Why the Alveolar Pressure (Palv) Curve Touches Baseline in Pressure Waveforms
The alveolar pressure curve returns to baseline at end-expiration because alveolar pressure equilibrates with the set positive end-expiratory pressure (PEEP) or atmospheric pressure when expiratory flow ceases and the respiratory system reaches its elastic equilibrium volume.
Physiologic Basis
The alveolar pressure waveform reflects the dynamic pressure changes within the alveoli throughout the respiratory cycle. At end-expiration, when there is no airflow and the respiratory muscles are relaxed, the alveolar pressure equals the end-expiratory pressure set on the ventilator (or zero if no PEEP is applied) 1.
During expiration, alveolar pressure decreases as gas flows out of the lungs down a pressure gradient from the alveoli to the airway opening 1.
At the end of expiration, when flow reaches zero, the pressure throughout the respiratory system equilibrates, and alveolar pressure returns to the baseline set by external PEEP or atmospheric pressure 1.
The baseline represents the elastic equilibrium volume of the respiratory system—the point where elastic recoil forces are balanced by the applied PEEP 1.
Important Exception: Intrinsic PEEP (Auto-PEEP)
A critical caveat occurs when expiratory time is insufficient for complete lung emptying, particularly in patients with airflow obstruction:
When intrinsic PEEP is present, end-expiratory alveolar pressure remains positive even without external PEEP, because the lungs have not fully decompressed to their elastic equilibrium volume 1.
The measured airway pressure may still touch baseline at the airway opening, but alveolar pressure remains elevated—this discrepancy occurs because the elevated alveolar pressure is "trapped" behind collapsed or narrowed airways 1.
Detection of intrinsic PEEP requires end-expiratory airway occlusion or simultaneous recording of flow and pleural pressure, as the standard pressure waveform at the airway opening will not reveal the elevated alveolar pressure 1.
In patients with COPD or asthma, intrinsic PEEP can reach 10-15 cm H₂O, creating a significant inspiratory threshold load that must be overcome before the ventilator can be triggered 1, 2.
Clinical Implications
Normal baseline return indicates adequate expiratory time and complete lung emptying to the set PEEP level 1.
Failure of the pressure curve to return to baseline before the next breath suggests dynamic hyperinflation and intrinsic PEEP 1.
Application of external PEEP (3-5 cm H₂O) can offset intrinsic PEEP in obstructive lung disease, reducing the work of breathing and improving patient-ventilator synchrony, but should never exceed the level of intrinsic PEEP 1, 2.
Common Pitfalls
Assuming baseline pressure equals alveolar pressure in all cases—this is only true when expiratory flow has ceased and airways remain patent 1.
Overlooking intrinsic PEEP in tachypneic patients with obstructive disease, where rapid respiratory rates shorten expiratory time and prevent complete exhalation 1, 2.
Misinterpreting expiratory muscle recruitment as intrinsic PEEP—active abdominal muscle contraction during expiration can elevate gastric pressure and mimic the pressure patterns of auto-PEEP, requiring careful waveform analysis to differentiate 1.