Dynamic intrinsic PEEP in acute exacerbations of COPD: association with respiratory mechanics and their early response to high-flow nasal cannula — a physiological study

Background During exacerbations of COPD (ECOPD), dynamic intrinsic positive end-expiratory pressure (PEEPi,dyn) acts as an inspiratory threshold load, thereby increasing the work of breathing. High-flow nasal cannula (HFNC) is increasingly used in this setting; however, its ability to counteract PEEPi,dyn may be limited in patients with marked dynamic hyperinflation. The relationship between baseline PEEPi,dyn, respiratory mechanics, inspiratory effort partitioning, and their early physiological response to HFNC has not been characterized in spontaneously breathing patients with ECOPD. Methods In this prospective physiological study, spontaneously breathing patients with acute hypercapnic respiratory failure due to ECOPD underwent physiological assessment at HFNC initiation (T0) and after 2 h of standardized HFNC delivered at 40 L/min (T1). Measurements included dynamic intrinsic positive end-expiratory pressure (PEEPi,dyn), corrected for the abdominal contribution; esophageal (Pes), transdiaphragmatic (Pdi), and nasal (Pnose) pressure swings; esophageal and transdiaphragmatic pressure–time products (PTPes and PTPdi); maximal diaphragmatic pressure-generating capacity (Pdi,sniff); the Pes/Pdi ratio; respiratory muscle ultrasound indices; expiratory tidal volume (Vte), expressed as absolute values and normalized to predicted body weight (Vte/PBW); respiratory rate (RR); and dyspnea. Associations between baseline PEEPi,dyn and physiological variables at T0, as well as between baseline PEEPi,dyn and changes in these variables from T0 to T1, were explored using Spearman’s rank correlation, with bootstrap 95% confidence intervals. Results Twelve patients underwent complete physiological assessment. At T0, higher PEEPi,dyn was associated with greater inspiratory effort, reflected by larger Pes and PTPes,min values (ρ=0.69 and 0.68) and larger Pdi and Pnose swings (ρ=0.69 and 0.82), and with lower maximal diaphragmatic pressure-generating capacity (Pdi,sniff; ρ=−0.77). At the same time point, higher PEEPi,dyn was also associated with a lower diaphragm thickening fraction (TF,di; ρ=−0.60), greater intercostal and sternocleidomastoid muscle recruitment (ρ=0.68 and 0.72), lower Vte and Vte/PBW (ρ=−0.80 and −0.69), and higher RR and dyspnea (ρ=0.71 and 0.61). Higher baseline PEEPi,dyn was associated with greater increases in Pes (ρ=0.69) and PTPes,min (ρ=0.84) from T0 to T1, greater extra-diaphragmatic muscle recruitment, a reduction in Vte/PBW (ρ=−0.65), and persistence or worsening of PEEPi,dyn (ρ=0.76). Conclusions In spontaneously breathing patients with ECOPD treated with HFNC, higher PEEPi,dyn was associated with a respiratory mechanical pattern characterized by greater inspiratory effort and dyspnea, redistribution of inspiratory muscle activity, and lower tidal volume. Higher baseline PEEPi,dyn was also associated with a less favourable early physiological response to HFNC, characterized by greater inspiratory effort, increased extra-diaphragmatic muscle recruitment, less favourable changes in Vte/PBW, and persistence or worsening of PEEPi,dyn.