Abstract:
A cardiopulmonary resuscitation method and apparatus that is adapted to performing high-impulse CPR includes providing a chamber having an expandable volume and a patient-contacting pad that moves as a function of volume of the chamber and supplying a controlled quantity of a fluid to the chamber. This results in increasing the chamber volume by a controlled amount, thereby compressing the patient&#39;s chest with the patient-contacting pad during a systolic phase.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to a method and apparatus for providing automated cardiopulmonary resuscitation (CPR) in the form of closed chest cardiac compression, preferably combined with pulmonary ventilation. 
     A practical mechanism for automating closed chest cardiac compression was first disclosed in U.S. Pat. No. 3,364,924 assigned to my assignee, Michigan Instruments, Inc. of Grand Rapids, Mich., and has been commercially exploited under the Thumper® Cardiopulmonary Resuscitation System. The system disclosed in U.S. Pat. No. 3,461,861 added the important function of pulmonary ventilation to the Thumper® system by supplying a ventilation cycle intermittently with a number of compression cycles according to the American Heart Association protocol. The waveform of the apparatus disclosed in the &#39;924 patent is shown at C in FIG.  14 . Waveform C generally resembles a damped exponential waveform. This is an improvement over, yet similar to, the sinusoidal waveform shown at M in FIG. 14 which is produced by manual closed chest cardiac compression. 
     In a number of articles, including that published by Maier, George W. et al. in Circulation, Vol. 1, 1984, entitled “The Physiology of External Cardiac Massage; High-Impulse Cardio-Pulmonary Resuscitation,” the disclosure of which is hereby incorporated herein by reference, a new form of CPR is proposed under the name “High Impulse CPR.” In high impulse CPR, the waveform more closely resembles a square wave, or impulse, rather than a sinusoidal form. A fast rise in the chest compression stroke that increases the area under the curve, as seen in curve H in FIG. 14, applies a greater amount of energy to the patient during the systolic phase. It was discovered that the high energy supplied by the high impulse CPR waveform significantly improved perfusion in the cardiovascular system of the patient. The development of high impulse CPR resulted from studies sponsored by my assignee, Michigan Instruments, Inc. 
     A commercial embodiment of a high impulse CPR has remained a long felt and unmet need in the art. The exponential acceleration curve necessary to produce the high impulse CPR effect must also be combined with the necessity for controlling the length of the compression stroke. Indeed, once the massage pad, which interfaces the apparatus to the patient, is exponentially accelerated to the selected depth of compression, it must abruptly decelerate and be held at the selected depth during the systolic phase. During the diastolic, or relaxation phase, the apparatus must retract the massage pad with sufficient acceleration to allow the patient&#39;s chest to return to its non-compressed state without interference by the apparatus. 
     The apparatus used to carry out the initial evaluation of high impulse CPR constituted a piston and a cylinder to which a compressed gas could be rapidly supplied and a fixed mechanical stop which limited the extent of the piston travel. While such apparatus was sufficient to demonstrate the benefit of high impulse CPR, it was not commercially viable. The use of a fixed mechanical stop was noisy and made adjustment of the compression stroke rather awkward. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for producing high impulse cardiopulmonary resuscitation (CPR) in a manner which achieves the long felt and unmet need for a commercial device of this type, particularly one that provides an adjustable depth of compression. 
     A method of performing cardiopulmonary resuscitation, according to an aspect of the invention, includes providing a chamber having an expandable volume and a patient-contacting pad that moves as a function of volume of the chamber and positioning the chamber with respect to the patient to bring the patient-contacting pad to alignment with the patient&#39;s chest. A controlled quantity of fluid is supplied to the chamber in order to increase the chamber volume by a controlled amount, thereby compressing the patient&#39;s chest during a systolic phase. It has been discovered that the seemingly contradictory requirements of rapidly accelerating the patient contacting pad, thereby compressing the patient&#39;s chest in a manner that achieves a controllable extent of compression depth, can be accomplished by this aspect of the invention. In particular, a very rapid acceleration of the compression stroke can be accomplished by rapidly supplying the fluid to the chamber. Control of the extent of compression depth can be achieved by controlling the quantity of the fluid supplied to the chamber. 
