CPR apparatus and method

Abstract

A CPR apparatus includes a chest compression unit and a means for mounting the chest compression unit on a patient. The chest compression unit includes a plunger disposed in a housing. At its one end extending from the housing the plunger has a compression member. The plunger is driven in a reciprocating manner by a reversible electromotor via a mechanism for translating rotational motion to linear motion or by a linear induction electromotor. The chest compression unit includes an electromotor control unit including a microprocessor, a first monitor for monitoring the position of the plunger in respect of the housing and a second monitor for monitoring the position of the plunger in respect of the mechanism for translating rotational motion to linear motion or the rotor of the linear induction electromotor. The monitored positions are communicated to the electromotor control unit. Also disclosed is a corresponding CPR method.

Claims

1. A cardiopulmonary resuscitation (CPR) apparatus, comprising: a chest compression unit comprising: a housing; a plunger disposed in the housing, the plunger having a compression member at one end of the housing that extends from the housing, wherein the plunger is configured to be driven in a reciprocating manner, and wherein the compression member is configured to deliver compressions of varying depths to a patient's chest; and a control unit that includes a microprocessor configured to cause the plunger to perform a series of CPR compressions including a plurality of soft start chest compressions and at least one compression at a full compression depth, the plurality of soft start chest compressions including a continuous increase from a first compression having a depth of less than the full compression depth to a final compression having a depth of the full compression depth, wherein during each compression the control unit is configured to drive the plunger until a desired compression depth is reached.

2. The CPR apparatus of claim 1, wherein the microprocessor is configured to determine the full compression depth based at least in part on an initial chest height of the patient.

3. The CPR apparatus of claim 1, wherein the continuous increase is linear.

4. The CPR apparatus of claim 1, wherein the continuous increase is nonlinear.

5. The CPR apparatus of claim 1, wherein the continuous increase extends over a period of 3 to 25 compressions.

6. The CPR apparatus of claim 1, wherein the continuous increase extends over a period of 5 to 15 compressions.

7. The CPR apparatus of claim 1, wherein following the plurality of soft start chest compressions the microprocessor is further configured to cause the plunger to perform a plurality of compressions having a constant compression depth or a plurality of compressions having a decreasing compression depth.

8. The CPR apparatus of claim 7, wherein the time at maximum compression depth during the plurality of soft start chest compressions is shorter than the time at maximum compression depth during the plurality of compressions having a constant compression depth or the plurality of compressions having a decreasing compression depth.

9. The CPR apparatus of claim 8, wherein the time at maximum compression depth during the plurality of soft start chest compressions is shorter by 50% or more than the time at maximum compression depth during the plurality of compressions having a constant compression depth or the plurality of compressions having a decreasing compression depth.

10. The CPR apparatus of claim 1, further comprising a mounting device configured to mount the chest compression unit on the patient.

11. A cardiopulmonary resuscitation (CPR) apparatus, comprising: a chest compression unit comprising: a housing; a plunger disposed in the housing, the plunger having a compression member at one end of the housing that extends from the housing, wherein the plunger is configured to be driven in a reciprocating manner, and wherein the compression member is configured to deliver compressions of varying depths to a patient's chest; and a control unit that includes a microprocessor configured to determine a full compression depth based at least in part on an initial chest height of the patient and to cause the plunger to perform a soft start chest compression, the soft start chest compression having a depth of less than the full compression depth, wherein during each compression the control unit is configured to drive the plunger until a desired compression depth is reached.

12. The CPR apparatus of claim 11, wherein the microprocessor is further configured to cause the plunger to perform a plurality of soft start chest compressions, the plurality of soft start compressions including a continuous increase in compression depth from a first compression depth having a depth of less than a full compression depth to the full compression depth.

13. The CPR apparatus of claim 12, wherein the continuous increase is linear.

14. The CPR apparatus of claim 12, wherein the continuous increase is nonlinear.

15. The CPR apparatus of claim 12, wherein the continuous increase extends over a period of 3 to 25 compressions.

16. The CPR apparatus of claim 12, wherein the continuous increase extends over a period of 5 to 15 compressions.

17. The CPR apparatus of claim 11, wherein the microprocessor is further configured to cause the plunger to perform a plurality of compressions having a constant compression depth or a plurality of compressions having a decreasing compression depth following the soft start chest compression.

