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Volume recruitment maneuvers are less deleterious than persistent low lung volumes in the atelectasis-prone rabbit
1Department of Anesthesia, Kingston General Hospital, ON, Canada.
Objectives:
To test whether the pulmonary risk of repeated volume recruitment is greater or less than the risk associated with unreversed atelectasis during 6 hrs of high-frequency oscillatory ventilation in the atelectasis-prone rabbit lung.
Design:
Prospective, controlled, randomized comparison over 6 hrs of ventilator management.
Setting:
Laboratory.
Subjects:
Twenty-eight adult New Zealand white male rabbits (weight 2.3 to 2.8 kg).
Background:
Controversy exists over whether high-frequency oscillatory ventilation should be used with volume recruitment maneuvers in the atelectasis-prone lung, or be used at low mean and peak pressures without volume recruitment to avoid the risks of even transient pulmonary overdistention. Potential risks and benefits accompany both alternatives.
Interventions:
We evaluated the pulmonary effects of three high-frequency oscillatory ventilation protocols in anesthetized rabbits made surfactant deficient by saline lavage, using animals ventilated with conventional positive-pressure ventilation with positive end-expiratory pressure as a reference group; n = 5 in each group. The three high-frequency oscillatory ventilation groups were ventilated for 6 hrs at 15 Hz (900 breaths/min), FIO2 = 1.0. The repeated stretch group received 15-sec sustained inflations at 30 cm H2O mean airway pressure every 20 mins, with maintenance mean airway pressure sufficient to keep PaO2 > 350 torr (46.7 kPa). The repeated deflation group was maintained at levels that produced PaO2 70 to 120 torr (9.3 to 16 kPa), with the endotracheal tube opened to atmospheric pressure for 15 secs every 20 mins. Animals in the repeated stretch after deflations group were managed as in the repeated stretch protocol but each sustained inflation was preceded by a 15-sec deflation to functional residual capacity. The conventional positive-pressure ventilation group was ventilated at rates of 30 to 100 breaths/min, keeping PaO2 70 to 120 torr (9.3 to 16 kPa). End-points included terminal functional residual capacity and a compliance index computed from respiratory system pressure-volume curves.
Measurements And Main Results:
After 6 hrs of ventilation, respiratory system compliance in the repeated stretch group had returned to control values (1.35 +/- 0.18 [SD] mL/kg/cm H2O). Respiratory system compliance was significantly less than this number in both the repeated deflation (0.89 +/- 0.08) and repeated stretch after deflations (1.24 +/- 0.22) groups (p < .05). Respiratory system compliance after 3 hrs of conventional positive-pressure ventilation decreased to 0.34 +/- 0.10 mL/kg/cm H2O. Functional residual capacity changes paralleled these changes of respiratory system compliance.
Conclusions:
These data demonstrate that the potential pulmonary risk of repeated lung stretch during volume recruitment is significantly less than the damage arising from the avoidance of such maneuvers in lungs in which alveolar recruitment is possible. We conclude that sustained inflations during high-frequency oscillatory ventilation produce the benefits of increased oxygenation for a given mean airway pressure plus decreased progression of lung injury.