Patent Publication Number: US-2020289367-A1

Title: Automated Chest Compression Device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority to U.S. patent application Ser. No. 14/885,952 filed Oct. 16, 2015, which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTIONS 
     The inventions described below relate to the field of CPR. 
     BACKGROUND OF THE INVENTIONS 
     Cardiopulmonary resuscitation (CPR) is a well-known and valuable method of first aid used to resuscitate people who have suffered from cardiac arrest. CPR requires repetitive chest compressions to squeeze the heart and the thoracic cavity to pump blood through the body. In efforts to provide better blood flow and increase the effectiveness of bystander resuscitation efforts, various mechanical devices have been proposed for performing CPR. In one variation of such devices, a belt is placed around the patient&#39;s chest and the belt is used to effect chest compressions, for example our commercial device, sold under the trademark AUTOPULSE®. Our own patents, Mollenauer, et al., Resuscitation Device Having A Motor Driven Belt To Constrict/Compress The Chest, U.S. Pat. No. 6,142,962 (Nov. 7, 2000); Sherman, et al., CPR Assist Device with Pressure Bladder Feedback, U.S. Pat. No. 6,616,620 (Sep. 9, 2003); Sherman, et al., Modular CPR assist device, U.S. Pat. No. 6,066,106 (May 23, 2000); and Sherman, et al., Modular CPR assist device, U.S. Pat. No. 6,398,745 (Jun. 4, 2002); Jensen, Lightweight Electro-Mechanical Chest Compression Device, U.S. Pat. No. 7,347,832 (Mar. 25, 2008) and Quintana, et al., Methods and Devices for Attaching a Belt Cartridge to a Chest Compression Device, U.S. Pat. No. 7,354,407 (Apr. 8, 2008), show chest compression devices that compress a patient&#39;s chest with a belt. Each of these patents is hereby incorporated by reference in their entirety. 
     These devices have proven to be valuable alternatives to manual CPR, and evidence is mounting that they provide circulation superior to that provided by manual CPR, and also result in higher survival rates for cardiac arrest victims. The devices provide Chest compressions at resuscitative rates and depths. A resuscitative rate may be any rate of compressions considered effective to induce blood flow in a cardiac arrest victim, typically 60 to 120 compressions per minute (the CPR Guidelines 2010 recommends 80 to 100 compression per minute), and a resuscitative depth may be any depth considered effective to induce blood flow, and typically 1.5 to 2.5 inches (the CPR Guidelines 2010 recommends about 2 inches per compression). 
     The AUTOPULSE® chest compression device uses a belt, which is releasably attached to a drive spool with the housing of the device. In a convenient arrangement, a spline is secured to the belt, and the spline fits into a slot in the drive spool of the device. The drive spool is accessible from the bottom, or posterior aspect, of the device. Before use, a fresh belt is fitted to the device, and this requires lifting the device to insert the spline into the drive spool. The patient is then placed on the housing of the device, and the belt is secured over the chest of the patient. Opposite ends of the belt are held together, over the chest of the patient, with hook and loop fasteners. The arrangement has proven effective for treating cardiac arrest victims and convenient to use. Other belt-based CPR compressions devices have been proposed, but not implemented in clinical use. Lach, Resuscitation Method and Apparatus, U.S. Pat. No. 4,770,164 (Sep. 13, 1988) secures a belt around a patient by threading it under a first roller, then under a second roller, over the patient, back under the first roller, and then to a large roller disposed on one side of the patient. The belt is secured to the roller with hook and loop fasteners, and is sized to the patient by the operator of the device. Kelly, Chest Compression Apparatus for Cardiac Arrest, U.S. Pat. No. 5,738,637 (Apr. 14, 1998) uses a belt that is bolted at its midpoint to the underside of a backboard, than secured to a scissor-mechanism on the patient&#39;s chest with hook and loop fasteners. Belt installation is not convenient in either device. A new, more convenient arrangement of the drive components and belt is disclosed in this application. 
