Mid-frequency ventilation: unconventional use of conventional mechanical ventilation as a lung-protection strategy

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

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

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).

Compliances between 10 mL/cm H(2)O and 42 mL/cm H(2)O 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 H(2)O. All the ventilators except the Hamilton Galileo had an optimum frequency near 50 cycles/min, whereas the predicted optimum frequency was 60 cycles/min.

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.