When performing CPR on an adult, the compression-to-breath ratio is:

In adult CPR, the universally accepted compression-to-breath ratio is 30:2. This means that for every thirty chest compressions, two rescue breaths are administered. The rationale behind this specific ratio is rooted in the need to provide a combination of effective circulation and oxygenation, maximizing blood flow to vital organs during cardiac arrest.

The importance of the 30:2 ratio lies in its balance. Chest compressions are critical for maintaining circulation, and the higher number of compressions in this ratio ensures that the heart has enough pressure to pump blood throughout the body, especially to the brain and other organs. After every 30 compressions, two breaths are given to provide oxygen, which is crucial since, during cardiac arrest, the person's breathing typically stops, leading to a deficit of oxygen in the blood.

Using other ratios, such as 15:2, 20:2, or 10:2, does not adhere to the current guidelines set by authoritative bodies such as the American Heart Association. These alternative ratios would reduce the number of compressions being delivered in the same period, potentially compromising the victim's survival chances by not maintaining adequate circulation. Thus, sticking to the 30:2 ratio is essential for effective CPR on adults.