18. The CPR apparatus of claim 17, wherein the time at maximum compression depth during the soft start chest compression is shorter than the time at maximum compression depth during the plurality of compressions having a constant compression depth or the plurality of compressions having a decreasing compression depth.

19. The CPR apparatus of claim 18, wherein the time at maximum compression depth during the plurality of soft start chest compressions is shorter by 50% or more than the time at maximum compression depth during the plurality of compressions having a constant compression depth or the plurality of compressions having a decreasing compression depth.

20. The CPR apparatus of claim 11, further comprising a mounting device configured to mount the chest compression unit on the patient.

Description

SHORT DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a first embodiment of the chest compression unit of the CPR apparatus of the invention, in a sectional view (except for some elements) through the axes A, B of the ball screw and electromotor shafts, with the plunger in a neutral, non-compressing state, the frame not being shown;

(2) FIG. 2 is the embodiment of FIG. 1, in the same view, with the plunger in an active, compressing state;

(3) FIG. 3 shows the ball screw nut with its holder of the embodiment of FIGS. 1 and 2, dismounted and in the same view;

(4) FIG. 4 shows the plunger of the embodiment of FIGS. 1 and 2, dismounted and in the same view;

(5) FIG. 5 illustrates the displacement of the plunger and other elements of the embodiment of FIGS. 1 and 2 when moving from the neutral state of FIG. 1 to the compressing state of FIG. 2, in the same view;

(6) FIG. 6 is a radial section C-C (FIG. 1) through the embodiment of FIGS. 1 and 2;

(7) FIG. 7 is a radial section E-E (FIG. 1) through a variant of the embodiment of FIGS. 1 and 2;

(8) FIG. 8 is perspective view of a first embodiment of the CPR apparatus of the invention;

(9) FIG. 9 is a partial sectional view of the chest compression unit of the CPR apparatus of FIG. 8;

(10) FIG. 10 is a control panel of the chest compression unit of the CPR apparatus of FIG. 8, in a top view;

(11) FIG. 11a is a diagram illustrating a selected compression/decompression cycle profile generated from a set of entered data;

(12) FIG. 11b is a corresponding diagram illustrating the recorded true stroke depth profile and the recorded motor control signal over the cycle of FIG. 11a;

(13) FIG. 11c is a corresponding diagram illustrating the recorded chest height and the motor current over the cycle of FIG. 11a;

(14) FIG. 12 is a diagram illustrating the profiles over a number of compression/decompression cycles during a soft compression upstart period;

(15) FIG. 13 illustrates a second embodiment of the chest compression unit of the CPR apparatus of the invention, in a sectional view corresponding to that of FIG. 1 except for some elements, with the plunger in a neutral, non-compressing state, the frame not being shown.

