Mechanical Ventilation II: Invasive Ventilation

Positive-pressure ventilation applies positive pressure directly to the airway, forcing air into the lungs and causing alveolar expansion during inspiration. After the preset volume or pressure is delivered, expiration occurs passively. This method requires an artificial invasive airway such as an endotracheal tube or tracheostomy to function effectively.

Volume-cycled ventilators deliver a preset air volume with each breath, ensuring consistent tidal volumes but risking barotrauma from excessive pressure. Pressure-cycled ventilators deliver airflow until a preset pressure is reached, but air volume varies with changes in airway resistance or lung compliance, potentially causing inconsistent ventilation and compromised breathing support.

High-frequency oscillatory ventilators deliver exceptionally high respiratory rates while maintaining low tidal volumes and high airway pressures. They are used to open collapsed alveoli in conditions like atelectasis and acute respiratory distress syndrome, and are believed to protect lungs from pressure-related injury during mechanical support.

Volume-cycled ventilators provide consistent, predictable breaths with preset air volumes, making them reliable for maintaining adequate ventilation. However, they may cause barotrauma because excessive pressure is sometimes needed to deliver the programmed volume, potentially damaging lung tissue and causing complications.

Pressure-cycled ventilators cycle off when a preset pressure is reached, regardless of how much air has entered the lungs. If airway resistance increases or lung compliance decreases, less air enters before the pressure limit is reached, resulting in lower tidal volumes and potentially inadequate ventilation for the patient.

Invasive mechanical ventilation requires an artificial airway, typically an endotracheal tube placed through the mouth or nose, or a tracheostomy created surgically in the trachea. These invasive airways allow direct connection of the mechanical ventilator to deliver positive pressure ventilation and support breathing in critically ill patients.

During passive exhalation, the ventilator cycles off after delivering air, allowing the elastic recoil of the lungs and chest wall to naturally expel air without active machine assistance. This passive process occurs in all positive-pressure ventilation modes and relies on the patient's natural lung elasticity to complete the breathing cycle.