Determinants of dynamic hyperinflation in a bench model.

Area of Science:

• Mechanical Ventilation
• Respiratory Physiology
• Pulmonary Mechanics

Background:

• High airway resistance (R(aw)) and minute ventilation (V(E)) are known to promote dynamic hyperinflation.
• Previous studies suggest a link between these factors, but a systematic examination of various mechanical parameters is lacking.

Purpose of the Study:

• To systematically investigate the effects of different mechanical ventilation parameters on dynamic hyperinflation.
• To determine the relative contributions of R(aw), V(E), respiratory system compliance (C(RS)), and flow dynamics to intrinsic positive end-expiratory pressure (PEEPi).

Main Methods:

• A ventilator-lung model was used to measure PEEPi.
• Parameters varied included R(aw), V(E), C(RS), and duty cycles/flow regimes.
• Intrinsic positive end-expiratory pressure (PEEPi) was measured under diverse mechanical conditions.

Main Results:

• Significant dynamic hyperinflation (PEEPi > 5 cm H(2)O) occurred at V(E) >= 15 L/min, even with low R(aw).
• Descending ramp flow resulted in higher PEEPi than constant flow at equivalent peak flows.
• Lower tidal volumes (0.6 L) led to greater PEEPi than higher tidal volumes (1.0 L) at equal V(E).

Conclusions:

• Dynamic hyperinflation can occur with high V(E) even at low R(aw), suggesting potential for occult hyperinflation in clinical settings.
• PEEPi was greater with higher respiratory rates and smaller tidal volumes compared to lower rates and larger tidal volumes at equivalent V(E).
• Clinicians should be aware of the potential for dynamic hyperinflation in patients with moderate R(aw) and V(E), even without obstructive lung disease.