Gas column pressure monitoring catheters

Details Gas column pressure monitoring catheters Gas column pressure monitoring catheters

2000-07-18

2004-01-06

Marmor, II, Charles A. (Department: 3736)

Surgery

Diagnostic testing

Measuring fluid pressure in body

C600S587000

Reexamination Certificate

active

06673022

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to air-based catheter systems for monitoring pressure in a mammalian body especially as applied to measuring intracranial pressure (ICP) in the brain.
2. Description of the Prior Art
An air-based catheter consists of a catheter with an air lumen that communicates with a bladder at or near its distal end and with a connector at or near its proximal end. The bladder volume of the catheter changes as pressure changes in accordance with P
1
V
1
=P
2
V
2
and thereby causes the pressure of the gas within the catheter to equal that of the external environment surrounding the bladder. The media in which the bladder senses pressure must be able to move toward or away from the bladder to cause the requisite change in bladder volume. Movement of the media is not a problem when the bladder is immersed in a flowable liquid such as blood. There is a problem when brain tissue is the media in that brain tissue has a limited ability to move. An air-based catheter designed to measure ICP must limit the movement required of the bladder so that the distance the bladder wall moves as it portrays the wave form of a heart beat does not exceed the intrinsic ability of brain tissue to move.
The prior art system, one made by Spiegelberg Gmbh, has a long air line that connects the catheter bladder to a bedside instrument. The air line significantly increases total system volume and thereby increases the volume change required of the bladder to reflect a given pressure change. Spiegelberg addresses the limited ability of brain tissue to move by purposefully underfilling the bladder. The underfilled bladder approximates a flat profile when placed in the brain. The (x)(y) planar area created by the flattened profile of the bladder reduces the movement required of brain tissue in the z dimension and does so in direct proportion to the x and y dimensions. The planar area chosen is one that limits the wall movement required of the bladder to a value compatible with the ability of brain tissue to move. The planar area is defined by the potential volume of the bladder and the degree to which it is underfilled. Spiegelberg uses a 425-ul bladder filled about 25% full. The underfilled bladder makes it possible for the Spiegelberg air-based catheter to measure pressure in the brain. An electromechanical pump is used to address the limited operating life of a bladder with a large surface area and small contained volume.
The volume of air in the bladder displaces brain tissue. It is therefore desirable that the air volume be as small as possible to limit tissue displacement. In the Spiegelberg system, most of the system volume that must be acted upon by the bladder is contained in the long air line that connects the intracranial catheter to a bedside instrument housing a pressure transducer. The connecting line represents about 80% of the total system volume. It is therefore responsible for 80% of the air volume required by the bladder to accomplish the required volume change. Bladder volume and therefore brain trauma in prior art are therefore largely defined by the volume of air in the air line that connects the catheter to a bedside instrument. Furthermore, the volume of air in the connecting air line largely defines the potential volume of the oversized bladder required to achieve the underfilled state necessary to limit bladder wall movement in the Z direction. The surface area of the oversized bladder increases air lost by diffusion and decreases the operating life of the catheter, i.e., the period of time after which air lost by diffusion must be replaced.
Brain trauma caused by the air volume within the bladder can be minimized if the air in the system outside the bladder is held to a minimum. If Vnb=volume of non-bladder air and Vb=volume of bladder air, then system volume=(Vnb+Vb). As system volume changes from V
1
to V
2
, V
1
−V
2
=(Vnb+Vb
1
)−(Vnb+Vb
2
). Vb is the only variable volume; therefore the change required of the bladder is least when the non-bladder air volume (Vnb) is zero. Accordingly, a design least demanding in terms of both bladder wall movement and volume of brain tissue displaced is one wherein the ratio of (system volume)÷(bladder volume) approaches 1:1. The ratio in the Spiegelberg device is 5:1, a ratio far from ideal. The 5:1 ratio calculation includes 550 ul of gas contained in the 154 cm long air line and approximately 125 ul of gas in the bladder for a total system volume of about 675 ul.
The patents and literature covering gas column pressure monitoring catheters describe systems designed for various pressure measurement applications. Catheters constructed according to these teachings, however, are not optimal for use in measuring intracranial pressure (ICP) in that they do not minimize brain trauma nor do away with the need for an electromechanical pump. The deficiencies, if addressed, would greatly improve the usefulness and safety of the catheter.
A number of prior art devices transmit physiological pressure through a gaseous medium. The following United Sates and foreign patents/patent publications have described pressure measuring catheters and other pressure transmitting systems wherein a gas is utilized as a pressure-transmitting medium in at least a portion of the system: U.S. Pat. No. 5,573,007 (Bobo), U.S. Pat. No. 2,840,069 (Squire et al), U.S. Pat. No. 4,227,420 (Lamadrid), U.S. Pat. No. 4,300,571 (Waldbilling), U.S. Pat. No. 4,314,480 (Becker), U.S. Pat. No. 4,648,406 (Miller), U.S. Pat. No. 4,841,984 (Armeniades et al.), U.S. Pat. No. 5,105,820 (Moriuchi, et al.), Patent publications: WO82/02657 (Ebert), WO86/03957 (Spiegelberg), WO90/11717 (Utah Medical Products, Inc.) Prior art pertaining specifically to a gas-column pressure-monitoring catheter that employs a flaccid bladder is explicitly covered in WO86/03957 (Spiegelberg) and U.S. Pat. No. 5,573,007 (Bobo).
The only prior art air-based catheter capable of measuring ICP in brain tissue is a device produced by Spiegelberg Gmbh in Germany. The Spiegelberg device addresses the matter of controlling bladder wall movement by using a partially filled bladder. The bladder has a potential volume of 425 ul net of the volume occupied by the catheter within. Once the bladder is inserted into the brain, an electromechanical pump defines ICP level. The pump then injects an amount of air into the bladder that ranges from 50 to 100 ul. The electromechanical pump deflates and refills the bladder on an hourly basis. The terms injected or filled as used hereinafter means the volume of gas inserted into the bladder where the pressure of the injected gas is equal to atmospheric pressure. The total air volume in the bladder consists of the volume in the uncollapsed portion of the bladder prior to air injection of about 50 ul plus the injected volume of 50-100 ul. The air volume of the bladder is therefore about 125 ul. The total volume of air in the bladder defines the brain tissue pushed aside and is correlated to the trauma imposed upon the brain.
The partially filled bladder assumes a flattened shape when positioned in the brain. If the area of the flattened bladder is described as (x)(y) and the movement of opposite walls of the bladder described as z, then z=the volume change divided by the area or (V
1
−V
2
)÷(x)(y). Z decreases as (x)(y) increases. The flat partially filled bladder used by the Spiegelberg device limits bladder movement by using a (x)(y) value such that z falls within the brain's ability to move.
The percentage the bladder must be underfilled to function in brain defines the absolute size of the bladder, the bladder surface area and in turn, the rate at which air is lost by diffusion. Air loss defines the system's operating life, i.e., the time the bladder can function before air lost by diffusion must be replaced. The fact that the Spiegelberg bladder must be substantially underfilled to function increases surface area and decreases

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