     Preferably, the chamber having an expandable volume is made up of a cylinder enclosing an adjustable piston which is connected with the patient-contacting pad, such as a rod. Most preferably, a compression spring is included in the chamber in order to rapidly return the piston to its retracted position at the beginning of the diastolic or relaxation phase. Indeed, by providing a spring of sufficient spring force, it is possible to provide a retraction force for active reshaping of the chest, as disclosed in my commonly assigned U.S. Pat. No. 5,743,864, the disclosure of which is hereby incorporated herein by reference. 
     According to a somewhat more detailed aspect of the invention, a cardiopulmonary resuscitation apparatus includes a chamber, a piston in the chamber, and a frame including a first portion adapted to be positioned posteriorly of a patient and a second portion supporting the chamber. The apparatus further includes a pressure source, a control valve assembly that is operative to selectively connect the pressure regulator to the chamber, and a timing circuit. 
     Preferably, the timing circuit selectively operates the control valve assembly to connect the pressure source to the chamber at the beginning of a systolic phase to accelerate the piston toward the patient to initiate chest compression and to disconnect the pressure source from the chamber and to seal the chamber during the remaining portion of the systolic phase. The pressure source is preferably a pressure regulator adapted to be supplied with a gas under pressure and producing a regulated pressure at an output. Also, preferably, the control valve assembly is operative to connect the pressure regulator output to the chamber. The timing circuit selectively operates the control valve assembly to supply regulated pressure from the pressure regulator output to the chamber for a controllable time period to move the piston to apply chest compression to a patient during a systolic phase. 
     These and other objects, advantages and features of this invention will become apparent upon review of the following specification in conjunction with the drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a cardiopulmonary resuscitation apparatus according to the invention; 
     FIG. 2 is an end view taken from II—II in FIG. 1; 
     FIG. 3 is a side elevation of an arm assembly of the apparatus in FIG. 1 with the cover removed to reveal internal details thereof; 
     FIG. 4 is a top plan view of the arm assembly in FIG. 3; 
     FIG. 5 is a schematic diagram of a pneumatic control system of the apparatus in FIG. 1; 
     FIG. 6 is a sectional view of a precision timing valve assembly; 
     FIG. 7 is a sectional view of a variable oscillatory relay latched ordinate numeration valve assembly; 
     FIG. 8 is a sectional view of a chest compression cylinder control valve assembly; 
     FIG. 9 is a sectional view of a system pressure regulator; 
     FIG. 10 is a diagram illustrating the chest compression cylinder control valve assembly during a systolic phase for which no compression has been selected; 
     FIG. 11 is the same view as FIG. 10 during the acceleration portion of the systolic phase; 
     FIG. 12 is the same view as FIG. 10 during the holding portion of the systolic phase; 
     FIG. 13 is the same view as FIG. 10 during the initial portion of the diastolic phase; 
     FIG. 14 is a diagram illustrating a comparison of the waveform produced by the apparatus in FIG. 1 with a waveform produced by prior methods; and 
     FIG. 15 is a block diagram of an alternative embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now specifically to the drawings, and the illustrative of embodiments depicted therein, a cardiopulmonary resuscitation system  20  includes a base  22 , a column  24  supported by the base, and a cardiopulmonary resuscitation arm assembly  26  adjustably supported along column  24  (FIGS.  1 - 4 ). Base  22  is configured to be positioned posteriorly of the patient and may be used by itself or in combination with a patient retention and support member as disclosed in U.S. Pat. No. 3,985,126, the disclosure of which is hereby incorporated herein by reference. Column  24  supports arm assembly  26  and also provides a pneumatic buffer tank  61  for the pneumatic system and houses a system pressure regulator  62 , as will be disclosed in more detail below. Arm assembly  26  is vertically adjustable along column  24  in order to accommodate the patient&#39;s chest diameter and may be adjusted by loosening a release handle  28 , repositioning the arm assembly, and retightening release handle  28  at the desired position of the arm assembly. A ventilation mask  30  and hose  32  provide controlled ventilation to the patient from an oxygen canister (not shown) connected to CPR system  20  by a connection hose  34 . In the illustrative embodiment, the oxygen supplied through hose  34  is also used to operate a pneumatic control system  52  which operates the closed chest compression portion of system  20 . However, the closed chest compression portion of system  20  could, alternatively, be operated from a different compressed gas, such as carbon dioxide or even from a hydraulic fluid source. 
