This experimental physiologic study is conducted to determine the effect of different mechanical ventilation settings (in terms of positive end-expiratory pressure) on patients receiving mechanical ventilation with expiratory flow obstruction and autoPEEP. The main question it aims to answer In patients with autoPEEP, what is the effect of applying external PEEP on gas exchange and the lung emptying pattern in patients with total ventilatory support? Participants must comply:

- Age > 17 years.

• Invasive mechanical ventilation (endotracheal tube or tracheostomy) in controlled mode, with SpO2 > 90%, for more than 12 hours.
• No contraindications for abdominal compression.
• Presence of autoPEEP greater than 3 cmH2O measured in mechanical ventilation with total support and ZEEP.
• Total mechanical respiratory support, without respiratory muscle activity (inspiratory or expiratory).

The study includes patients with controlled mechanical ventilation and autoPEEP. By programming different levels of external PEEP, its effect on respiratory mechanics, gas exchange, and pulmonary emptying constants will be determined. Once included in the study, participants remain on the established mechanical ventilation schedule.Subsequently, they will remain for 5 minutes at each programmed PEEP level, during which the variables under study will be recorded. They will remain at 3 PEEP levels for 15 minutes before taking arterial blood gas samples. The variables under study will be recorded and stored in a respiratory monitor and an electrical impedance tomograph, and subsequently analyzed offline.

Inclusion Criteria:

Age > 17 years. Invasive mechanical ventilation (endotracheal tube or tracheostomy) in controlled mode, with SpO2 > 90%, for more than 12 hours.

No contraindications for abdominal compression. AutoPEEP greater than 3 cmH2O measured during mechanical ventilation with full support and ZEEP.

Full mechanical respiratory support, with no respiratory muscle activity (inspiratory or expiratory).

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Exclusion Criteria:

Cardiovascular instability: Hypotension (MAP < 60 mmHg), hypertension (SBP > 180 mmHg), heart rate < 40 bpm or > 150 bpm.

Refusal to participate in the study by a family member (without signed informed consent).

Evidence of air leak: Pneumothorax, subcutaneous emphysema, pneumomediastinum. Intracranial hypertension (ICP > 20 mmHg). Pregnant patient. Body Mass Index greater than 40 kg/m².