in the plenum terminating at an end adjacent the inner surface of the diaphragm;

(b) providing a first length of tubing which extends from a location outside the human body to the sensor and is in communication with the plenum therein;

(c) providing a second length of tubing which extends from a location outside the human body to the sensor and is in communication with the exhaust tube therein;

(d) providing gas through the first tubing at a substantially constant flow rate into the plenum in the housing;

(e) withdrawing gas through the exhaust tube when the pressure outside the sensor is less than the pressure in the plenum such that the diaphragm is spaced away from the opening to the exhaust tube and transmitting the gas flow through the second tubing, and substantially cutting off the flow of gas into the exhaust tube when the pressure in the plenum is lower than the pressure outside the sensor such that the diaphragm is pressed against the opening of the exhaust tube;

(f) measuring the pressure in the first tubing which provides the gas flow to the sensor to provide an estimate of the pressure within the plenum of the sensor which is itself substantially equal to the pressure outside the sensor; and

(g) providing a substantially constant negative pressure to the second tubing which transmits the gas from the pressure sensor thereby to aid the withdrawl of any gas that is introduced into the sensor and reduce the possibility of gas being accidentally injected into the human body if the sensor develops a leak, and to allow the sensor to record negative pressure.

15. The method of claim 14 including the step of comparing the flow of gas into the sensor with the flow of gas from the sensor and cutting off the flow of gas when the flow rates are unequal, which is an indication that a leak has developed in the system.