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HFOV - an overview of High Frequency Oscillatory Ventilation

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Background

High frequency oscillatory ventilation (HFOV) is a type of mechanical ventilation that uses a constant distending pressure (mean airway pressure [MAP]) with pressure variations oscillating around the MAP at very high rates (up to 900 cycles per minute). This creates small tidal volumes, often less than the dead space. In conventional ventilation large pressure changes (the difference between PEEP and PIP) create physiological tidal volumes and gas exchange is dependent on bulk convection (expired gas exchanged for inspired gas). HFOV relies on alternative mechanisms of gas exchange such as molecular diffusion, Taylor dispersion, turbulence, asymmetric velocity profiles, Pendelluft, cardiogenic mixing and collateral ventilation.1 The large pressure changes and volumes associated with conventional ventilation have been implicated in the pathogenesis of ventilator induced lung injury (VILI) and chronic lung disease (CLD).2 Animal studies suggest that HFOV may reduce lung injury.3

Indication

At present HFOV is only indicated as a rescue therapy

  1. Failure of conventional ventilation in the term infant (Persistent Pulmonary Hypertension of the Newborn [PPHN], Meconium Aspiration Syndrome [MAS]).4,5 NB: The evidence for HFOV in term infants with severe pulmonary dysfunction is not strong.6
  2. Air leak syndromes (pneumothorax, pulmonary interstitial emphysema [PIE])7
  3. Failure of conventional ventilation in the preterm infant (severe RDS, PIE, pulmonary hypoplasia) or to reduce barotrauma when conventional ventilator settings are high.

HFOV is not as yet proven to be of benefit in the elective or rescue treatment of preterm infants with respiratory dysfunction and may be associated with an increase in intraventricular haemorrhage.8 Furthermore, caution is needed when HFOV is used as high airway pressures may result in impaired cardiac output causing hypotension requiring inotropic support or volume expansion. Some infants poorly tolerate the extra handling involved in switching ventilators or may not respond to HFOV. If there is no improvement with HFOV, consider reverting to conventional ventilation.

In this unit HFOV is delivered using the SensorMedics Oscillator or the Drager VN500.

Terminology

Frequency High frequency ventilation rate (Hz = cycles per second, i.e. 10Hz = 10 cycles/sec = 600 cycles/min)
MAP  Mean airway pressure (cmH2O)
Amplitude delta P or power is the variation around the MAP
Oxygenation 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 In HFOV oxygenation can be separated from ventilation as they are not dependent on each other as is the case with conventional ventilation. Ventilation or CO2 elimination is dependent on amplitude and to a lesser degree frequency.

Prescribing

Treatment is initiated and adjusted on medical staff orders as prescribed on the Level 3 chart.

Initial settings on HFOV

Optimal lung volume strategy (aim to maximise recruitment of alveoli)
• MAP in 2-3 cmH2O above the MAP on conventional ventilation
• MAP in 1-2 cmH2O steps until oxygenation improves

• Set frequency according to underlying lung pathology. For mature infants starting at 10Hz is appropriate.
  For preterm infants with HMD higher frequencies (12-15 Hz) are recommended.

• Consider recruitment manoeuvres after discussion with consultant
Low volume strategy (aim to minimise lung trauma) 
• Set MAP equal to the MAP on conventional ventilation
• Set frequency according to underlying lung pathology. Higher frequencies are lung protective.

• Adjust amplitude to get an adequate chest wall vibration
  • Obtain an early blood gas and adjust settings as appropriate.
  • Obtain chest radiograph to assess inflation

Making adjustments once established on HFOV

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

* Consider recruitment manoeuvres - discuss with consultant
** Changes in frequency should only be made in discussion with attending Neonatologist+

Chest Radiograph

  • Initial chest radiograph at 1-2 hrs to determine the baseline lung volume on HFOV (aim for 8 ribs).
  • A follow-up chest radiograph in 4-6 hours is recommended to assess the expansion.
  • Thereafter repeat chest radiography with acute changes in patient condition.

