Unilateral mechanical asymmetry: positional effects on lung volumes and transpulmonary pressure

Ventilated patients with asymmetry of lung or chest wall mechanics may be vulnerable to differing lung stresses or strains dependent on body position. Our purpose was to examine transpulmonary pressure (P TP) and end-expiratory lung volume (functional residual capacity (FRC)) during body positioning changes in an animal model under the influence of positive end-expiratory pressure (PEEP) or experimental pleural effusion (PLEF).

Fourteen deeply anesthetized swine were studied including tracheostomy, thoracostomy, and esophageal catheter placement. Animals were ventilated at V T = 10 ml/kg, frequency of 15, I/E = 1:2, and FIO2 = 0.5. The animals were randomized to supine, prone, right lateral, left lateral, and semi-Fowler positions with a PEEP of 1 cm H2O (PEEP1) or a PEEP of 10 cm H2O (PEEP10) applied. Experimental PLEF was generated by 10 ml/kg saline instilled into either pleural space. P TP and FRC were determined in each condition.

No significant differences in FRC were found among the four horizontal positions. Compared to horizontal positioning, semi-Fowler's increased FRC (p < 0.001) by 56% at PEEP1 and 54% at PEEP10 without PLEF and by 131% at PEEP1 and 98% at PEEP10 with PLEF. Inspiratory or expiratory P TP showed insignificant differences across positions at both levels of PEEP. Consistently negative end-expiratory P TP at PEEP1 increased to positive values with PEEP10.

FRC did not differ among horizontal positions; however, semi-Fowler's positioning significantly raised FRC. P TP proved insensitive to mechanical asymmetry. While end-expiratory P TP was negative at PEEP1, applying PEEP10 caused a transition to positive P TP, suggestive of reopening of initially compressed lung units.