Abstract:
A driver for a pneumatic ventricular assist device (VAD) is powered by pressurized air, oxygen or any other gas commonly available in hospital rooms, intensive care units and operating rooms. The driver can provide both blood-ejecting pressure (systole) and blood-filling vacuum (diastole) to the VAD. The driver is controlled by a computer/digital controller by means of pressure and volume sensors, and electromechanical valves. Ventricular pumping is performed by a single spring-loaded piston or bellows. The computer can actively regulate maximum systolic ventricular pressure, maximum diastolic vacuum, cycling rate and/or ejection volume (depending on the operating mode). The driver is also capable of automatically and periodically venting the drive line to eliminate condensation and foul air. The absence of a motor or electrical pump make the device small, reliable, easy to handle, and less expensive.

Description:
[0001]    This application claims priority from a Provisional Application, Serial No. 60/470,711, filed May 15, 2003. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to medical equipment, and, more particularly, to machines for powering pneumatic ventricular assist devices.  
         BACKGROUND  
         [0003]    Ventricular assist devices (“VAD”) are used to help supplement the heart&#39;s pumping action both during and after certain kinds of surgery, in situations where a complete cardiopulmonary bypass (using a heart-lung machine) is neither needed nor advisable in light of the serious side effects associated therewith. Ventricular assist devices typically comprise a pair of cannulae or other tubing and some sort of pump operably connected to the cannulae. In use, the cannulae are attached to either the left side of the heart (a left ventricular assist device) or to the right side of the heart (a right ventricular assist device) “in parallel,” i.e., the pump supplements the heart&#39;s pumping action but does not completely bypass it, and the pump is activated. Alternatively, a pump may be directly implanted into the body.  
           [0004]    Originally, ventricular assist devices were air powered, wherein fluctuating air pressure, provided by a simple mechanical air pump machine, was applied to a bladder-like sac. The bladder had input and output valves, so that blood would enter the bladder through the input valve when the pressure on the bladder was low, and exit the bladder through the output valve when the pressure on the bladder was high. Unfortunately, these pneumatic ventricular assist devices were complicated, and used expensive mechanical valves that were prone to failure, subject to “clogging,” and that caused blood trauma or damage because of hard, metal edges and the like.  
           [0005]    To overcome these problems, smaller, more reliable ventricular assist devices have been in use and/or development. These include axial flow pumps for temporary insertion directly into the heart, and peristaltic or centrifugal pumps. The former are based on the Archymides&#39; Principle, where a rod with helical blades is rotated inside a tube to displace liquid. In use, a catheter-mounted, miniature axial flow pump is appropriately positioned inside the heart, and is caused to operate via some sort of external magnetic drive or other appropriate mechanism. With high enough RPM&#39;s, a significant amount of blood can be pumped. In the case of peristaltic pumps, blood is moved by the action of a rapidly rotating impeller (spinning cone or the like), which causes the blood to accelerate out an exit. Both of these categories of ventricular assist devices are generally reliable and implantable, but are very expensive, not particularly durable, and are not useful in situations where a patient needs a true pulsating blood supply. Specifically, axial and peristaltic pumps are typically left on in a continuous operation mode, where a steady stream of blood is supplied on a continuous basis, as opposed to the natural rhythm of the heart, which acts on a periodic, pulse-producing basis. In addition, such pumps are still largely in the developmental or trial phase.  
           [0006]    Because of the inherent performance limitations of these ventricular assist devices, pneumatic devices would seem to be a good choice for providing pulsing pulmonary augmentation. However, as mentioned above, pneumatic ventricular assist devices are prone to failure and can cause blood damage and clotting. Moreover, the driver units for operating the pneumatic ventricular assist devices are motor-based (therefore, generally mechanically unreliable), and can only offer a simple cyclical pressure mode of operation, i.e., a repeating minimum and maximum pressure applied to the VAD bladder, which cannot be adjusted for particular patient conditions.  
           [0007]    Accordingly, a primary object of the present invention is to provide a driver for pneumatic ventricular assist devices that is more reliable, that has no electrical pump or motor, and that provides a greater degree of operational flexibility and customization.  
