This study aims to determine whether a protective mechanical ventilator strategy generates a reduction in the Bohr´s dead space in patients with moderate or severe acute respiratory distress syndrome (ARDS). Commonly used ventilatory strategies in the clinical practice were applied sequentially to assess their impact. Data obtained from volumetric capnography will be recorded after each ventilatory strategy is applied

Protective mechanical ventilation strategies are currently the cornerstone of treatment for patients undergoing mechanical ventilatory support. Among them, we can mention the reduction in tidal volume, the reduction in driving pressure, the PEEP setting, and the respiratory rate reduction. All of these strategies are aimed at preventing ventilator-induced lung injury. Few clinical studies have evaluated the effects of protective mechanical ventilation strategies on dead-space. This study was designed to evaluate dead space and alveolar ventilation with a sequence of protective ventilatory strategies, keeping PEEP levels constant. The ventilatory strategy was developed to reduce tidal volume, set an end-inspiratory pause, and reduce the frequencies of lung tissue impact.

Baseline:

All the patients kept the variables under study constant for 60 minutes. Each of the participants was studied for a period of 150 minutes. PEEP programming was set with transpulmonary end-expiratory pressures to maintain between 0 and 5 cmH2O and was remained constant throughout the study.

Protocol design:

Baseline: Vt 7 ml/kg/PBW Phase I: Vt 6 ml/kg/PBW. Phase II: Vt 5 ml/kg/PBW. Phase III: end-inspiratory pause prolongation until achieving I:E ratio equal to 1, maintaining a constant Vt level (5 ml/kg/PBW) Phase IV: Respiratory rate reduction by 20% of basal conditions, maintaining constant VT level (5 ml/kg/PBW).

After 30 minutes of continuous monitoring, minute ventilation, mechanical power, variables obtained by volumetric capnography, and arterial blood gases were recorded in each study phase.

This study will be performed in the Intensive Care Unit of a University Hospital.

Inclusion Criteria:

• Age older than 18 years
• The requirement of invasive mechanical ventilation for more than 4 hours and less than 72 hours
• Severe hypoxemia (PAFI < 200) secondary to ARDS

Exclusion Criteria:

• Heart failure stage IV
• Chronic obstructive pulmonary disease (COPD) on home oxygen
• Bronchopulmonary fistula
• Hypovolemic Shock with active hemorrhage
• Gastrointestinal bleeding
• Oesophageal Varices
• Nasopharynx surgery, recent oesophageal or gastric surgery
• Massive Pulmonary Thromboembolism
• Catastrophic respiratory failure requiring urgent extracorporeal life support
• Respiratory acidosis. Hydrogen potential (pH) is less than 7.20 and PaCO2 higher than 60 mmHg at baseline.