     CPR arm assembly  26  includes a patient interface, such as a massage pad  36 , or the like, which may be of the type disclosed in commonly assigned U.S. Pat. No. 4,570,615 entitled CARDIO-PULMONARY RESUSCITATOR PAD, the disclosure of which is incorporated by reference herein. The massage pad provides a conforming interface between the CPR system and the patient&#39;s sternum. Arm assembly  26  additionally includes, a compression depth gauge  38 , which provides the operator an indication of the depth of compression which is being achieved. Compression depth is controllable by a compression depth input device  40  mounted on a control panel  42 . Control panel  42  additionally includes a run/stop input device  44  and a ventilation volume input device  46 . CPR system  20  additionally includes a pressure indicator, such as a pop-up column pressure indicator  48 , to indicate to the operator the presence of sufficient operating pressure in the system. A flexible pressure hose  50  interconnects the portion of the pneumatic circuit  52  of system  20  supported by column  24  to the portion of the pneumatic system supported by arm assembly  26 . The flexible nature of hose  50  facilitates the adjustability of arm assembly  26  along column  24 . 
     CPR system  20  includes a fluid-based control system  52  (FIG.  5 ). As previously set forth, fluid control system  52  is preferably operative operated by oxygen, but could, alternatively, be operated by some other gas or non-gas fluid. Control system  52  includes a supply system  54  made up of a quick connect connector  56  for coupling with a source of oxygen or other fluid, such as through hose  34 , and a filter  58  to remove dust and other particles from the control fluid. A pressure release valve  60  is provided to limit over pressure conditions from damaging system components. 
     A system pressure regulator  62  produces a high flow rate of fluid at a controlled pressure. Preferably, system pressure regulator  62  regulates pressure from an unregulated pressure source to a regulated pressure that is greater than or equal to one-half of the source pressure level that is relatively close to supply pressure. System pressure regulator  62  includes a base  174  defining an inlet chamber  176  which is connected with an outlet chamber  178  by a control passage  180  (FIG.  9 ). A control valve  184  is positioned to regulate the flow through control passage  180 . Control valve  184  includes a poppet  186  which selectively closes control passage  180 . Poppet  186  is moved away from control passage  180  by a sensing diaphragm  182  which is connected with poppet  186  by a stem  188 . Stem  188 , in the illustrated embodiment, is hollow to thereby transmit the pressure of inlet chamber  176  to sensing diaphragm  182 . A compression spring  190  biases sensing diaphragm  182  toward the open position of control valve  184  such that an increase in pressure in inlet chamber  176  causes sensing diaphragm  182  to tend to compress spring  190  causing poppet  186  to seal control passage  180 . As pressure decreases in inlet chamber  176 , the decrease in pressure in stem  188  allows the bias of spring  190  to flex diaphragm  182  thereby causing poppet  186  to be removed from control passage  180 . Poppet  186  is biased in the direction of diaphragm  182  by a bias spring  192  and includes an O-ring  194  made from an oxygen compatible material, such as Viton, which seals the interface between poppet  186  and control passage  180 . Spring  190  is of a length that it is compressed a small percentage of its length during normal operation of pressure regulator  62 . Spring  190  is, therefore, operated in a linear region of the spring-force curve. A compression adjustment device, such as screw  196 , allows the pre-tension of spring  190  to be adjusted. This adjusts the operating point of control valve  184  thereby allowing the output pressure of system regulator  62  to be adjusted. Pressure relief valve  60  is mounted to base  174  in fluid connection with inlet chamber  176 . In the illustrated embodiment, system regulator  62  is positioned with column  24 . This is advantageous because it conveniently accommodates the length of spring  190  and combines a housing for system regulator  62 , the function of buffer column, and the support of arm assembly  26  in one convenient assembly. 