         SUMMARY  
         [0008]    A driver for a pneumatic ventricular assist device (VAD) is powered by pressurized air, oxygen or any other gas commonly available in hospital rooms, intensive care units and operating rooms. The driver can provide both blood-ejecting pressure (systole) and blood-filling vacuum (diastole) to the VAD. The driver is controlled by a computer/digital controller by means of pressure and volume sensors, and electromechanical, computer-controlled valves. Ventricular pumping is performed by a single spring-loaded piston or bellows inside a pump cylinder. The computer can actively regulate maximum systolic ventricular pressure, maximum diastolic vacuum, cycling rate and/or ejection volume (depending on the operating mode). The driver is also capable of automatically and periodically venting the drive line to eliminate condensation and foul air. The absence of a motor or electrical pump make the device small, reliable, easy to handle, and inexpensive.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    These and other features, aspects, and advantages of the present invention will become better understood with respect to the following description, appended claims, and accompanying drawings, in which:  
         [0010]    [0010]FIG. 1 is a schematic diagram of an air-pressure powered driver for pneumatic ventricular assist devices, according to the present invention;  
         [0011]    [0011]FIGS. 2A &amp; 2B are schematic diagrams of a portion of the air-pressure powered driver in operation; and  
         [0012]    [0012]FIGS. 3A-3C are various views of the air-pressure powered driver as implemented as a wheeled, portable cart. 
     
    
     DETAILED DESCRIPTION  
       [0013]    With reference to FIG. 1, an air pressure powered driver  10  for driving a pneumatic ventricular assist device  12  (VAD) includes a console unit  14  and a pressurized air/gas unit  16 , which includes one or more backup tanks  18  of pressurized air and a gas input connector  20  that attaches to a facility-wide pressurized air line  22 . The console unit  14  includes a computer or other electronic controller  24 , a pump cylinder  26  with a spring-loaded, sealed, air-movement member  28  (piston or bellows), an inlet pressure valve  30 , and a cylinder venting valve  32 , both of which are attached to the pressurized air source  16  and the source (or input) end of the pump cylinder  26 . The pump cylinder driveline  34  (i.e., the output of the pump cylinder  26 ) is attached to the ventricular assist device  12 . In use, at the beginning of a cycle, the computer  24  opens the inlet pressure valve  30  to compress the bellows and spring  28  and raise the systolic pressure in the VAD  12  (active systole). Once the maximum desired drive line pressure is achieved, as measured by a driveline pressure sensor  36  electrically connected to the computer  24 , the inlet pressure valve  30  is closed and the computer  24  waits (passive systole) until the desired blood volume is ejected from the VAD (volume-limited mode) or the systolic time has elapsed (frequency-limited mode). Diastole begins by opening the cylinder venting valve  32 . The compressed spring inside the bellows  28  then creates a vacuum for the blood-filling phase of the cycle (i.e., as the spring pushes the bellows outwards, the air pressure in the driveline  34 , connected to the VAD  12 , decreases). Once the desired vacuum level is reached, as measured by the driveline pressure sensor  36 , a vacuum regulating valve  38  (attached to the driveline  34 ) opens to let air into the VAD/inner piston space/driveline  34  insuring that the desired vacuum level is not exceeded. The computer  24  then waits for the desired blood volume to fill the VAD  12  (volume-limited mode) or until the diastolic time has elapsed (frequency-limited mode).  
         [0014]    As noted above, the driver  10  utilizes controlled pressurized air for operating the VAD  12 , as supplied to the console  14  from the pressurized air unit  16 , and not a motor-driven pump or the like. The pressurized air unit  16  may be separate from the console  14 , or attached thereto, e.g., as part of a mobile cart or the like (see FIGS. 3A-3C). The primary source of pressurized air is the pressurized air, oxygen or other gas supply  22  found in most hospital rooms, intensive care units, and operating rooms, which is connected to the unit  16  by the connector  20 . The inlet pressure needs to be several times greater than the maximum systolic pressure desired, which is about 5 psi. Standard hospital oxygen and air supplies are regulated for fifty psi of pressure, which may be regulated by a regulator/alarm unit  40  positioned between the console  14  and the tanks  18  and supply line  22 . The tanks  18  are provided as a backup in case the main supply line  22  is shut off, or where portability is needed. A selector valve  42 , either computer-controlled or manual, is provided for selecting between the supply line  22  and tanks  18 . The regulator/alarm unit  40  may be configured to emit an alarm if the input pressure into the regulator/alarm unit  40 , i.e., the line pressure or tank pressure, falls or drops below a certain level.  
