GB2295458A - DVT prevention apparatus and monitoring method - Google Patents

Abstract

An inflatable garment for applying compression to the limb of a patient has two inflatable chambers (1, 2) on respective limbs of a patient. Both the lower and upper chambers are directly connected sequentially to a common source of inflation. A non-return valve (9) prevents any deflation of the first chamber when beginning to inflate the second chamber. A differential pressure sensor (31, 32) connected between garments on the two limbs of a patient indicates by a display device 34 the polarity of the pressure reading which of the garments is being monitored. <IMAGE>

Description

DVT PREVENTION APPARATUS AND METHOD The present invention relates to a DVT prevention apparatus and method, in particular, an apparatus which stimulates the fibrinolytic system and prevents stasis in traumatised or otherwise immobilised patients by urging venous blood from the lower extremities.

Deep vein thrombosis (DVT) and pulmonary embolism have long been recognised as significant complications in the treatment of medical and surgical patients.

In the United States, pulmonary embolism is known at present to be responsible for more than 200,000 deaths each year. Many of these deaths occur in patients suffering from serious disease, but a very large number occur in patients who have comparatively minor ailments and who might have led normal lives.

Equally significant is the estimate that nearly a million people suffer from chronic leg ulcers due to previous deep vein thrombosis.

Current knowledge suggests that the prevention of thrombosis in the calf will also prevent the development of proximal vein thrombosis and hence prevent major pulmonary embolus. But a number of thrombi start ab initio in the proximal veins, and it is to prevent this small proportion of potentially fatal emboli that compression of thigh, in addition to that of the calf, is applied in high risk cases.

The precise physiological reasons for the protection that external pneumatic compression provides against deep vein thrombosis are not exactly known.

However, studies and clinical trials have proved that external intermittent compression, when applied to the lower limb with a boot like structure, is effective i the reduction of the incidence of DVT.

The original studies focused on the problem of stasis and blood stagnation. One set of researchers applied a pulsating pressure wave of 40 mm Hg at a rate of 10 mm Hg/sec for a period of approximately 10 seconds, and another set of researchers used a much longer compression time of approximately 60 seconds and a slower pressure rise time of approximately 1 mm Hg/sec.

It has also been suggested that compression applied at a slower rate to the legs of patients undergoing surgery stimulates fibrinolysis during the postoperative period, but that the effect was much reduced in cancer patients.

On the other hand, it has been found that the faster rate of compression even in a knee length garment is still effective in patients with a malignant disease.

More recent research showed that despite ignoring possible stasis in the legs which were not treated, intermittent compression of the arms alone maintained blood fibrinolytic activity at levels sufficient to prevent DVT in the legs.

This important contribution was later (1980) confirmed by others who agreed that the induction locally of increased fibrinolysis may be a significant part of the protective action of intermittent compression techniques but also that compression applied to the thigh in addition to the calf further increased the fibrnolytic activity.

The pumping cycle of typical equipment now manufactured uses a compression time of 10 seconds at a pressure rise time of 8 - 12 mm Hg/Sec to provide a maximum pressure of 40 mm Hg. The decompression phase is usually approximately a minute to allow the veins to refill.

It is also accepted that the increase in fibrinolytic activity during intermittent compression therapy plays a dominant part in the explanation that both uniform and sequential compression in the calf is equally effective in the prevention of thrombosis in the calf in spite of haemodynamic studies showing that sequential graded compression provides greater pulsatility.

It is further accepted that a minority of patients do not respond to calf compression alone and are found to have proximal thrombosis in the thigh which may even lead to fatal embolism. Compression of the thigh as well as the calf is recommended for high risk patients.

The urging of blood flow from the lower extremities using sequential compression requires that the lower chambers are always at a higher or at least equal pressure to the upper chamber. During the compression cycle, it is thought that reflux can have an undesirable side effect. Prevention of reflux is normally achieved by ensuring that communication exists between adjacent chambers during the inflation of an upper chamber. In these designs the sequential filling oL succeeding chambers can result in a temporary drop of pressure in the lower chambers.

