Surgery – Respiratory method or device – Means for supplying respiratory gas under positive pressure
Patent
1993-10-18
1996-07-30
Burr, Edgar S.
Surgery
Respiratory method or device
Means for supplying respiratory gas under positive pressure
12820421, 12820426, 12820524, 417 42, A61M 1600
Patent
active
055402224
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The present invention relates to piston-based ventilators for ventilating the lungs of patients.
BACKGROUND OF THE INVENTION
Commercially-available piston-based ventilators operate to deliver a preset tidal volume at a preset pattern and rate of flow. Several models are available which satisfactorily perform this function. With such ventilators, however, the patient cannot independently influence the flow rate or volume once a cycle is triggered. In certain applications, it is more desirable to utilize ventilators with which the controlled variable is pressure, as opposed to volume or flow, output. Here, the control system of the ventilator is designed to alter pressure according to specified functions. Although this approach does not guarantee a specific tidal volume, it does allow the patient the freedom to influence his/her breathing pattern during the supported breath. This results in greater comfort.
Prior art exists for piston-based ventilator designs which are intended to deliver pressure according to specified functions (e.g. U.S. Pat. No. 4,448,192 to Stawitcke, Younes, J. Applied Physiology 62:2491-2499, 1987). In all these prior art designs, pressure is controlled according to a servo feedback mechanism in which the actual pressure is compared with the desired pressure, an error signal is generated, and this error signal is used to drive the motor which, in turn, alters the pressure in the piston, thereby minimizing the error signal and maintaining the actual pressure as close as possible to the desired function. This approach, however, has two major disadvantages:
1. By definition, an error (difference between actual and desired pressure) must exist in order for the motor to respond. This error theoretically can be minimized by increasing the gain of the error signal. However, because in ventilated patients, pressure in the tubing is invariably highly variable, and because of obligate delays between the generation or change in error signal and the actual change in pressure, a high error gain would be associated with wide fluctuations in the command signal to the motor and hence oscillations. Thus, if the error gain is set such that a difference in pressure of 1 cm H.sub.2 O between actual and desired pressure results in a piston pressure output of 30 cm H.sub.2 O, a relatively small variation in the actual pressure signal (e.g. .+-.1 cm H.sub.2 O) would initiate oscillations of considerable amplitudes. This consideration makes it necessary to use relatively low error gain or substantial filtering techniques, both of which result in poor responsiveness of the device. Thus, if a patient pulls harder, which tends to reduce system pressure, the compensation by the device will be slow and incomplete, thereby causing actual pressure to deviate substantially from desired pressure.
2. This type of control makes it progressively more difficult to regulate adequately pressure at points more distal from the piston and, hence, at points closer to the patient. This is because all tubing used to connect piston to patient's airway displays substantial compliance and resistance. There are, therefore, obligate delays in pressure transmission from piston to patient. The farther out (from the piston) the point used for pressure feedback (used to generate the error signal) is located, the greater the delay between a change in piston pressure in response to a change in error signal and those changes being detected at the site used for pressure feedback. This delay would tend to result in the motor overcompensating with a pressure overshoot which, once the overshoot reaches the pressure monitoring site, will unnecessarily reduce the error signal resulting in an undershoot. A tendency for oscillation is again created which is greater the closer the pressure feedback site is to the patient's airway, and also greater the higher the compliance and resistance of the tubing between piston and pressure monitoring site. To offset this instability, the response must be damped with more damping required
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patent: 3918447 (1975-11-01), Inkster et al.
patent: 4336590 (1982-06-01), Jacq et al.
patent: 4587967 (1986-05-01), Chu et al.
patent: 4617637 (1986-10-01), Chu et al.
patent: 5107830 (1992-04-01), Younes
patent: 5265594 (1993-11-01), Olsson et al.
patent: 5365922 (1994-11-01), Raemer
Burr Edgar S.
Deane, Jr. William J.
University of Manitoba
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