Surgery – Respiratory method or device – Means for supplying respiratory gas under positive pressure
Reexamination Certificate
2000-09-29
2003-06-17
Lo, Weilun (Department: 3761)
Surgery
Respiratory method or device
Means for supplying respiratory gas under positive pressure
C128S204180
Reexamination Certificate
active
06578575
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to a method for optimization of ventilator function aiming at that desired therapeutic goals should be reached. The optimization therefor is achieved on the basis of investigated physiological properties of the respiratory system.
2. Description of the Prior Art
Mechanical ventilation may cause lung damage. High pressures lead to damage denoted barotrauma. Ventilation at low pressures and volumes may lead to collapse and re-expansion of lung compartments during expirations and inspirations, respectively. This phenomenon may be denoted Recorex. Recorex may lead to lung trauma because of shear forces between zones of collapsed and aerated lung parenchyma. Studies of lung mechanics and adaptation of mechanical ventilation to lung physiology allow reduction of barotrauma and Recorex. The elastic pressure volume diagram, the P
el
/V curve, can be recorded, for example with an electronically controlled ventilator.
With one or several P
el
/V curves recorded under different circumstances one can judge whether the tidal volume, Vt, as a whole or in-part falls within the pressure/volume range within which barotrauma and Recorex are minimal. Under guidance of this information one may change the pattern of ventilation in such a way that an optimal P/V range is used to ventilate the lungs. One may for example increase the positive end expiratory pressure, PEEP, if the lung volume is to low. An alternative is to increase the frequency or to reduce the time for expiration in relation to the time for inspiration so that the lungs during expiration do not empty to a degree such that elastic forces exerted by the thoracic cage and the lungs are equilibrated. Thereby, it is accomplished that the alveolar pressure at the end of expiration is positive, which is denoted auto-PEEP. This implies that the lung volume is increased so that Recorex is avoided. If one rather would find that the pressure should be diminished during inspiration, one may decrease ventilation and thereby lower the peak airway pressure in order to decrease the risk for barotrauma.
Either an increase of PEEP or a lowering of the peak airway pressure frequently lead to a decrease of pulmonary gas exchange which may be deleterious. A lowering of the oxygenation of the blood can be counteracted by an increase in the fraction of oxygen inspired gas. This is, however, associated with risks. Another effect is that CO
2
-elimination is decreased which implies that carbon dioxide is retained in the body. Lately one has frequently accepted this effect, denoted permissive hypoventilation, however, without improved clinical results. Several physiological mechanisms are dependent on pH and thereby on the partial pressure of CO
2
in arterial blood, PaCO
2
. Accordingly it is important, not only to exercise control over airway pressures but also over gas exchange so that PaCO
2
is maintained within suitable limits.
By recording of a so called single breath test for CO
2
one may estimate to what extent a change of Vt and minute ventilation, Vmin, will lead to a change of CO
2
elimination expressed in ml/min. By measuring, or estimating, how much CO
2
elimination changes at a change of breathing pattern one can during the following breaths estimate to what extent the change will lead to a change of PaCO
2
.
In order to improve the results of mechanical ventilation at grave lung disease both barotrauma and Recorex should be avoided. In order to get around that the gas exchange is unduly affected so that CO
2
retention or hypoxia develops one may better use the Vt. by flushing the connecting tubes, particularly the tracheal tube with unspoiled gas during the later part of expiration. Thereby one may decrease the dead space so that one may decrease Vt without the risk for reduced gas exchange.
The pulmonary exchange of carbon dioxide and oxygen are coupled to one another because for each volume of oxygen taken up and consumed through metabolism a thereto proportional volume of carbon dioxide is produced and eliminated. What has been said above about carbon dioxide is accordingly paralleled by a corresponding phenomenon for oxygen. Technically it is more difficult to make fast and accurate determination of oxygen concentration than of carbon dioxide concentration. Particularly, at high inspired oxygen concentrations it is very difficult to accurately measure oxygen uptake. For these reasons carbon dioxide is below focussed upon in discussions about gas exchange although most aspects are relevant also with respect to oxygen.
Strategies for avoidance of lung damage following mechanical ventilation, and even to protect the lungs against damage, is denoted protective lung ventilation, PLV.
U.S. Pat. No. 4,917,080 desribes a method for controlling a ventilating apparatus. In a first simulator the characteristics of the ventilating apparatus is simulated and in a second simulator patient parameters are simulated. An adjustment to the ventilating apparatus can first be processed in the first simulator and the output is coupled to the second simulator to derive an effect of the adjustment as new patient data from the second simulator. The adjustment may then be switched to the ventilating apparatus or entered manually.
SUMMARY OF THE INVENTION
GB 2 093 218 discloses a respirator comprising a screen and controlled by a microprocessor. On the screen curves reflecting the result of new respiratory conditions can be shown. This simulation can be carried out while ventilation takes place with pre-established parameters. If the new values are satisfactory, the operator can request the change in the ventilation of the patient.
One objective of the present invention concerns new principles for determination of how PLV should be performed on the basis of the physiological characteristics of the individual patient and in consideration of the demands of an adequate gas exchange. The invention is founded on measurements of respiratory mechanics and also of gas exchange. The measurements are followed by a mathematical characterisation of these physiological characteristics. Thereafter a computer performs an analysis of the mode of ventilation by the ventilator with the goal that the therapeutic goal defined by the responsible operator (doctor or therapist) is reached. In a typical case the goal is defined as a combination of PLV and adequate gas exchange.
Methods for studies of the mechanics of the respiratory system are previously known, for example through the Swedish patent C2 506521 and Repiratory Mechanics During Mechanical Ventilation in Health and Disease,” C. Svantesson, Thesis, Department of Clinical Physiology, Lund University (1997), ISBN 91-682-2766-9 and “A Single Computer-Controlled Mechanical Insufflation Allows Determination of the Pressure-Volume Relationship of the Respiratory System,” Svantesson et al., Journal of Clinial Monitoring and Computing (1999). Accordingly, one may use a computer-controlled ventilator to study the mechanics of the respiratory system. A pressure and volume range extending beyond the range of the tidal volume at the current setting of the ventilator, Vt, may be studied. A modification of one expiration by prolongation of the time for expiration and/or by reducing PEEP allows a volume range below the Vt to be studied. A volume range above the Vt may be studied by increasing of the volume of gas that is insufflated during the inspiration following the modified expiration.
The three segments of the sigmoid Pel/V curve are described by an equation in which the elastic recoil pressure is related to the volume, V. The reference volume for V is the lowest observed volume during the measurement. The intermediate segment which begins at the volume Vlip and the pressure Plip is linear and has, accordingly, a constant slope corresponding to a value of compliance denoted Clin. The lower and the upper segments are non-linear. The slopes of these segments (compliance) approach asymptotically zero when the curves are extrapolated towards a low and
Lo Weilun
Mitchell Teena
Schiff & Hardin & Waite
Siemens Elema AB
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