Expiratory volume clamping: A new method to assess respiratory mechanics in sedated infants

During breathing under sedation via a two-way valve, airflow (V̇), volume (ΔV), and airway pressure (P) were recorded in eight normal (N) infants, seven with reversible obstructive airway disease (ROAD), and seven with chronic lung disease (CLD). Intermittently, expiratory volume clamping (EVC) was applied, involving selective occlusion of the expiratory valve for three to five breaths. The latter produced cumulative increases in ΔV that, due to progressive recruitment of the Hering-Breuer reflex, were accompanied by increasing expiratory plateaus in P (i.e., apneas). The resultant passive inflation ΔV-P relationships were closely approximated by the expression: ΔV = aP² + bP + c, wherein a represented the pressure-related changes in chord compliance (Crs), b the Crs at P = 0, and c the difference between the dynamic end-expiratory and relaxation volumes of the respiratory system. Relative to N, the ROAD and CLD infants had significantly reduced weight-specific values of a/kg, their b/kg values were increased, whereas the c/kg measurements did not significantly vary. Moreover, for each subject we determined 1) the net Crs/kg obtaining at P = 20 cmH₂O (i.e., Crs₂₀/kg), an estimate of the net deflation compliance; 2) the passive respiratory time constant (τ(rs)) based on the slope of the expired ΔV/V̇ relationship; and 3) the respiratory system conductance (Grs/kg). Relative to N, the mean Crs₂₀/kg was significantly reduced only in the infants with CLD and, due to increases in τ(rs), both patient groups depicted significantly diminished values of Grs/kg, suggesting the presence of airways obstruction. Based on these observations, the new EVC technique described herein provides a simple and practical approach to noninvasively quantify the passive respiratory mechanics in infants and, thereby, evaluate the presence of abnormalities in their pulmonary mechanics.