GB2162430A - Minute volume divider for connection to an anaesthesia machine - Google Patents

Abstract

A minute volume divider includes a control device which is manually operated by a rotary knob 1 for varying the frequency of delivery of tidal volumes of anaesthetic gas. In order to provide a direct reading of the frequency and also a ready of the tidal volumes in dependence upon the minute volume as determined by an anaesthesia machine to which the divider is connected, the knob 1 has a disc 3 which is fixed to the knob and moves on a surface 2 as the knob is turned. The disc 3 carries an index mark 5 and a logarithmic tidal volume scale 6 which is calibrated in millilitres. A shroud 4, which extends from the surface 2 over a part of the edge of the disc 3, carries a scale 7 which is also logarithmic and indicates the frequency per minute of the delivery of the tidal volumes by the divider. A further logarithmic scale 8 is provided on the surface 2 and indicates the minute volume in litres per minute delivered to the divider by the anaesthetic machine. By setting the index knob 5 to the required tidal volume frequency on the scale 7, the magnitude of each tidal volume for any given minute volume on the scale 8 can be read from the scale 6. <IMAGE>

Description

SPECIFICATION Minute volume divider for connection to an anaesthesia machine Anaesthesia machines deliver a steady flow of anaesthetic gas at a rate which is known as the minute volume. However, many patients during anaesthesia do not breathe adequately and need their lungs to be ventilated artificially. This is known as controlled ventilation.

To provide this, before the gas is administered to a patient, the minute volume flow of anaesthetic gas is divided into a series of tidal volumes which are delivered intermittently and each of which is the volume of gas which enters the patient's lungs in each period of inspiration. The steady minute volume flow of gas from the anaesthesia machine is divided into the tidal volumes by a piece of apparatus which is known as a minute volume divider.

The minute volume divider of gas delivered by the anaesthesia machine is determined by the adjustment of the anaesthesia machine itself and each of the tidal volumes is determined by adjustment of the minute volume divider. Most minute volume dividers have an adjusting knob which varies the tidal volumes, and which is not calibrated. Generally when an anaesthetic is being administered to a patient, a predetermined frequency of delivery of the tidal volumes is necessary and this rate is of course dependent upon the frequency of inspiration of the patient.In consequence to obtain a required frequency of delivery, a predetermined minute volume of gas is delivered to the minute volume divider by the anaesthesia machine and the tidal volume is adjusted until the frequency, which is timed by means of a stop watch or other timing device and is indicated by the inflation and deflation of a visible bag or bellows, is correct.

This method of adjusting or determining the frequency of delivery of the tidal volumes is rather complicated and it is therefore desirable to provide a minute volume divider with a control which directly adjusts the frequency of delivery of the tidal volumes, although this has not been done in the past. When a frequency control is provided, it can be directly calibrated to provide an indication of the frequency of delivery of the tidal volumes, but the minute volume divider itself cannot provide a direct indication of the tidal volumes corresponding to any particular frequency because this depends not only upon the frequency, but also upon the minute volume.

The minute volume is determined by the anaesthesia machine and not by the minute volume divider, and accordingly there is no input of the magnitude of the minute volume into the minute volume divider unless the minute volume divider is provided with a flow meter for this purpose. The provision of such a flow meter appreciably increases the complexity and accordingly the cost of the minute volume divider.

The aim of the present invention therefore is to provide a minute volume divider with a control device which varies the frequency of the delivery of the tidal volumes and is calibrated to provide a direct reading of the frequency and also a reading of the tidal volumes in dependence upon the minute volume as determined by an anaesthesia machine to which the minute volume divider is connected.

To this end, according to this invention, in a minute volume divider which includes a control device which is manually operated by a rotary knob for varying the frequency of delivery of the tidal volumes, the knob carries an index mark, and three circular or partcircular logarithmic scales are provided and centred on the axis of rotation of the knob, the first one of the scales being on a stationary surface and being calibrated so that the position of the index mark along it indicates the frequency, and the other two scales registering with each other and one being on a stationary surface and the other being on a surface which turns with the knob, one of the other scales being calibrated to indicate the tidal volume and the other to indicate the minute volume and the scales and the index mark being relatively positioned such that when the knob is turned to set the index mark against a required frequency on the first scale, the other two scales are set so that for any minute volume read from one of the other two scales, the magnitude of each tidal volume can be read from the other one of the other two scales or, for any tidal volume read from the other of the other two scales, the required minute volume to be supplied to the divider can be read from the one of the other two scales.

With this simple calculating device incorporated with the rotary knob which operates the frequency control device, when two of the three variables are predetermined, the magnitude of the third variable can be immediately read from the appropriate scale.

