Pressure monitors and tube assemblies

Abstract: A monitor 10 for measuring the pressure in the cuff 2 on a tracheal tube 1 is clipped onto a coupling 7 at the end of an inflation indicator balloon 8. A silicon strain gauge pressure sensor 14 is urged resiliently against one side of the balloon, the other side of which rests on a base plate 12. The sensor 14 provides an output to a numeric display 18 representative of pressure in the balloon 8 and hence in the cuff 2.

Description: This invention relates to a cuffed medico-surgical tube assembly including a cuff pressure monitor and a cuffed medico-surgical tube of the kind for insertion into a body cavity and having a flexible inflation indicator located externally of the body communicating with the cuff and inflated on inflation of the cuff, the pressure monitor having an electrical pressure sensor and a display on which is provided a display representation indicative of pressure in the cuff. Such features are known from US-A-4,134,407. The invention is more particularly concerned with assemblies including a cuffed endotracheal tube or tracheostomy tube. Tracheal tubes are used to transmit anaesthetic or ventilation gases to a patient, such as during surgery, or to provide an airway to the trachea when the patient is breathing spontaneously. These tubes often have a cuff around the tube close to the patient end, which is inflated to seal with the trachea so that gas flow is confined within the tube. The cuff is inflated and deflated via a small-bore lumen extending along the tube within its wall, which opens into the cuff close to the distal or patient end and is connected to one end of an inflation line close to the proximal or machine end of the tube. The other end of the inflation line extends outside the patient and has a connector and an inflation indicator in the form of an inflatable balloon the interior of which communicates with the interior of the inflation line. The cuff is initially deflated and, after insertion of the tube into the trachea, is inflated by means of a syringe or similar device coupled to the connector, which administers a measured volume of air. This causes inflation of the cuff and the indicator. The indicator provides visual evidence to the clinician of the state of inflation of the cuff. These tubes may remain in place for some time and the pressure within the cuff can change during this time. The pressure may increase because of diffusion of anaesthetic gases through the wall of the cuff. Alternatively, the pressure may decrease because of leakage. Although the inflation balloon indicates large changes in pressure of the cuff, it is not sensitive to small changes in pressure. It is desirable to be able to maintain the cuff at the correct pressure because too high a pressure can lead to damage to the tracheal lining, whereas too low a pressure can allow leakage of gas between the tube and the trachea. Pressure gauges are available which can be coupled to the connector at the machine end of the inflation line but these are generally bulky and expensive. Their size and weight make them unsuitable for attachment to a tube long term. The pressure gauge can be incorporated into the syringe used to inflate the cuff, as described in EP-A-0396353. It is an object of the present invention to provide an alternative pressure monitor that can be used to monitor cuff pressure. According to one aspect of the present invention there is provided an assembly of the above-specified kind, characterised in that the monitor includes a U-shape housing extending around and supported by the inflation indicator, that the housing has a plate extending on one side of the inflation indicator, and that a spring mounted on the housing resiliently urges the pressure sensor in contact with the opposite side of the inflation indicator so that change in inflation of the indicator thereby causes a change in the output of the pressure sensor and hence a change in the display representation. The inflation indicator is preferably a flexible balloon. The monitor may be removable from the inflation indicator and may have a clip that fastens onto a coupling at one end of the inflation indicator. The pressure sensor preferably includes a silicon strain gauge. The display is preferably a numeric display and may be a liquid crystal display. A cuff pressure monitor and a tube assembly including the monitor in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side elevation view of the assembly; Figure 2 is a sectional side elevation view of the monitor to a larger scale; and Figure 3 is a transverse section along the line III - III of Figure 2. With reference first to Figure 1, there is shown a conventional cuffed endotracheal tube 1 having an inflatable cuff 2 extending around the tube close to its patient end 3. The interior of the cuff 2 communicates with an inflation lumen 4 extruded along the length of the tube 1 within its wall. Towards the machine end 5 of the tube, one end of an inflation line 6 is connected into the lumen 4. The inflation line 6 is flexible and, in use extends out of the patient's mouth. At its other end, the inflation line 6 has a coupling 7, which may include a valve (not shown), and an inflation indicator 8 in the form of a flexible balloon or envelope sealed to one end of the coupling. The interior of the inflation indicator 8 communicates with the inflation line 6 and cuff 2 so that the indicator is inflated when the cuff is inflated. As so far described, the tube is conventional. With reference now also to Figures 2 and 3, the pressure monitor 10 is removably clipped onto the outside of the coupling 7 at the machine end of the inflation line 6 and encompasses the inflation indicator 8. The pressure monitor 10 is contained in a plastic housing 11 of U-shape in section which has a flat base-plate 12 and an upper assembly 13 spaced from the base-plate by a short distance. The upper assembly 13 includes an electrical pressure sensor 14, such as including a silicon strain gauge, which is carried on a resilient, spring support 15 and projects downwardly towards the base-plate 12. The electrical output of the pressure sensor 14 is supplied to a processor 16 which performs various conventional calibration, scaling and calculation operations on its input and provides display driver signals on line 17 to a multi-segment liquid crystal display LCD 18. Other displays could be used. To the left of the upper assembly 13, the housing 11 is formed into a spring clip 19 which overlies and projects downwardly towards the base plate 12. The pressure monitor 10 also has a panel 20 on its side on which can be written pressure readings taken at different times. Alternatively, each pressure reading could be stored electronically within the processor 16. In use, the pressure monitor 10 is clipped onto the coupling 7, so that the spring clip 19 firmly locates on the coupling, and so that the inflation indicator 8 lies between the base plate 12 and the upper assembly 13. The pressure sensor 14 is urged vertically downwards by the support 15 to contact the upper surface of the exterior of the inflation indicator 8. The output of the pressure sensor 14 depends on the pressure within the inflation indicator 8, and hence on the pressure within the cuff 2. The pressure monitor 10 can be clipped onto the coupling and inflation indicator either before or after the cuff 2 is inflated, following intubation into the trachea. The display 18 can provide an indication of pressure in the cuff 2 on any standard pressure scale, or on an arbitary numeric scale. Alternatively, the indication of pressure could be provided on a non-numerical display, such as, for example, a bar graph display. Preferably each tube has its own pressure monitor 10 which is clipped in place when the tube is inserted and remains in place until the tube is removed. The nurse or anaesthetist can check the pressure reading periodically and, if the cuff pressure is too high or low, air can be inserted into or withdrawn from the cuff using a syringe inserted into the coupling 7 in the usual way. This operation of correcting the pressure in the cuff can be carried out whilst the monitor is in place. Alternatively, the pressure monitor 10 could be carried around in the pocket of the nurse or other user and clipped in place on different tubes as and when they are being checked. The pressure monitor 10 could include a visual or audible alarm that signals when the cuff pressure exceeds predetermined limits. The pressure monitor is not limited to use on tracheal tubes but could be used on other medico-surgical tubes having a cuff that is inflated to seal the tube with the body cavity and a flexible inflation indicator communicating with the cuff.

