Final answer:
The immediate danger of excessive ventilation after cardiac arrest is Respiratory alkalosis, resulting from hyperventilation that reduces CO2 levels and increases blood pH.
Step-by-step explanation:
The immediate danger of excessive ventilation during the post-cardiac arrest period for patients who achieve Return of Spontaneous Circulation (ROSC) is Respiratory alkalosis. When ventilation is high, this can lead to an increased respiratory rate while the volume of air per breath decreases, causing more carbon dioxide (CO2) than necessary to be exhaled. This results in low blood CO2 levels and an increase in blood pH, a condition known as hyperventilation. It is essential to manage the patient's breathing carefully post-cardiac arrest to avoid complications such as Respiratory alkalosis, which can adversely affect the patient's recovery.