Am. J. Respir. Crit. Care Med., Volume 162, Number 1, July 2000, 319-323

Biologically Variable or Naturally Noisy Mechanical Ventilation Recruits Atelectatic Lung

W.ALAN C.  MUTCH, STEFAN HARMS, M. RUTH GRAHAM, STEPHEN E. KOWALSKI, LINDA G. GIRLING, and GERALD R. LEFEVRE

Department of Anaesthesia and Neuroanaesthesia Research Laboratory, Faculty of Medicine, University of Manitoba, Winnipeg, Manitoba, Canada

Biologically variable mechanical ventilation (bv)using a computer-controller to mimic the normal variability in spontaneous breathingimproves 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 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/f^a) (West, B. J., M. Shlesinger. Am. Sci. 1990;78:40-45). In a porcine model of atelectasisright lung collapse with one-lung ventilationwe studied if bv (n = 7) better reinflates the collapsed lung compared with conventional monotonously regular control mode ventilation (c; n = 7) over a 5-h period. We also investigated the influence of sigh breaths with c (s; n = 8) with this model. Reinflation of the collapsed lung was significantly enhanced with bvgreater Pa_O2 (502 ± 40 mm Hg with bv versus 381 ± 40 mm Hg with c at 5 h; and 309 ± 79 mm Hg with s; mean ± SD), lower Pa_CO2 (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). bv resulted in an 11% increase in measured tidal volume (VT_m) over that seen with c by 5 h (14.7 ± 1.2 ml/kg versus 13.2 ml/kg). The respiratory rate variability programmed for bv demonstrated an inverse power law frequency distribution ( y  1/f^a) 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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