Dialysis pressure monitoring with clot suppression

Details Dialysis pressure monitoring with clot suppression Dialysis pressure monitoring with clot suppression

2000-07-25

2003-02-04

Sykes, Angela D. (Department: 3762)

Surgery

Blood drawn and replaced or treated and returned to body

Body inserted tubular conduit structure

C604S004010, C210S645000, C210S741000, C422S082130, C600S504000

Reexamination Certificate

active

06514225

ABSTRACT:

BACKGROUND OF THE INVENTION
Arterial and venous blood pressures in hemodialysis and other extracorporeal tube sets have traditionally been measured indirectly via a blood/air interface and air column communicating with a pressure measuring transducer. Such interface is typically located in an air trap chamber. The air column typically is contained within and communicates between various components: the top of a chamber, a pressure monitor tube (PMT), a dialysis machine tubing and a pressure measuring transducer housing within the dialysis machine. Also known are blood/air interfaces without a chamber where a PMT communicates with a blood tube at a “T” connection.
Each air column typically comprises a sterile chamber/PMT portion and an unsterile machine portion. A sterility barrier (or transducer protector), capable of transmitting air pressure while maintaining sterility, separates the sterile chamber/PMT portion from each unsterile machine portion. Typically the sterility barrier is a hydrophobic membrane permeable to air flow but not to aqueous liquids. Typically, the air column has a large cross section in the air trap chamber (10-35 mm ID) and a narrow cross section in the PMT (0.5-3.5 mm ID).
Such blood/air interfaces have numerous problems. First, blood exposed to air activates a clotting cascade, usually in direct proportion to the blood/air interface surface area and to the degree of stagnation of the blood at the interface. Anticoagulants such as heparin are required to counteract such clotting tendency. Anticoagulants are costly and have numerous side effects for the patient.
Second, air in conventional air trap chambers can escape and enter the patient, even if no air enters the chamber in the incoming blood flow. That is, if and when the blood/air interface falls below the blood inlet (e.g., a downspout) the incoming blood flow causes cavitation at the interface and entrains air emboli in the downward blood flow, such that the air may escape the air trap chamber.
Third, air trap chambers often comprise over 20 percent of the size, weight and cost of the entire blood tubing set.
The fourth problem relates to blood/air interface level changes in the chamber due to pressure reductions (where liquid level goes down) and pressure increases (where liquid level goes up). Such level changes promote a risk of inhibiting accurate pressure measurements and may promote air emboli passing to the patient. The degree of blood/air interface level change is in direct proportion to the total volume of air in both the tube set and pressure machine portions according to Boyle's law. Machine air volumes typically vary from 2 cc to more than 10 cc, depending on manufacturer and model. PMT air volumes typically range from 0.5 cc to 6 cc. Chamber air volumes depend on the blood level chosen by the clinician, but typically range from 3 cc to 20 cc. With the blood pump off, blood pressures are zero, and the arterial and venous blood/air interfaces are at the level in the chamber initially chosen by the clinician. But when blood flows increase to typical speeds (e.g. 450 ml/min with 15 G AVF needles), pre-pump pressures drop as much as −300 mmHg or more and post-pump pressures increase as much as +500 mmHg.
In the positive pressure case, the air volume may be compressed as much as 40 percent or more ((1160 mmHg−760 mmHg)/760 mmHg). If the chamber/PMT air volume is less than 40 percent of the total air volume, the blood/air interface level can rise into the PMT until it is stopped by the transducer protector. Blood is thus trapped in the transducer protector and typically clots, and the machine transducer is no longer able to accurately measure pressure. This is a highly dangerous situation. In the negative pressure case, the air volume may be expanded as much as 65 percent or more (760 mmHg/(760 mmHg−300 mmHg)). If the chamber blood volume is less than the expansion air volume, the blood/air interface may fall until it empties the chamber and passes air to the patient, causing air emboli, an even more dangerous situation.
A fifth problem relates to dialysis tube sets and dialyzers requiring priming with physiologic fluid to eliminate unwanted air prior to processing blood through the circuit. In typical prior art chambers, an initial saline prime creates a saline/air interface in an upper portion of the chamber at a position chosen by the physician. When blood flow starts, however, saline is completely displaced by blood due to the excellent mixing in these chambers. Typically, the blood inlet is close to the saline/air interface or points at the saline/air interface. Thus, the saline/air interface quickly becomes a blood/air interface.
Other prior art chambers have long blood inlet downspouts or other arrangements to enter the chamber well below the fluid level in the chamber and pointed away from the fluid level. For example, Fresenius AG has an air trap chamber designed to promote a blood/saline or plasma/air interface with the blood inlet directed transversely and located well below the interface of the chamber. In these chambers, the initial saline/air interface may be set well above the blood inlet to the chamber. Blood is slightly heavier than saline (the cellular elements more so than plasma), so when blood enters the chamber (especially at low or moderate flow common in Europe and Japan), blood tends not to invade the stagnant (saline) area above the inlet level. This is often sufficient to stratify into a blood/saline/air interface or even stratify into a blood/plasma/air interface since plasma is almost the density of saline and will rise above blood's cellular elements if relatively undisturbed. As saline-to-air contact initiates no clotting-cascade, and plasma-to-air contact has few if any initiators for clotting, this design is thought to provide clotting protection over a normal blood/air interface.
In practice, however, this approach has had little practical value. Dialysis has many events that cause abrupt pressure changes: peristaltic pump action at high flows (a large pressure pulse where flow instantaneously slows, stops or even reverses with each roller stroke); pump stoppages due to alarms; patient movements, patient coughing, line kinking, inadvertent clamping, etc. These pressure changes cause the fluid level to rise or fall rapidly. In those chambers which have large cross sectional areas, “plug flow” does not occur. Instead, blood “burps” up into the stagnant plasma or saline layers, and displaces some or all of the plasma and/or saline. Now blood is in a stagnant area of the chamber with a blood/air interface, and significant clotting is created. “Plug flow” is the movement of two fluids in a tube as separate but intact bodies, such that an interface separating the two fluids is maintained. Plug flow is easier with relatively small ID tubes than larger tubes.
Sixth, a greater destroyer of a stable blood/plasma or saline interface in air trap chambers is air bubbles entrained in the incoming blood flow. These bubbles rise to the surface, passing through any saline or plasma layers because the cross sectional area of these chambers is much larger than the diameter of these bubbles. If bubbles enter a tube small enough that the bubble bridges from wall to wall, frictional forces stop the bubble from rising further, unless convective forces push on the fluid column. The bubble locks the fluid above the bubble from mixing with the fluid below the bubble (as artfully employed by clinical analyzers). Bubbles of less diameter than the tube they are carried in, however, will freely rise. Due to the non-airfoil shape of these bubbles, they drag up blood in their wake into the plasma/saline layers. It takes relatively few bubbles to completely displace essentially all plasma or saline with blood creating a stagnant blood/air interface. As above, this blood is now subject to the stagnation clotting cascade mechanism.
The prior art also includes Cobe machines that mate with a sterile cassette tube set with a non-porous, diaphragmatic sterility

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