5 The present invention relates to an arrangement for measuring two-way or bidirectional respiration gas flows w ithout increasing the dead space. Such an arrangement includes a hose which is connected to the breathing passages of a patient and which , via a Y-piece , is connected to a tube for inspiration gas and a tube for -|Q expiration gas, said tubes being connected in turn to a respi rator, or breathing apparatus; and further includes a vent ri device arranged in each of said tubes.

The known art wi ll be described with reference to Figure I of • _j 5 the drawing, while the invention wi ll be described with reference to Figure 2 thereof.

Background Art

0 Patients under intensive care and anaesthetized patients are often connected to equipment which assists with their breathing. This is illustrated in Figure 1 .

A hose 1 is connected to the breating passages of the patient. 5 That end of the hose 1 which extends away from the patient is connected to a Y-piece 2, to which is connected two tubes 3 and 4. Unidirectional flow for alternating inspiration gas is effected through the one tube 3 and for expiration gas through the other tube 4. The tubes are, in turn, connected to a respirator or breathing 0 apparatus not shown.

It is of great clinical interest to be able to measure the respiration flows as close to the patients as possible. Alternative solutions include the insertion of a flow meter for bidirectional flows between 5 the hose and Y-piece , or to insert two flow meters for unidirectional

flows between said Y-piece and the apparatus tubes (B).

The known technique employs various kinds of flow meter, for example: 1 ) Respiration flow meters according to Fleisch, incorporating inserts having narrow, mutually parallel channels. The pressure drop across the inserts is proportional to the flow. The flow meter is symmetrical, and measures in both directions.

2) Vortex meters in which a rod is placed perpendicular to the gas flow, so as to generate vortices. The vortices move as swiftly as the gas and can be detected with the aid of ultrasonic devices. The vortex meter is intended for flow in one direction only.

3) Venturi tubes, which are provided with a constriction, such that the gas moves at a faster rate and the lateral pressure falls. Downstream of the constriction there is an elongate gradual transition to the "general" diameter of the gas tube. The difference is lateral pressure between a location immediately upstream of the constriction to the narrowest location thereof is proportional to the square of the flow. The total drop in pressure over the whole of the venturi tube is much lower than the pressure difference measured in the first instance. Thus, the lateral pressure in the constricted area of the tube is lower than the lateral pressure upstream and downstream of the constriction. Consequently, the flow can be measured by determining the difference is lateral pressure between the narrowest part of the constriction and the lateral pressure downstream of the venturi tube. The differential pressure thus measured is 20-30% lower than the difference in pressure which prevails simultaneously between an upstream location and the narrowest location of the constriction. The venturi tube is intended for flow in one direction.

The above described apparatus possess various properties, which are summarized below.

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The f low meter to Fleisch is highly sensitive to condensation and particles of foreign matter, while this particular sensitivity is low in the case of vortex and venturi type flow meters. When a transducer is positioned in the flow meter according to Fleisch, at A in Figure 1 , the dead space is greatly increased, whi le the inclusion of two transudcers at B in Figure 1 does not result in this undesirable effect. This latter is also true of transducers placed at B in the vortex and venturi methods. The measuring area becomes insufficient with the vortex meter, but is sufficient with the Fleisch and venturi meters. All three meters have a small resistance to flow , their resistance being smallest with the venturi device. The degree of sensitivity to the configuration of the supply lines is high in the vortex apparatus , but low in the other two.

Routines for setting the arrangement to zero are necessary in all methods, except for the vortex method.

The object of the present invention is to improve the aforementioned techniques for measuring the bidi rectional respiration f lows of anaesthetized patients or patients under intensive care.

To this end the invention is mainly characterized in that a pressure-differential meter having two inlets or inputs is connected to the inlet of each venturi 's device, that a magnet valve is arranged to connect the two inlets together periodically; and that an electronic means is arranged to ca lculate the measured value and to control setting of the arrangement to zero, said electronic means to this end being electrical ly connected to the pressure- differential meter and to the magnet values.

Preferred embodiment

The invention is illustrated schematically in Figure 2 and relates to a venturi-tube meter, which offers many advantages when used as a respi ration flow meter, as will be gathered from the resume above. In order to reduce the noxious clearance, it is necessary to use separate meters for the inspiration and expiration flows. As illustrated in said Figure, a tube 1 connected to the breathing passages of a patient is connected to an inspiration tube 3 and an expiration tube 4, via a Y-piece 2. In each of these tubes there is arranged a venturi means, the latter being arranged to co-act with a common transducer means in the form of a pressure differential meter 8 and to indicate the flow electrically. The meter 8 is connected to the venturi means in the tube 3 via a first line 5, and to the venturi means in the tube 4 via a second line 6. An annular chamber 5 'and 6* connects the lateral orifices together at the narrowest location of the venturi means. Connected across the two inputs of the meter 8 is a short line 10. Zero-setting of the transducer is essential for obtaining a correct measurement. In the i llustrated embodiment, simple zero-balancing' of the pressure differential meter 8 is effected by intermittently (e.g. each hour) connecting both inlets of the meter to the same pressure, by means of a magnet valvue 7 (by rotating the valve body 90 , as shown by the arrow in Figure 2).

An electronic means 9 (a microprocessor) for calculating the measured value and for controlling the zero-setting of the transducers is arranged to control the setting of the valve 7 and to lineate the measured pressure-differential signal, so that it can be presented as a calibrated flow signal. Flow will be present alternately in either the inspiration tubes 3 or the expiration tube 4. The pressure signal will have different characteristics, depending upon which of the two tubes 3 , 4 is through-passed by gas.