High Frequency Ventilation
(HFV)

 

 Reviewed by Malcolm Battin
January
2001
Clinical Guidelines Back Newborn Services Home Page
Background Terminology Initial Settings on HFV Making Adjustments once Established on HFV
Chest Radiographs Weaning Other Related Documents References

Background

High frequency ventilation (HFV) is defined by the ‘high frequency’ (2.5-15 Hz) and low tidal volume (0.5-5 mL/kg). The tidal volume is barely greater than the dead space hence alternative mechanisms of gas transport are required to explain the effect of HFV 1.

Indications for high frequency ventilation include

  1. Rescue following failure of conventional ventilation (PPHN, Meconium).2,3

  2. Air leak syndromes (pneumothorax, pulmonary interstitial emphysema) 4

  3. To reduce barotrauma when conventional ventilator settings are high.

HFV is not as yet proven to be of benefit in the elective treatment of respiratory distress syndrome 5 . Furthermore, caution is needed when HFV is used as high airway pressures may result in impaired cardiac output causing a low BP requiring inotropic support or volume expansion. Also some infants poorly tolerate the extra handling involved in switching ventilators or may not respond to HFV. If no improvement with HFV consider reverting to conventional ventilation.

Terminology

Frequency
  • High frequency ventilation rate (Hz, cycles per second)
MAP
  • Mean airway pressure (cmH2O)
Amplitude
  • delta P or power is the variation around the MAP
Oxygenation is dependent on MAP and FiO2
  • MAP provides a constant distending pressure equivalent to CPAP. This inflates the lung to a constant and optimal lung volume maximising the area for gas exchange and preventing alveolar collapse in the expiratory phase. 
  • Ventilation is dependent on amplitude and to lesser degree frequency. Thus when using HFV CO2 elimination and oxygenation are independent.

Initial settings on HFV
  If you are using HFOV on the Babylog 8000plus, click here.

Optimal lung volume strategy
(aim to maximise recruitment of alveoli).
  • Set MAP 2-3 cmH2O above the MAP on conventional ventilation
  • MAP in 1-2 cmH2O steps until oxygenation improves
  • Set frequency to 10 Hz
Low volume strategy
(aim to minimise lung trauma)
  • Set MAP equal to the MAP on conventional ventilation
  • Set frequency to 10 Hz
  • Adjust amplitude to get an adequate chest wall vibration.

Making adjustments once established on HFV

Poor Oxygenation Over Oxygenation Under Ventilation Over Ventilation
Increase FiO2 Decrease FiO2 Increase Amplitude Decrease Amplitude
Increase MAP
(1-2cmH2O)		 Decrease MAP
(1-2cmH2O)		 Decrease Frequency
 (1-2Hz)
if Amplitude Maximal		 Increase Frequency
 (1-2Hz)
if Amplitude Minimal

Chest Radiograph

Weaning

  • Reduce FiO2 to <40% before weaning MAP (except when over-inflation is evident).
  • Reduce MAP when chest x-ray shows evidence of over-inflation (>9 ribs).
  • Reduce MAP in 1-2cm increments to 8-9.
  • In air leak syndromes (low volume strategy), reducing MAP takes priority over weaning the FiO2.
  • Wean the amplitude in 4cm H2O increments.
  • Do not wean the frequency
  • Consider switching to conventional ventilation when MAP <10cm H2O, Amplitude 20-25 and blood gases satisfactory.
  • Suction is indicated for diminished chest wall movement indicating airway or ET tube obstruction or if there are visible/audible secretions in the airway.
  • Avoid in the first 24 hours of HFV, unless clinically indicated.
  • Avoid hand-bagging during the suctioning procedure
    • use PEEP protector and continue with patient on the ventilator.
    • increase FiO2 following the suctioning procedure.
    • MAP may be temporarily increased 2-3cm H2O until oxygenation improves.

References

1 Chang H K. Mechanisms of gas transport during ventilation by HFO. J Appl Physiol 1984 ; 56: 553-63
2 Clark RH et al. Prospective, randomized comparison of HFO and conventional ventilation in candidates for ECMO. J Pediatr.1994;124: 447-54
3 Kohe et D, et al. High-frequency oscillation in the rescue of infants with persistent pulmonary hypertension. Crit Care Med. 1988; 16: 510-6
4 Clark RH et al. Pulmonary interstitial emphysema treated by HFOV. Crit Care Med 1986; 14: 926-30
5 Bhuta T, Henderson-Smart D. Elective high frequency oscillatory ventilation vs conventional ventilation in preterm infants with acute pulmonary dysfunction. (Cochrane Review) In: The Cochrane Library, Issue 2. Oxford: Update Software; 1998. Updated quarterly



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