         [0015]    An inlet pressure sensor  44 , in fluid communication with the console&#39;s pressurized air input line  46  and electrically connected to the computer  24 , may be provided to issue a signal to the computer  24  to warn the user if the inlet pressure drops due to a supply failure.  
         [0016]    The computer  24  can be of any appropriate design or configuration. In one exemplary embodiment, the computer  24  comprises a microcontroller or microprocessor  50  and associated standard components (RAM, I/O bus, etc.), a video controller  52  and display  54  operably connected to the microcontroller  50 , a communications bus or port  56  (e.g., USB, Ethernet) for external access to the microcontroller, and an A/D and D/A converter  58  or other sensor/valve interface or control unit. The computer  24  also includes a speaker  60  for sounding alarms or the like.  
         [0017]    Remaining components will be described with respect to the operation of the air-pressure powered driver  10 .  
         [0018]    Pneumatic ventricular assist devices work by applying air pressure to a bladder or sac effectively attached in parallel to a patient&#39;s heart. Specifically, when pressure is applied to the sac, blood in the sac is ejected. When the air pressure against the sac is reduced, the sac expands, causing blood to enter the sac. When appropriate directional valves are employed, this creates a pulsing or cyclical blood flow. According to the present invention, with reference to FIGS. 2A and 2B, this action is accomplished using computer-controlled valves, a source of pressurized air, and the pump cylinder with spring-loaded bellows or piston.  
         [0019]    As shown in FIG. 2A, at the beginning of a cycle, the computer  24  opens the inlet pressure valve  30 . This causes air to enter into the inlet side of the pump cylinder  26 , which compresses the bellows and spring  28  (it should be noted that the inlet side of the cylinder is sealed or separate from the outlet side). Compressing the bellows  28  causes the pressure of the air/gas in the driveline  34  to increase, which in turn compresses the VAD bladder or sac  70 , forcing blood out of the sac, through a VAD outlet valve  72 , and into the patient&#39;s bloodstream.  
         [0020]    Once the maximum desired pressure in the driveline  34  is achieved, as measured by the driveline pressure sensor  36 , the inlet pressure valve  30  is closed and the computer  24  waits (passive systole) until the desired blood volume is ejected from the VAD  12  (volume-limited mode) or the systolic time has elapsed (frequency-limited mode). If the diastolic vacuum has not been established or is below the desired level (i.e., the driveline pressure is above the desired diastolic vacuum level), the computer  24  causes the vacuum regulating valve  38  to open momentarily to let a small amount of air escape the driveline  34  at the end of the systolic period.  
         [0021]    As shown in FIG. 2B, diastole begins by opening the cylinder venting valve  32 . The compressed spring inside the piston cylinder or bellows will then create a vacuum for the blood-filling phase of the cycle. Specifically, as pressurized air is let out of the cylinder  26 , there is no longer enough pressure to counteract the spring in the bellows  28 . The spring forces the bellows/piston  28  outwards, increasing the effective volume of the driveline  34  and reducing the air pressure therein. This causes the VAD bladder  70  to expand, drawing in blood through a one-way VAD inlet valve  74 . Once the desired vacuum level is reached, as measured by the driveline pressure sensor  36 , the vacuum regulating valve  38  is opened to let air into the driveline  34  insuring that the desired vacuum level is not exceeded. If the desired vacuum level is not reached then it will be adjusted for the next cycle by opening the vacuum regulating valve  38  as discussed above. The computer  24  then waits for the desired blood volume to fill the VAD (volume-limited mode) or until the diastolic time has elapsed (frequency-limited mode).  