DESCRIPTION OF PREFERRED EMBODIMENTS

(16) The CPR apparatus of the invention comprises a chest compression unit of which a first embodiment is shown in FIGS. 1 and 2. The chest compression unit 1 is mounted in a frame (not shown) and comprises a generally cylindrical plunger 33 (FIG. 4) disposed co-axially in a cylindrical housing 4. The housing 4 has a proximal open end and a distal end closed by a top wall 27. The plunger 33 comprises a cylindrical main section 3 and a distal terminal section in form of an inwardly bent circular flange 17. At its proximal end the cylindrical section 3 of the plunger 33 protrudes from the proximal opening of the housing 4. A chest compression disk 5 provided with a polymer suction cup 6 on its proximal face is affixed to the cylindrical section 3 at the proximal end thereof. From the neutral, unloaded state illustrated in FIG. 1 the plunger 33 can be displaced axially in respect of the housing 4 by ball screw means comprising a ball screw nut 7 mounted on a ball screw shaft 8. The ball screw nut 7 is firmly held by distal wall sections 13, 14, 30 of a generally rotationally symmetric ball screw holder 32. A proximal terminal radial flange 16 of the holder 32 transmits the displacement of the ball screw nut 7 and thus the holder 32 to a distal face of the compression disk 5 by a first spring coil 19 mounted between the distal face of the compression disk 5 and a proximal face of the flange 16. A second spring coil 18 mounted between a proximal face of the nut screw holder's 32 flange 16 and a distal face of the plunger's 33 flange 17 serves for maintaining contact of the first spring coil 19 with the disk 5 and the flange 6. The displacement of the ball screw nut 7 with its holder 32 will now be explained by reference to a shaft 8 with right hand threads. By rotating the shaft 8 clockwise (as seen by a person looking at the shaft from a distal direction) the nut 7 and the holder 32 are displaced in a distal direction; anti-clock-wise rotation causes displacement in the opposite, proximal direction. The distal terminal portion of the holder 32 comprising wall elements 13, 14, 30 is integral with an oblong cylindrical section 15 of smaller outer and inner diameter ending in the aforementioned radially outwardly extending proximal flange 16. The ball screw/nut holder assembly 7, 32 constitutes an actuator that is displaceable along the ball screw shaft 8 and thus along the cylinder axes of the housing 4 and the plunger 33. From a position distally of the housing top wall 27 the ball screw shaft 8 extends through a central opening in the top wall 27 into the housing 4. The ball screw shaft 8 is mounted centrally in the top wall 27 by means of a ball bearing 26. Inside of the housing 4 the ball screw shaft 8 extends via the ball screw nut 7 into the lumen of the nut holder 32 and from there into the lumen of the plunger 33.

(17) The ball screw shaft 8 centers the nut holder 32 and the plunger 33 in the housing 4. In addition, the nut holder 32 and the plunger 33 can be kept centered in the housing 4 by spacer means such as linear ball bearings 20, 21 cooperating with corresponding bearings disposed in corresponding radial planes. This arrangement is shown in FIG. 6 for the plunger 33 centered by ball three bearings 20, 20a, 20b disposed at angles of 0, 120° and 240° and mounted at the inner cylindrical face of the housing 3. The ball screw nut/holder assembly 7, 32 is secured against rotation in the housing 4 by the balls of the bearings 20, 20a, 20b running in shallow, axially extending groves 31, 31a, 31b, respectively, in the outer face of the plungers 33 cylinder portion 3; alternatively, the locking can be accomplished by slide bearings arranged between the inner cylindrical face of the housing 3 and the outer face of the plungers 33 cylinder portion or by other suitable means. By a similar arrangement (not shown) the nut holder 32 can be prevented from rotating in the housing 4. It is also possible to lock the nut holder and the plunger to prevent their rotation in respect of each other; this arrangement is illustrated for the combination of a nut holder and a plunger shown in FIG. 7. The oblong cylindrical section 115 of the nut holder has a profile 161 interlocking with a corresponding profile 151 of the distal opening in the plungers flange 117 section; reference numbers 104 and 108 refer to the cylinder wall of the housing and the ball screw shaft, respectively.

(18) At its end protruding from the top wall 27 of the housing 3 the ball screw shaft 8 carries a toothed pulley 9 cooperating with a toothed belt 10 driven by the pulley 12 mounted on the shaft 11 of a reversible electromotor 2 powered by a rechargeable lithium ion battery 28. The electromotor 2 is firmly mounted at the housing by means of a motor holder 25. Alternatively the electromotor can be mounted on a base 29 at which the housing is mounted. The electromotor is controlled by a control unit 24 comprising microprocessor means. The position of the plunger 3 in respect of the housing 4 is monitored by a position sensor 22, 23 in electrical contact P, Q; P′, Q′ with the control unit 24. Displacement of the nut holder 32 in a proximal direction makes the flange 16 act on the proximal end of the first coil spring 19 which transmits the compression force via the plunger 33 to the chest of the patient. The increase of resistance offered against additional compression offered by the chest causes the first coil spring to be increasingly compressed. The arrangement of the first coil spring 19 provides for determination of the force by which the patient's breast is compressed in the following manner. A means 22, 23 for detecting the position of the plunger 33 is arranged between the plunger 33 and the housing 4 in form of a foil potentiometer 22 on which a wiper 23 acts. The foil potentiometer 22 is affixed in an axial direction to the inner face of the housing 4, whereas the wiper 23 is affixed to the outer face of the plunger 33 opposite to the foil potentiometer 22. To bring down wear the wiper can take the form of a spring-loaded ball or a spring-loaded axially rounded wheel. The resistance in the foil potentiometer varies in a linear manner with the position of the wiper 23. The resistance of the potentiometer and thus the position of the plunger 33 is continuously monitored by the control unit. The position of the nut holder 32 and thus the ball screw nut 8 is monitored by the aforementioned control unit. The differences in position correspond to a force that can be calculated by taking into consideration the spring constant of the first coil spring 19, optionally also taking into consideration the spring constant of the second coil spring 18, and be used to adjust the compression depth continuously. A limiter 35 limits the compression of the first coil spring 19.

