Fig. 5: Concept of corner frequency in high-frequency ventilated neonates, according to Dorkin et al.^19,53

Impedance (Z) is plotted against frequency (f) following \({|}Z{|} = \sqrt {R^2 + \left( {2\pi \cdot f \cdot I - \frac{1}{{(2\pi \cdot f \cdot C)}}} \right)^2}\) with the specific variables resistance (R), compliance (C), and inertance (I). The solid line represents the impedance/frequency relationship in a healthy lung; the dotted line represents elevated airway resistance; the dashed line reduced compliance. Smaller impedance with increasing frequency represents a lower pressure cost of ventilation and smaller V_O. The resonance frequency (f_o) with the lowest impedance can be calculated by \(f_O = \frac{1}{2\pi \sqrt {I \cdot C}}\) and is marked with open rings. The corner frequency (f_c) is inversely related to the time constant of the respiratory system (\(f_c = \frac{1}{2\pi \cdot R \cdot C}\)), above f_c (marked with squares) the additional fall in lung impedance is small and the dampening of ΔP_O increases. Thus, low compliance lung disease (e.g., RDS) might benefit from higher frequencies (>10 Hz) and high resistance lung disease from a lower frequency (<10 Hz). However, increasing frequency is limited by decreasing ventilator performance.⁵⁷