     In operation, system regulator  62  repeatedly opens and closes control valve  184  as a function of inlet pressure as sensed by diaphragm  182 . The duty cycle between opening and closing of control valve  184  causes an adjustment of the pressure in outlet chamber  178 . The configuration of system regulator  62  allows a sufficiently regulated outlet pressure at a level that is within one order-of-magnitude of the inlet pressure level at a relatively high flow rate. By way of example, in the illustrated embodiment, pressure regulator  62  produces a nominal output pressure of between 48 psig and 63 psig from an input pressure of between 50 psig and 90 psi at a flow rate of at least 100 liters per minute and, preferably, at least 125 liters per minute. System pressure regulator  62  produces an output to a conduit  64  which is supplied to other portions of the fluid control system, as set forth below. 
     Fluid control system  52  additionally includes a timing circuit, such as timing and control section  66 . Timing and control section  66  receives regulated pressure from a timing circuit regulator  68 , which is supplied from conduit  64  and produces, in the illustrative embodiment, an output pressure of approximately 30 psig on line  70 . Timing and control section  66  additionally includes a ventilation and control supply regulator  72 , which is supplied from conduit  64  and produces an output, in the illustrative embodiment, of approximately 30 psig on an output line  74 . Timing and control section  66  includes a pneumatic oscillator circuit  76 , which is supplied from timing circuit regular  68  through a resistor R 1  alternatingly to a pneumatic capacitor C 1  and C 2  in order to control, respectively, the diastolic and systolic phases of the compression cycle. Pneumatic capacitors C 1  and C 2  are connected to a timing valve assembly  78  (FIG.  6 ), which is made up of a spool  80  having two stable positions that are maintained by a pair of spring-biased detent assemblies  82  and  84 . Capacitors C 1  and C 2  are connected to opposite sides of a piston  86  thereby allowing the spool  80  to be moved between the position illustrated in FIG. 6, in which detent assembly  82  is engaged with a recess  88 , and a position in which spool  80  is moved to the right of the position illustrated in FIG. 6, in which detent assembly  84  engages recess  88 . In the position illustrated in FIG. 5, compressed air is supplied through resistor R 1  directly to capacitor C 2  during the diastolic phase. Capacitor C 2  and resistor R 1  are sized in order to provide an approximately 375 millisecond time period for the diastolic phase cycle. At the end of the diastolic phase, the pressure developed in capacitor C 2  is sufficient to move the spool  80  to the left from the position illustrated in FIG. 5 in order to begin the systolic phase. 
     When spool  80  is in the position opposite that as illustrated in FIG. 5, capacitor C 2  is vented to atmosphere and valve portion  90   b  connects the pressure of line  70  to a line  92  connected with a chest compression cylinder control valve assembly  94  to initiate the systolic phase, or chest compression cycle, as will be described in more detail below. This also connects capacitor C 1  through valve portion  90   a  to line  70  through resistor R 1 . This causes capacitor C 1  to charge with pressure according to a time constant which regulates the systolic phase of the waveform which is nominally set to 375 milliseconds according to the American Heart Association protocol. At the end of the systolic phase, the pressure built up in capacitor C 1  moves spool  80  to the position illustrated in FIG.  5  and the diastolic phase begins. To initiate the diastolic phase, a valve reset line  96  is pressurized by valve portion  90   b  to apply a pressure through stop input  44  to the valve reset ports  142  and  144  of valve  94 , thus resetting the valve  94  in a reset state. 
