What condition is Nancy trying to prevent by breathing into a paper bag during a panic attack?

During a panic attack, an individual may hyperventilate, leading to excessive exhalation of carbon dioxide (CO2). This decrease in CO2 levels results in a condition known as respiratory alkalosis. By breathing into a paper bag, Nancy is essentially re-inhaling some of the expelled CO2, which helps to restore the balance of gases in her blood. This practice increases the concentration of CO2, thereby helping to alleviate the symptoms of hyperventilation and reduce the alkalinity of the blood, preventing or correcting respiratory alkalosis. The act of breathing into the bag allows the person to regain normal breathing patterns and stabilize their body's pH levels, effectively addressing the physiological response to the panic attack.