(19) The CPR apparatus of the invention may furthermore comprise a safety CPR control unit 90 independent of the electromotor control unit, the safety control unit 90 comprising a microprocessor 91, a plunger position monitoring probe 92 in electric communication with the microprocessor 91, a temperature monitoring probe 93, and optionally an electric audio alarm 94, the safety CPR control unit 90 being energized by the battery 28 energizing the electromotor 2 or a separate battery, the CPR control unit 90 being capable of reversing the electromotor 2 and stopping it when a temperature or positional limit stored in the microprocessor 91 is exceeded.

(20) The basic operation principles of CPR apparatus of FIGS. 1 and 2 will now be explained in more detail with reference to FIG. 5 in combination with FIGS. 1 and 2. In this figure only elements of the apparatus essential for the explanation are shown. At the left hand the apparatus is shown in the neutral, unloaded state at the start of a compression cycle. At the right hand the apparatus is shown in an active state at end of the compression phase. While the position of the housing is remaining fixed, the plunger 33 and the ball screw nut holder 32 are displaced distally when going from the neutral state to the active state. Prior to administration of compression in CPR the plunger 33 is lowered by making the electromotor 2 rotate the ball screw shaft 8 in an appropriate direction, such as counter-clockwise in case of a shaft 8 with right-handed threads, until the suction cup 6 abuts the chest of the patient at the sternum region. The plunger 33 is now in the neutral or unloaded state, from which the administration of CPR to the patient is started. To allow the plunger 33 be brought into this position merely by rotating the ball screw shaft 8 should have an appropriate length, such as a length of 12 cm or more, preferably of about 15 to 18 cm. Stopping the downward movement of the plunger 33 at the neutral or unloaded state can be controlled by the operator or automated by monitoring the position of the plunger 33 in respect of the nut holder 32 or a corresponding positional relationship. As soon as a decrease in axial distance between them is detected, that is, as soon as for instance a decrease in distance o in FIG. 5 is sensed, the control unit 24 stops the electromotor 2 driving the ball screw shaft 8.