     Another feature of our AUTOPULSE® CPR chest compression device is the ability of the control system to hold the compression belt at the height of compression. The AUTOPULSE® can operate to perform compression in repeated compression cycles comprising a compression stroke, a high compression hold, a release period, and an inter-compression hold. No other automated CPR chest compression device is capable of holding compressions at a high threshold of compression. The method of operating the AUTOPULSE® device to accomplish compressions in cycles of compression, hold, and release is covered by our previous patent, Sherman, et al., Modular CPR assist device to hold at a threshold of tightness, U.S. Pat. No. 7,374,548 (May 20, 2008). The holding periods are accomplished with a brake operably connected to the motor drive shaft of the device, which can be energized to stop the drive shaft to lock the belt in place about the patient. A new, more energy-efficient braking system is disclosed in this application. 
     On occasion, a chest compression device must be used on a patient at the same time that doctors want to take x-rays of the patient&#39;s chest. This is not possible if the radiopaque metal components of the chest compression device (the motor and drive train) are located directly under the load distributing portion of the compression belt, which overlies the patient&#39;s chest and heart when properly installed, so that the radiopaque component are also located under the heart. This means that radiopaque component are in the field of view of the x-ray machine. 
     SUMMARY 
     The devices and methods described below provide for a belt-driven chest compression device in which the compression belt is readily replaceable. The chest compression device includes a platform which houses drive components, and a compression belt which is connected to the drive components through releasably attachable couplings near the upper surface of the device. Removal and replacement of the belt may be accomplished while a patient is disposed on the housing. This arrangement helps avoid twisting of the belt and facilitates removal and replacement of the belt. Installation of the belt is simpler than our prior AUTOPULSE® device, and is tensioned upon installation by the user. To ensure that compression cycles start from an optimum low level of tightness, without slack, the control system of the device may control the device to loosen the belt upon start-up and thereafter draw the belt to the slack take-up position, or to tighten the belt upon start-up while monitoring an indicator of tightness (motor current, load on a load cell, strain on the belt), and conditionally tighten the belt to a slack take-up position (if the belt is loose initially) or reverse and loosen the belt and then tighten the belt while monitoring an indicator of tightness, to tighten the belt to a slack take-up position (if the initial tightness exceeds the desired tightness of a slack take-up position). 
     A brake is used to provide the holding periods during operation of the device. The brake comprises a parking pawl, with a pawl and park gear arrangement, with a park gear fixed to a component in the drive train, and a pawl operable to obstruct the park gear. 
     The arrangement of components in the device provides for a radiolucent region of the device, which underlies the heart of the patient when the device is installed properly on a cardiac arrest victim. For example, the compression belt may be driven by laterally located drive spools, which extend superiorly in the device to drive train components disposed superiorly to the compression belt (and, thus, superiorly to the heart of the patient when the device is installed). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the CPR chest compression device installed on a patient. 
         FIG. 2  is a perspective view of the CPR chest compression device, illustrating the connection between the compression belt and intermediate straps at a point above the housing. 
         FIG. 3  illustrates the single-piece compression belt which may be used in the compression device of  FIG. 1 . 
         FIG. 4  is a perspective view of drive train of the compression device, including the motor and drive shaft, drive belts, and secondary or planetary drive spools. 
         FIG. 5  is an end view of drive spool, drive belts, and secondary drive spools. 
         FIGS. 6, 7, 8, 9 and 10  illustrate alternative drive trains for rotating the drive spools. 
         FIGS. 11, 12 and 13  illustrate improved braking mechanisms for use with the drive train of  FIG. 4  and other chest compression devices. 
         FIG. 14  illustrates another embodiment of a CPR chest compression device installed on a patient. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTIONS 
       FIG. 1  shows the chest compression device fitted on a patient  1 . The chest compression device  2  applies compressions with the compression belt  3 . The chest compression device  2  includes a belt drive platform  4  sized for placement under the thorax of the patient, upon which the patient rests during use and which provides a housing  5  for the drive train and control system for the device. The control system, embedded anywhere in the device, can include a processor and may be operable to control tightening operation of the belt and to provide output on a user interface disposed on the housing. Operation of the device can be initiated and adjusted by a user through a control panel  6  and a display operated by the control system to provide feedback regarding the status of the device to the user. 
