Abstract:
A chest compressor includes a piston ( 14 ) that moves in downward and upward strokes, with the piston undergoing a smooth reversal at the bottom of the downward stroke. A compression spring such as a wave spring ( 60 ), is positioned to engage the piston only near the end of its downward stroke, to smoothly reverse the piston motion, limit downward force on the patient at the end of the stroke, and avoid a downward pulse due to the momentum of the downwardly-moving piston. A stop ( 90, 92 ) is latchable in an inward position to allow reduction in the piston stroke by engaging an outward flange ( 56 ) on the piston before the piston has moved fully downward.

Description:
BACKGROUND OF THE INVENTION 
     An automatic chess compressor can be carried by an emergency worker and applied to a patient to stimulate blood circulation and breathing. Such a chest compressor usually includes a piston that moves up and down in a cylinder (assuming that the patient is reclined so his chest faces upward). The cylinder is held closely over the patient&#39;s chest as by a strap wrapped around the patient&#39;s chest area. The chest compressions may occur at a frequency such as ½ to one second apart, so the piston moves downward rapidly in each stroke. 
     A rapidly downwardly-moving piston tends to impart a downward pulse to the patient at the bottom of the stroke, which is not desirable. It is possible to use a spring that is fixed to the piston to reduce the downward force as the piston moves down. However, the spring force applied along most of the piston movement tends to slow piston movement so additional energy is required to move the piston. A spring has the advantage of more rapidly raising the piston after each chest compression, but it is found that resilience of the chest is sufficient to rapidly raise the piston after each downward stroke. 
     In most cases, the patient&#39;s chest should be compressed by about one to two inches in each stroke, to benefit blood circulation and breathing while avoiding harm to the patient&#39;s chest (e.g. by breaking a rib). The piston actually must apply a downward stroke of about two to four inches to produce a patient chest compression of about one to two inches because the backup such as the strap wrapped about the patient, presses into the patient and takes up some of the compression. For children and small adults, it is desirable to reduce the chest compression to near the minimum, while for adults of normal to large size it desirable to use chest compressions close to the maximum. The chest compressor should be easily and rapidly convertible between different compression distances. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, a chest compressor is provided of the type that comprises an actuator that includes a piston that moves vertically (when the patient&#39;s chest faces upward) to repeatedly compress the patient&#39;s chest, wherein piston motion is gradually reversed near the bottom of its downstroke while minimizing the energy absorbed to create such reversal. The piston moves during most of its downstroke without having to overcome the force of a spring that would slow its downward motion. A spring, such as a wave spring, first engages the piston near the bottom of its downstroke and at that time the spring rapidly slows downward motion of the piston and avoids a large downward force at the end of the stroke. 
     The length of piston stoke is easily varied by the emergency worker, so the stroke length can be changed for different patients. This is accomplished by providing one or more stops that can be moved into the path of the piston. In a telescoping piston arrangement wherein an outer piston part lies within an inner piston part, the stop(s) engage only the outer piston part. The compression spring that reverses piston movement, lies at the bottom of the outer piston part to engage an outward flange at the top of the inner piston part near the end of the piston downstroke. 
     The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a chest compressor of the present invention showing stops in phantom lines. 
         FIG. 2  is an isometric view of a wave spring of the chest compressor of  FIG. 1 . 
         FIG. 3  is a graph showing an idealized path of a piston of the invention, and showing how the momentum of the piston could produce a pulse in the absence of the spring of the present invention. 
         FIG. 4  is a sectional view of a chest compressor of another embodiment of the invention which includes only one piston part. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a chest compressor  10  mounted on a patient P to repeatedly compress the patient&#39;s chest C. The chest compressor includes a cylinder  12  (which is not necessarily cylindrical) whose function is to guide a piston  14  in vertical movement. In the following discussion it is assumed that the patient lies in a reclined position with his/her chest facing upward. A frame  20  is attached to the cylinder and it holds a strap  22  that encircles the patient&#39;s upper torso and holds a stabilizer  28  that resists tipping. The piston repeatedly moves from an upward position shown at  14  to a downward position shown at  14 A, at a rate such as a stroke every ½ to one second. 
     In the chest compressor illustrated, the piston  14  is moved downward by compressed fluid such as compressed air or oxygen that flows through a control into the cylinder though a tube  24  and out though another tube  26 , both tubes being connected to a top plate  30  of the cylinder. Applicant finds that sufficiently rapid upward movement is produced by the resilience of the patient. Means other than compressed gas can be used to move the piston up and down, including an electrically powered mover such as a solenoid or motor(s). 