(21) At start the first coil spring 19 is in an extended state whereas the second coil spring 18 is in a compressed state. During compression of the patient's chest proximal face of the plungers 33 suction cup 6 moves from L.sub.1 to L.sub.2 over a distance l, whereas the proximal flange face of the ball nut holder 32 moves from M.sub.1 to M.sub.2 over a distance m. Due to the increasing resistance of the patients chest against compression met by the plunger 33 its displacement l is smaller than the displacement m of the ball screw nut holder 32, the difference being made up by the compression length of the first coil spring 19, the difference between the distance o between points O.sub.1,O.sub.2 of the proximal face of the ball screw nut holder 32 and the distal face of the distal terminal face of the plunger 33 and the corresponding distance p between points P.sub.1, P.sub.2. While the electromotor displaces the ball screw nut holder 32 over a distance m, a compression depth of only l is obtained due to the damping effect of the first coil spring 19, m−l=o−p. Since the displacement l of the plunger 33 is monitored by the linear potentiometric position sensor 22, 23 and the displacement m of the ball screw nut holder 32 is monitored by an encoder or a Hall probe, the compression length o−p of the first coil can be determined. Since the coil spring constant of the first coil spring 19 is known, the compression force exerted on the patient can be determined for any position, and the displacement be controlled by the motor control unit so that a desired compression force is administered to the patient. The first coil spring 19 has a spring constant of about 100 N/mm; it is arranged to be essentially uncompressed in the unloaded, neutral state of the apparatus. The second coil spring 18 has a spring constant of about 0.15 N/mm; it is arranged to be sufficiently compressed in the loaded state to enable it to displace the plunger in a distal direction during the decompression phase. During retraction of the plunger 33 the distance o increases until the plunger 33 does no longer exert a pressure on the patients chest. At this moment, that is, as soon as the monitoring means detect that the distance o does no longer change, retraction of the plunger 33 is stopped. Since the resilient nature of the human chest and the height of the sternum above the back plate does change, that is, decreases during CPR, it is important that the neutral state of the plunger 33 be adapted to that change to make the plunger 33 always start from a neutral unloaded state. Additionally, the depth of compression, which is appropriately about 50 mm for an adult person, can be varied during CPR, for instance by taking into account the aforementioned anatomical changes monitored by the sensing means of the apparatus of the invention, which can be stored in the memory of the control unit.

(22) FIG. 8 illustrates a preferred embodiment of the apparatus of the invention 100 comprising a chest compression unit attached to a frame. The frame comprises two legs 41, 42 swivelingly (at 43, 44) mounted at opposite ends of a back plate 40. One of the legs 42, alternatively both legs 41, 42, is additionally mounted releaseably (at 44) at the back plate 40 to allow the frame to be easily applied around the chest of a patient. Attheir other ends the legs 41, 42 are swivelingly mounted by hinges 45; 47, 48 at opposite ends of a base 53 at which the chest compression unit of the apparatus of the invention is mounted. The base 53 forms the uppermost portion of the frame and is positioned at a substantial distance above the chest at the sternum region when mounted to a patient. The compression unit is of similar functional design as the one of FIGS. 1 and 2. The power source 51 and the control unit 52 are however disposed at opposite sides of the housing 58. The housing 58 is mounted at the base 53 in a perpendicular relationship.

(23) In contrast to the rigid mounting of the housing 4 at the base 29 in the embodiment of FIGS. 1 and 2 the housing 58 is mounted at the base 53 displaceably in a manner that it can be locked in a desired axial position. While various locking means are conceivable the locking is accomplished here by an iron cylinder 61 toothed at its one base with which it faces a correspondingly toothed, axially extending face 60 of the housing 58. The cylinder 61 is mounted displaceably in a bore of a sturdy socket 62 enclosing the housing 58 and firmly mounted at the base 53. The bore with the cylinder 61 extends in a direction perpendicular to the cylinder axis of the housing 58. In a locked position the toothed base of the cylinder 61 is pressed against toothed face 60 of the housing 58 by a spring coil. In an unlocked position the iron cylinder 61 is withdrawn against the force of the spring 64 coil from the toothed face 60 of the housing 58 by an energized electromagnet coil 63 into the lumen of which it extends with its non-toothed end. This arrangement allows the housing 58 with the plunger 65 to be displaced in a downward proximal direction towards the chest of the person to receive CPR treatment until contact between the sternum region and the compression disk/suction cup assembly 57, 56 is obtained. Then the housing 58 is locked with the base 53 by energizing the coil 63, and administration of CPR can start. A handle 59 in form of a circumferential flange attached to the housing 58 near its distal end facilitates the axial displacement of the housing 58 in an unlocked state.

(24) The housing, the electromotor, the entire transmission of the driving force from the electromotor to the ball nut shaft, and the control unit 24 are partially or fully enclosed by a protective cover 50. The power source 51, a 24 V lithium ion battery, is disposed in a pocket of the cover 50, in which it is held by a snap connection (not shown). An exhausted battery thus can be easily replaced by a charged one. A female connector mounted on the cover 50 allows the motor to be powered by 10-32 V DC, which is available in an ambulance or from a medically certified 90-264 V AC aggregate that provides 24 V DC.