     The belt includes a wide load-distribution section  7  at the mid-portion of the belt and left and right belt ends  8 R and  8 L (shown in the illustration as narrow pull straps  9 R and  9 L), which serve as tensioning portions which extend from the load distributing portion, posteriorly relative to the patient, to drive spools within the housing. The left and right belt ends are secured to intermediate straps  10 R and  10 L, with loops  11 R and  11 L (for example, square loops, as illustrated). When fitted on a patient, the load distribution section is disposed over the anterior chest wall of the patient, and the left and right belt ends extend posteriorly over the right and left axilla of the patient to connect to their respective lateral drive spools shown in  FIG. 2 . 
       FIG. 2  shows the chest compression device in isolation, including the belt drive platform and housing. As illustrated in  FIG. 2 , the intermediate straps  10 R and  10 L are secured at one end to the loops, and secured at the other end to planetary drive spools  12 R and  12 L disposed laterally on either side of the housing. The planetary or lateral drive spools are in turn driven by a motor also dispose within the housing, through various belts and gears described below. The intermediate straps are attached to the planetary or lateral spools such that, upon rotation of the spools, the intermediate straps are pulled posteriorly, spooled upon the lateral spools, thereby drawing the compression belt downward to compress the chest of the patient. The intermediate straps can be fixed to the planetary or lateral drive spools in any suitable manner. The intermediate straps may be flexible and floppy, or they may be self-supporting (that is, they remain in vertical orientation, without other support, when the platform is horizontal) so long as they are still flexible enough so they may be wrapped around the drive spools. 
     The belt  3 , as shown in  FIG. 3 , comprises the load distribution section  7  and left and right belt ends  8 R and  8 L in the form of left and right pull straps  9 R and  9 L. The load distribution section is sized and dimensioned to cover a significant portion of the anterior surface of a typical patient&#39;s chest. The pull straps are narrow, relative to the load distribution section, to limit material requirements of the associated spools, but the belt ends may be made in the same width as the load distribution section. Corresponding hook sections and loop sections ( 13 R,  13 L) on the left and right belt ends secure the compression belt to the loops ( 11 R,  11 L) and thus to the intermediate straps  10 R and  10 L. The pull straps are fitted through the loops, folded together and secured with hook and loop fasteners or other releasable attachment system (that is, attachment systems that can be operated to quickly attach and detach the two parts without tools). The hook and loop fasteners together with the loops provide a convenient means for releasably securing the compression belt to the intermediate straps, in conjunction with double loop sliders illustrated in  FIG. 1 , but other convenient means of releasably attaching the belt ends to the intermediate straps may be used (such as matching center release buckle components (seat belt buckles), side release buckles (back pack buckles) cam buckles, belt buckles, etc. may be used). One size belt may be used for patients of various sizes, or belts of various sizes can be provided for use with the device depending on the size of the patient. The initial tightness of the belt is established by a CPR provider who pulls the straps through the double loop sliders and attaches hook and loop segments together (the system may establish a slack take-up position for the belt, as described below, after the CPR provider has secured the belt to the buckles). The belt is preferably a one-piece belt, but can be provided as a two-piece belt with overlapping load-distribution sections which can be applied by first laying one side over the patient&#39;s chest and next laying the other side over the first side, and securing the two sections together (with, for example, corresponding hook and loop fasteners). A bladder may be incorporated into the load-distribution section  7 . 
     The belt ends may be attached directly to the drive spools, using a spline and slot arrangement disclosed in our prior U.S. Patent, Quintana, et al., Methods And Devices For Attaching A Belt Cartridge To A Chest Compression Device, U.S. Pat. No. 8,740,823 (Jun. 3, 2014). The belt ends may be attached directly to the drive spools using any suitable fastener, clamp or connecting means. 
     The drive spools have a first segment engaging the drive belts, and a second segment, extending inferiorly from the first segment, which engages the intermediate straps or belt ends. The space between the drive spools, on a corresponding coronal plane and inferior to the drive belts, is unoccupied by drive train components or other radiopaque components and thus constitutes the radiolucent window mentioned above. 
