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United States Patent m
 APPARATUS FOR DETERMINING THE
VOLUME OF A BELLOWS RESERVOIR FOR
MEDICATION IN AN IMPLANTABLE
 Inventor: Goeran Palmskog, Jaerfaella, Sweden
 Assignee: Siemens Elema AB, Soma, Sweden
 Appl. No.: 231,311
 Filed: Apr. 22,1994
 Foreign Application Priority Data
Apr. 22, 1993 [SE] Sweden 9310344
 Int. CI.6 A61M 11/00; A61M 1/00;
 U.S. CI 604/891.1; 604/93; 604/118;
128/DIG. 13; 128/903
 Field of Search 604/891.1, 93,
604/151, 153, 65-67, 246, 118; 128/DIG. 12,
DIG. 13, 903
 References Cited
U.S. PATENT DOCUMENTS
4,360,019 11/1982 Portner et al. .
4,373,527 2/1983 Fischell 604/891.1
4,395,259 7/1983 Prestele et al 604/67
4,443,218 4/1984 DeCant, Jr. et al 604/67
FOREIGN PATENT DOCUMENTS WO89/01795 4/1993 WIPO.
Primary Examiner—John D. Yasko
Assistant Examiner—Adam J. Cermak
Attorney, Agent, or Firm—Hill, Steadman & Simpson
In a method and apparatus for determining the volume of a bellows reservoir for medication in an infusion system which is implantable in a patient, the pressure in the reservoir is measured and the corresponding volume is determined from a predetermined relationship between pressure and volume. A pressure gauge is disposed for measuring the pressure in the bellows reservoir, and a signal corresponding to the measured pressure is supplied to a memory unit wherein a corresponding volume is determined from the measured pressure value by means of the predetermined relationship between pressure and volume being stored in the memory unit.
6 Claims, 2 Drawing Sheets
U.S. Patent Apr. 16,1996 sheet i of 2 5,507,737
U.S. Patent Apr. 16,1996 Sheet 2 of 2 5,507,737
APPARATUS FOR DETERMINING THE VOLUME OF A BELLOWS RESERVOIR FOR MEDICATION IN AN IMPLANTABLE INFUSION SYSTEM
BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention is directed to a method and to an 10
apparatus for determining the volume of a bellows reservoir for medication in an implantable infusion system.
2. Description of the Prior Art
In an implantable infusion system, wherein medication to be dispensed is contained in a reservoir, there is a need to 15 continuously determine the volume of medication in the reservoir. This is for at least two reasons, first, it must be ensured that the medication volume in the reservoir does not become too low before a medication refilling procedure is undertaken, otherwise air, which is always present in the reservoir, would then be pumped into the patient. Second, because the pressure in the bellows reservoir decreases as the medication volume becomes less than 2-3 ml, if the medication volume reaches such a low level the pump must operate with a larger pressure difference, resulting in reduced medication volume per pump stroke, leading to cavitation in the pump. Such cavitation is detrimental to medications like insulin, whose molecules are "fragile."
Moreover, it is desirable to be able to determine the 30 volume of the medication in the bellows reservoir during a filling procedure, in order to avoid overfilling the reservoir. The bellows reservoir is contained in the implantable unit in a chamber which creates appropriate pressure conditions for siphoning medication from a refilling apparatus which is 35 temporarily placed in fluid communication with the implanted unit during the refilling procedure. If the bellows reservoir is overfilled to the extent that the bellows wall comes into contact with the wall of the chamber, the conditions for siphoning are no longer present. 40
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and an apparatus for continuously determining the volume of a bellows reservoir for medication during operation of an 45 infusion system, as well as during emptying and filling of the reservoir.
The above object is achieved in a method for determining the volume of a bellows reservoir for medication in an 5Q implantable infusion system by measuring the pressure which is exclusively present in the reservoir, and determining the corresponding volume from a predetermined relationship between pressure and volume.
The above object is also achieved in an apparatus which 55 includes a pressure gauge disposed for measuring the pressure exclusively in the reservoir, the pressure gauge generating an output signal corresponding to the measured pressure, which is supplied to a memory unit in which a predetermined relationship between pressure and volume is go stored. Based on the stored relationship, the memory unit generates an output representing the volume corresponding to the measured pressure.
As used herein, the references to measuring the pressure which is exclusively present in the reservoir are meant to 65 exclude measurements, for example, of the pressure prevailing generally within the implantable housing. Measuring the
pressure which is exclusively present in the reservoir means undertaking a measurement of the pressure which exists within the reservoir itself, rather than a measurement of the pressure prevailing in the "environment" of the reservoir.
In a further embodiment of the method, the predetermined relationship between pressure and volume is determined by simultaneous measurement of the pressure versus volume curves which arise during filling and emptying of the reservoir. Using these curves, the relationship of volume and pressure can then be determined.
The pressure gauge is preferably disposed in the filler port of the reservoir, and may be a piezoelectric gauge.
The memory unit in which the predetermined relationship between volume and pressure is stored, is preferably disposed in an external programmer, outside of the patient and separate from the implanted unit, which communicates with the implanted unit, at least to the extent of receiving output signals from the pressure gauge, by telemetry.
DESCRIPTION OF THE DRAWINGS
FIG. 1 shows typical pressure-volume curves which arise when emptying residual insulin from, and filling fresh insulin into, an implantable insulin infusion system.
FIG. 2 is a side sectional view through an implantable unit of an infusion system constructed in accordance with the principles of the present invention, the infusion system also including a schematically-indicated external programmer.
DESCRIPTION OF THE PREFERRED
FIG. 1 shows typical pressure-volume curves which arise when emptying residual insulin from, and filling fresh insulin into, an implantable insulin infusion system, of the type shown in FIG. 2. The pressure can be recorded by means of a pressure gauge introduced with a cannula through the septum 6 to about the same level as the pressure gauge 18 of the apparatus according to the invention, described below, during emptying and filling of carefully determined volumes of insulin. This calibration procedure has been verified for fifteen reservoirs.
The relationship between pressure and volume can obviously vary for different types of reservoirs. The example illustrated in FIG. 1 relates to a bellows reservoir having a volume of approximately 24 ml.
As can be seen in FIG. 1, the pressure curve which arises when emptying the reservoir has a plateau at about 400 mbar. The level for this plateau is dependent on the temperature at which the emptying takes place. For an infusion system implanted for animal tests, the temperature was 38° C, in which case the plateau will be situated at about 700 mbar.
When 2-3 ml of insulin remain in the reservoir, the pressure within the reservoir begins to decrease below the plateau pressure, and to empty the last 2-3 ml of insulin from the reservoir the pressure must be lowered to less than 100 mbar. This appears to be the case for all examined bellows reservoirs of the type in question. At a pressure of 50 mbar, there is still 0.5-1 ml of insulin remaining in the reservoir. A pressure gauge which has a pressure threshold of approximately 600 mbar will, for normal body temperature, identify when the volume of the residual insulin drops to 2-3 ml, which is suitable in practice. If the volume of residual insulin drops below this level, the pump must