# Feedback Devices for Cardiopulmonary Resuscitation: A Narrative Review
## 2 Classification of Feedback Devices for Cardiopulmonary Resuscitation
### Audio-Visual Feedback Devices
A comparison study with the Zoll PocketCPRTTM showed the CPRmeterTM to perform better in detecting inclinations during compression [16]. The second-generation CPRmeterTM 2 can be combined with the CPRmeterTM app to promote quality improvement initiatives by sharing details of the CPR, such as compression depth, compression frequency, time duration, and chest compression scores. After the resuscitation is completed, the CPRmeterTM application can report, share, and export statistical data to enable a more detailed analysis of the resuscitation details. The Zoll PocketCPRTTM is a small, battery-powered device used for CPR made in the United States. Its appearance is depicted in Figure 2a. It utilizes an accelerometer to measure the frequency and depth of compression and can be placed on the victim's chest to provide audio prompts to guide the rescuer in performing CPR, if desired. The feedback on compression depth is provided through the flashing of LED lights, with the metronome increasing the LED flashing rate to reflect the correct compression depth and rate according to the CPR guidelines [12]. However, a study on the Zoll PocketCPRTTM demonstrated a high proportion of inadequate compression points during CPR usage, which can be attributed to the device's lack of a pressure sensor and reliance on an accelerometer, which cannot detect incorrect compression angles. Additionally, the time from the start of measurement to the first recorded chest compression was significantly increased when using the Zoll PocketCPRTTM, potentially reducing the success rate of resuscitation in practical applications [16]. Despite its limitations, the Zoll PocketCPRTTM remains a promising device for improving CPR performance. Cardio First AngelTM (CFA), on the other hand, shown in Figure 2b, is an entirely mechanical feedback device designed to assist with cardiopulmonary resuscitation (CPR) by providing audio feedback on chest compression. Intriguingly, this device does not require any power source. It relies on a specially designed spring system that emits a sound upon achieving the correct compression depth from 50-60 mm. Additionally, the spring mechanism produces a second sound when the chest fully rebounds, thus confirming both the downward and upward motion of a correct compression. Comparative studies have been conducted to evaluate the effectiveness of standard chest compression performed without the aid of CFA versus those assisted by the CFA device on resuscitation mannequins. Notably, significant improvements were observed in the compression depth when the CFA-assisted method was employed [21]. Furthermore, a clinical trial involving 80 participants demonstrated that the use of CFA substantially increased rates of ROSC and concurrently decreased the risk of rib fractures. These results underscore the potentially transformative impact of the CFA device in improving CPR efficacy and safety [20]. The True-CPR developed by Physio-Control is a smart compression feedback device from the United States that can provide real-time CPR feedback, displaying compression depth and rate on a highly visible dial. The external appearance of the True-CPR device is depicted in Figure 2c. In addition, the device provides metronome prompts for cardiac resuscitation and ventilation guidance to assist rescuers in adhering to recommended guidelines. Unlike other feedback devices that use accelerometers, True-CPR is the only device to use Tri-Axis Field Induction (TFI), a proprietary technology based on physical control. TFI uses a three-dimensional magnetic field to accurately measure the distance between two objects (chest pad and back pad) during the resuscitation process, enabling accurate measurement of chest compression depth and providing high-quality real-time feedback. True-CPR compensates for misalignment between the chest and back pads, ensuring accurate measurement of compression depth even when misaligned [24]. This feature greatly improves the ability to measure compression depth on flexible surfaces.