Biomedical subjects
Publications and source records attributed to E Delavault.
We describe a servo-controlled piston pump driven by a stepping motor. The analogue controller is a single non-linear second-order feedback loop with adjustable speed and acceleration limits. This system, designed to simulate slowly-moving, active and non-linear systems, can be used as a low (0-6.5 litre s-1) flow volume generator in calibration procedures.
A computerized system for occlusion pressure measurement during a rebreathing test is described. The system is implemented on an Apple II microcomputer. A set of programs allows calibration, data acquisition during the experiment, and fast automatic processing of the various parameters of ventilation and occlusion pressure versus end tidal PCO2. The use of a limited memory system is made possible by an electronic interface which allows preprocessing of the mouth pressure. In addition, that device drives a new simple electromagnetic valve with low flow resistance and dead space.
The sensitivity of the respiratory center following a single 0.3 mg X kg-1 iv dose of droperidol was determined in eight healthy volunteers by using carbon dioxide (CO2) rebreathing and mouth occlusion pressure measurements (P0.1). There were no significant differences in the mean slopes of the minute ventilation/partial pressure of CO2 (VE/PCO2) and log P0.1/PCO2 relationships between control measurements and those made 30, 60, 90, 150, and 240 min after droperidol administration. However, individual variations in response were present, one subject showing significant (approximately 50%) depression of respiratory drive.
The usual impedance measurement by Fourier analysis leads to undetectable errors when applied to nonlinear fluid systems where a nonlinear pressure drop p = K X V2 adds to the linear pressure-flow relationship, but processing series of impedance data with various input levels allows the measurement of such systems. This new method has been tested on simulations, nonlinear resistances, and intubation tubings. It gives accurate estimates of the nonlinear coefficient K together with the true linear impedance of the measured system. This way, the usual respiratory impedance measurement may be applied to intubated patients, as the respiratory impedance of the subject can be separated from the nonlinear behavior of the endotracheal tube or cannula.
The respiratory effects of premedication with i.v. injection of diazepam have been assessed in 10 healthy patients awaiting minor operative procedures. Measurements were recorded before and 60 min after administration of diazepam 0.14 mg kg-1. Mouth occlusion pressure (P 0.1) was used as an index of neuromuscular inspiratory drive. Minute-ventilation (VE), respiratory frequency (f) and mean inspiratory flow rate (VT/TI) were significantly reduced after diazepam. During carbon dioxide rebreathing the slopes of VE, f, VT/TI and P 0.1 with PACO2, were significantly reduced. These results confirm that i.v. diazepam produces significant respiratory depression in healthy subjects. We conclude that diazepam used under clinical conditions depresses the respiratory centre.
The "oclusion pressure" technique is widely used to test the response of respiratory centers to CO2. The graphic treatment of the test necessitates a fast recording of the mouth pressure signal, thus using a great deal of paper and requiring a tedious interpretation. The device described here controls an electromagnetic valve closing it during expiration and opening it after a given time delay, following the onset of inspiration. During that time only, the mouth pressure signal goes through an analog switch, the highest signal value is then equal to the occlusion pressure. This device allows the use of a slow recorder without loss of information. In addition, it has the advantage of generating very short respiratory occlusions, thus sparing the subject being tested from interferences in subsequent respiratory cycles. It can be used as a pretreatment unit to be associated with a microprocessor.
The measurement of respiratory impedance by means of the forced oscillations technique depends on the frequency response of various measurement devices: pressure transducers, flowmeter, filters, etc. In the range of frequencies where measurements were done (0--100 Hz) it is difficult to have the same response (amplitude and phase) for mouth pressure and flow transducers. It is possible to identify the relative frequency response of both devices. This can be done by comparison of measurements made on a well-defined mechanical system with a theoretical model representing this system. The method also avoids separate frequency calibrations of pressure and flow. Its validity depends only on the accuracy of the model of impedance of the standard system used.
We have studied the effect of spontaneous breathing on the measurement of total respiratory impedance by the oscillation method, that is using forced random pressure input. Free breathing of the subject induces parasite signals on the pressure and flow measured at the mouth. These induced random signals are correlated through a loudspeaker box with side tubing and flowmeter impedance. The respiratory impedance of the subject, calculated by the cross-power spectrum method, is shown to be systematically biased. This bias is obvious in a calculation model, experimentally verified by an analogue electrical experiment. The validity of the method depends on the respiratory noise level which affects the correlation between the forced pressure input and related flow output. The impedance of the apparatus should thus be minimized in order to decrease the respiratory noise. In this manner, the bias would be shifted to lower frequencies and the accuracy of the measurement near the resonant frequency of the respiratory system would be improved.
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