In the field of hemodialysis and other techniques where blood is removed from a patient for processing and then returned, it is important to periodically assess the blood flow rate through a fistula, graft, or catheter to monitor the onset of stenosis. This is often accomplished by the reading of access pressures through the venous and arterial fistula needles, for example. Early detection of stenosis in a fistula, graft, implantable port, or a catheter can permit low cost repairs to be made. On the other hand, if these problems are ignored or not detected, the cost of the revision or replacement of the fistula, graft, implantable port, or catheter can be very high and burdensome to the patient.
As shown in Omachi U.S. Pat. No. 5,454,374, access pressures may be determined through volumetric manipulations involving the determination of a pressure head height of blood in a visual manner. Alternatively, clinicians rely on a stop flow methodology using a blood flow line, which has technical problems. Also gauges on the dialysis machine are used to estimate pressure, which technique is imprecise. In these latter techniques, head height issues contribute to inaccuracy even when the machines provide a desirable, electronically dampened pressure reading.
It would be desirable to directly read the access pressure through a fistula needle, for example, before a blood tubing set is attached, which blood tubing set communicates between a fistula set in communication with the patient""s vascular system and a membrane dialyzer. In doing this, it is important to isolate the sterile fistula needle set from a connected, unsterile pressure gauge. Also, there is a need to dampen the pressure pulse, which is naturally provided by the pulsatile flow of blood in the patient, to provide a mean pressure reading which is not strongly subject to inaccuracy due to head height variations. Ideally, the reading could be taken at about the level of the heart.
Furthermore, there is a need for such a device to control the flow of blood into the fistula set as pressure is measured, so that it does not enter into contact with a microporous barrier, which is typically present to protect the patient from bacterial contamination. Also, the system needs to have a substantial length to allow easy positioning and reading of the pressure gauge.
By this invention, the above needs are met in an inexpensive manner that is easy to use with relatively little training, so that patients can be monitored to detect the early onset of stenosis in a fistula, graft, implantable port, or catheter which is permanently implanted in connection with the vascular system of the patient.
By this invention, a method is provided of measuring access pressure through a first tube which is flow-connected to the vascular system of a patient. The method comprises the steps of: connecting one end of pressure tubing to an outer end of the first tube, with a member blocking the flow of blood through at least part of the pressure tubing while permitting the passage of air therethrough, to suppress or damp pressure pulses or oscillations through the tubing. One connects the other end of the pressure tubing to a pressure gauge. Thus, upon opening access through the first tube to the vascular system, blood flows into the first tube and compresses the air in the connected first tube and pressure tubing, plus the connected gauge, causing pressure from the vascular system to be readable by the gauge while the pressure pulses are attenuated in a simple, nonelectronic manner.
Preferably, the xe2x80x9cmemberxe2x80x9d mentioned above is a microporous member, typically a microporous block or plug positioned within or adjacent to the pressure tubing and capable of providing the damping or attenuation of the pulsatile nature of the pressure from the patient""s cardiovascular system at the gauge.
Preferably, the internal volume of the pressure tubing is less than the internal volume of the first tube. As the result of this, pressurized blood entering an empty first tube as the pressure is read does not advance completely through the first tube before it is halted by compression of the initial air in the first tube and the pressure tubing, as well as the residual volume of air within the pressure gauge. This may be accomplished by providing pressure tubing which has a connector at each end, the tubing having a single lumen of reduced diameter from normal flexible tubing, which lumen diameter is typically no more than about one third of the outer diameter of the tubing. Thus, the internal volume of the pressure tubing can be less than the internal volume of the first tube even if the length of the pressure tubing is greater than the length of the first tube, which situation is preferred so that there is adequate tube length to conveniently hold a pressure gauge and to position it at approximately the level of the patient""s heart and to read it with ease, and also to reduce the chance that the fistula needle connection to the patient""s fistula is disturbed as the pressure gauge is connected and handled.
Preferably, the set which defines the pressure tubing carries a microporous member which is capable of preventing the passage of bacteria therethrough. This can be a second microporous member if desired, above and beyond the microporous plug described above which suppresses pressure oscillations through the pressure tubing, thus attenuating the pressure pulses. A conventional 0.2 micron bacterial filter may be used. This uniquely provides both flow blocking and aseptic conditions with commercially available materials.
Alternatively, the microporous member may be a plug which has a bacteria blocking capability similar to conventional 0.2 micron bacterial filters. Also, a membrane-type bacterial filter may have pores that are small enough to provide the desired attenuation of pressure pulses through the pressure tubing, to facilitate reading of the gauge.
Also, if desired, the pressure tubing may have a bore which is sufficiently narrow and of a length to provide the desired pressure pulse attenuation through the tubing without the need for a porous plug so that, typically, only a bacteria blocking filter membrane is provided, as needed, to protect the patient from bacterial contamination through connection to a nonsterile pressure gauge.
Thus, the objectives described above may be achieved by a structurally simple, inexpensive method and apparatus, as described herein.