What causes poor oxygenation in a patient with Acute Respiratory Distress Syndrome (ARDS) on mechanical ventilation?

From the Guidelines

Poor oxygenation in ARDS patients on mechanical ventilation is primarily caused by widespread inflammation in the lungs that damages the alveoli and capillaries, leading to increased permeability of the alveolar-capillary membrane, resulting in pulmonary edema and the accumulation of protein-rich fluid in the alveoli, as well as surfactant dysfunction, atelectasis, and ventilation-perfusion mismatch 1. The inflammatory process in ARDS leads to a range of pathophysiological changes that impair gas exchange and contribute to poor oxygenation. Key factors include:

• Increased permeability of the alveolar-capillary membrane, allowing fluid and proteins to accumulate in the alveoli
• Surfactant dysfunction, leading to alveolar collapse (atelectasis) and further ventilation-perfusion mismatch
• The damaged lung tissue becomes stiff and non-compliant, making it difficult to ventilate effectively
• In severe cases, fibrosis may develop, creating a thickened barrier to oxygen diffusion Mechanical ventilation itself can sometimes worsen the situation through ventilator-induced lung injury if excessive pressures or volumes are used. Management focuses on lung-protective ventilation strategies with low tidal volumes (4–8 ml/kg predicted body weight) and lower inspiratory pressures (plateau pressure < 30 cm H2O), as well as the use of higher PEEP without lung recruitment maneuvers (LRMs) in patients with moderate to severe ARDS, and the consideration of prone positioning, neuromuscular blockers, and venovenous extracorporeal membrane oxygenation (VV-ECMO) in selected cases 1. Some key recommendations for managing ARDS include:
• Using mechanical ventilation strategies that limit tidal volume and inspiratory pressures
• Using higher PEEP without LRMs in patients with moderate to severe ARDS
• Considering prone positioning, neuromuscular blockers, and VV-ECMO in selected cases
• Avoiding prolonged LRMs in patients with moderate to severe ARDS.

From the Research

Causes of Poor Oxygenation in ARDS Patients on Mechanical Ventilation

The causes of poor oxygenation in patients with Acute Respiratory Distress Syndrome (ARDS) on mechanical ventilation can be attributed to several factors, including:

• Ventilator-induced lung injury (VILI) due to high tidal volumes or inadequate positive end-expiratory pressure (PEEP) settings 2, 3
• Inadequate lung recruitment, leading to ventilation-perfusion mismatch 2, 3
• Severe hypoxemia, defined as a PaO2/FiO2 ratio < 100 mmHg, which is associated with high mortality rates 2
• Ineffective mechanical ventilatory strategies, such as insufficient use of low FiO2 or failure to optimize PEEP 3
• Presence of other organ failures, which can exacerbate the severity of ARDS and worsen oxygenation 2

Factors Affecting Oxygenation in ARDS Patients

Several factors can affect oxygenation in ARDS patients on mechanical ventilation, including:

• The level of PEEP, which must be optimized to maintain lung recruitment while minimizing the risk of VILI 2, 3
• The use of prone positioning, which can improve oxygenation by promoting a more homogeneous distribution of ventilation 2, 4
• The application of neuromuscular blocking agents (NMBAs), which can help maintain patient-ventilation synchrony and improve oxygenation 2
• The use of extracorporeal membrane oxygenation (ECMO) as a rescue therapy in severe ARDS cases 2, 5, 6

Mechanical Ventilatory Strategies in ARDS

Evidence-based mechanical ventilatory strategies in ARDS aim to prevent secondary lung injury and improve patient outcomes, including:

• Utilization of low tidal volumes and low plateau pressures to minimize barotrauma 3
• Optimization of PEEP to maintain or improve lung recruitment 3
• Use of low FiO2 to prevent injury related to oxygen free radicals 3
• Application of prone ventilation to improve oxygenation and reduce the risk of VILI 3, 4