     Timing and control section  66  additionally includes a cycle counter, such as a fluid-based cycle counting circuit  98 , to control the relationship between chest compression cycles and ventilation cycles. Counting circuit  98  includes a valve assembly  100  (FIG. 7) and a pneumatic capacitor C 3  which is charged through a pneumatic resistor R 2 . Valve assembly  100  includes a detent assembly  102 , which provides a stable position for a spool  104 , and a piston  106 , which, when supplied with sufficient pressure from capacitor C 3 , moves spool  104  to the right, as illustrated in FIG. 7. A return spring assembly  108  returns spool  104  to the position illustrated in FIG. 7 when the pressure is vented from capacitor C 3 . Valve assembly  100  additionally includes a quick dump relay  110 . Quick dump relay  110  includes a port  112  which is connected with valve portion  90   b  of timing valve assembly  78 . 
     In operation, circuit  98  begins in the illustration illustrated in FIG. 5. A first valve portion  114   a  conducts the fluid from resistor R 1  to pneumatic capacitor C 2  bypassing a pneumatic resistor R 3 . A second valve portion  114   b  opens and closes the circuit between line  74  and patient demand valve device  116 . Each time the timing circuit  76  goes through one cycle pressurizing line  92  during the systolic phase, a quantity of fluid is added to capacitor C 3  through a resistor R 2 . Capacitor C 3  and resistor R 2  are sized in order to accumulate a quantity of fluid sufficient to create a number of cycles of circuit  76  equal to the ratio of chest compressions to ventilation cycles desired, as set forth by guidelines such as the American Heart Association protocol. When the number of chest compressions is sufficient to build a sufficient pressure in capacitor C 3 , piston  106  moves an actuating force for spool  104  within spool  104  to the state illustrated in FIG.  5 . This connects patient demand valve  116  to supply line  74  through valve portion  114   b  in order to initiate a patient&#39;s ventilation cycle. Simultaneously, capacitor C 3  is vented to atmosphere through valve portion  114   b  and valve portion  114   a  connects resistor R 3  in the circuit leading to capacitor C 2 . Resistor R 3  slows the charging of capacitor C 2  thereby prolonging the diastolic cycle during which the patient is undergoing a ventilation cycle, which is in keeping with the American Heart Association protocol. Quick dump relay  110  ensures that circuit  98  will not move to a ventilation state during a systolic phase. During a systolic phase, the pressure in line  92  keeps circuit  98  in the state illustrated in FIG.  5 . Once the pneumatic oscillating circuit  76  moves to the diastolic phase, pressure is relieved from quick dump relay  110  which allows the pressure therein to quickly dump to atmosphere thereby allowing circuit  98  to rapidly move to a ventilation position opposite that shown in FIG.  5 . 
     Patient demand valve  116  is commercially available as marketed by Allied Health Care under Model No. L535-011. Patient demand valve  116  is supplied with ventilation oxygen through a ventilation flow rate control needle valve; namely, ventilator volume input  46 . Resistor R 3  increases the length of the relaxation phase during which ventilation occurs in the illustrated embodiment from approximately 375 milliseconds to approximately 1.5 seconds. 
     In the illustrated embodiment, the fluid connections between the various components making up timing and control system  66  are formed as channels in a pneumatic logic block to which the various components are mounted. As is known in the art, such channels may be machined in the face of a block of material and isolated from each other and from atmosphere by gasket material placed between the channels and a cover placed over the channels and gasket material. Also, in the illustrated embodiment, one or more capacitors C 1  through C 5  are provided in whole or in part by cavities formed in a block, such as the logic block, preferably on a portion of the logic block opposite the portion forming the fluid connection channels. 
