I would classify the Zoll Lifevest as a class 3 device because it has really high risk and it is used to help sustain life. The lifevest device begins to apply high energy shock during life threatening scenarios of ventricular tachycardia or ventricular fibrillation. However, this device will send electric shocks when it detects an abnormal rhythm even if it is not life threatening. This device requires clinical trials to justify its claim and show that the device does less harm to the human body compared to benefits.
Some obstacles that Zoll Lifecor encountered when trying to get FDA approval for the lifevest might have been based on safety. The lifevest has many safety precautions such as an alarm every time the device is about to execute an electric shock. There also is a button the user can click if the alarm goes off and the patient does not require the defibrillator. FDA would have scrutinized the clinical trials to make sure that the device was safe.
People with cardiac issues such as arrhythmia or previously had a cardiac arrest can use this device on an daily basis. This device bridges the market gap between having an implanted device. Many patients need defibrillators and there are invasive procedures to implement them into the human body, but this lifevest allows the patient to have a defibrillator without needing to go through surgery.

References
http://www.startribune.com/wearable-defibrillator-can-bridge-the-gap-to-implanted-device/232122331/