(25) A top face of the control unit 52 is provided with a means for input of instructions to the electromotor control unit. The input means is, for instance, a touch-sensitive polymer film panel 54. The panel 54 comprises a number of input keys 55 and may also comprise indicators, such as LED indicators, for battery status and other functions. By exerting pressure on a particular area an electrical contact is temporarily closed to send an electric signal to the control unit. Since the apparatus of the invention is used in emergency situations, it is important that the operator can rely on a simple choice of instructions.

(26) A panel comprising a polymer foil 54 with touch sensitive areas or buttons 70, 72, 74 76, 79 for entering a preferred pattern of instructions to the apparatus of the invention is shown in FIG. 10. The buttons 70, 72, 74, 76 are paired with LEDs 71, 73, 75, 77, respectively, indicating whether a button has been activated by touching it.

(27) By pressing the adjustment button 70 for a short time (<0.5 seconds) the apparatus is set to a plunger adjustment state. In the plunger adjustment state the position of the plunger with the suction cup in respect of the patient can be adjusted. This adjustment is accomplished by, for instance, means functionally corresponding to the means illustrated in FIG. 8 for locking/unlocking the housing 58 in respect of the base 53. On activating the adjustment state the zero position data in the memory of the control unit is erased. In a plunger adjustment prior to the administration of CPR, the suction cup of the plunger is made to abut the sternum region of the patients chest without exerting any pressure on it. By pressing the adjustment button 70 for a longer time (>0.5 seconds) the apparatus is switched on or off, depending on its state.

(28) By pressing the locking or pausing button 72 the housing 58 is positionally locked in respect of the base 53. This locking position is stored as the zero (displacement) position in the memory of the control unit. As long as the driving of the plunger is not activated the plunger remains locked with the housing 58. The locking position can be activated during CPR treatment, for instance during defibrillation of the patient or for other reasons.

(29) By pressing the active mode button 74 the apparatus is put into the continuous operating mode, in which it performs continuous compressions at a rate of 100 compressions per minute, which is preferred. The control unit may though be programmed for any other desired continuous compression rate. Alternatively, by pressing the activation 30:2 button the apparatus is put into a discontinuous operating mode, in which it performs 30 compressions at a chosen rate, in particular at a rate of 100 compressions per minute, followed by a pause of 3 seconds in which no compressions are administered. This cycle of 30 compressions/3 sec pause is continued until stopped temporarily by the operator by pressing the pausing button 72 or by pressing the adjustment button 70 to allow the plunger, if desired, to be withdrawn prior to dismounting the chest compression apparatus from the patient.

(30) The charging state of the battery is monitored by light indicators 78. If the battery charge is so low that the battery should be replace the rightmost one of charging state indicators 78 is lighted and a buzzer arranged in the apparatus does emit a buzzing sound. The emptied battery is exchanged for a charged one by pressing the pause button 72, changing the battery, and pressing the active mode button 74 to resume administration of CPR from the stored zero position. The buzzer can be switched off for 60 seconds by pressing the buzzer silencing button 79.

(31) A warning light 80 is set to warn for a variety of malfunctions, such as a software conflict, insufficient battery power, a sensing means failure, etc.

(32) FIG. 11b illustrates the motor power supply control signal (by pulse width modulation, PWM) and the recorded compression/decompression profile over a compression/decompression cycle obtained by entering a set of data (length of cycle sec); stroke depth (cm); time (sec) T.sub.1 from start of cycle to maximum stroke depth; T.sub.2−T.sub.1=time at maximum stroke depth; T.sub.3−T.sub.2=time from maximum stroke depth to zero stroke depth) and so as to generate the desired compression/decompression profile illustrated in FIG. 11a. FIG. 11c illustrates the motor current (A) and the recorded chest height (mm) over the compression/decompression cycle of FIG. 11a.

