What is the relationship between autopeep (intrinsic positive end-expiratory pressure) and resistance in patients with obstructive lung diseases?

Relationship Between Auto-PEEP and Airway Resistance

Auto-PEEP (intrinsic PEEP) develops as a direct consequence of increased airway resistance combined with insufficient expiratory time, creating a positive feedback loop where higher resistance leads to greater air trapping and higher auto-PEEP levels.

Fundamental Pathophysiologic Mechanism

The relationship between auto-PEEP and resistance is directly proportional and mechanistically linked. Auto-PEEP occurs when the time required to decompress the lungs to elastic equilibrium volume exceeds the available expiratory time before the next inspiration 1. This phenomenon is driven by:

• Increased airway resistance that impedes expiratory gas flow, preventing complete lung emptying before the next breath 1
• High pulmonary compliance combined with elevated resistance creates the most pronounced gas trapping and auto-PEEP generation 2
• The difference between alveolar pressure and proximal airway pressure at end-expiration defines auto-PEEP, which occurs specifically due to impedance to expiratory gas flow 2

Clinical Evidence of the Resistance-Auto-PEEP Relationship

Strong correlations exist between resistance and auto-PEEP in stable COPD patients. In a study of 96 stable COPD patients, pulmonary flow resistance (RL) correlated significantly with dynamic auto-PEEP (r = 0.69, p < 0.001) 3. Additionally:

• Auto-PEEP correlated inversely with FEV1 (r = -0.56, p < 0.001), indicating that greater airway obstruction promotes higher auto-PEEP 3
• PaCO2 correlated directly with auto-PEEP (r = 0.6, p < 0.001), suggesting that dynamic hyperinflation contributes to chronic hypoventilation 3
• This relationship is most pronounced in conditions with both elevated airway resistance and high pulmonary compliance, such as COPD and asthma 2

Clinical Consequences of the Resistance-Auto-PEEP Interaction

Auto-PEEP creates an inspiratory pressure threshold load that must be overcome before ventilation can begin. The inspiratory muscles must fully counterbalance auto-PEEP before the ventilator can be triggered during assisted mechanical ventilation 1. This means:

• High levels of auto-PEEP (such as those in acute COPD exacerbations or asthma) significantly increase the magnitude of patient effort to trigger the ventilator 1
• When the achieved decrement in pleural pressure is smaller than the level of auto-PEEP, ineffective or "wasted" efforts occur without triggering the ventilator 1
• Auto-PEEP may decrease cardiac output by impairing respiratory muscle perfusion, contributing to respiratory muscle dysfunction 1

Ventilatory Factors That Modulate the Relationship

Ventilator settings directly influence auto-PEEP generation in the presence of increased resistance. Key factors include:

• High driving pressures and prolonged inspiratory times significantly increase gas trapping, especially when both airway resistance and pulmonary compliance are elevated 2
• Reduced expiratory time increases total PEEP, while increased expiratory time decreases it 4
• Higher respiratory rates and larger tidal volumes during mechanical ventilation or CPR can generate substantial auto-PEEP levels in patients with increased resistance 5

Therapeutic Implications

Application of external PEEP can counterbalance auto-PEEP and reduce inspiratory work, but only up to a certain level. The American Thoracic Society recommends:

• External PEEP at 60-85% of measured auto-PEEP can improve patient-ventilator interaction and reduce inspiratory effort during assisted ventilation 1, 6
• The change in total PEEP when external PEEP is applied correlates inversely with preexisting auto-PEEP levels (r = -0.84), meaning external PEEP is less additive when auto-PEEP is already high 4
• In mechanically ventilated COPD patients, reducing minute ventilation and increasing expiratory time limits dynamic hyperinflation, accepting controlled hypercapnia if necessary 6, 7

Common Pitfalls to Avoid

• Do not assume proximal airway pressure reflects alveolar pressure in patients with high resistance—the impedance to expiratory flow creates a pressure gradient that masks auto-PEEP at the airway opening 2
• Resistance measurements during quiet breathing provide information about uncompressed airway caliber but do not replace spirometry for assessing obstruction severity 1
• External resistance application can paradoxically counteract airways compression and reduce flow limitation in select mechanically ventilated COPD patients, but this must be assessed individually using iso-volume pressure-flow curves 8