The present invention provides a method of monitoring pressures in alternately inflating compression means on respective limbs of a patient, comprising monitoring the differential pressure between corresponding compression means on the two limbs and determining from the polarity of the differential pressure value which of the compression means is being currently monitored.

The present invention also provides limb compression apparatus, comprising first inflatable compression means for one of a pair of limbs of a patient, second inflatable compression means for the other of said pair, means for alternately inflating said first and second compression means in successive cycles of inflation and deflation so that when one of said compression means is inflated, the other is deflated, a differential pressure sensor connected between said first and second compression means to provide an output signal having a first polarity when said first compression means is inflated and a second polarity when said second compression means is inflated and monitoring means responsive to the polarity of said output signal to determine which of the two said compression means is at a pressure currently represented by the modulus of said output indication.

Preferred embodiments of the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings, of which: Figure 1 depicts a schematic representation of an apparatus which may be used with embodiments of the present invention; Figure 2 depicts the rotor of a rotary valve assembly for use in the apparatus depicted in Figure 1; Figures 3a and 3b depict the opposite faces of the stator of a rotary valve assembly for use in the apparatus depicted in Figure 1; Figure 4 depicts a schematic representation of an apparatus according to a preferred embodiment of the present invention; Figure 5 is a graph which shows the areas of a normal inflation curve during the compression cycle of the apparatus in Figures 1 and 4; and Figures 6 and 7 depict examples of controller for use with the apparatus in Figures 1 and 4.

Figure 1 depicts schematically, apparatus which may be used in the present invention. A first inflatable garment 1 and a second inflatable garment 2 are provided for enveloping the lower and upper regions of a limb respectively. There is a controller 3 for sequentially inflating the garments 1 and 2 and pressure supply lines 4a and 4b located between garment 1 and the controller 3 and garment 2 and controller 3 respectively. The controller 3 comprises a compressor 5, a regulator 6 and a timer 7 for controlling the inflation and deflation periods of the individual garments 1 and 2.

The garments 1 and 2 are connected to pressure supply lines 4a and 4b through valves 8a and 8b respectively. The valves 8a,8b are controlled by the timer 7. A non-return valve 9 is located in pressure supply line 4a and an optional restrictor 10 is located in pressure supply line 4b.

During an inflation cycle, the timer 7 controls vale 8a to apply a pressure pulse from the controller 3 to the lower calf by opening valve 8a to allow flow to garment 1. The valve 8a allows inflation via inflation port 11a with the deflation port 12a closed.

Approximately four to five seconds later the timer 7 controls valve 8b to apply a second pressure pulse to the upper thigh, for example, via garment 2.

The second pressure pulse lasts for approximately seven to ich seconds and inflation occurs via inflation port llb with the deflation port 12b closed.

During the total inflation cycle of approximately twelve seconds, the pressure in the lower calf garment 1 is maintained through the non-return valve 9 which ensures that inflation of the upper thigh garment 2 can only occur by air flow through the compressor 5 and not by reflux of air from garment 1. The restrictor 10 which is optional, serves to ensure that during inflation, the pressure in the upper thigh garment 2 does not exceed the pressure in lower calf garment 2.

However, when a steady state is reached, both garments will be at the same pressure.

At the end of the cycle, the timer 7 controls valves 8a and 8b to deflate the garments 1 and 2 simultaneously, exhausting to atmosphere via deflation ports 12a and 12b. The inflation and deflation is depicted graphically in Figure 5 which is described later.

In one option, it is possible to ensure that the maximum pressure achieved by garment 2 is always less than that in garment 1. In order to achieve this garment 2 is provided with a bleed valve 13 so that flow through the restrictor 10 resulting in a pressure drop is maintained even when garment 2 reaches a steady state.

Figures 2 and 3a, 3b depict the two parts of a rotary valve assembly for use with the apparatus of Figure 1 and forming the valves 8a and 8b. The rotary valve assembly comprises a rotor 21 as depicted in Figure 2 and a stator 22 as depicted in Figures 3a and 3b. Figure 3a shows the face A which actually contacts the face of the rotor 21 shown in Figure 2. Figure 3b shows the face B which is visible when the rotor 21 and stator 22 are assembled together.