     In use, a CPR provider will apply the compression device to a cardiac arrest victim. The CPR provider will place the cardiac arrest victim on the housing  5 , and secure the belt ends  8 R and  8 L to the respective left and right intermediate straps (or directly to the drive spools), with the patient already on the anterior surface of the housing, so that there is no need for access to the bottom surface of the device. Where the compression belt is a one-piece belt, at least one of the belt ends is secured to its corresponding intermediate strap after the patient is placed on the platform. With the belt in place, the CPR provider initiates operation of the chest compression device to repeatedly compress the chest of the patient to a depth and at a rate suitable for resuscitation. If the belt must be replaced after the patient is placed on the platform, the CPR provider can readily detach the compression belt from the intermediate straps and install a new compression belt by securing the belt end of the new compression belt to the intermediate straps. This can be done without removing the patient from the housing, which saves a significant amount of time compared to prior art systems and minimizes the delay in initiating chest compressions attendant to belt replacement. With the belt in place, the CPR provider initiates operation of the device to cause repeated cycles of tightening and loosening of the belt about the thorax of the patient. Should the belt become damaged, or twisted during use (the front-loading device should make twisting less likely), the CPR provider interrupts operation of the device to replace the belt, detaches the right belt end from the right intermediate strap or right drive spool, and detaches the left belt end from left intermediate straps or the left drive spool, while the patient remains on the platform. 
     The benefits of the compression belt and intermediate straps arrangement, with a releasable attachment to the intermediate straps, can be achieved in combination with the benefits of additional inventions described below, or they may be achieved in isolation. 
       FIG. 4  is a perspective view of drive train of the compression device, including the drive shaft, drive belts, and planetary drive spools, which operably connects the motor  20  and its motor shaft to the compression belt. The drive train comprises a first drive shaft  21  (in this case, an extension of the motor shaft or the output shaft of any reduction gears) and a first gear  22  (a sun gear) which in turn is fixed to the first drive shaft. The first/sun gear engages a second/planetary gear  23  which in turn is fixed to a second drive shaft  24 . (The motor shaft, first and second drive shafts, gears and drive spools are supported in a channel beam which extends across the device, providing support for the components and the housing.) Rotation of the first drive shaft  21  in one direction results in counter-rotation (rotation in the opposite direction) of the second drive shaft  24 . The first and second drive shafts thus rotate in opposite directions. The first and second drive shafts  21  (left) and  24  (right) are connected to the first and second lateral drive spools  12 R and  12 L through drive belts  25 R and  25 L, such that rotation of the first and second shafts results in rotation of the first and second lateral drive spools, which in turn spool the intermediate straps to cause tightening of the compression belt about the chest of the patient. As illustrated in  FIG. 4 , the drive shafts may comprise toothed wheels (driving pulleys) and the drive spools may comprise toothed wheels (driven pulleys), and the drive belt is a toothed drive belt. The motor shaft can be connected to the first drive shaft  21  directly or through reduction gears in a gear box  26 . A brake  27  may be operably connected to the drive train at any appropriate point, and several embodiments of preferred brakes are shown in more detail in  FIGS. 11, 12 and 13 . 
     As depicted in  FIG. 4 , the drive shafts  21  (left) and  24  (right) are disposed asymmetrically about the inferior/superior centerline of the device, but the drive spools may be disposed symmetrically. The belts provide a convenient linkage between the toothed wheels, and may be replaced with comparable components such as chains, with corresponding sprockets on the drive shafts ( 21 ,  24 ) and first and second lateral drive spools  12 R and  12 L, or worm gears interconnecting drive shaft (or shafts) with the lateral drive spools. 
