Biologically Variable or Naturally Noisy Mechanical Ventilation Recruits Atelectatic Lung

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Abstract
Biologically variable mechanical ventilation (V˙bv)—using a computer-controller to mimic the normal variability in spontaneous breathing—improves gas exchange in a model of severe lung injury (Lefevre, G. R., S. E. Kowalski, L. G. Girling, D. B. Thiessen, W. A. C. Mutch. Am. J. Respir. Crit. Care Med. 1996;154:1567–1572). Improved oxygenation with V˙bv, in the face of alveolar collapse, is thought to be due to net volume recruitment secondary to the variability or increased noise in the peak inspiratory airway pressures (Ppaw). Biologically variable noise can be modeled as an inverse power law frequency distribution (y ∝ 1/fa) (West, B. J., M. Shlesinger. Am. Sci. 1990;78:40–45). In a porcine model of atelectasis—right lung collapse with one-lung ventilation—we studied if V˙bv (n = 7) better reinflates the collapsed lung compared with conventional monotonously regular control mode ventilation (V˙c; n = 7) over a 5-h period. We also investigated the influence of sigh breaths with V˙c (V˙s; n = 8) with this model. Reinflation of the collapsed lung was significantly enhanced with V˙bv—greater PaO2 (502 ± 40 mm Hg with V˙bv versus 381 ± 40 mm Hg with V˙c at 5 h; and 309 ± 79 mm Hg with V˙s; mean ± SD), lower PaCO2 (35 ± 4 mm Hg versus 48 ± 8 mm Hg and 50 ± 8 mm Hg), lower shunt fraction (9.7 ± 2.7% versus 14.6 ± 2.0% and 22.9 ± 6.0%), and higher respiratory system compliance (Crs) (1.15 ± 0.15 ml/cm H2O/kg versus 0.79 ± 0.19 ml/cm H2O/kg and 0.77 ± 0.13 ml/cm H2O/kg)—at lower mean Ppaw (15.7 ± 1.4 cm H2O versus 18.8 ± 2.3 cm H2O and 18.9 ± 2.8 cm H2O). V˙bv resulted in an 11% increase in measured tidal volume (Vtm) over that seen with V˙c by 5 h (14.7 ± 1.2 ml/kg versus 13.2 ml/kg). The respiratory rate variability programmed for V˙bv demonstrated an inverse power law frequency distribution (y ∝ 1/fa) with a = 1.6 ± 0.3. These findings provide strong support for the theoretical model of noisy end-inspiratory pressure better recruiting atelectatic lung. Our results suggest that using natural biologically variable noise has enhanced the performance of a mechanical ventilator in control mode.

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