WO1980000917A1

WO1980000917A1 - Servo-control of arterial oxygen tension

Info

Publication number: WO1980000917A1
Application number: PCT/GB1979/000186
Authority: WO
Inventors: P Collins; N Levy
Original assignee: Department Paediatrics Med; P Collins; N Levy
Priority date: 1978-11-13
Filing date: 1979-11-09
Publication date: 1980-05-15
Also published as: EP0020557A1; NO793677L

Abstract

An apparatus for controlling the oxygen concentration in a respirable mixture of air and oxygen issuing from a mixer (6) for delivery to a patient (4) in a chamber such as an incubator (2) includes a device, for example, an intravascular electrode for generating a signal corresponding to the oxygen concentration in the blood of the patient (4). The signal is received by a control unit (12) having a shaft (14) which enters the mixer (6), the angular position of which shaft (14) controls the percentage of oxygen in the gas mixture issuing from the mixer.

Description

SERVO - CONTROL OF ARTERIAL OXYGEN TENSION

This invention relates to apparatus for automatically controlling the concentration of oxygen in the output from gas-mixing apparatus, so as to control the arterial oxygen tension in patients breathing the gas.

The control of arterial oxygen tension is particularly important in the case of sick, preterm babies in order to avoid the Scylla of hypoxia and the Charybdis of hyperoxia.

When the arterial oxygen tension is continuously monitored in sick, preterm babies, marked fluctuations in it can be seen to occur in short periods, although intermittent analysis of the blood gas may indicate a steady state of the oxygen tension. Such fluctuations may occur spontaneously, or as a result of routine nursing or other procedures. Changes in oxygen tension, together with alterations in heart rate and blood pressure may be an additional cause of perinatal morbidity. The correction of these fluctuations by the manual adjustment of the inspired oxygen concentration requires constant observation and consumes both time and manpower.

The present invention aims at providing apparatus which automatically keeps the arterial oxygen tension of patients within chosen limits for a greater proportion of time than with manual control.

According to the present invention, an apparatus for controlling the oxygen concentration in a respirable mixture of air and oxygen issuing from a gas mixer for delivery to a patient, comprises a device for generating a signal corresponding to the oxygen concentration in the blood of the patient connected to the device, an control unit adapted to receive the signal and to alter the oxygen concentration in the respirabie gas issuing from the mixer in one or more discrete amounts when the signal exceeds a preselected limit.

The present invention will now be described by way of example with reference to the accompanying drawing, in which:

Figure 1 is a diagrammatic view of the apparatus of the present invention used for monitoring and controlling the arterial oxygen tension in a preterm baby, and

Figure 2 is the curve of current (I) against voltage (V) obtained in the oxygen sensor included in the apparatus shown in Figure 1.

The apparatus shown in Figure 1 includes a chamber or incubator 2 containing the patient 4. The incubator is intended to be supplied under pressure with a mixture of air and oxygen issuing from a mixer 6 adapted to receive separate supplies of air and oxygen through inlets 8. The incubator 2 is vented to atmosphere through the normal gaps in its construction, because it is not designed to be gas-tight.

A control unit 12 is coupled to the mixer through a shaft 14, the angular position of which controls the percentage of oxygen in the gas mixture passing to the incurator 2. The shaft 14 is connected to a servomotor so that the shaft is able to be driven through steps, with the size of each step (or angular rotation of shaft 14) being such that it results in a 5% change in the oxygen concentration.

The control unit 12 is intended to be responsive to the arterial oxygen tension in the patient 4, as sampled either by means of an intravascular electrode, for example, an indwelling umbilical catheter-tip electrode, or transcutaneously using a heated skin-surface electrode. The electrode is connected to any oxygen meter included in the control unit. The reading of meter is checked at least every four hours by a formal blood gas analysis, and adjustment made as necessary.

The control unit 12 has knobs (not shown) or other devices by means of which the upper and lower limits for the arterial oxygen tension can be pre-set. In one series of experiments, the lower limit was set at 7.3kPa (55mm Hg) and the upper limit at 10.7kPa (80mm Hg).

