Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2000-12-15
2002-09-24
Walberg, Teresa (Department: 3742)
Surgery
Diagnostic testing
Cardiovascular
C600S549000, C600S561000, C600S481000, C600S505000
Reexamination Certificate
active
06454720
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the measurement of one or more physical parameters using a medical probe.
STATE OF PRIOR ART
Medical probes or catheters are used to dispense a fluid inside the body of a patient and/or to measure certain physical parameters inside the body. The parameters may be parameters linked to dispensing fluid, such as the flow-rate or chemical composition, or simply environmental parameters such as pressure, temperature, humidity or pH. The fluid may be a liquid, for example, as when taking intravascular or urinary measurements, or gaseous as when taking pulmonary measurements.
For measuring distal pressure, i.e. at the end of the probe inside the body, the standard method consists in transferring the measurement to a proximal location, i.e. to the outside of the probe in order to measure the pressure with a standard electronic sensor that does not have to be miniaturised. Two main techniques can be used for the interface that enables the pressure to be transmitted: using a liquid mandrel or a gaseous mandrel. These techniques create problems, however, in terms of reliability, user friendliness and accuracy. The direct use of gas as an interface causes damping of the measured signal and is unreliable as the patient's secretions can block the pressure reading. The liquid-mandrel technique has the advantage of being based on the use of an incompressible interface. It can be used to measure liquids but also to measure gas pressures when a suitable interface is used, as described in U.S. Pat. No. 4,813,431. There is, however, a risk of air bubbles appearing in the column and is therefore potentially dangerous for the patient. In this event steps are required to remove the bubbles. Furthermore, the high density of water in the liquid column creates a difference between the pressure read and the internal pressure that is dependent on the variation in altitude between the two points. International patent 95/22,280 describes a method using laser measurements to estimate this difference but the technique requires a device that is expensive and far from practical to use.
Probes and catheters are used for different applications in the medical sector but on the whole the requirements and technical problems are identical. A particularly significant example is that of mechanical ventilation.
The mechanical ventilation of a patient in intensive care may continue over several weeks, even several months. Specialist s often use “pressurised” ventilation techniques in which the ventilator must reach a level of pressure (controlled pressure) or facilitate patients' inspiration (assisted pressure). The respirator is guided by the patient's reactions that it receives via flow-rate- or pressure-sensors. The aggressiveness of the ventilation techniques is preferably limited in order to prevent the patients' state of health from worsening, accelerate their recovery and gradually encourage patients to breath on their own again. When patients still have or have regained their breathing reflexes they trigger gas to be dispensed from the respirator. It is therefore essential for sensors, particularly pressure sensors, to be sensitive and reliable in order for mechanical ventilation to be suited to patients' actual requirements.
At present the pressure sensors are located on the external circuit that connects the patient to the mechanical respirator. The signals emitted by the sensors do not reflect the actual conditions in vivo due to the difference caused by the connecting parts, particularly the intubation probe. This phenomenon is particularly noticeable in the phase during which the probe is removed from the patient when the flow rates are immediately increased. The gaseous mandrel technique is marketed by all the probe producers but it is too unreliable for use in controlling the respirator directly.
Direct, reliable measurements of the pressure inside the respiratory tracts result in significant progress for the safety of patients and open the way to developing ventilation techniques with improved performance characteristics that will reduce the average stay in intensive care units and will therefore have a positive effect on hospital costs. The usefulness of such techniques is increased with the new, high frequency ventilation methods used on adults and particularly young children, for whom no accurate routine monitoring means are currently available.
Another example concerns urodynamics. Certain urology examinations require liquid to be injected into the patient's bladder and the changes in pressure to be monitored. At present catheters fitted with instruments that include electronic-pressure sensors, which are also very fragile, are expensive. They are therefore mainly reserved for research applications. Consequently, measuring the physiological liquid-supply pressure is preferred. The technique is, however, difficult to implement and unreliable due to the problems caused by removing bubbles and the pressure difference resulting from the height of the column of injected liquid.
Another example concerns the probes used in the cardiovascular sector. The arterial pressure is measured outside the body at the proximal end of a catheter filled with pressurised physiological liquid. This technique requires the use of a pocket of pressurised serum and a control valve that take up space around the patient unnecessarily. This application illustrates the use of a catheter containing a pressure sensor but that does not dispense a fluid.
A technique is therefore required, particularly for measuring pressure, that integrates the electronic sensor directly at the end of the probe or catheter in the actual place where the measurement is necessary. The requirement extends to measuring several parameters at the end of a catheter.
For reasons of hygiene the probes are preferably used once only. Ideally they should therefore be produced for a moderate cost price. They are produced using extrusion techniques that enable the cost objectives to be reached, even for medium-sized production runs. The techniques are restricting in terms of materials and shapes as they require the probe to be symmetrical around the longitudinal axis. Techniques are known that enable probes to be produced with an end in a different material from that of the probe body (see for example U.S. Pat. No. 3,890,976 and international patent 94/00174). Techniques are known for measuring pressure at the end of a probe using an electronic sensor that is implanted directly in the probe and that is connected to the outside of the patient's body by leads. They are used to measure intratracheal pressure (see international patent 94/22518) or arterial pressure (see international patent 97/17888). Multiplexing techniques have been described to enable a single pair of leads to be used (see U.S. Pat. No. 4,432,372). These techniques, however, lead to significant increase in cost that arises from the sensor being fastened inside the probe and the electrical connections that are not directly compatible with the extrusion techniques. Their use is therefore limited to only a few applications.
In general, and particularly concerning pressure, the production of microsensors is dependent on the microtechnology that is currently being developed through the progress made in the microelectronics industry. The techniques that are being developed today enable mechanical functions to be integrated into electronic components and miniature electronic sensors to be designed. Compared to standard mechanical sensors the new sensors are more sensitive, more reliable and multi-purpose because they are capable of being connected to a signal processing unit.
For pressure sensors in particular, various techniques for measuring the distortion of a membrane have been proposed for use in these systems. Piezoresistive techniques are used to measure the distortion of a piezoresistive part placed on the surface of a membrane. The pressure is determined by measuring variations in resistance. Today this tech
Clerc Jean-Frederic
Perruchot François
Renard Stephane
Commissariat a l'Energie Atomique
Dahbour Fadi H.
Krebs Robert E.
Thelen Reid & Priest LLP
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