In gravity drainage mode, the patient’s intrathoracic pressure is determined by which factor?

• A. The ambient room temperature
• B. The float ball water column level only
• C. The suction control setting
• D. The patient's respiratory rate

Answer: (B)

In gravity drainage, the chest tube system uses a passive water-seal with no suction. The pressure transmitted back to the intrathoracic space is set by the hydrostatic pressure of the water column in the seal, which is determined by how high the float-ball/water column rises. That height defines the maximum negative pressure that can be generated in the pleural space, so the intrathoracic pressure is governed by the water-column level.

Ambient room temperature isn’t a controlling factor for this pressure, and the suction control setting isn’t used in gravity mode. The patient’s respiratory rate reflects their own effort but does not set the mechanical pressure in the gravity-drainage system.