     In the arrangement of  FIG. 4 , a single motor is used to drive both drive shafts and both drive spools, without a direct connection to the compression belt, which is one system which enables the anterior releasable attachment system for the compression belt. In this arrangement, the motor  20 , battery  28 , and control system are located superiorly to the portion of the lateral drive spools  12 R and  12 L to which the intermediate straps or belt ends are secured (in our current AUTOPULSE® compression device, the motor drive shaft is located on the same transverse plane as the lateral spindles) thus leaving an open, unoccupied space in the inferior portion of the device which is devoid of radiopaque components. This open, unoccupied space is located beneath (posterior to) the load distributing band. Thus, when the compression device is installed on the patient, this unoccupied space is located under the heart of the patient, and provides a clear, radiolucent window for imaging the heart with fluoroscopy, x-rays or CT scanning. When installed on the patient, motor and drive shafts which drive the belts are located superiorly to the region of the housing underlying the compression belt, corresponding to the region of the patient&#39;s heart, and the drive spools, though they extend inferiorly into the superior/inferior level of the heart, are laterally displaced from the centerline of the housing (and, correspondingly, from the centerline of the patient&#39;s body). The benefits of the drive train illustrated in  FIG. 4  can be obtained in combination with the front-loaded compression belt of  FIG. 1 , or with other belt attachment mechanisms. Also, the benefits of the radiolucent window can be achieved with other arrangements of the drive train, so long as the drive train components are displaced from the area of the platform which underlies the patient&#39;s heart during use (for example, two motors may be used, with one motor operably connected to each drive spool, or directly to each drive shaft). 
       FIG. 5  is an end view of the drive shaft (from the inferior end of the device), drive belts, and secondary drive spools shown in  FIG. 4 , including the drive shafts  21  (left) and  24  (right), lateral drive spools  12 R and  12 L, drive belts  25 R and  25 L and the motor  20 . During the compression stroke, the motor is operated to turn each drive spool sufficiently to pull the intermediates straps downward to the extent necessary to achieve compression at the desired depth. This may vary with the diameter of the drive spools. Preferably, the drive spools  12 R and  12 L are about 0.75″ (2 cm) in diameter, and rotate about 2.5 rotations on each compression stroke (drive spool  12 R will rotate counter-clockwise when viewed from the inferior view of  FIG. 5  and drive spool  12 L will rotate clockwise, in this arrangement) to pull the intermediate straps downwardly (posteriorly, relative to a patient laying supine on the housing) about 1 to 2 inches (2.5 to 5 cm) to obtain a chest compression of the desired depth of 2 inches (5 cm). The drive spools  12 R and  12 L may be made with a larger diameter, such that it takes less rotation, such as half of a complete rotation, to spool the intermediate straps only partially around the drive spools, to pull the intermediate straps downward to the extent necessary for adequate compression. In this arrangement, the intermediate straps can be made of a fairly stiff material, such that they are self-supporting and stand vertically above the housing when not attached to the belt. 
     The drive train can be varied, while still achieving the benefits of arrangement which permits attachment of the belt to the drive train from the front or side of the housing. For example, as shown in  FIG. 6 , the linkage between the drive spools can be provided with a rack and pinion system, with drive pinions (toothed wheels)  31 R and  31 L, and right and left racks  32 R and  32 L and right and left driven pinions  33 R and  33 L. (Various arrangements can be used to properly rotate the drive spools, including a single pinion with a reversing gear at one of the drive spools, or disposition of the belt end/intermediate strap on opposite sides of the drive spools, as shown in  FIG. 8 .) As shown in  FIG. 7 , the linkage between the drive shafts can drive the left and right drive shafts and the left and right drive spools  12 R and  12 L through drive straps  34 R and  34 L. The drive straps in this system spool about the drive shafts, and also about the left and right drive spools  12 R and  12 L (a single drive shaft may be used in this embodiment). 
     In operation, rotation of the drive shafts will result in spooling of the drive straps  34 R and  34 L on the drive shafts  31 R and  31 L, which will result in rotation of drive spools  12 R and  12 L, and thus result in tightening of the compression belt. This system may use the natural resilience of the chest to expand the compression belt in the release phase of the compression cycle, while the motor operates to allow unspooling of the drive straps  34 R and  34 L about the drive shafts  31 R and  31 L coincident with the spooling of the drive straps  34 R and  34 L about the drive spools  12 R and  12 L. 
       FIG. 8  shows a drive train in which both the right and left belts are driven by a single drive shaft, with each drive belt causing rotation of its associated drive spool in opposite directions, with one of the drive spool/intermediate strap connections disposed on the inside (medial) portion of the drive spool to ensure that rotation of the drive spool results in spooling of the intermediate strap on the drive spool. Each of these drive trains can be used in a system in which the compression belt is releasably or permanently attached to the drive train from the front of the device, or the side of the device, thus allowing installation, removal and replacement of the belt while the patient is on the platform. (Analogous to the usage relating to automobiles, the drive train is the group of components that operate to deliver power to the belt, exclusive of the motor). 
