GB2160343A

GB2160343A - Wheelchair cushion monitor and alarm apparatus

Info

Publication number: GB2160343A
Application number: GB8518220A
Authority: GB
Inventors: Brian Andrews; Christopher Anthony Bar; Martin William Ferguson-Pell
Original assignee: University of Strathclyde
Current assignee: University of Strathclyde
Priority date: 1981-07-31
Filing date: 1985-07-19
Publication date: 1985-12-18
Also published as: GB8518220D0; GB2160343B

Abstract

Apparatus for reminding patients with limited movement to perform movements at regular intervals to reduce the risk of pressure sores is described. A load sensor (11) positioned beneath the patient senses the pressure exerted by the weight of the patient. A movement interval timer (MIT) (14) monitors the output of the sensor (11) during a predetermined period. If the pressure is not relieved during this period a signal is fed to the input (13a) of an alarm (13) and to a push-up error counter (PEC) (15), incrementing the counter by one. If the pressure on the sensor (11) is relieved, the MIT (14) is reset. Successive signals resulting from timed periods without movement are fed to the alarm (13) and the PEC (15) and if the accumulated number of signals exceeds a predetermined number a flip-flop (16) is enabled actuating the alarm (13). The alarm signal continues until the patient moves to relieve the load on the sensor, resetting the MIT (14) and PEC (15). <IMAGE>

Description

SPECIFICATION Wheelchair cushion monitor and alarm apparatus The present invention relates particularly, but not exclusively, to monitors and alarms for use in reminding patients with limited movement to perform movements at regular intervals in order to reduce the risk of developing pressure sores. The invention is particularly suited for use with wheelchair bound patients.

The development of pressure sores is principally dependent on the amount of movement of the patient at the support surface and on the magnitude of pressure developed thereat.

Knowledge of the amount of movement, the pressure magnitude and the time over which the movement and/or magnitude is maintained can indicate whether the patient is likely to develop pressure sores; the less movement and the greater the pressure time product the more likely the patient is to develop pressure sores.

There have been proposed several devices which monitor the movement and the pressure magnitude over a period of time and which provide an alarm if an insufficient amount of movement occurs or if a particular level of pressure has been exceeded for too long (Fordyce and Simons (1968), U.S. Public Health Rep. 83, 527528; Malament et al (1975), Arch. Phys. Med. Rehab, 56, 161165; and Chawla, Andrews and Bar (1978), Paraplegia 16, 413-419).

In addition, in the Chawla device the pa-tient was given an opportunity to prevent the audible alarm by performing a push-up in a predetermined time after a visual indication was presented. A counter was also used which actuated the alarm if a push-up of the required duration was not achieved. A further counter also counted the time by which the patient exceeded the predetermined time, called the error time.

The results obtained with these devices indicate that despite the alarm signal, which may be audible or visual, wheelchair patients do not tend to relieve pressure at regular intervals. In addition, the relatively rapid frequency of the alarm reminders (usually at a predetermined interval up to 20 mins.) is tiresome for the patient and is often socially embarrassing.

Furthermore these devices showed that although push-ups of sufficient duration were performed these tended to be at irregular intervals.

An object of the present invention is to obviate or mitigate the abovesaid disadvantages.

According to a first aspect of the present invention there is provided apparatus for monitoring the pressure developed between two surfaces in contact, said pressure being represented by a variable parameter which may have either a first or second value, said first value corresponding to a pressure of magnitude greater than zero and said second value corresponding to zero pressure, said apparatus comprising a sensor responsive to variations of said parameter, timing means connected to the output of said sensor for monitoring the value of said parameter and generating a first signal if said parameter does not change from said first value to said second value within a predetermined period of time, said timing means being reset if said change of value does occur within said predetermined period, counting means connected to the output of said timing means for continuously accumulating the number of said first signals generated by said timing means, comparing said accumulated number with a preset number and generating a second signal if said accumulated number exceeds said preset number, and alarm means connected to the output of said counting means, said alarm means being activated upon receipt of said second signal, said alarm means being deactivated and said timing means and said counting means being reset if said parameter changes its value to said second value.