The oxygen sensor was operated in accordance with the position of the flat, constant-current, portion of the characteristic curve, as shown in Figure 2. Over a certain voltage range, the current remains constant irrespective of voltage variations, and was altered only in proportion to detected changes in oxygen concentration in the blood being analysed. When the measured oxygen concentration fell outside the pre-set range, the sensor caused the shaft 14 to be rotated in the appropriate direction and by such an amount that the oxygen concentration was altered by 5% in the direction causing the arterial oxygen tension to move back towards the nearer limit.

The apparatus was intended to sample the oxygen concentration at intervals of one minute. If the reading taken one minute after an oxygen change indicated that the arterial oxygen tension was still outside limits, a further change of 5% and in the same direction was made. This process was repeated a third time, resulting in an overall change of 15% in the oxygen concentration supplied to the patient. As soon as the third successive change was made, and irrespective of its success in bringing the oxygen concentration back within limits, an alarm associated with control unit 12 was activated. This was in case manual intervention was necessary to prevent either hypoxia or hyperoxia.

The unit 12 also included means for providing a pre-set concentration of oxygen in the gas issuing from πrixer 6 in the event of a mains failure. Means were also provided by which any desired inspired oxygen concentration could be produced by manually controlling the mixer.

With the above sampling rates and sizes of changes (5%) in the oxygen concentration, it has been found that the number of episodes during which the arterial oxygen tension was outside the above chosen limits was more on servo-control than on manual control, but the proportion of time spent outside these limits was less with servo-control than with manual control. Overall, the arterial oxygen tension was outside the set limits for 15.2% of the time when on servo-control, and for 31.1% of the time when on manual control. Fran the above information it will be appreciated that the optimum sampling intervals, and the size of the change in the inspired oxygen concentration have yet to be determined. Ideally there should be fewer excursions outside the set limits with servo-control than with manual control, as well as ensuring that the total time spent outside these limits was significantly less with servo-control than with manual control.

Although the apparatus as used included an umbilical arterial catheter-tip electrode, with a matched oxygen meter, it is within the purview of the present invention to use a transcutaneous electrode with a similarly-matched oxygen meter to achieve the same desired effect. Any continuously-reading sensor of arterial oxygen tension may be used in the equipment. The oxygen -enriched air which is regulated by the control unit may be supplied to the patient directly into an incubator, head-box or face mask, or indirectly, as by means of a mechanical ventilator.

Claims

1. An apparatus for controlling the oxygen concentration in a respirable mixture of air and oxygen issuing from a gas mixer for delivery to a patient, comprising a device for generating a signal corresponding to the oxygen concentration in the blood of the patient connected to the device, a control unit adapted to receive the signal and to alter the oxygen concentration in the respirabie gas issuing from the mixer in one or more discrete amounts when the signal exceeds a preselected, limit.

2. An apparatus as claimed in Claim 1, in which the control unit is connected to the mixer and includes a shaft which enters the mixer, the angular position of which shaft controls the percentage of oxygen in the gas mixture issuing from the mixer.

3. An apparatus as claimed in Claim 2, in which the control unit includes a servo-motor connected to the shaft for rotating the shaft in preselected discrete angular steps.

4. An apparatus as claimed in Claim 3, in which each preselected angular step of the shaft results in a 5% change in oxygen concentration.

5. An apparatus as claimed in any one of Claims 1 to 4, in which the signal generating device is an intravascular electrode.

6. An apparatus as claimed in any one of Claims 1 to 4, in which the signal generating device is a heated skin-surface electrode.

7. An apparatus as claimed in any one of Claims 1 to 6, in which the control unit includes means for preselecting upper and lower limits of arterial oxygen tension in the patient.

8. An apparatus for controlling the oxygen concentration in a respirable mixture of air and oxygen, including a device for generating a signal corresponding to the oxygen concentration in the blood of a patient connected to the device; a control unit connected to the mixer of air and oxygen and adapted to alter the oxygen concentration, in the gas issuing from the mixer, in a limited number of timed steps when the said signal goes outside the the chosen maximum and minimum limits corresponding to maximum and minimum limits of arterial oxygen tension, and means for supplying the respirable mixture to the patient.

9. An apparatus for controlling the oxygen concentration in a respirable mixture of air and oxygen issuing from a gas mixer for delivery to a patient, constructed, arranged and adapted to operate substantially as hereinbefore described with reference to and as illustrated in the Figures of the accompanying drawing.