AttendMe development preview. Illustrative model; independent clinical review pending.
Fig 1Underwater seal drain1 2 breathing · L leak · S suction · C clampcolumn +3.0 cm · no bubbling
Water in the drain tube: it shows pleural pressureBubbles from the tip: air leaving the chestVent line to the suction regulatorThe drainage tube ends 2 cm under the water, so air can leave the chest but cannot return. Breathing on their own, pleural pressure falls on inspiration and the water climbs the tube; on a ventilator the swing reverses. Bubbles mean air is leaving the pleural space. Suction at −20 cm H₂O damps the swing; a clamp or kink stops it. Illustrative pressures, not clinically reviewed.