According to yet another aspect of the present invention there is provided a method of monitoring the pressure developed between two surfaces in contact, said pressure being represented by a variable parameter which may have either a first or second value, said first value corresponding to a pressure of magnitude greater than zero and said second value corresponding to zero pressure, said method comprising, detecting the value of said parameter by sensing means, monitoring the value of said parameter over a predetermined period of time by timing means, transmitting a signal from said timing means to counting means if said parameter does not change from said first value to said second value before the end of said predetermined period of time, or resetting said timing means if said change of value does occur before the end of said predetermined period, continuously accumulating the number of said signals and comparing said accumulated number with a preset number by counting means, activating an alarm if said accumulated number exceeds said preset number, and de-activating said alarm and resetting said timing means and said counting means if said change of value occurs.

The apparatus is particularly envisaged for use in wheelchair cushions.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 is a block diagram of one circuit for use with the apparatus according to the first aspect of the present invention; Fig. 2 is a block diagram similar to Fig. 1 with some additional circuit components; Fig. 3 is a schematic diagram of the apparatus according to the present invention shown in place in a wheelchair cushion; Fig. 4 is a schematic diagram similar to Fig.

5 showing an alternative arrangement in the wheelchair cushion; Referring now to Fig. 1 of the drawings, apparatus for determining the movement of an individual with respect to a support surface comprises a pressure actuated switch 11, an electronic circuit 12 and an audible alarm 13.

The electronic circuit 12 has a movement interval timer (MIT) 14, connected to the output of the sensor 11. The output 14a of the MIT 14 is connected to one input 13 a of the alarm 13 and to one input 15a of a pushup error counter (PEC) 15. The PEC 15 has an output 15b connected to one input 16a of a flip-flop 16. The other input 16b of the flipflop is fed from one output 17a of a mode timer (MT) 17. Another output 17 b of the mode timer 17 is connected to another input 1 sic of the PEC 15. The output from the flipflop 16 is connected to the other input 1 3b of the alarm 13.

In use there are two conditions; when the switch is loaded, i.e. in tne on-position and when the switch is not loaded, i.e. in the offposition. In the first condition the MIT 14 is actuated and begins to count for a predetermined period, for example, 15 minutes. If there is no movement during the counting an output signal is fed to the input 13a from the MIT 14 at the end of the counting period. If there is movement of a sufficient duration during the counting which interrupts the sensor output the timer is reset. The output signal is also fed to the PEC 15 and when the PEC receives the signal the counter is incremented by 1. After subsequently timed periods without a movement, successive output signals are fed to the alarm 13 and to the PEC 15, the counter accumulating the number of output signals and continuously comparing the accumulated number with a preetermined value.If the predetermined value is exceeded the PEC provides an output signal which enables the flip4lop 16. When this occurs the flip-flop 16 sends an output to the alarm 13 which actuates the alarm to produce an audible signal. The audible signal continues until the individual moves to relieve the load on the sensor, by doing a 'push-up' for example. When this occurs the MIT 14 is reset and the PEC 15 is also reset by the MT.

Thus this circuit provides an indication of the interval between movements, i.e. of the length of time for which the individual has been supported.

As shown in Fig. 2, a general reset timer (GRT) 18 can be connected to the sensor output. In the pressure relieved condition, or when the wheelchair is not in use, if this condition is maintained for a short preset time, called the push-up valid interval, e.g. 5 seconds, the MIT 14 is reset and the GRT 18 enabled. The GRT 18 causes the system to be reset if the pressure relieved state is maintained for a continuous preset period, e.g. 3 minutes.

