Mid-Frequency Ventilation: Unconventional Use of Conventional Mechanical Ventilation as a Lung-Protection Strategy

BACKGROUND: Studies have found that increasing the respiratory frequency during mechanical ventilation does not always improve alveolar minute ventilation and may cause air-trapping.

OBJECTIVE: To investigate the theoretical and practical basis of higher-than-normal ventilation frequencies.

METHODS: We used an interactive mathematical model of ventilator output during pressure-control ventilation to predict the frequency at which alveolar ventilation is maximized with the lowest tidal volume (V_T) for a given pressure. We then tested our predicted optimum frequencies and V_T values with various lung compliances and higher-than-normal frequencies, with a lung simulator and 5 mechanical ventilators (Dräger Evita XL, Hamilton Galileo, Puritan Bennett 840, Siemens Servo 300 and Servo-i).

RESULTS: Compliances between 10 mL/cm H₂O and 42 mL/cm H₂O yielded V_T between 4.1 mL/kg (optimum frequency 75 cycles/min) and 6.0 mL/kg (optimum frequency 27 cycles/min). The intrinsic positive end-expiratory pressure at the optimum frequency was always less than 2 cm H₂O. All the ventilators except the Hamilton Galileo had an optimum frequency near 50 cycles/min, whereas the predicted optimum frequency was 60 cycles/min.

CONCLUSIONS: With these ventilators and pressure-control ventilation, alveolar minute ventilation can be optimized with higher-than-normal frequency and lower V_T than is commonly used in patients with acute respiratory distress syndrome. We call this strategy mid-frequency ventilation.