High-frequency oscillatory ventilation (HFOV) as a mode of mechanical ventilation has an unknown effect on bronchial blood flow (Qbr), conductance (Cbr), and airway dimensions. We chose to study HFOV effects in a chronic sheep model where Qbr and airway hemicircumference were continuously measured. In four adult sheep a pulsed-Doppler flow probe (20 MHz) was placed around the bronchial artery and a pair of transit time ultrasonic transducers (Triton Technology Inc., San Diego) were sutured and glued on opposing sides of the left lingular lobe bronchus (third-generation airway) to measure airway hemicircumference. After complete recovery, sheep were anesthetized (propofol: 150 μg/kg/h and valium: 0.5 mg/kg/h), intubated and ventilated using HFOV (Sensormetics 3100B, Viasys) set at frequency 5 Hz, I:E = 0.33, FiO2 0.5, amplitude (delta pressure) 50 to 60 cm H2O. The mean airway pressure was initially set at 10 cm H2O for 10 minutes and then continued for 10 minutes at each of 15, 20, 25, and 30 cm H2O. A final pressure of 40 cm H2O for 40 seconds was applied to simulate a recruitment maneuver. As the airway pressure increased progressively from 10 to 40 cm H2O, airway caliber increased in parallel to 105% of baseline (p < .05). Arterial pressure fell from 121 to 113 mm Hg (NS), while bronchial blood flow decreased progressively to 39% of baseline (from 4.23 to 1.63 kHz) and conductance fell to 42% of baseline (from 0.036 to 0.015 kHz/mm Hg) (p < .05). HFOV requires a choice of mean airway pressure settings often determined by oxygenation requirements but would appear to have the effect of decreasing total bronchial blood flow and increasing airway caliber with yet to be determined clinical ramifications.
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