Relative effects of negative versus positive pressure ventilation depend on applied conditions

Abstract

Purpose: Comparisons of negative versus positive pressure ventilation have imperfectly matched the pressure-time profile or the lung volume history, or have incompletely applied in vivo negative pressure to include the complete thoracic wall and abdomen.

Hypothesis: Negative pressure exerts the same pattern of lung distension as positive pressure when the pressure-time and volume history profiles are identical and the application of negative pressure is over the whole lung.

Methods: (1) In isolated (ex vivo) and (2) intact (in vivo) mouse lungs (n = 4/group) (sealed chamber enclosing either the whole lung or whole mouse except for external airway opening), identical and inverse-tidal, square-wave pressure-time profiles were obtained with positive and negative pressure ventilation. (3) Following an identical volume history, surfactant-depleted rabbits (n = 7) were randomly assigned to sustained, static equivalent positive versus negative pressures. (4) Surfactant-depleted anesthetized rabbits (n = 10) with identical volume histories were randomized to positive versus negative ventilation with identical pressure-time characteristics.

Results: Matched positive and negative pressure time profiles in ex vivo and in vivo mice resulted in identical tidal volumes. Identical (negative vs. positive) sustained static pressures resulted in similar PaO(2) and end expiratory lung volumes. Positive and negative ventilation with identical volume histories and pressure time characteristics showed no difference in oxygenation or lung volumes. Historical comparisons suggested better oxygenation with negative pressure when the volume history was not identical.

Conclusions: These data do not support major biological differences between negative and positive pressure ventilation when waveforms and lung volume history are matched.

Fig. 1

Tidal ventilation in ex vivo mouse lungs: cyclic ventilation using reciprocal and identical pressure–time profiles with positive and negative pressure inflation yields similar volume–time profiles (a) that can be superimposed (b). Cyclic ventilation using positive versus negative inflation pressures of 5, 7.5, and 10 cmH₂O resulted in identical mean tidal volumes at each level of inflation pressure (c)

Fig. 2

Tidal ventilation of in vivo mouse lungs: cyclic ventilation using reciprocal and identical pressure–time profiles with positive and negative pressure inflation yields similar mean tidal volumes with peak inspiratory inflation pressures of 5, 7.5, and 10 cmH₂O. End-expiratory pressures of 3 cmH₂O were used in all cases. EEP end-expiratory pressure

Fig. 3

Static inflation in in vivo surfactant-depleted rabbit lungs: lungs were inflated with static application of positive or negative pressure (range 5–20 cmH₂O). The relationship between EELV and oxygenation (PaO₂) was indistinguishable between positive and negative pressure inflation

Fig. 4

Tidal ventilation of in vivo surfactant-depleted rabbit lungs: PaO₂ was similar at various levels of EELV following comparable (square-wave) positive versus negative pressure ventilation. In contrast, comparison with historical negative pressure data where the pressure–time profile was non-square-wave (and non-identical), demonstrated higher PaO₂ at mid-range levels of EELV (*P < 0.03, ANOVA)

Fig. 5

Schematic of positive versus negative pressure ventilation under quasistatic conditions: initial conditions at end-exhalation (a); positive pressure ventilation with 40 cmH₂O of airway pressure applied (b); and negative pressure ventilation with −40 cmH₂O pressure applied to the body surface (c). For full description see Appendix C (supplementary material)