Claims: A cuffed medico-surgical tube assembly including a cuff pressure monitor (10) and a cuffed medico-surgical tube (1) of the kind for insertion into a body cavity and having a flexible inflation indicator (8) located externally of the body communicating with the cuff and inflated on inflation of the cuff, the pressure monitor (10) having an electrical pressure sensor (14) and a display (18) on which is provided a display representation indicative of pressure in the cuff (2), characterised in that the monitor (10) includes a U-shape housing (11) extending around and supported by the inflation indicator (8), that the housing (11) has a plate (12) extending on one side of the inflation indicator (8), and that a spring (15) mounted on the housing resiliently urges the pressure sensor (14) in contact with the opposite side of the inflation indicator (8) so that change in inflation of the indicator (8) thereby causes a change in the output of the pressure sensor(14) and hence a change in the display representation. An assembly according to Claim 1, characterised in that the inflation indicator is a flexible balloon (8). An assembly according to Claim 1 or 2, characterised in that the monitor (10) is removable from the inflation indicator (8). An assembly according to any one of the preceding claims, characterised in that the monitor (10) has a clip (19) that fastens onto a coupling (7) at one end of the inflation indicator (8). An assembly according to any one of the preceding claims, characterised in that the pressure sensor includes a silicon strain gauge (14). An assembly according to any one of the preceding claims, characterised in that the display is a numeric display (18). An assembly according to any one of the preceding claims, characterised in that the display is a liquid crystal display (18).

Inventors: CRAWLEY BRIAN, TURNBULL CHRISTOPHER STRATTON, CRAWLEY, BRIAN, TURNBULL, CHRISTOPHER STRATTON
Applicants: SMITHS INDUSTRIES PLC, SMITHS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1991
Main classification: A61B5
IPC codes: A61M16, A61M25, A61B5