(33) The electromotor control unit of the CPR apparatus comprises software for recording the initial chest height unaffected by compression, that is, the distance between the skin area above the sternum on which the compression member is applied and the back plate in a direction perpendicular to a support or back plate on which the patient rests with his or her chest. In a zero compression depth setting mode the plunger with the compression member is displaced in a downward direction by the electromotor until the face of the compression member facing the chest of the patient is abutting but not compressing the chest above the sternum. During a further downward movement, such as a movement of a few mm, the compression member experiences an increasing a resistance by the chest tissues against compression. This resistance is detected by a change in the ratio of displacement of the first monitoring means and the second monitoring means. Once such a change is detected the displacement is stopped; for positional fine tuning the plunger/compression member may be retracted for the distance during which it has experienced an increasing resistance. Upon retraction the plunger/compression member is set at the exact zero compression depth. Alternatively setting of the zero compression depth can be controlled manually by the operator. The recorded zero compression depth or initial chest height is stored as a reference in a memory of the electromotor control unit. In particular, it is stored in a permanent memory to allow the battery of the apparatus to be changed without loss of data. To compensate for a variation of chest height between patients the electromotor control unit comprises software for setting the full compression depth to a given fraction of the measured initial chest height. The given fraction may be made vary in a linear or non-linear manner between patients with a large chest and patients with a small chest. By this feature of the invention the patient will receive compressions of a depth appropriate to his or her chest anatomy so as to avoid compressions putting the integrity of the tissues of the chest at risk or compressions of insufficient depth.

(34) In another preferred embodiment of the invention the electromotor control unit of the CPR apparatus comprises software for a soft start of compressions. A soft start of compressions is a characterized by a continuous linear or non-linear increase from a compression depth of zero mm to a full compression depth, such as a full compression depth of 55 mm reached after seven compressions of linearly increasing depth (FIG. 12). FIG. 13 illustrates a second preferred embodiment of the chest compression unit of the CPR apparatus of the invention, which differs from the first preferred embodiment illustrated in FIGS. 1-5 substantially by the combination of reversible electromotor means and mechanical means for translating rotational motion to linear motion having been exchanged for linear electromotor means. The linear motor means comprise a linear motor of known design, such as one disclosed in U.S. Pat. Nos. 4,460,855 and 5,091,665. Suitable linear motors are manufactured by NTI AB, Spreitenbach, Switzerland and their U.S. subsidiary LinMot, Inc. The chest compression unit 201 is mounted in a frame (not shown) and comprises a generally cylindrical plunger corresponding to the plunger 33 of FIG. 4 disposed co-axially in a cylindrical housing 204. The housing 204 has a proximal open end and a distal end closed by a top wall 227. The plunger comprises a cylindrical main section 203 and a distal terminal section in form of an inwardly bent circular flange 217. At its proximal end the cylindrical section 203 of the plunger protrudes from the proximal opening of the housing 204. A chest compression disk 205 provided with a polymer suction cup 206 on its proximal face is affixed to the cylindrical section 203 at the proximal end thereof. From the neutral, unloaded state illustrated in FIG. 13 the plunger can be displaced axially in respect of the housing 204 by tubular linear induction motor means comprising a “stator” and a “rotor”. The stator comprises a longitudinal series of coaxial permanent magnets 243 in, for instance, a NSNS . . . sequence regularly interspaced by a series of non-conducting cylindrical spacers 244 of same diameter. The magnets 243 and the spacers 244 are enclosed in a cylindrical shell 242 of an insulating material. At its one, distal end the stator 242, 243, 244 is centrally mounted in the top wall 227 of the housing 204 in a manner so as to extend along the axis xA-xA of the housing 204 and from there into the lumen of the plunger 233. The rotor comprises a longitudinal series of regularly interspaced cylindrical metal coils 241 centered at the axis xA-xA and enclosing, at a short distance, the stator 242, 243, 244, which is freely axially displaceable within the cylindrical void defined by the coils 241 of the rotor. The rotor, which is referred to in the following by the reference number 241 of the coils is enclosed by a ferromagnetic shielding tube 245. A similarly centered rotor casing 246 surrounds the shielding tube 245 at a distance, the void 247 between the tube 245 and the rotor casing 246 being used for housing electrical circuitry. Electrical connection between the linear motor and a motor control unit 224 is provided by a multi-lead cable 249 comprising an extendable coil portion 248 disposed between the top wall 227 of the housing and the linear motor casing 246 to compensate for axial displacement of the rotor 241. Not shown are Hall sensor or photocell means disposed between the stator and the rotor for detecting their relative position. The rotor casing 246 and, thereby, the rotor 241 is secured at wall sections 213, 230 of a generally rotationally rotor holder 213, 214, 230 functionally corresponding to the ball nut screw holder 32 of the embodiment of FIGS. 1-5. A proximal terminal radial flange 216 of the rotor holder transmits the axial displacement of the rotor 241 to a distal face of the compression disk 205 via a first spring coil 219 mounted between the distal face of the compression disk 205 and a proximal face of the flange 216. A second spring coil 218 mounted between a proximal face of the rotor holders flange 216 and a distal face of the plungers 233 proximal flange 217 serves for maintaining contact of the first spring coil 219 with the disk 205 and the terminal flange 216. Appropriate, including alternate, energizing of one or more of the rotor coils 241 displaces of the rotor 241 in a distal direction. Opposite energizing displaces the rotor 241 in an opposite, proximal direction. Wall section 230 is integral with an oblong cylindrical section 215 of smaller outer and inner diameter ending in the radially outwardly extending proximal flange 216. The rotor holder 213, 214, 230 constitutes an actuator, which is displaceable along the stator 242, 243, 245 in either direction by appropriate energizing and thus along the coincident cylinder axes xA-xA of the housing 204 and the plunger 203, 217.

