Specific compliance and gas exchange during high-frequency oscillatory ventilation.

OBJECTIVES To evaluate the use of specific compliance (static compliance/functional residual capacity) to adjust mean airway pressure, resulting in optimal gas exchange during high-frequency oscillatory ventilation in a surfactant-deficient newborn piglet. DESIGN Prospective controlled animal study. SETTING Laboratory. SUBJECTS Eight newborn piglets at 5 days of age. BACKGROUND High-frequency oscillatory ventilation enables the use of relatively high mean airway pressures without the lung damage associated with conventional positive pressure ventilation. Mean airway pressures can be increased, resulting in static lung expansion that approaches total lung capacity with its negative impact on venous return. Therefore, knowledge of lung volume is important for safe patient management. A simple, noninvasive technique to enable the clinician to determine the optimal mean airway pressure likely would improve patient management. INTERVENTIONS The lungs were lavaged after placement of central catheters and tracheostomy to lower respiratory system compliance and worsen ventilation perfusion matching. The animals were ventilated with high-frequency oscillatory ventilation at the same mean airway pressure as before lung lavage. Mean airway pressures then were increased in a step-wise fashion up to 30 cm H2O or until clinical deterioration occurred. All other ventilator variables, Fio2, frequency, and pressure amplitude were constant throughout the experiment. MEASUREMENTS AND MAIN RESULTS Before lavage and at each level of mean airway pressure after lung lavage, respiratory system compliance and functional residual capacity were measured. Additionally, central arterial pressure, central venous pressure, heart rate, arterial blood gas, and pulse oximetric saturation were recorded. Lung lavage significantly lowered respiratory system compliance (static as well as specific compliance) and worsened ventilation perfusion matching as evidenced by an increase in Paco2 and a decreased arterial to alveolar oxygen ratio. With increasing mean airway pressures, static/specific compliance improved and then peaked before declining, functional residual capacity increased, and blood gas improved until reaching the flat portion of the pressure-volume relationship of the lung. Optimal gas exchange as reflected by the highest arterial to alveolar oxygen ratio and lowest Paco2 at constant ventilation was found at a mean airway pressure that maintained the functional residual capacity and static respiratory system compliance at the same level as the preinjury levels ("normalized" functional residual capacity and respiratory system compliance). CONCLUSIONS These results suggest that specific compliance measurement that incorporates static respiratory system compliance and functional residual capacity during high-frequency oscillatory ventilation can be used to adjust mean airway pressure and achieve "normalized" functional residual capacity, static compliance, and gas exchange. These measurements may provide a simple method to optimize lung volume in a surfactant-deficient patient during high-frequency oscillatory ventilation.