         [0022]    The blood volume in the VAD  12  can be measured directly by a sensor in the VAD chamber (not shown). The blood volume in the VAD blood sac need not be measured directly, however, allowing for a simpler VAD design, but can be indirectly calculated by the computer  24  (calibrated to the VAD and driveline deadspace) by using Boyle&#39;s law (assuming a constant temperature, P 1 ·V 1 =P 2 ·V 2 ) and measuring the displaced volume in the pump cylinder  26  and driveline and barometric pressures. The barometric pressure and displaced volume can be measured by having, respectively: (i) a barometric pressure sensor  80  operably attached to the computer  24 ; and (ii) a distance sensor  82  (LED, other optical sensor, or the like) in the pump cylinder  26  and operably connected to the computer  24 , that measures the distance from one end of the pump cylinder to the bellows (or another appropriate measurement).  
         [0023]    A safety pressure relief valve  84  is attached to the driveline  34  to insure that maximum VAD/driveline pressure (e.g., 5 psi) is never exceeded, which could lead to air leaks in the VAD  12 .  
         [0024]    Periodically or at user selected times, the driver  10  has the capability of venting the driveline  34  to prevent excess condensation and remove fouled air. This is accomplished at the end of the diastolic period by opening a driveline venting valve  86 , positioned between the driveline  34  and the pressurized air input line  46 , for a short time.  
         [0025]    The VAD/inner-cylinder/driveline space  34  is pressurized with fresh air. Excess pressure is vented by the pressure relief valve  84 . Then the vacuum regulating valve  38  is opened to vent the system.  
         [0026]    The computer  24  has the capability of controlling the entire process according to user selectable or manufacturer&#39;s preset parameters such as desired stroke volume, rate, VAD output, systolic to diastolic ratio, maximum diastolic volume, minimum systolic volume, maximum systolic pressure, and/or maximum diastolic vacuum. The computer, through its sensors, also has self diagnostic capabilities and can trigger warnings and alarms to the user. Finally, the computer may also have the capability of storing or relaying the operational status and performance of the driver to remote locations (nurses&#39; station, doctor&#39;s office) via network or wireless communications  56 .  
         [0027]    Although the VAD pumping action is primarily effectuated using pressurized air, the computer, valves, and sensors are electrically powered, via a standard power supply  30  (attached to a wall outlet), generator, battery power system, or the like (not shown).  
         [0028]    Silencers or mufflers  88  may be attached to the outputs of the valves  32 ,  38 , for minimizing noise as pressurized air is periodically let out of the driver&#39;s air lines.  
         [0029]    An emergency foot pump or bellows  90  may be operably attached to the driveline  34 , via a manual selector valve  92  and/or connector  94 . In an emergency (i.e., complete loss of pressurized air and/or electrical power), the foot bellows  90  are pumped manually, causing a variable pressure to be applied to the VAD  12 . Preferably, the air volume displaced by the bellows  90  is configured to generally match the displacement volume required for operating the VAD pumping sac  70 .  
         [0030]    [0030]FIGS. 3A-3C show how the air-pressure powered driver  10  can be implemented as a portable cart.  
         [0031]    Although the air-pressure powered driver has been described as having separate air inlet and pump venting valves  30 ,  32 , respectively, a unitary air distribution device could be used instead, i.e., a computer-controlled device with three states: (i) “closed;” (ii) open to ambient (possibly through a muffler); and (iii) open to air input line  46 . This is also the case for the valves  38 ,  84 ,  86  on the driveline  34 . Thus, the term “air distribution device,” as used herein, refers both to: stand-alone, discreet valves; multi-state valves; or a combination of the two.  
         [0032]    Although the air-pressure powered driver has been illustrated as having a spring-loaded bellows or piston in the pump, a different biasing mechanism other than a spring could be used instead (polymer members, motor units, constructing the bellows out of a deformable material with a material memory, etc.). Accordingly, the term “biased air movement member” incorporates any bellows, pistons, or the like biased with a spring or other suitable device.  
         [0033]    Since certain changes may be made in the above-described air-pressure powered driver for pneumatic ventricular assist devices, without departing from the spirit and scope of the invention herein involved, it is intended that all of the subject matter of the above description or shown in the accompanying drawings shall be interpreted merely as examples illustrating the inventive concept herein and shall not be construed as limiting the invention.