High-Frequency Oscillatory Ventilation

Introduction

• High-frequency ventilation (HFV) is an unconventional mechanical ventilation strategy that delivers extremely small tidal volumes (smaller than anatomic dead space) at supra-physiologic respiratory rates.¹
• This technique was developed to minimize ventilator-induced lung injury while maintaining adequate gas exchange.
• There are three primary types of HFV: HFOV, high-frequency jet ventilation, and high-frequency percussive ventilation.
• HFOV employs a reciprocating piston-driven diaphragm to generate active inspiration and expiration. The piston moves forward and backward at a precisely controlled frequency set by the clinician, creating alternating positive and negative pressures superimposed on a constant mean airway pressure (mPaw).
• Other rarely used types of HFV include high-frequency positive pressure ventilation, which is an obsolete technique and is delivered using a conventional ventilator where respiratory rates are set at maximum limits, and high-frequency flow interruption, in which gas is delivered as rapid, discrete pulses by intermittently interrupting a continuous high pressure gas flow.
• Both inspiration and expiration are active processes, meaning the ventilator actively pulls gas out of the lungs during the expiratory phase rather than relying on passive elastic recoil.

Mechanism of Gas Exchange in HFOV

• Traditional mechanical ventilation relies on bulk convection (the movement of gas molecules into and out of the alveoli via tidal breathing). HFOV achieves gas exchange through multiple simultaneous mechanisms that enhance CO₂ elimination and oxygen delivery despite tidal volumes smaller than anatomic dead space.¹
• HFOV uses a constant distending pressure (mean airway pressure) with small tidal volumes (about 1–3 mL/kg, less than anatomic dead space) oscillated at 5–15 Hz, so gas exchange depends on several non-conventional mechanisms rather than simple bulk tidal breaths. Various mechanisms contribute to gas transport during HFOV (Figure 1):¹
  • Bulk convection and turbulence: Oscillatory flow still generates net movement of gas towards and away from alveoli; turbulence in the larger airways enhances mixing beyond simple laminar flow.
  • Pendelluft: Differences in regional compliance and resistance cause asynchronous filling and emptying between lung units, so gas shifts between regions during oscillation and improves mixing in dependent areas.
  • Asymmetric velocity profiles and radial mixing: Inspiratory and expiratory velocity profiles differ, producing shear and radial mixing that augment axial gas movement within conducting airways.
  • Taylor dispersion: Oscillatory laminar flow with velocity gradients across the airway lumen increases effective longitudinal dispersion of gas, enhancing CO₂ removal and O₂ delivery at very small tidal volumes.
  • Collateral ventilation and cardiogenic mixing: Collateral channels permit gas movement between neighboring units, and cardiac motion further perturbs parenchymal gas, adding another layer of mixing, especially in injured, heterogeneous lungs.

Indications and Contraindications

• HFOV is rarely used as a first line. Rather, it is a rescue modality that is used when conventional mechanical ventilation fails to achieve adequate oxygenation or ventilation.² Indications include:
  • Neonatal indications:
    • Meconium aspiration
    • Persistent pulmonary hypertension
    • Pulmonary interstitial emphysema
    • Pulmonary hypoplasia
  • Pediatric indications
    • Alveolar hemorrhage
    • Ventilator-associated lung injury
    • Large air leak with inability to keep lungs open
    • Pediatric acute respiratory distress syndrome (ARDS)
    • Refractory hypoxemia
    • Bronchopulmonary fistulae
  • Adult indications
    • It may be considered only as a rescue strategy in adults with severe, refractory hypoxemic ARDS despite optimized mechanical ventilation (low VT, high positive end-expiratory pressure (PEEP), prone positioning), primarily when extracorporeal membrane oxygenation is unavailable. HFOV is not recommended as the primary mode of ventilation in adults with ARDS. Randomized controlled trials have shown no benefit and possible harm when compared to conventional mechanical ventilation.¹
• Absolute contraindications to HFOV do not exist. However, several conditions are considered relative contraindications or potential hazards, in which HFOV may be less effective or potentially harmful.² These include:
  • Higher intrathoracic pressures
  • Right ventricular preload; require volume administration ± inotropic support
  • Pneumothorax
  • Migration/displacement of endotracheal tube
  • Bronchospasm
  • Airway obstruction from mucus plugging, secretions, hemorrhage, or clot
  • Pneumomediastinum
  • Subcutaneous emphysema
  • Refractory respiratory acidosis
  • Neonatal intraventricular hemorrhage
  • Increased pulmonary capillary wedge pressure