The stator 22 has the inflation ports lla and llb for connection to garments 1 and 2 and the ports 14a and 14b for connection to the compressor output from the controller 3. A further pair of ports lla' and llb' can also be provided for connection to a second pair of garments 1' and 2' for use on another limb as shown in Figure 4. In Figures 3a and 3b the ports lia, 11b and lla', llb' are shown spaced by 1800.

In operation, the timing of the pressure pulses to be applied to the garments 1 and 2 is controlled by rotation of the rotor 21 relative to stator 22 in the direction of the arrow X shown in Figure 2. At one point in the rotation cycle, a closed channel 23 in the rotor 21 begins to interconnect channels 14a and ila in the stator 22 so that inflation fluid (air) from the compressor 5 is supplied to inflate garment 1. After a short time, further rotation of the rotor 21 brings a second closed channel 24 of the rotor into alignment so as to begin interconnecting channels 14b and 11b of the stator 22 thereby applying air also to the second garment 2.The two garments remain connected to the compressor 5 until, with further rotation of the rotor, channels 23 and 24 no longer connect channels 14a and 14b to channels lla and llb respectively. Thereafter still further rotation of the rotor 21 brings che channels 11a and llb into alignment with channels 25 and 26 respectively of the rotor which permit air in the garments to exhaust to atmosphere through apertures in the rear of the rotor 21 (corresonding to ports 12a and 12b of the valves in Figure 1).

Whilst garments 1 and 2 are connected to atmosphere, the second set of ports lla' and llb' are sequentially connected by channels 23 and 24 so that the second pair of garments 1' and 2' are inflated from ports 14a and 14b.

The rotary valve 21 which is driven from a synchronous timer motor provides a complete cycle time of approximately ninety seconds per revolution.

Visible and audible failure alarms can be incorporated in the schematic representation of the apparatus as depicted in Figure 4. In this arrangement, a pair of calf and thigh garments 1,2 are located on leg X and a pair of calf and thigh garments 1',2', are located on leg Y. The alarms comprise sensors in the form of differential pressure transducers 31 and 32 connected between calf garments 1 and 1' and thigh garments 2 and 2'. The output from each transducer 31,32 is fed to a microprocessor 33 connected to the controller 3. A display device 34 indicates the pressure value in each sleeve and will indicate the alternate compression of each leg X,Y, by the output polarity of the transducers 31,32.

Microprocessor 33 may be programmed to activate an alarm if the pressure in either leg X,Y does not exceed a preset value, if a garment 1,2,1' or 2 fails to inflate due to leakage, if a garment inflates too slowly implying it is applied too loosely to the limb, if it inflates too quickly implying it is applied too tightly, if there is a substantially immediate pressure rise on beginning inflation, implying a garment is blocked, and also in the event of failure of the controller 3.

The microprocessor 33 will compute the rate of inflation in each garment and will detect any deviation from a standard as illustrated in Figure 5. Figure 5 depicts graphically the pressure variation in any one of the garments 1,2,1' and 2' with time. The full curves A and B in the graph represent inflation of garments 1 and 2 respectively and curve C represents subsequent simultaneous deflation. Normal deviation from these curves is represented by shaded areas.

Within the shaded areas the deviation is considered to be normal but outside the shaded areas, the deviation is considered abnormal and the transducers 31, 32 will be set to activate an alarm.

Figure 6 is a schematic representation of another form of controller which is not an embodiment of the present invention. In this apparatus the differential pressure transducers 31 and 32 in Figure 4 are replaced by two gauge pressure transducers 35 and 36, which measure the pressure in the inflated calf garments (1,1') and thigh garments (2,2') respectively.

The apparatus in Figures 4 and 6 would work equally as well if garments 1 and 1' are placed on the calf of each leg X,Y only without any ioss of the pre-calibrated compression pressure characteristics.

