Which of the following is an essential component of CPR?

The essential component of CPR is chest compressions. This aspect is crucial because it creates blood flow to the heart and brain during cardiac arrest. Chest compressions help to manually pump blood through the body when the heart is unable to do so, delivering oxygen to vital organs.

Rescue breathing, while important in some CPR techniques, is not always used for every individual. In cases of a witnessed collapse, especially in adults, focusing on high-quality chest compressions may be more effective and encouraged. Defibrillation, while critical in the treatment of certain heart conditions, is a separate procedure that comes into play when using an automated external defibrillator (AED). Evaluation refers to assessing the situation and responsiveness but is not a direct action taken during CPR.

Hence, chest compressions are the fundamental action that determines the effectiveness of CPR, making it the correct answer.