Patent Application
20080119766
The invention relates generally to apparatus for treating cardiac arrest and, more specifically, chest compression devices.
Sudden cardiac arrest is a leading cause of death in developed countries in the Western World, like United States and Canada. To increase the chance for survival from cardiac arrest, important aspects are CPR (Cardio Pulmonary Resuscitation) and heart defibrillation given in the first few critical minutes after the incident. CPR is performed to ensure a sufficient flow of oxygenated blood to vital organs by external compression of the chest combined with rescue breathing. Heart defibrillation is performed to re-establish normal heart rhythm by delivery of external electric shock.
The quality of CPR is essential for survival. Chest compressions must be given with a minimum of interruptions, and be of sufficient depth and rate. Manually performed chest compressions is an extremely exhausting task, and it is practically impossible to give sufficient quality manual CPR during transportation of a patient.
Many different types of automatic chest compression devices have been developed to overcome this, based on a wide variety of technical solutions. Some devices comprise a piston which presses the patient's chest down with a given frequency and a given force. These devices comprise hydraulic/pneumatic mechanisms to provide a reciprocating movement for the piston. Other devices comprise a belt embracing the chest and a rotating motor with a spindle being engaged and disengaged.
Chest compressions given by automatic devices have the potential to be more forceful than manual compressions. There is a balance between 1) giving optimal blood flow to vital organs and 2) limiting the impact to the chest, to avoid internal injuries as a result of the external force being applied to the patient. Previously known automatic chest compression devices are designed mainly with respect to 1), and in many cases do not provide a satisfactory balance between 1) and 2).
The invention comprises a chest compression device that permits control of the compression characteristics. In some embodiments, this is achieved by providing the device with an electric motor and a controller. Such embodiments may also comprise a transmission mechanism for transferring mechanical energy between the motor and a compression element. Other advantageous features of the invention are mentioned in the appended claims.
In some embodiments, a satisfactory quality for chest compressions (frequency, speed and force) has been achieved using motors that are able to accelerate very rapidly and at the same time are able to provide high power in short periods of time. These requirements may be fulfilled by servo motors. In some embodiments, the servo motors have low rotational inertia and are adapted for high peak power.
In some embodiments, control of the motor is performed by a controller. Use of an electric motor with a controller enables full control of compression with respect to most or all of important factors, such as compression depth, compression force, compression frequency, duration of compressions, rate of relieving and applying pressure, etc. In some embodiments, control of these factors is performed by controlling the waveform of the compressions.
By having control of the compression waveform as applied to the patient, it is possible to achieve an improved balance for each patient/recipient and for each stage in the treatment. In this way the pulse pattern of the compression/decompression can be adapted to the individual patient at different stages in treatment, thus leading to improved therapy concerning both blood flow and avoidance of internal injuries.
The controller may further provide the ability to extract and log chest compression data from the system controller, enabling clinical studies and optimization of the system. Internal injuries could be related to for instance the depth profile of the compression piston, etc. Logging data would enable research into this topic and others.
The invention will now be described by means of examples illustrated in the drawings, wherein:
As mentioned previously, the motor 1 may advantageously fulfill certain requirements regarding: a) kinetic energy at max speed, b) peak power, c) efficiency (at a given power), d) weight and dimensions.
Limited kinetic energy provides dynamic performance that is, the ability to freely select a displacement profile for the compression element without high power consumption. Limited kinetic energy also provides improved safety if there is a fault in the electrical power system causing all the kinetic energy to be released into the patient's chest. In some embodiments, the limit for the kinetic energy of the motor is about 4 J (breast stiffness 200N×displacement 0.02 m=4 J).
In one example, peak power, with for example a maximum force of 550N transferred to a patient and a maximum retraction speed for the compressing element of 0.63 m/s is: P=550N×0.63 m/s=347 W. This is the power necessary, in one embodiment, at the patient's end, and losses in the transmission mechanism may advantageously be taken into consideration. This leads to a peak power for the motor in one embodiment of the invention of 400 W-600 W.
In one embodiment of the invention, substantially free return of the patient's chest to a non-compressed position is permitted by retracting the compressing element at high speed (e.g. 0.63 m/s). In another embodiment a substantially free return of the chest to an uncompressed position is permitted by means of the transmission mechanism (e.g. by mechanically disconnecting the motor from the compression element). Where the transmission mechanism is disconnected to permit return of the chest, the maximum return speed requirement may be ignored and a motor with a peak power of. 300 W-500 W has been found to be adequate.
