Introduction Ventilator-induced diaphragmatic dysfunction is a common complication in critically ill patients receiving invasive mechanical ventilation and is associated with prolonged ventilatory support, delayed weaning and increased length of stay in the intensive care unit (ICU). Strategies that preserve respiratory muscle function may therefore improve recovery in this population. Photobiomodulation therapy combined with a static magnetic field (PBMT-sMF) has been proposed as a non-invasive intervention capable of modulating biological processes involved in muscle function. However, high-quality evidence in mechanically ventilated ICU patients remains limited.

Methods and analysis This is a multicentre, randomised, triple-blind, placebo-controlled clinical trial. Critically ill patients requiring invasive mechanical ventilation will be randomly allocated to receive PBMT-sMF or placebo. The intervention will be applied once daily to predefined anatomical sites over the respiratory muscles during ICU stay. The primary outcome is ICU length of stay, defined as days from ICU admission to ICU discharge. Secondary outcomes include diaphragm thickness by ultrasonography, time to successful weaning from mechanical ventilation, hospital length of stay and selected physiological and laboratory parameters. Data will be analysed using appropriate statistical methods.

Ethics and dissemination Ethics approval has been obtained by the Research Ethics Committee of the coordinating institution Associação Dr. Bartholomeu Tacchini (approval number: 7.525.651). Informed consent will be obtained from legally authorised representatives and from participants if capacity is regained. Findings will be disseminated through peer-reviewed publications and scientific conferences.

Trial registration number NCT06454006.