Parameters and Settings

• HFOV requires adjustment of four primary parameters to achieve optimal ventilation and oxygenation: mPaw, inspired oxygen (FiO₂), amplitude (ΔP), and frequency (Hz) (Figure 2).
• Oxygenation optimization
  • mPaw and FiO₂ are used to manipulate oxygenation:
  • mPaw is the time-weighted average pressure delivered throughout the respiratory cycle. It determines lung recruitment.
    • mPaw is adjusted by modifying the inspiratory flow rate and the expiratory back-pressure valve (similar to PEEP adjustment)
    • The initial mPaw is typically set 3-5 cmH₂O above the mPaw used during conventional mechanical ventilation (maximum 45-60 cmH2O).
  • Excessive mPaw causes overdistention, increases intrathoracic pressure, impairs venous return, and increases the risk of barotrauma.
  • FiO2 is adjusted to maintain the desired SpO₂ and PaO₂ levels.
• Ventilation optimization
  • Amplitude and frequency are used to manipulate ventilation. Tidal volume is directly proportional to amplitude and inversely proportional to frequency.
  • Amplitude (ΔP or Power): It represents the pressure difference generated by piston excursion and is the primary determinant of delivered tidal volume. The piston’s forward movement (inspiration) and backward movement (expiration) determine the magnitude of pressure swings. Increasing the voltage across the piston increases amplitude and piston displacement, delivering larger tidal volumes above the mean airway pressure.
  • Amplitude should be adjusted to achieve visible “chest wall vibrations” or “chest wiggle,” which is a clinical indicator of adequate ventilation.²
  • Amplitude is commonly initiated at 20-30 cmH2O in neonates, depending on patient size and lung mechanics.
  • Frequency (Hz) is the oscillation rate set directly by the clinician, measured in Hertz (Hz). Frequency selection influences the balance between ventilation and oxygenation.
  • Frequency is typically set between 8-15 Hz in neonates, 6-10 Hz in pediatrics, and 3-6 Hz in adults.
    • Higher frequencies (10-15 Hz) improve oxygenation but minimize ventilation (because tidal volumes decrease as frequency rises, due to shortened inspiratory time). Such frequencies can be set if permissive hypercarbia is a goal.³
    • Lower frequencies (3-8 Hz) increase tidal volumes, therefore improving ventilation.
  • Inspiratory Time Fraction: Inspiratory time is typically set at 33% of the respiratory cycle (I:E ratio approximately 1:2). It is rarely adjusted clinically.

Intraoperative Use of HFOV

• HFOV can be safely continued intraoperatively for patients who require surgery while receiving this support modality.
• However, this imposes many challenges to the anesthesiologist. These include:⁴
  • Inhalational anesthetic agents cannot be used with HFOV, and total intravenous anesthesia is required.
  • End-tidal CO₂ monitoring is not possible during HFOV. Frequent arterial blood gas measurements or transcutaneous CO₂ monitoring are required to track ventilation adequacy.
  • HFOV generates significant noise that can hinder clinical examination, particularly auscultation of heart sounds and lung fields.
  • Many anesthesiologists lack familiarity with HFOV management, necessitating consultation with pulmonary/critical care specialists and respiratory therapists, as well as careful coordination.