What is the function of the high-negativity float valve located at the top of the water-sealed chamber?

• A. It prevents any negative pressure from forming
• B. It vents air to the room on each breath
• C. It allows the patient to draw as much intrathoracic pressure as needed during inspiration
• D. It seals the chest drain shut during expiration

Answer: (C)

The main idea is that this valve serves as a safety relief that does not block normal inspiratory efforts. During inspiration the intrapleural pressure becomes more negative to expand the lung. The high-negativity float valve allows that negative pressure to be reached without the chest drainage system resisting it. If intrapleural pressure becomes dangerously negative, the valve opens to vent to the room, preventing injury from excessive negative pressure. In this way, the patient can generate the necessary negative pressure to draw air into the chest, while the system still maintains one-way flow overall.

If you were thinking of the other options: the device is not designed to prevent any negative pressure from forming entirely, nor does it continuously vent every breath, and it does not seal the chest drain shut during expiration.