       FIG. 9  shows a drive train similar to the drive train of  FIG. 5 , in which the lateral drive spools  12 R and  12 L of  FIG. 5  are replaced with sprocketed spools  35 R and  35 L. The sprocketed spools engage corresponding perforations in the intermediate straps, and pull the intermediate straps downward when rotated in a first direction, thus tightening the belt, and push the intermediate straps upward when rotated in the opposite direction, thus loosening the belt. Corresponding tensioning spools  36 R and  36 L are provided immediately adjacent to the sprocketed spools  35 R and  35 L, to force the perforated intermediate straps into engagement with a sprocket of the sprocketed spools. 
     In each of the drive trains illustrates in  FIGS. 5 through 9 , levers may be used in lieu of a large diameter drive spool, and would function to pull the intermediate straps posteriorly. Levers attached to the intermediate straps, driven by the same mechanisms proposed for the lateral drive spools, will pull the intermediate straps posteriorly to tighten the belt. 
       FIG. 10  shows a drive train for driving the compression belt using a ring gear and pinion. In this system, the ring gear  37  takes the place of the rack of the drive train of  FIG. 6  described above, to transfer power from the motor and drive shaft to the lateral drive spools. In this system, drive pinion  31  drives the ring gear, in alternating clockwise and counterclockwise rotations, which in turn drive the driven pinions  33 R and  33 L and their translating output pinions  38 R and  38 L, which in turn drive the drive spools  12 R and  12 L in back and forth rotations to pull down and push up, or spool and unspool, the intermediate straps  10 R and  10 L (not shown). The ring gear is preferably located superiorly to the inferior portion of the drive spools which engage the intermediate straps, so that, when a patient is disposed on the device, with the belt properly positioned over the thorax, the ring gear does not lie in the region of the housing which underlies the patient&#39;s heart. 
     Finally, the drive spools can be replaced with any convenient lever mechanism, driven through appropriate linkages by the motor, and operable to pull the intermediate straps downwardly and push the intermediate straps upwardly (or at least allow upward motion on recoil of the patient&#39;s thorax), while obtaining the benefit of maintaining an empty space in the “heart” region of the housing. The spools, however, are a convenient implementation of a levering mechanism. 
     The compression device preferably operates to provide cycles of compression which include a compression down-stroke, a high compression hold, a release period, and an inter-compression hold. The hold periods are accomplished through operation of a brake operable to very quickly stop the rotating components of the drive train. Any brake may be used, including the cam brake or wrap spring brake previously proposed for use in a chest compression device, or the motor can be stalled or electronically balanced to hold it during hold periods.  FIG. 11  illustrates an improved braking mechanism that may be used with the drive train of  FIG. 4 . The braking mechanism comprises a parking pawl mechanism, similar to parking pawls used in automotive transmissions. The parking pawl  41  and associated park gear (a notched wheel or ratchet wheel)  42  can be located at any point in the drive train or motor shaft, with the park gear non-rotatably fixed to any rotating component, and is shown in  FIG. 11  fixed to the motor shaft  21 , between the motor  20  and the gear box  26 . The pawl  41  is operated by a solenoid actuator  43  and solenoid plunger  44  or other actuator (for example, a motor may be used to swing the pawl into contact with the park gear), which is fixed relative to the drive shaft. To brake and stop the drive train the control system operates the solenoid to force the pawl into interfering contact with the park gear, and to release the drive train the control system operates the solenoid to withdraw the pawl from the park gear. Preferably, the pawl is spring-biased away from the park gear, so that if the solenoid fails the pawl will be withdrawn from interference with the park gear. In this case, the solenoid is operated to force the pawl toward the park gear during the entire hold period. Alternatively, the pawl is shifted by action of a spring into interfering contact, and remains in interfering contact until the solenoid is powered to withdraw the pawl, so that battery power is not needed to hold the pawl in interfering contact. Alternatively, the pawl may be unbiased, so that, after being shifted by action of the solenoid into interfering contact, it remains in its interfering position until withdrawn, so that battery power need not be consumed to hold the brake in position (but may be applied to hold the brake in position), and is only applied to shift the pawl into interfering contact with the park gear and withdraw the pawl. 