(35) The stator 242, 243, 244 centers the rotor 241 and the plunger 203, 217 in the housing 204. In addition, the rotor 241 and the plunger 203, 217 can be kept centered in the housing 204 by spacer means such as linear ball bearings 220, 221 co-operating with corresponding bearings disposed in corresponding radial planes. This arrangement corresponds to that shown in FIG. 6 for the first preferred embodiment of the invention.

(36) The linear electromotor is powered by a rechargeable lithium ion battery 228. The linear electromotor is controlled by the control unit 224 comprising microprocessor means. The position of the plunger 203, 217 in respect of the housing 204 is monitored by a position sensor 222, 223 in electrical contact xP, xQ; xP′, xQ′ with the control unit 224.

(37) Displacement of the rotor 241in a proximal direction makes the flange 216 act on the proximal end of the first coil spring 219, which transmits the compression force via the plunger 203, 217 to the chest of the patient. The increase of resistance against additional compression offered by the chest causes the first coil spring 219 to be increasingly compressed. The arrangement of the first coil spring 219 provides for determination of the force by which the patients breast is compressed in the following manner. The aforementioned means 222, 223 for detectingthe position of the plunger is arranged between the plunger 203, 217 and the housing 204 in form of a foil potentiometer 222 on which a wiper 223 acts. The foil potentiometer 222 is affixed in an axial direction to the inner face of the housing 204, whereas the wiper 223 is affixed to the outer face of the plunger 203,217 opposite to the foil potentiometer 222. To bring down wear the wiper 223 can take the form of a spring-loaded ball or a spring-loaded axially rounded wheel. The resistance in the foil potentiometer varies in a linear manner with the position of the wiper 223. The resistance of the potentiometer and thus the position of the plunger 203, 217 is continuously monitored by the control unit. The position of the rotor 241 is monitored by the control unit 224. The differences in position correspond to a force that can be calculated by taking into consideration the spring constant of the first coil spring 219, optionally also taking into consideration the spring constant of the second coil spring 218, and be used to adjust the compression depth continuously. The first and second coil springs fully correspond functionally to the first and second coil springs 19, 18, respectively, of the embodiment of FIGS. 1-5. A stroke limiter 235 limits the compression of the first coil spring 219. An operator interface 270 comprising a keyboard and a display allows an operator to enter parameter values for a selected mode of CPR, and to monitor the CPR procedure based on these values. Reference no. 229 designates a portion of the CPR apparatus frame to which the chest compression unit of FIG. 13 is mounted.

(38) The basic operation principles of the chest compression unit of the CPR apparatus of FIG. 13 correspond to those of the unit of FIGS. 1-5 to which reference is made.