In the practice of medicine there are many instances in which accurate measurement of patient blood pressure is required. In some instances, it is necessary to obtain accurate blood pressure measurements from particular locations within a patient's body, or “in-vivo.” Among those instances in which it is necessary to obtain accurate in-vivo blood pressure measurements are procedures involving the use of an in-vivo balloon or in-vivo balloon-like construct. (In the interest of brevity the term “balloon” will be used throughout this description to denote balloons and balloon-like constructs.)
One type of procedure that uses an in-vivo balloon is intra-aortic balloon (IAB) therapy. By way of illustration, further background will be provided in the context of IAB therapy.
Intra-aortic balloon pump therapy is frequently prescribed for patients who have suffered a heart attack or some other form of heart failure. In such therapy, a thin balloon is inserted through an artery into the patient's aorta. The balloon is connected through a series of tubes to a complex drive apparatus which causes the balloon to inflate and deflate repeatedly in time with the patient's heartbeat, thereby removing some of the load from the heart and increasing blood supply to the heart muscle during the therapy period.
The inflation/deflation apparatus supplies positive pressure for expanding the balloon during an inflation cycle and negative pressure for contracting the balloon during a deflation cycle. An IAB apparatus is shown schematically in FIG. 1. In the FIG. 1 apparatus, an intra-aortic balloon (IAB) 10 is surgically inserted into a patient's aorta and is connected through a catheter 12 having a small diameter lumen, a connector 11, and an extender 14 having a relatively large diameter lumen to a pneumatic isolator 18 divided by a pliant membrane 20 into a primary side 22 and a secondary side 24. Accordingly, all elements to the left of membrane 20 in FIG. 1 are referred to as being on the “primary side” of the FIG. 1 apparatus, and all elements to the right of membrane 20 in FIG. 1 are referred to as being on the “secondary side” of the FIG. 1 apparatus.
The entire volume between membrane 20 and balloon 10 is completely filled with a gas, such as helium, supplied by a gas source 26. The gas source is coupled to the secondary side of the isolator via a fill/purge line 15. A gas pressure sensor 25 is provided for monitoring the gas pressure within the secondary side of the IAB apparatus. For purposes of discussion, the gas present within the secondary side of the IAB system is referred to as the “shuttle gas.” Accordingly, pressure sensor 25 is the “shuttle gas pressure sensor” and it measures “shuttle gas pressure.”
A positive pressure source 28 is connected through a solenoid valve 30 to the input or primary side 22 of isolator 18. Similarly, a negative pressure source 32 is connected through a solenoid valve 34 to the input or primary side 22 of isolator 18. The primary side 22 of isolator 18 is also connected through a solenoid valve 36 to a vent or exhaust port 38. In such systems, the isolator, gas source, negative and positive pressure sources, vent port and their associated valves together comprise a reusable drive unit, and the extender, catheter and balloon are disposable so as to accommodate sterility concerns.
During an inflation cycle, solenoid valve 30 is opened to permit positive pressure from positive pressure source 28 to enter primary side 22 of isolator 18. This positive pressure causes membrane 20 to move toward secondary side 24, thereby forcing the shuttle gas in the secondary side to travel toward and inflate balloon 10. For deflation, solenoid valve 30 is closed and solenoid valve 36 is opened briefly to vent the gas from primary side 22 to atmosphere, after which valve 36 is closed. Solenoid valve 34 is then opened, whereupon negative pressure source 32 creates a negative pressure on the primary side 22 of isolator 18. This negative pressure pulls membrane 20 toward primary side 22, whereby the shuttle gas is drawn out from the balloon.
Maximum patient benefit is achieved when the timing of IAB inflation and deflation is correct. To meet this requirement, the patient's blood pressure waveform must be accurately monitored. The monitored signal is then analyzed for key cardiac events.
Accordingly, the IAB system as shown in FIG. 1 includes a pressure sensor 40 proximal to the front end of the balloon for the purpose of monitoring a patient's blood pressure during IAB therapy. Sensor 40 can be a fiber optic sensor that measures pressure by observing how light is reflected from a diaphragm which moves in response to pressure changes. The optical signal generated by sensor 40 is passed back to a monitor outside of the patient's body via a fiber optic line 13 that passes through the balloon 10, catheter 12 and connector 11 (connector 11 being a pneumatic and fiber optic connector suitable for accommodating both a fiber connection and a pneumatic connection between the catheter and extender). The optical “pressure” signal transmitted through line 13 is converted into an electrical signal by converter module 17.