Volume recruitment and oxygenation in pulmonary edema: a comparison between HFOV and CMV

Differential effect of recruitment maneuvres on pulmonary blood flow and oxygenation during HFOV in preterm lambs

The effects of lung volume recruitment manouvres on pulmonary blood flow (PBF) during high-frequency oscillatory ventilation (HFOV) in preterm neonates are unknown. Since increased airway pressure adversely affects PBF, we compared the effects of two HFOV recruitment strategies on PBF and oxygenation index (OI). Preterm lambs (128 ± 1 day gestation; term ∼150 days) were anesthetized and ventilated using HFOV (10 Hz, 33% tI) with a mean airway pressure (Pao) of 15 cmH2O. Lung volume was recruited by either increasing Pao to 25 cmH2O for 1 min, repeated five times at 5-min intervals (Sigh group; n = 5) or stepwise (5 cmH2O) changes in Pao at 5-min intervals incrementing up to 30 cmH2O then decrementing back to 15 cmH2O (Ramp group; n = 6). Controls ( n = 5) received constant HFOV at 15 cmH2O. PBF progressively decreased (by 45 ± 4%) and OI increased (by 15 ± 6%, indicating reduced oxygenation) in controls during HFOV, which was similar to the changes observed in the Sigh group of lambs. In the Ramp group, PBF fell (by 54 ± 10%) as airway pressure increased ( r2 = 0.99), although the PBF did not increase again as the Pao was subsequently reduced. The OI decreased (by 47 ± 9%), reflecting improved oxygenation at high Pao levels during HFOV in the Ramp group. However, high Pao restored retrograde PBF during diastole in four of six lambs, indicating the restoration of right-to-left shunting through the ductus arteriosus. Thus the choice of volume recruitment maneuvre influences the magnitude of change in OI and PBF that occurs during HFOV. Despite significantly improving OI, the ramp recruitment approach causes sustained changes in PBF.

Peripheral airways injury in acute lung injury/acute respiratory distress syndrome

PURPOSE OF REVIEW

Peripheral airways are less than 2 mm in diameter and comprise a relatively large cross-sectional area, which allows for slower, laminar airflow. They include both membranous bronchioles and gas exchange ducts, and have been referred to in the past as the 'quiet zone', partly because these structures were felt to contribute little to lung mechanics, and partly because they are difficult to study directly.

RECENT FINDINGS

Recent studies suggest that peripheral airway dysfunction plays a significant role in acute respiratory distress syndrome, which may be exacerbated by injurious mechanical ventilation strategies. The presence of elevated airways resistance, intrinsic positive end-expiratory pressure or a lower inflection point on a pressure-volume curve of the respiratory system may indicate presence of impaired peripheral airway function. In-vitro animal and human studies have begun to elucidate the signaling mechanisms responsible for stretch and shear mediated cellular injury.

SUMMARY

Understanding the pathophysiology of peripheral airway dysfunction in acute respiratory distress syndrome and mechanical ventilation continues to evolve. Greater insight into the signaling mechanisms involved in cellular injury and repair will lead to further alterations in mechanical ventilation strategies, and may lead to specific treatment options.

Bench-to-bedside review: distal airways in acute respiratory distress syndrome

Distal airways are less than 2 mm in diameter, comprising a relatively large cross-sectional area that allows for slower, laminar airflow. The airways include both membranous bronchioles and gas exchange ducts, and have been referred to in the past as the 'quiet zone', in part because these structures were felt to contribute little to lung mechanics and in part because they were difficult to study directly. More recent data suggest that distal airway dysfunction plays a significant role in acute respiratory distress syndrome. In addition, injurious mechanical ventilation strategies may contribute to distal airway dysfunction. The presence of elevated airway resistance, intrinsic positive end-expiratory pressure or a lower inflection point on a pressure–volume curve of the respiratory system may indicate the presence of impaired distal airway function. There are no proven specific treatments for distal airway dysfunction, and protective ventilation strategies to minimize distal airway injury may be the best therapeutic approach at this time.

Invited review: lung edema clearance: role of Na(+)-K(+)-ATPase

Acute hypoxemic respiratory failure is a consequence of edema accumulation due to elevation of pulmonary capillary pressures and/or increases in permeability of the alveolocapillary barrier. It has been recognized that lung edema clearance is distinct from edema accumulation and is largely effected by active Na(+) transport out of the alveoli rather than reversal of the Starling forces, which control liquid flux from the pulmonary circulation into the alveolus. The alveolar epithelial Na(+)-K(+)-ATPase has an important role in regulating cell integrity and homeostasis. In the last 15 yr, Na(+)-K(+)-ATPase has been localized to the alveolar epithelium and its contribution to lung edema clearance has been appreciated. The importance of the alveolar epithelial Na(+)-K(+)-ATPase function is reflected in the changes in the lung's ability to clear edema when the Na(+)-K(+)-ATPase is inhibited or increased. An important focus of the ongoing research is the study of the mechanisms of Na(+)-K(+)-ATPase regulation in the alveolar epithelium during lung injury and how to accelerate lung edema clearance by modulating Na(+)-K(+)-ATPase activity.

Oxygenation response to a recruitment maneuver during supine and prone positions in an oleic acid-induced lung injury model

Prone position and recruitment maneuvers (RM) are proposed as adjuncts to mechanical ventilation to open up the lung and keep it open. We studied the oxygenation response to a RM (composed of a 30-s sustained inflation at 60 cm H(2)O airway pressure) performed in prone and supine positions in dogs after oleic acid- induced lung injury using an inspired O(2) fraction of 0.60. In one group (n = 6) first supine then prone positions were examined after a RM at 8 cm H(2)O and 15 cm H(2)O of positive end-expiratory pressure (PEEP). In the second group (n = 6) the sequence of positions was reversed. Prone positioning after supine position always improved oxygenation, whereas the decrement in Pa(O(2)) was relatively small when dogs were returned to the supine position. Oxygenation improved in both groups after a RM, and the improvement was sustained (after 15 min) in the prone position at 8 cm H(2)O of PEEP, but 15 cm H(2)O of PEEP was required in supine position. Our results suggest that a RM improves oxygenation more effectively with a decreased PEEP requirement for the preservation of the oxygenation response in prone compared with supine position.