     Fluid control system  52  additionally includes a chest compression control assembly  120 . Chest compression control valve assembly  120  includes a control valve assembly, such as chest compression cylinder valve assembly  94 , and a controlled volume device  122  in the form of a piston  124  in a cylinder  126 . Control volume device  122  additionally includes a return spring  128  in order to return piston  126  to its retracted or released position at the end of a compression, or systolic, phase. In the illustrated embodiment, spring  128  has a spring force of three pounds or greater. Piston  124  is connected through patient massage pad  36  by a connecting rod  130 . 
     Chest compression cycle control valve assembly  94  includes a first valve assembly  131 , including a first spool  132 , and a second valve assembly  133 , including a second spool  134 , both in a common housing  136  (FIG.  8 ). Valve assembly  94  includes a detent assembly  138  for use with first spool  132  and a return spring  140  for use with second spool  134 . A port  142  which connects with line  94  provides a reset for valve  131 . A port  144 , which is connected with line  94  through run/stop input  44 , provides a reset for valve assembly  133 . An input port  146  is connected through conduit  64  to system pressure regulator  62 , and an outlet port  148  is connected with control volume device  122 . A bracket  150  on housing  136  provides a mount for patient demand valve  116 . Valve assembly  131  has a passage  152  connected with input port  146  and a passage  154  connected with valve assembly  133 . Valve assembly  133  has a vent passage  156  connected with atmosphere, a vent passage  157  connected with atmosphere and a passage  158  connected with output port  148 . Valve assembly  131  has a channel  160  formed in spool  132 . Valve assembly  133  has a first channel  162  and a second channel  164  formed in spool  134 . When a particular channel is aligned with a pair of passages, it provides a path through the valve. 
     Chest compression control  120  additionally includes a capacitor C 4  connected with a control port  166  of valve assembly  132  and a capacitor C 5  connected with a control port  68  of valve assembly  134 . Capacitor C 4  is connected through compression depth input control  40 , which is a variable resistor, to line  92 . Capacitor C 5  is connected through a fixed resistor R 5  to line  92 . Check valve  170  is provided in parallel with depth control resistor  40 . Check valve  172  is provided in parallel with resistor R 5 . 
     When the timing and control system  66  pressurizes line  92  at the beginning of the systolic phase, capacitors C 4  and C 5  begin to fill. If the user adjusts depth input control  40  to a zero compression setting, which corresponds to a minimum restriction condition, capacitor C 4  charges faster than capacitor C 5  causing valve  131  to set sooner than valve  133 . If the user sets depth input control  40  to a defined extent of compression, which corresponds to a more restricted condition, capacitor C 5  charges faster than capacitor C 4  causing valve  133  to set before valve  131  sets. At the end of the systolic phase and at the beginning of the diastolic phase, timing and control system  66  vents line  92  and pressurizes line  94 . This causes capacitors C 4  and C 5  to rapidly depressurize through respective check valves  170 ,  172  and resets valves  132  and  134  to the position illustrated in FIG.  8  through reset ports  142  and  144 , respectively. If run/stop switch  44  is placed in a “stop” position, valve  133  is held in a reset position by a continuous pressure from line  74 . 
     Operation of chest compression control system  120  can best be understood by reference to FIGS.  10 - 12 . FIG. 10 illustrates the condition wherein the user adjusts compression input device  40  to a zero compression setting. In such setting, it is important that there be no perceivable movement in pressure pad  36 . In such setting, the relative lack of restriction by compression input device  40  causes capacitor C 4  to set valve  131  before capacitor C 5  sets valve  133 . When valve  131  is set, channel  160  switches to a blocked state, thus preventing system pressure regulator  62  from being connected with control volume device  22 . When capacitor C 5  causes valve  133  to set after valve  131  has set, channel  164  is momentarily connected with vent passage  156 , thereby venting any pressure buildup in Turbin valve  94 . In this manner, when channel  162  interconnects passage  154  and passage  158  upon the subsequent setting of valve  133 , there will be no pressure charge that can perceivably move pressure pad  36 . At the end of the systolic phase, valves  131  and  133  are reset without any substantial compression stroke occurring. 