Furthermore, the circuits shown in Figs. 1-4 could be easily modified to be actuated only when a particular level of pressure is exceeded. Thus a pressure/time profile could be monitored for each individual, the alarm being actuated when a particular value of the pressure/ time integral is exceeded. This arrangement could be implemented by the insertion of a pressure monitoring circuit PMC (not shown) between the sensor and the timer or counter, with the PMC having a threshold so that an output signal is provided when the threshold is exceeded. An example of such a PMC would be an operational amplifier wired for differential amplifier operation with one input being adjustable to set a variable thresh- old.

Referring now to Fig. 3 the apparatus is shown inserted in a wheelchair cushion 21, the cushion 21 having a cut-out 22 in which the electronics are located and being connected to the sensor 12 by leads 23. An alternative arrangement is shown in Fig. 4 with the sensor and electronics located in the one unit, in the centre of the cushion 21.

It may be understood that several modifications may be made to the apparatus as hereinbefore described without departing from the scope of the invention. For example, the sensor can be of any type responsive to weight or any other parameter which varies underload such as temperature. The alarm could be visual or audio/visual. The circuitry hereinbefore described is designed to operate on power consumption CMOS technology (complementary-metal-oxide-semiconductor) although it could be based on any other suitable logic system, e.g. TTL.

Thus there is provided apparatus for determining the quality and quantity of movement of an individual with respect to a support surface. The combination of the portable autonomous device and the facility to switch between monitor and alarm modes depending on the movement history of the patient gives the apparatus according to the present invention advantages in that patients find it less tiresome and it produces a better patient response reducing the risk of pressure sores developing.

Claims

1. A method of monitoring the pressure developed between two surfaces in contact, said pressure being represented by a variable parameter which may have either a first or second value, said first value corresponding to a pressure of magnitude greater than zero and said second value corresponding to zero pressure, said method comprising, detecting the value of said parameter by sensing means, monitoring the value of said parameter over a predetermined period of time by timing means, transmitting a signal from said timing means to counting means if said parameter does not change from said first value to said second value before the end of said predetermined period of time, or resetting said timing means if said change of value does occur before the end of said predetermined period, continuously accumulating the number of said signals and comparing said accumulated number with a preset number by counting means, activating an alarm if said accumulated number exceeds said preset number, and de-activating said alarm and resetting said timing means and said counting means if said change of value occurs.

2. Apparatus for monitoring the pressure developed between two surfaces in contact, said pressure being represented by a variable parameter which may have either a first or second value, said first value corresponding to a pressure of magnitude greater than zero and said second value corresponding to zero pressure, said apparatus comprising, a sensor responsive to variations of said parameter, timing means connected to the output of said sensor for monitoring the value of said parameter and generating a first signal if said parameter does not change from said first value to said second value within a predetermined period of time, said timing means being reset if said change of value does occur within said predetermined period, counting means connected to the output of said timing means for continuously accumulating the number of said first signals generated by said timing means, comparing said accumulated number with a preset number and generating a second signal if said accumulated number exceeds said preset number, and alarm means connected to the output of said counting means, said alarm means being activated upon receipt of said second signal, said alarm means being deactivated and said timing means and said counting means being reset if said parameter changes its value to said second value.

3. Apparatus as claimed in claim 5 including resetting means for resetting said timing means and said counting means if said parameter remains at said second value for a period of time exceeding a predetermined period.

4. Apparatus as claimed in claim 2 or 3, wherein said sensor is a pressure-actuated switch.

5. Apparatus as claimed in claim 2 or 3 wherein said sensor is a pressure transducer.

6. Apparatus as claimed in claim 2, 3, 4 or 5 wherein said alarm means generates an audible and/or visual signal when activated.

7. Apparatus as claimed in any of claims 2 to 6 wherein said pressure is exerted by the weight of a person sitting upon said supporting surface.

8. Apparatus substantially as hereinbefore described with reference to Figs. 3 and 4 or to Figs. 5 and 6 of the accompanying drawings.