Weaning

  • Reduce FiO2 to <40% before weaning MAP (except when over-inflation is evident).
  • Reduce MAP when chest radiograph shows evidence of over-inflation (>9 ribs).
  • Reduce MAP in 1-2cm H2O increments to 8-10 cm H2O.
  • In air leak syndromes (low volume strategy), reducing MAP takes priority over weaning the FiO2.
  • Wean the amplitude in 2-4cm H2O increments.
  • Do not wean the frequency
  • Discontinue weaning when MAP 8-10 cm H2O and Amplitude 20-25
  • If infant is stable, oxygenating well and blood gases are satisfactory then infant could be extubated to CPAP or switched to conventional ventilation. Discuss with consultant.

Suctioning

  • Suction is indicated for diminished chest wall movement (chest wobble), elevated CO2 and/or worsening oxygenation suggesting airway or ET tube obstruction, or if there are visible/audible secretions in the airway.
  • Avoid in the first 24 hours of HFOV, unless clinically indicated.
  • In-line suctioning must be used (see Suction Protocol for full procedure)
  • On the sensormedics, press the STOP button briefly while quickly inserting and withdrawing suction catheter (PEEP is maintained)

Possible adverse effects

  1. Hyperinflation may result and manifest by decreased cardiac output recognised by: tachycardia; decreased peripheral pulse; peripheral shutdown; decreased blood pressure and desaturation. 
  2. Pneumothorax signs may be gradual occurring over several hours. Indications are deterioration of blood gas and saturation levels and decreased vibrations on affective side. 
  3. Increased risk of dislodgment of ET tube due to the short rigid vibrating tubing. 
  4. The association between HFOV and IVH remains open. Studies report varying IVH rates of this multifactorial complication. 
  5. Airway damage. In line suction is always to be used for infants on HFOV.

References

  1. Jane Pillow. High frequency oscillatory ventilation: Mechanisms of gas exchange and lung mechanics. Crit Care Med 2005; 33 (3 suppl.): S135-S141
  2. Attar MA, Donn SM. Mechanisms of ventilator-induced lung injury in premature infants. Semin Neonatol 2002; 7: 353-360
  3. McCulloch PR, Forkert PG, Froese AB. Lung volume maintenance prevents lung injury during high frequency oscillatory ventilation in surfactant-deficient rabbits. Am Rev Respir Dis 1988;137(5):1185-92.
  4. Clark RH et al. Prospective, randomized comparison of HFO and conventional ventilation in candidates for ECMO. J Pediatr.1994;124: 447-54
  5. Kohelet D, Perlman M, Kirpalani H, et al. High-frequency oscillation in the rescue of infants with persistent pulmonary hypertension. Crit Care Med. 1988 May;16(5):510-6.
  6. Henderson-Smart DJ, De Paoli AG, Clark RH, Bhuta T. High frequency oscillatory ventilation versus conventional ventilation for infants with severe pulmonary dysfunction born at or near term. Cochrane Database Syst Rev. 2009(3):CD002974.
  7. Clark RH et al. Pulmonary interstitial emphysema treated by HFOV. Crit Care Med 1986; 14: 926-30
  8. Henderson-Smart DJ, Cools F, Bhuta T, et al. Elective high frequency oscillatory ventilation versus conventional ventilation for acute pulmonary dysfunction in preterm infants. Cochrane Database Syst Rev. 2007(3):CD000104.
  9. Bhuta T, Henderson-Smart DJ. Rescue high frequency oscillatory ventilation versus conventional ventilation for pulmonary dysfunction in preterm infants. Cochrane Database Syst Rev. 2000(2):CD000438.
  10. van Velzen A, De Jaegere A, van der Lee J, et al. Feasibility of weaning and direct extubation from open lung high-frequency ventilation in preterm infants. Pediatr Crit Care Med. 2009 Jan;10(1):71-5.
  11. Text Merenstein GB, Gardner SL, editors. Handbook of Neonatal Intensive Care. 6th ed. St. Louis, Missouri: Mosby; 2006.
  12. SensorMedics Company High Frequency Oscillatory Ventilator. Operator's manual.
  13. NW Nursing and Midwifery Suctioning Policy and Nitric Oxide Policy

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Document Control

  • Date last published: 13 October 2018
  • Document type: Clinical Guideline
  • Services responsible: Neonatology
  • Owner: Newborn Services Clinical Practice Committee
  • Editor: Sarah Bellhouse
  • Review frequency: 2 years