For example, if a patient is to be supplied with a predetermined minute volume of anaesthetic gas at a predetermined frequency, the knob is turned until the index mark is set against the predetermined frequency on the first one of the logarithmic scales. The predetermined minute volume is then read from one of the other two scales and against this reading the required tidal volume may be read from the third scale.

Similarly, if the patient is to be supplied with a predetermined minute volume of a gas and predetermined tidal volumes of gas, the two readings of the minute volume and the tidal volumes on the second two scales are set against each other and in so doing the knob will be turned to provide the required frequency of delivery of the tidal volumes. This frequency will then be indicated by the index mark on the first scale.

Finally, in a similar manner if a predetermined frequency and predetermined tidal volumes are required, the necessary minute volume of gas to be supplied to the machine can at once be read directly from the remaining scale.

Preferably the scale which indicates the tidal volumes is on a surface which turns with the knob and the scale which indicates the minute volume is on the same stationary surface as the scale which indicates the frequency. In this case, the frequency scale preferably extends around a semi-circular arc on one side of the axis of rotation of the knob and the minute volume scale extends around a semi-circular arc on the opposite side of the axis of rotation of the knob. The tidal volume scale is then extend preferably provided on a circular disc which is fixed to and rotates with the knob over the surface on which the frequency and minute volume scales are provided. The tidal volume scale can then around an arc of rather more than a semi-circle as is generally necessary to provide the necessary range of variation of the tidal volume using a logarithmic scale.

An example of a control knob of a control device of a minute volume divider in accordance with the invention together with its associated scales is illustrated diagrammatically in the accompanying drawing in which a knob 1 is rotatably mounted on a flat face 2 of a casing of the divider. A circular disc 3 is fixed to and rotates with the knob 1 on the flat face 2. An arcuate shroud 4 projects from the flat face 2 of the casing of the divider and projects over the lower half of the edge of the disc 3.

The disc 3 carries an index mark 5 and a logarithmic tidal volume scale 6 which is calibrated in millilitres. The shroud 4 carries a fixed scale 7 which is again logarithmic and indicates the frequency per minute of the delivery of the tidal volumes by the divider. A further fixed logarithmic scale 8 is scribed on the surface 2 and indicates the minute volume in litres per minute delivered to the minute volume divider by an anaesthetic machine.

With the knob and the scales set as shown, it will be seen that the minute volume divider is set to deliver tidal volumes at a frequency of 12 per minute and accordingly if the minute volume is six litres per minute, each tidal volume will be 500 ml. Other tidal volumes corresponding to various minute volumes at the set frequency of 12 per minute can be read directly from the two scales 6 and 8.

Claims (6)

1. A minute volume divider for connection to an anaesthesia machine, the divider including a control device which is manually operated by a rotary knob for varying the frequency of delivery of tidal volumes of anaesthetic gas, wherein the knob carries an index mark, and three circular or part-circular logarithmic scales are provided and centred on the axis of rotation of the knob, a first the scales being on a stationary surface and being calibrated so that the position of the index mark along it indicates the frequency, and the other two scales registering with each other and one being on a stationary surface and the other being on a surface which turns with the knob, one of the other scales being calibrated to indicate the tidal volume and the other to indicate the minute volume and the scales and the index mark being relatively positioned such that when the knob is turned to set the index mark against a required frequency on the first scale, the other two scales are set so that for any minute volume read from one of the other two scales, the magnitude of each tidal volume can be read from the other of the two scales or, for any tidal volume read from the other of the other two scales, the required minute volume to be supplied to the divider can be read from the one of the other two scales.

2. A minute volume divider according to Claim 1, in which the scale which is calibrated to indicate the tidal volume is on the surface which turns with the knob and the scale which is calibrated to indicate the minute volume is on a stationary surface.

3. A minute volume divider according to Claim 2, in which the frequency scale extends around a part-circular arc on one side of the axis of rotation of the knob and the minute volume scale extends around a part-circular arc on the opposite side of the axis of rotation of the knob.

4. A minute volume divider according to Claim 3, in which the tidal volume scale is on a circular disc which is fixed to and rotates with the knob over the stationary surface on which the minute volume scale is provided.

5. A minute volume divider according to Claim 4, in which the stationary surface on which the frequency scale is provided is a part-circular arcuate shroud which is mounted on the stationary surface on which the minute volume scale is provided and which extends over an edge portion of the disc, and the index mark comprises a radial line on the disc.

6. A minute volume divider according to Claim 1, constructed substantially as described with reference to the accompanying drawing.