Experimental studies on lung mechanics, gas exchange and oxygen delivery under open lung conditions Mechanical ventilation with decelerating versus constant inspiratory flow
Authors: Markström A. M. 
Published: January 2009

The treatment of patients with acute respiratory failure poses an ever‐present clinical challenge since conventional ventilation sometimes causes further impairment of lung function. Thus, it is important to identify characteristics of different ventilatory patterns and ventilatory strategies that affect airway pressures, lung volumes, pulmonary gas exchange and hemodynamics. In order to mimic acute respiratory failure in the patient, we have used an experimental animal injury model, characterized by highly unstable alveoli.

The basic prerequisite for this thesis was a positive end‐expiratory pressure (PEEP) sufficient to keep the lung open throughout the whole ventilatory cycle. Different inspiratory flow patterns were studied by using a successful recruitment procedure together with ventilation performed using PEEP levels below, at, and above the inflection‐point pressure. To elucidate the effect of prolonged inspiration time, sufficient PEEP was supplied to prevent end‐expiratory collapse already during the reference mode with an inspiration/expiration ratio (I:E ratio) of 1:1. When an I:E ratio of 2:1 or above was used, both studies (one with mean airway pressure constant and the other with total PEEP constant) showed impaired hemodynamics, but displayed unaffected arterial oxygen tension. Decelerating inspiratory flow delivers the major part of the tidal volume during early inspiration under sustained constant inspiratory pressure. This reduces serial dead space and improves alveolar ventilation. In all studies, decelerating inspiratory flow showed increased carbon dioxide elimination, i.e. improved alveolar ventilation, but decelerating inspiratory flow was found to be no better for alveolar recruitment or in oxygenation.

In an attempt to explain radiological observations when studying the relationship of pulmonary radiological appearance to end‐expiratory pressure, Staubs' concept of quantal alveolar behavior was used. This states that the individual alveolus fills essentially independently of its neighbors and has a propensity to be either air‐filled and expanded, or completely fluid‐filled and collapsed. Stepwise sequential deflation of the lung from full expansion, followed by stepwise inflation of the lung, produced a pressure/attenuation loop. This hysteresis phenomenon implies that the attenuation of the lung relates to the set PEEP, but also to the immediately preceding structural status. For identical end‐expiratory pressures the attenuation on the hysteresis down‐slope invariably had a higher value than that of the up‐slope. Thus, the pressure at which alveolar collapse takes place differs from that of expansion.

In conclusion, ventilation with decelerating inspiratory flow revealed lower peak inspiratory and higher mean airway pressure than did constant inspiratory flow. No differences in end‐inspiratory pressure were seen. Decelerating inspiratory flow delivers the major part of the tidal volume during early inspiration, which reduces serial dead space and improves alveolar ventilation. The decelerating inspiratory flow was found to be no better than constant inspiratory flow for alveolar recruitment or oxygenation. When the lung was ventilated with PEEP sufficient to prevent end‐expiratory collapse, an inspiration‐to‐expiration ratio at and above 2:1 impaired hemodynamics. Our results suggest that, in order to obtain and maintain open lung conditions, the lung must be recruited and PEEP reduced to the level just above the alveolar collapse pressure.