It has been found that various chemical compounds, such as, for example, nitric oxide (NO), administered during a patient inspiratory phase may provide beneficial effects.
For example, NO presents some lung vasodilator properties that may be helpful for respiratory distress conditions such as respiratory distress syndrome of newborn.
Apparatus for delivering such gaseous chemical compounds have therefore been designed to deliver the compounds during the patient's inspiratory phase.
One such apparatus is described in Canadian Patent Application No. 2,106,696, filed on Sep. 22, 1993 and published on Mar. 25, 1994 and naming Robert Briand and Marie-Hélène Renaudin as inventors. In this document, Briand et al. describe an apparatus for delivering controlled doses of NO to the respiratory system of the patient without conventional pre-mixing of the NO with oxygen supplied by a ventilator device. The apparatus therefore includes means for detecting the beginning of a patient inspiratory phase and to open an electromagnetic valve for a predetermined duration to supply a controlled dose of NO. The duration and the pressure of the NO supplied dose is adjusted so as to obtain the desired NO concentration with respect to the average inhalation volume of the patient. The NO dose supplied is therefore not directly related to the inhalation volume of the patient. Of course, there is no NO injection during the expiration phase.
A major drawback of the apparatus described by Briand et al. is the automatic opening of the electromagnetic valve for a predetermined duration each time the beginning of an inhalation phase is detected. Indeed, if the patient repetitively draws short breaths, harm may be caused by the high concentration of NO injected to the patient.
In an article entitled: “Comparison of two administration techniques on inhaled nitric oxide on nitrogen dioxide production”, published in Canadian journal of Anaesthesiology 1995, Vol. 42: 10, pages 922–927, Dubé et al. describe an injection system for delivering NO during inspiratory phase. In this injection system, an electronic circuit detects the beginning and the end of each inspiration by processing a flow signal supplied by a ventilator. At the beginning of the inspiratory phase, the electronic circuit opens a solenoid valve and NO is injected into the respiratory line. At the end of the inspiratory phase, the electronic circuit closes the solenoid valve and the injection of NO is stopped.
FIG. 1 of the appended drawings is a graph of the inspiratory gas flow 20 vs time for a conventional ventilator when the ventilator is in a first mode. When it is in this mode, the flow of inspiratory gas is constantly delivered for a predetermined duration (inspiratory phase 22) and the patient then expires (expiratory phase 24). In the injection system proposed by Dubé et al., when the gas flow reaches a predetermined threshold level 26, a solenoid valve is open, delivering NO to the patient. The line 28 illustrates the injected flow of NO in the inspiration circuit over time. It is to be noted that the scale is different for the flow of inspiratory gas 20 and the flow 28 of NO. Indeed, line 28 illustrating the flow of NO is shown scaled up for illustrative purposes.
Since the solenoid valve used by Dubé et al. is of the type fully open/fully closed, the flow 28 of NO is constant when the valve is open. As can be seen from FIG. 2, the concentration 29 of NO is essentially constant over time during the inspiratory phases. When the inspiratory gas flow 20 falls below the threshold level 26, the solenoid valve is closed, stopping the flow of NO.
FIG. 3 is a graph of the inspiratory gas flow 30 vs time for a conventional ventilator when the ventilator is in a second ventilating mode. When it is in this mode, the flow of gas is not constantly delivered for a predetermined duration but follows a particular curve during the inspiratory phase 32 and the patient then expires (expiratory phase 34). In the injection system proposed by Dubé et al., when the gas flow reaches a predetermined threshold level 36, the solenoid valve is open delivering NO to the patient. The line 38 illustrates the flow of NO over time. Again, it is to be noted that the scale is different for the flow of inspiratory gas and the flow 38 of NO. Indeed, line 38 illustrating the flow of NO is shown scaled up for illustrative purposes.
Since the solenoid valve used by Dubé et al. is of the type fully open/fully closed, the flow of NO is constant when the valve is open. As can be seen from FIG. 4, the concentration of NO (line 39) is not constant over time during the inspiratory phases but varies inversely with the flow of gas since the flow of NO is constant. When the inspiratory Gas flow 30 falls below the threshold level 36, the solenoid valve is closed.
A drawback of the injection system of Dubé et al. is that, in certain cases, the NO concentration is not constant during the inspiratory phase.
Canadian patent application No. 2,133,516 filed on Oct. 3rd, 1994 and naming Bathe et al. as inventors describes a nitric oxide (NO) delivery system monitoring the inspiratory gas flow of a patient and controlling a proportional valve to allow a calculated flow of NO to enter the inspiratory gas flow. The delivery system calculates the flow of NO in order to maintain a constant, user programmable, NO concentration in the inspiratory gas.
A drawback of the delivery system of Bathe et al. is that, while the delivery system may be programmed so that the concentration of NO in the inspiratory gas flow is constant, there are no provisions to modify the concentration of the NO during a particular inspiratory phase, or to program the variation of the concentration of NO over a number of successive inspiratory phases in view of gradually increasing or decreasing the concentration of NO supplied to the patient.