High efficiency leads to long battery life and little generation of heat. In one embodiment of the invention, motor 1 has an efficiency of about 75%, however motors with other efficiencies may also be used.
Weight and dimensions are limited in an embodiment of the device adapted for portable use. In said embodiment the motor's weight may be limited to 500 grams.
Other relevant parameters of the motor may include average power, voltage (insulation strength), motor constants (rpm/V, etc), durability, radial and axial load on bearing. Average power may be controlled to avoid overheating a motor. In one embodiment of the invention the motor 1 has an average power greater than 100 W.
Motor 1 can e.g. be a brushless DC motor (for example a motor with a peak power equal or higher than 400 W and efficiency higher than 75%, or, for example, a motor with a peak rating up to 500 W and 150 W average rating, such as a brushless Minebae 40S40A) or it can be a DC motor with brushes. If transistors provide the commutation, any variant or combination of block commutation or sinusoidal commutation might be used. Motor 1 may comprise a controller structure with feed forward.
Intermediate storage of energy may advantageously be provided in embodiments of the device which comprise batteries not complying with the above mentioned criteria, energy storage in capacitors may help to acheive the 600 W peak power requirement. If boost circuitry is used to achieve a substantially constant battery current during the compression cycle, the battery heat dissipation can be reduced and batteries with less power handling capability than the A123 system may be used.
Another possibility (not shown) is to provide a power source adapted for connection to AC or DC mains with a small 100 W power supply if the high power lithium ion battery (or batteries) is connected in parallel with the supply. The battery will provide the peak power needed for the device operation while the power supply will ensure that the battery does not discharge. Using batteries in stead of capacitors as an energy storage will ensure that the device operation is not interrupted if the power supply is disconnected for a short period when moving the patient from one room to another etc. In one embodiment of this invention capacitors are used instead of batteries.
A combination of the above mentioned embodiments is also possible.
A motor power control circuit may be activated in case of an error situation. The circuit may cut the supply to the motor e.g. by opening the battery high side connection to the bridge circuitry. The motor power control 20 may be activated by: a) a motor controller circuit 25, b) manually (emergency stop 22), c) the main controller 12, d) a low battery voltage signal, e) low/high regulated 5V and 3.3V (not shown), and/or d) hardware shutdown as a consequence of high peak current. If the motor controller 25 fails and the bridge current rises, the main controller 12 may initiate a shut down. A hardware solution may be used if faster shutdown is needed. Some embodiments of the invention can comprise only one or a selected group of the above mentioned activating inputs. In one embodiment, substantially all input lines to the motor power control 20 have to be activated in order for the switch to turn “on” and allow compressions of the patient.
As mentioned above, in one embodiment, the battery 10 delivers power to the motor 1 via the motor power controller 20 and the three phase bridge 21. The bridge circuit 21 can have an energy storage capacitor (not shown) which may aid compression element return in an error mode. The bridge 21 comprises high side transistors (not shown) which preferably run at 100% duty cycle in order to achieve block commutation of the motor 1. In one embodiment of the invention battery voltage is limited to 30V and the bridge can comprise mosfets with a breakdown voltage of 60V.
The motor controller circuit 25 drives the motor in accordance with a drive profile, that is a determined sequence of digitally modulated pulses with a determined shape. Circuit 25 will encompass all the necessary drive algorithms needed.
Outputs from the controller 25 may include:
The motor controller may comprise software for performing the following tasks:
Motor controller 25 controls operation of motor 1 by controlling operation of the three phase bridge 21. As a safety measure, the device may be adapted to proceed in such a way that if battery 10 is suddenly removed, the main controller 12 notices the removal and immediately initiates a controlled shut down.
Safe termination of operation may be limited to turning off bridge 21 thus allowing the compression element 5 (
During start up the main processor 12 preferably controls all the device's parts. When the system is “good to go” a signal will be given to the motor controller 25. The software may comprise drive algorithms in order to safely drive the motor/device in various states of operation, illustrated in
As one can see the device according to the invention permits performance of controlled, swift and effective CPR. The use of an electric motor permits also easy adaption of the compression parameters to different patients and different situations.