     In the second situation illustrated in FIGS. 111 and 12, the user has dialed in a compression on the compression depth input control  40 . As previously set forth, such manipulation of input  40  causes an increase in the restriction thereof which causes capacitor C 4  to charge slower than capacitor C 5 . This causes valve  133  to set prior to valve  131  setting. When valve  133  sets prior to valve  131  setting, channel  162  connects passage  154  with passage  158 . Passage  154  is connected through channel  160  to the output of system regulator  62  causing the relatively high output volume of system regulator  62  to be applied to controlled volume device  122 . This results in displacement of piston  124  for the duration of time that valve  133  is set and valve  131  is not yet set. When the capacitor C 4  charge is sufficient to set valve  131  (FIG.  12 ), system pressure regulator  62  is disconnected from control volume device  122  eliminating any further fluid volume being added to control volume device  122 . Valves  131  and  133  remain in such position until reset at the end of the systolic phase by line  94  being pressurized and line  92  being vented. During such period when valves  131  and  133  are set, valve  133  seals port  148  which causes the fluid supplied to control volume device  122  to remain sealed therein. This holds the compression stroke at its full extended position until valves  131  and  133  are reset (FIG. 13) at the end of the systolic phase by the pressure on line  96  and elimination of pressure on line  92 . 
     A cardiopulmonary resuscitation system disclosed herein is capable of accelerating the patient&#39;s chest to a compression depth of at least 3 centimeters and, preferably, at least 8.5 centimeters in a rise time T r  (FIG. 14) of less than 100 milliseconds and, preferably, less than 60 milliseconds and, most preferably, approximately 50 milliseconds. Furthermore, the CPR system is capable of maintaining that compression during the remaining portion of the compression phase and quickly releasing the compression thereby allowing the patient&#39;s chest to reshape without interference from the CPR system. Although the invention is illustrated with the piston being returned by a return spring, with minor modification to the fluid control system, it would be possible to actively return the piston using fluid pressure. It would also be possible to adjust the spring force of spring  128  to provide at least a greater amount of active return of the pressure pad. 
     Alternative techniques are available for supplying a fixed quantity of a fluid to the controlled volume device. In a cardiopulmonary resuscitation system  20 ′, illustrated in FIG. 15, a chamber C is provided separate from the controlled volume device  122  and pressurized to a particular pressure P during the relaxation phase by a pressure source (not shown). During the systolic phase, a valving arrangement V connects chamber C with controlled volume device  122 . The gas in chamber C quickly equalizes in the controlled volume device providing a controlled quantity of fluid in the controlled volume device. While CPR system  20 ′ would be fully functional, it would require a chamber C having a volume that is quite large with respect to the overall size of the cardiopulmonary resuscitation system  20 ′. 
     Although the controlled volume device is illustrated as a piston operating in a cylinder, other controlled volume devices could be used, such as bladder-type devices, bellows, gas bags, and the like. Other devices could be used with cardiopulmonary resuscitation system  20 ,  20 ′, such as ECG parameter monitoring devices of the type disclosed in commonly assigned U.S. Pat. Nos. 5,077,667 and 5,683,424, the disclosures of which are hereby incorporated herein by reference, as well as automatic fibrillator devices, and the like. As previously set forth, the invention could be utilized to perform cardiopulmonary resuscitation with active reshaping of the chest as disclosed in commonly assigned U.S. Pat. No. 5,743,864, the disclosure of which is hereby incorporated herein by reference. Additionally, although the invention is illustrated as an entirely fluid-based system, particular functions could be alternatively carried out by electrical or electronic control systems. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the principles of the invention, which is intended to be limited only by the scope of the appended claims.