     The piston illustrated in  FIG. 1  is a telescoping piston which includes a plurality of piston parts that lie one within the other, with first and second, or outer and inner piston parts  40 ,  42  being shown. The piston has an axis  44 . The cylinder has a radially inwardly-projecting (with respect to the axis  44 ) flange  50  at its bottom. The outer piston part  40  has top and bottom flanges  52 ,  54 , and the inner piston part  42  has a top flange  56 . When the piston reaches nearly its lowest position at  14 A, the inner piston flange  56  approaches the outer piston part lower flange at  54 A. If there were no spring to slow the downward movement of the inner piston part near the end of the downward stroke, the downward momentum of the inner piston part would result in an extra downward pulse applied to the patient, which might injure the patient without increasing blood circulation or breathing. 
     In accordance with one aspect of the invention, applicant positions a compression spring  60 , in the form of a wave spring, in the path of the inner piston part  42 . The wave spring  60  shown in  FIG. 2 , has four raised parts  61 - 64  and four corresponding lower parts  66  between the raised parts.  FIG. 1  shows that the spring lower parts  66  rest on the bottom flange  54  of the outer piston part and the spring is preferably held thereagainst as by adhesive or by a holddown  70 . Although the wave spring is of small radial width, the bottom flange  54  of the outer piston part is provided with a larger radial width than the cylinder bottom flange, to hold the wave spring. 
     In normal operation, the outer piston part  40  moves downward until its top flange  52  is stopped by the cylinder flange  50 , and the inner piston part has begun to move down within the outer piston part. The inner piston part top flange  56  engages the wave spring  60  and compresses it against the outer piston part bottom flange  54 . Such compression of the wave spring as to  60 A, slows downward movement of the inner piston part to avoid a large force at the end of piston downward movement, and even helps reverse the piston direction of motion. In the prior art, springs were provided that were continuously connected to the piston to continually urge it upward. As a result, greater energy (e.g. higher pressure air) was required to rapidly move the piston in a full downward stroke. In the present invention, the compression spring engages the piston to slow its downward movement only near the end of downward piston movement, so energy is absorbed from the piston only along a small portion of its stroke. 
       FIG. 1  shows that the outer piston part has a stroke length A between its upward position and a position where its top outer flange  52  is stopped by the cylinder flange  50 . The inner piston part has a maximum stroke length B between its top flange  56  lying against the top plate and its top flange  56  fully compressing the wave spring against the outer piston part flange  54 . (The outer piston part can be provided with an internal flange to assure that the inner piston part always lies below the top of the outer piston part). The lengths A and B are each about two inches, and the distance A+B is about four inches. The wave spring has an uncompressed height H that is about 20 millimeters (about 0.8 inch). Therefore, the wave spring does not engage and begin slowing the inner piston part top flange  56  until the outer piston part has moved down completely (about 2.0 inches) and the inner piston part has moved down along a partial stroke length (2 inches−0.8 inch=1.2 inch) so the complete piston has moved down 3.2 inches out of a maximum stroke length of 4.0 inch. 
       FIG. 3  is an idealized graph  82  of motion of the bottom  80  of the piston, with the piston moving down and up about every one-half to one second. Without the wave spring, the momentum of the rapidly moving piston inner part would produce a short pulse at  84  that might harm the patient as by breaking a rib, and which is such a short pulse that it creates no appreciable increase in blood circulation or breathing. With the wave spring, the bottom of the piston follows the path indicated at  86 , which produces a more gradual reversal. 
     As mentioned above, a combined piston stroke of about four inches is desirable for full size adults, while a piston stroke of about two inches is desirable for a child or small adult. The emergency worker can rapidly decide the length of piston stroke that is appropriate for a particular patient. Applicant allows the emergency worker to quickly adjust the stroke length by providing at least one pair of stops  90 ,  92  ( FIG. 1 ). Each stop includes a pin  94  that is mounted on a double click mechanism, of the type used in pens to project and retract the pen tip. When the emergency worker first pushes in the stop  90 , the pin is latched (temporarily held) in the inward position wherein the pin lies in the path of the top flange  52  of the outer piston part Then the outer piston part cannot move down below the pin and the combined downward stroke length is about 2 inches (with the spring slowing the stroke near its bottom). Another pair of such stops can be provided at a lower location to allow only about one inch of outer piston part movement to limit the stoke length to three inches. 
       FIG. 4  shows a chest compressor actuator  100  with a single piston part  102 . The wave spring  60  rests on an internal flange  104  of the actuator and is compressed by an external flange  106  of the piston part  102  of the actuator piston, only after about two-thirds of the total downward stroke. 
     Thus, the invention provides a chest compressor of the type in which a piston moves up and down (relative to a patient with an upwardly facing chest), wherein a spring is provided that engages the piston to slow its downward movement during a downward stroke, only after the piston has completed a majority of its downward stroke. The spring is preferably a compression spring, and is preferably a wave spring. The spring is useful for a chest compressor having a piston with only one piston part, or a piston with a plurality of telescoping piston parts. The downward stroke can be reduced from its maximum, by providing one or more stops that are moveable into the path of an outward flange of a piston or piston part, to prevent the piston or piston part from moving down along its full downward stroke. 
     Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.