Signs of Lung Overdistension:
- Decreased Lung Sounds: A significant reduction in breath sounds on auscultation may indicate over-distension of the lung tissue or excessive air trapping. This is because over-distended alveoli can become less functional and contribute to decreased overall lung sound intensity.
- Elevated Plateau Pressures: Increased plateau pressures measured during mechanical ventilation can suggest that the alveoli are over-distended. Plateau pressures significantly above 30 cm H2O may indicate excessive PEEP or other ventilation settings causing lung overdistension.
- Development of Barotrauma: Clinical signs such as subcutaneous emphysema (air under the skin causing a crackling sensation) or pneumothorax (air in the pleural space) can indicate that excessive PEEP is causing barotrauma, leading to damage and air leakage.
- Decreased Oxygenation: A sudden drop in oxygen saturation (SpO2) or partial pressure of oxygen (PaO2) despite appropriate PEEP levels might suggest that the lung tissue is over-distended and less capable of efficient gas exchange.
Signs of Decreased Compliance:
- Increased Work of Breathing: The patient may exhibit signs of increased respiratory effort, such as accessory muscle use or increased respiratory rate, which can be indicative of decreased lung compliance. This often occurs when the lungs become stiffer and less able to expand.
- Elevated Driving Pressures: An increase in driving pressure (the difference between plateau pressure and PEEP) may signal reduced lung compliance. Elevated driving pressures typically reflect a stiffening of the lung tissue, requiring adjustment of PEEP to optimize lung recruitment without causing damage. Driving pressure is the ventilator variable most strongly tied to outcome: in ARDS, each 7 cm H2O increase raised the relative risk of death by about 41% even when tidal volume and plateau pressure were within protective limits (Amato 2015), and in surgical patients each 1 cm H2O increase raised the odds of postoperative pulmonary complications by about 16%, while a PEEP change that raised driving pressure tripled them (Serpa Neto 2016). A driving pressure of 15 cm H2O or less is the usual ceiling, and the PEEP level that minimizes it is the practical target.
- Increased Ventilator Settings: If adjustments to tidal volume or respiratory rate are needed to maintain adequate ventilation, it may be a sign of decreased lung compliance. This often necessitates reevaluating PEEP settings to match better the patient’s changing lung mechanics.
By continuously monitoring these signs, healthcare providers can effectively adjust PEEP levels to ensure optimal ventilation while minimizing the risks associated with over-distension and decreased lung compliance.
| Finding | Points toward | Why it appears |
|---|---|---|
| Decreased breath sounds on auscultation | Lung overdistension | Overdistended alveoli become less functional, lowering overall lung sound intensity; may also reflect air trapping |
| Plateau pressure significantly above 30 cm H2O | Lung overdistension | Suggests excessive PEEP or other ventilator settings are distending the alveoli |
| Subcutaneous emphysema or pneumothorax | Barotrauma from overdistension | Excessive PEEP causing alveolar damage and air leakage |
| Sudden fall in SpO2 or PaO2 despite appropriate PEEP | Lung overdistension | Overdistended tissue is less capable of efficient gas exchange |
| Increased work of breathing (accessory muscle use, rising respiratory rate) | Decreased compliance | Stiffer lungs are less able to expand, raising respiratory effort |
| Rising driving pressure (plateau pressure − PEEP) | Decreased compliance | Reflects stiffening lung tissue; PEEP needs adjustment to recruit without causing damage |
| Need to raise tidal volume or respiratory rate to maintain ventilation | Decreased compliance | Signals changing lung mechanics and a need to reevaluate PEEP |
