This is a randomised controlled trial evaluating whether controlled air introduction into pleural space (air entrainment) during pleural effusion drainage reduces pain, improves patient satisfaction, and facilitates more effective drainage in patients with non-expandable lung (NEL). NEL lung is a common complication in patients with malignant or chronic pleural effusions, where the lung fails to fully re-expand after fluid removal due to pleural disease or fibrosis. In these patients, drainage often creates excessive negative pressure within the pleural cavity, leading to pain, vasovagal episodes, early termination of drainage, and the need for repeated procedures. This study investigates a simple, safe, and low-cost intervention using a standard 3-way tap attached to the drainage system. By intermittently opening the tap to atmospheric air during drainage, air enters the pleural cavity in a controlled fashion, reducing negative pressure and potentially reducing pain, improving drainage tolerance, and minimising the need for repeated procedures. Pleural effusion drainage is a common procedure in patients with advanced malignancy or chronic pleural disease. In patients with NEL, fluid removal creates a vacuum effect within the pleural space due to the inability of the lung to fully re-expand. This negative pressure is a key driver of severe procedural pain, vasovagal symptoms, and premature cessation of drainage. It may also necessitate multiple drainage procedures over a short period. Currently, there are limited strategies to mitigate this problem, often relying on stopping the procedure prematurely or on analgesia, which does not address the underlying cause. This trial evaluates the introduction of atmospheric air into the pleural space during drainage as a pragmatic, low-cost solution. The technique uses standard equipment - a 3-way tap - allowing air to be introduced safely and intermittently during drainage to reduce the vacuum effect. Patients undergoing therapeutic pleural drainage with an indwelling catheter or chest drain will be randomised in a 2:1 ratio to: Standard drainage care (control group) Drainage with intermittent controlled air introduction (intervention group) Air entrainment will be performed by briefly opening the 3-way tap to atmospheric air during drainage up to five times, based on patient discomfort and operator discretion. This aims to equalise pleural pressures, reduce pain, and improve drainage outcomes. Randomisation is weighted 2:1 towards the intervention group to maximise the number of patients who may benefit, following favourable preliminary data. Both patients and outcome assessors will be blinded to group allocation. Outcomes collected Primary Outcomes: Patient-reported pain scores during drainage - Pain will be assessed using the Visual Analogue Scale (VAS), ranging from 0 to 10 cm, where 0 indicates no pain and 10 indicates worst imaginable pain. Higher scores represent a worse outcome. Secondary Outcomes: Volume of pleural fluid drained Number of pleural drainage procedures required Time interval between drainage procedures Incidence of complications (e.g., pneumothorax, re-expansion pulmonary oedema, infection) Reasons for incomplete drainage, including the presence and characteristics of non-expandable lung Patient-reported satisfaction with the drainage procedure