     Various parking pawl mechanisms may be used. As illustrated in  FIG. 12 , another suitable parking pawl mechanism includes the park gear  42 , the solenoid plunger  44  and pawl  41  which directly engages the park gear and serves as the pawl. To brake and stop the drive train the control system operates the solenoid to force the pawl into interfering contact with the park gear, and to release the drive train the control system operates the solenoid to withdraw the pawl from the park gear. As illustrated in  FIG. 13 , another suitable parking pawl mechanism includes the park gear  42 , a sliding pawl  45 , and cam  46 . The cam is turned with a rotary solenoid  47 , which engages the follower  48  to push the pawl into interfering contact with the park gear. The cam may have an eccentric profile, however the portion of the cam lobe in contact with the follower when the cam is in the locked and/or unlocked position is circular (for example, a non-circular cam lobe with an isodiametric top radius, where a radius of a contact point with the follower is a substantially fixed radius relative to the cam shaft) so that forces applied to the cam by the follower will not cause the cam to rotate. This allows the cam lobe portions associated with locking and unlocking to maintain a stable position. The follower rests on an equal radial segment or portion of the cam lobe during engagement of the pawl with the park gear to maintain a stable position and minimize disengagement force to release the park gear. If the motor is powered in the locked position, the power required to rotate the cam to unlock the pawl is constant, minimized and/or decreasing. Once the pawl is forced into interfering contact with the park gear, no battery power is required to hold the pawl in interfering contact with the park gear. Power is required to disengage the pawl, but no battery power is required to hold the pawl away from the park gear. The pawls of the braking mechanisms are controlled by the control system, which is further programmed to operate the solenoid to force the pawl into interfering contact with the pawl gear to brake the drive train, and thus hold the compression belt at a set threshold of tightness during a period of the compression cycle, such as the high compression hold period of the compression cycle or the inter-compression hold period of the compression cycle. Once the pawl is forced into interfering contact with the park gear, no battery power is required to hold the pawl in interfering contact with the park gear. Power may be required to disengage the pawl, but no battery power is required to hold the pawl away from the park gear. 
     In use, a CPR provider will apply the device to a cardiac arrest victim, and initiate operation of the device. In applying the device, the CPR provider will secure each belt end to its corresponding intermediate belt (or directly to a corresponding drive spool). Initial tightness of the belt is not critical, as the control system will operate to cinch the belt to achieve an appropriate tightness for the start of compressions. After placement of the belt, the CPR provider initiates operation of the device through the control panel. Upon initiation, the control system will first test the tightness of the belt. To accomplish this, the control system is programmed to first loosen the belt (the intermediate straps will be set to a position to provide enough band length to accommodate this, and can be initially partially spooled) to ensure that it is slack, then tighten the belt until it sensed that the belt is tight to a first, low threshold of tightness (a slack-take up position or pre-tensioned position). The control system will sense this through a suitable system, such as a current sensor, associating a spike in current drawn by the motor with the slack take-up position. When the belt is tight to the point where any slack has been taken up, the motor will require more current to continue to turn under the load of compressing the chest. The expected rapid increase in motor current draw (motor threshold current draw), is measured through a current sensor, a voltage divider circuit or the like. This spike in current or voltage is taken as the signal that the belt has been drawn tightly upon the patient and the paid-out belt length is an appropriate starting point. (The exact current level which indicates that the motor has encountered resistance consistent with slack take-up will vary depending on the motor used and the mass of the many components of the system.) An encoder measurement at this point is zeroed within the system (that is, taken as the starting point for belt take-up). The encoder then provides information used by the system to determine the change in length of the belt from this pre-tightened or “pre-tensioned” position. 
     Various other means for detecting slack take-up may be used. The control system can also determine the slack-take up position by analyzing an encoder scale on a moving component of the system (associating a slow down in belt motion with the slack take-up position), a load sensor on the platform (associating a rapid change in sensed load with the slack take-up position), or with any other means for sensing slack take-up. 