The pressure in the thigh chambers 2 and 2' is controlled by comparing the output of the two transducers 35 and 36. During the whole compression cycle the non-return valve 9 will maintain the pressure in chamber 1 monitored by transducer 35. During the inflation period of chambers 2, 2' the pressure is controlled and monitored by transducer 35. The final steady state pressure in chambers 2, 2' is pre-set from the outputs of transducers 35 and 36. An alarm is activated if during the inflation of chamber 1, the electrical outputs from transducers 35 and 36 differ from preset values. The display of pressure reading from transducer 36 will only commence five seconds after the start of the chamber 1 inflation. The transducers 35 and 36 are referenced to zero pressure prior to a measurement by exhausting the pressure ports through exhaust ports 25 and 26 to atmosphere.

A further arrangement which does not embody the invention is shown schematically in Figure 7. Here pump 5 supplies compression air via a mechanical pressure regulator 6 which can be adjusted by control 37 to provide a desired maximum pressure in the lines 4a and 4b feeding the rotary valve stator 22. A gauge pressure sensor 38 senses the pressure on a common output line from the regulator 6 feeding lines 4a and 4b. Alternatively the sensor 38 may sense the pressure in the line 4b itself. In a further arrangement which is an embodiment of the invention, instead of gauge pressure sensor 38, a differential pressure sensor is connected between the lines from ports 11b and llb' to upper chamber 2 on the two legs X and Y.

Microprocessor 33 receives signals indicative of sensed pressure from the gauge sensor 39 Dr the differertia sensor). Pump 5 can be energised and de-energised under the control of the microprocessor 33. Also timer 7 is formed as a motor or actuator to drive the rotor 21 of the rotary valve and signals on a line 39 may indicate to the microprocessor 33 the current position of the timer/motor 7 in the inflation/deflation cycle for the chambers 1 and 2.

In operation, during inflation of a chamber 1, pump 5 runs continuously and the maximum pressure reached by the chamber 1 is controlled by the regulator 6. When the rotary valve first connects line 4b to port 11b (or lob'), the pressure in lines 4a and 4b as sensed by sensor 38 drops to a low level from the previous maximum. Non-return valve 9 maintains the chamber 1 at the maximum pressure.

This drop in pressure from the regulated maximum may be detected by the microprocessor to indicate that inflation of a chamber 2 is now starting. In this case it may be possible to dispense with line 39.

As chamber 2 inflates, the pressure sensed by sensor 38 rises. The microprocessor 33 is programmed to respond to the sensed pressure reaching a predetermined value corresponding to a desired maximum pressure for chamber 2 (which may be less than that for chamber 1 set by the regulator 6) by de-energising the pump 5 so that this sensed pressure is maintained until further rotation of the rotary valve connects both chambers 1 and 2 to exhaust.

Conveniently, the microprocessor 33 records the maximum regulated pressure achieved during inflation of chamber 1 (as indicated by sensor 38) and is programmed to control pump 5 on subsequent inflation o' chamber 2 to provide a maximum pressure which is a preset traction say 80%) of said recorded maximum regulated pressure. The microprocessor 33 may include a control input allowing the desired fraction to be set up to a maximum of 100%. The pressures achieved on successive inflation cycles of chambers 1 and 2 may be displayed by the microprocessor on the display 34.

Claims (2)

CLAIMS:

1. A method of monitoring pressures in alternately inflating compression means on respective limbs of a patient, comprising monitoring the differential pressure between corresponding compression means on the two limbs and determining from the polarity of the differential pressure value which of the compression means is being currently monitored.

2. Limb compression apparatus, comprising first inflatable compression means for one of a pair of limbs o a patient, second inflatable compression means for the other of said pair, means for alternately inflating said first and second compression means in successive cycles of inflation and deflation so that when one of said compression means is inflated, the other is deflated, a differential pressure sensor connected between said first and second compression means to provide an output signal having a first polarity when said first compression means is inflated and a second polarity when said second compression means is inflated and monitoring means responsive to the polarity of said output signal to determine which of the two said compression means is at a pressure currently represented by the modulus of said output indication.