CPR is a technique used by a rescuer in an emergency situation to get oxygen into a victims blood when that persons heart has stopped beating and/or they are not breathing spontaneously. When performing CPR the rescuer creates blood circulation in the victims body by periodically compressing the victims chest.
The American Heart Association (AHA) recommends that the rescuer press down on the sternum with a force sufficient to depress it between 1.5 and 2.0 inches. The current recommended rate for these periodic depressions is 100 times a minute, and 30 chest compressions should be given for every two rescue breaths. Chest compressions produce blood circulation as the result of a generalized increase in intrathoracic pressure and/or direct compression of the heart. The guidelines state “blood circulated to the lungs by chest compressions will likely receive enough oxygen to maintain life when the compressions are accompanied by properly performed rescue breathing.” A victim can be kept alive using CPR provided the rescuer(s) are able to continue delivering properly performed chest compressions and rescue breaths.
Administering CPR is a challenging and physically demanding procedure which is performed under stressful life and death circumstances. Performing chest compressions and rescue breaths is a also a physically demanding task, and can be difficult to properly coordinate. The quality of chest compressions and rescue breaths delivered to a patient can degrade for a number of reasons, including fatigue, lack of visual references, and rescue situation stresses. As rescuers become fatigued, they may not realize that they are compressing a patient's chest with inadequate force. The more fatigued a rescuer becomes, the less he may be compressing a patient's chest, and the more likely the effectiveness of the CPR is reduced.
To be most effective, the rescuer must attempt to keep the chest compressions uniform both in terms of the time between successive chest compressions and the amount of force used for each compression. Keeping uniform intervals for chest compressions is difficult the longer the CPR must be administered as the stresses associated with a rescue situation can cause the rescuer's sense of time to be distorted. Keeping the chest compressions uniform in terms of force is difficult not only because of fatigue, but also because it is difficult for the rescuer to estimate the force being applied based on the distance which the chest is being compressed. Much of the difficulty in estimating the distance which the chest is being compressed stems from the relative position of the rescuer and the victim. When performing chest compressions, the rescuer positions his or her shoulders directly above the victim's chest, and pushes straight down on the sternum. In this position, the rescuer's line of sight is straight down at the victim's chest. With this line of sight, the rescuer has no visual reference point to use as a basis for estimating the distance that he or she is compressing the chest.
The aforementioned problems may be compounded by a number of factors, such as when the length of time that CPR is being administered increases, and when the rescuer is not accustomed to rescue situations (for example when CPR is being performed by a lay person or a relatively inexperienced rescuer).
A number of devices have been proposed to assist a rescuer in applying CPR, as described, for example, in U.S. Pat. No. 6,125,299 to Groenke et al. Most of these devices measure either the force applied to a patient's chest, or measure the acceleration of the patient's chest (or rescuer's hand), or both. The measured force may be compared to a known desired value, and a prompt may be issued from the device instructing a rescuer to compress the patient's chest harder or softer. Displacement of a patient's chest can be calculated by double integrating a measured acceleration, and a prompt may be issued from the device instructing a rescuer to compress the patient's chest harder or softer. Many prior art devices also measure the frequency of chest compressions given, and are able to prompt a rescuer to increase or decrease the rate of compressions being administered.
Although measuring acceleration is an acceptable method of determining chest compression during CPR, the method is not without its flaws. For example, signal error, external acceleration error, and drift error in the compression starting points can all create inaccuracies in chest compression measurement. External acceleration error can arise from use of the accelerometer in a moving vehicle such as an ambulance, or from unusual patient attitudes, such as partially sitting up.
For these reasons, there is a need in the art for a practical device that more accurately measures the compression of a patient's chest during CPR, and provides feedback to a rescuer in the event that the displacement and frequency of chest compressions falls outside a preset criteria. A device of this type will provide rescuers with coaching which will enable them to deliver chest compressions consistently and beneficently.