     As an alternative mode of operation, the control system can be programmed to initially tighten the belt while detecting the load on the belt through a motor current sensor, and, upon detecting a load in excess of a predetermined threshold, loosening the belt to slack and then tightening the belt to detect the slack take-up position, or, upon detecting the load below the predetermined threshold, continue to tighten the belt to the slack take-up position. 
     Once the slack take-up position is achieved, the control system associates the belt position with the slack take-up position. This can be achieved by detecting an encoder position of an encoder, and associating the encoder position with the slack take-up position of the belt, or detecting the position of a compression monitor fixed to the belt and associating this position with the slack take-up position of the belt. If the encoder position is used to track the unspooled length of the belt, which corresponds to the desired compression depth, the control system will be programmed to operate the motor and brake to provide repeated compression cycles which include tightening the belt to a high threshold of tightness (based upon the length of belt spooled on the lateral drive spool, which corresponds to the compression depth achieved), holding the belt tight momentarily at the high threshold, loosening the belt, and holding the belt at the slack take-up position momentarily, where the slack take-up position has been determined in reference to the encoder position. If a compression monitor, such as a compression monitor  50  of  FIG. 14 , is used to track the compression depth achieved by the compression device, the control system will be programmed to operate the motor and brake to provide repeated compression cycles which include tightening the belt to a high threshold of tightness (based on the compression depth as measured by the compression monitor, or determined from signals generated by the compression monitor), holding the belt tight momentarily at the high threshold, loosening the belt, and holding the belt at the slack take-up position momentarily, where the slack take-up position has been determined in reference to the compression monitor zero point which was associated with the slack take-up position. 
     Where a compression monitor, such as the compression monitor  52  of  FIG. 14 , is used to determine the compression state achieved by the system and provide feedback for control of the system, a compression sensor, such as compression sensor  50  of  FIG. 14 , can comprise an accelerometer based compression monitor such as the compression monitor described in Halperin, et al., CPR Chest Compression Monitor, U.S. Pat. No. 6,390,996 (May 21, 2002), as well as Palazzolo, et al., Method of Determining Depth of Chest Compressions During CPR, U.S. Pat. No. 7,122,014 (Oct. 17, 2006), or the magnetic field based compression monitor described in Centen, et al., Reference Sensor For CPR Feedback Device, U.S. Pub. 2012/0083720 (Apr. 5, 2012). The compression monitor typically includes sensors for generating signals corresponding to the depth of compression achieved during CPR compressions, and associated hardware/control system for determining the depth of compression based on these signals. The components of the compression monitor system may be incorporated into the belt, or the sensors may be incorporated into the belt while the associated hardware and control system are located elsewhere in the device, or integrated into the main control system that operates the compression belt. While controlling the device to perform repeated cycles of compression, the control system may use the compression signals or depth measurement provided by the compression sensor (e.g., compression sensor  50  of  FIG. 14 ) or compression monitor (e.g., compression monitor  52  of  FIG. 14 ) to control operation of the device. The control system can operate to tighten the belt until the depth of compression achieved by the system, as determined from the compression signals, indicates that the compression belt has pushed the anterior chest wall downward (in the anterior direction, toward the spine) to a desired predetermined compression depth (typically 1.5 to 2.5 inches). The desired depth is predetermined in the sense that it is programmed into the control system, or determined by the control system, or input by an operator of the system). 
     The control system may comprise at least one processor and at least one memory including program code with the memory and computer program code configured with the processor to cause the system to perform the functions described throughout this specification. The various functions of the control system may be accomplished in a single computer or multiple computers, and may be accomplished by a general purpose computer or a dedicated computer, and may be housed in the housing or an associated defibrillator. 
     While the preferred embodiments of the devices and methods have been described in reference to the environment in which they were developed, they are merely illustrative of the principles of the inventions. The elements of the various embodiments may be incorporated into each of the other species to obtain the benefits of those elements in combination with such other species, and the various beneficial features may be employed in embodiments alone or in combination with each other. Other embodiments and configurations may be devised without departing from the spirit of the inventions and the scope of the appended claims.