- Poster presentation
The diaphragm has an expiratory braking effect in spontaneously breathing lung injured animals as shown by electrical diaphragmatic activity
Intensive Care Medicine Experimentalvolume 3, Article number: A572 (2015)
Albeit defined as an inspiratory muscle, the diaphragm interacts with the respiratory system also during expiration. The expiratory load on the diaphragm derives from lung volume and hence, possibly from lung collapse. We hypothesized that the diaphragm has a braking effect during the whole expiration, preserving lung patency during SB in collapse prone lungs. We investigated this hypothesis by measuring the expiratory electrical diaphragmatic activity (EAdiexp) in the modulation of expiratory flow (V'exp) in a porcine model.
Mild acute respiratory distress syndrome (ARDS) was induced in 7 anesthetized, tracheostomized pigs by repeated lung lavages, targeting a PaO2/FiO2 of 250 mmHg. After stabilization, the animals were converted to spontaneous breathing (SB) and underwent a decremental continuous positive airway pressure (CPAP) trial of 15, 12, 9, 6, 3 and 0 cmH2O, while EAdiexp and the expiratory trans-diaphragmatic pressure (Pdiexp) were measured. In two of the pigs EAdiexp and Pdiexp were assessed also during controlled mechanical ventilation (CMV) after muscle relaxation using the same positive end-expiratory pressure (PEEP) as for CPAP. For each studied condition, lung volume was assessed by profiting of sulfur hexafluoride (SF6) washin/washout technique. The V'exp slope before the expiratory peak flow and the expiratory volume (Volexp) at 50% and 75% of expiratory time were estimated. Statistical significance of expiratory EAdiexp/Pdiexp was assessed by F-tests (α = 0.05). Between SB and CMV, comparisons were performed at the same lung volume, which was verified by applying the Kolmogorov-Smirnov test (α=0.05).
When CPAP was decreased, EAdiexp increased until a CPAP of 6 cmH2O and then remained unaltered (although elevated) with further decrease in CPAP. EAdiexp and Pdiexp were tightly correlated as confirmed by an R2 > 0.82 (p < 0.01). Lung volumes were comparable during SB and CMV when exposed to the same CPAP/PEEP levels. V'exp had a slower initial increase (before reaching its peak) during SB than during CMV. By decreasing CPAP/PEEP, V'exp rise was markedly more delayed during SB than during CMV at the same lung volumes indicating a delayed emptying of the lungs, which was also shown by that Volexp at 50% and at 75% of expiration were higher during SB than during CMV.
The findings suggest that the diaphragm plays an important role during expiration. The EAdiexp modulates diaphragmatic expiratory mechanical “braking” activity, possibly to protect against reductions in lung volume and atelectasis formation. The EAdiexp can be useful in setting CPAP/PEEP during SB in mild ARDS conditions.
The School of Anesthesia and Intensive Care of Medicine, Bari University, Italy; The Swedish Heart and Lung Foundation.