Abstract
Background: Optimisation of oscillatory frequency (f) in HFOV is still an open issue. The f at which the pressure cost of achieving flow is minimal is the resonant frequency (f0) (Venegas & Fredberg 1994). However, to date no tools are available for the assessment of f0 at bedside.
Objectives: We hypothesized that f0 would optimize gas exchange while minimizing the mechanical stress to the lung. In infants receiving HFOV, characterizing lung mechanics and gas exchange at different f.
Methods: We studied 14 infants (gestational age=29±4 wks; postnatal age=5±8 days; body weight=1.35±0.8 Kg). f = 5, 8, 10, 12 and 15 Hz were tested in random order. Tidal volume (Vt) was adjusted to keep DCO2(=Vt2f) constant. For each frequency respiratory system reactance (Xrs) was measured by the forced oscillation technique without suspending ventilation (Dellacà et al 2013) and used to compute f0 as the point of zero crossing of the Xrs vs. f curve. Transcutaneous partial pressure of oxygen (PtrO2) and carbon dioxide (PtrCO2) ware continuously monitored.
Results: Xrs increased from -53.6±29.6 to 10.86±1.9 cmH2O*s/L and always crossed the zero in the studied range of frequency. On average f0 was 10.9±1.9 Hz. At f0 PtrCO2 was 43±7 mmHg and PtrO2/FiO2 was 172±69 mmHg. Neither PtrCO2 nor PtrO2/FiO2 changed with oscillatory frequency: PCO2 ranges from 41±7 at 5Hz to 42±7 mmHg at 15 Hz, while PtrO2/FiO2ranges from 177±61 at 5 Hz to 172±88 mmHg at 15 Hz.
Conclusions: f0 was evaluated for the first time in infants receiving HFOV. Setting f at f0 did not result in better gas exchange than other f but has the potential to minimize the mechanical stress applied to the lung to achieve it.
- Copyright ©ERS 2015