High Frequency Oscillator Ventilation (HFOV) a new strategy in the treatment

of patients with the Acute Respiratory Distress Syndrome and low lung

compliance pathologies

Amanda Alves

Interna Internato Complementar Medicina Interna

S. Medicina, Hospital Garcia de Orta

Almada, Portugal

Abstract

Several lung protective ventilator strategies for treating patients with ARDS have been investigated. These strategies aim to reverse atelectasis by increasing alveolar recruitment, without overdistension of the existing healthy alveolar units. Both derecruitment and overdistension appear to play an active role in ongoing acute lung injury (ALI). Such ventilator strategies, using conventional modes of ventilation, have been limited by the risk of barotrauma, haemodynamic compromise and severe carbon dioxide (CO₂) retention.

HFOV is a ventilatory mode in which high frequency, low amplitude, pressure oscillations of gas at 5-15 Hz (up to 900 breaths/min), are generated in the airways, resulting in high mean airway pressures and low tidal volumes (V_t) of 1-2ml/Kg. Both the inspiratory and expiratory phases of ventilation are active. A high bias flow of fresh gas is applied (20-60L/min).Gas exchange is achieved utilising sub-deadspace tidal volumes, and as such may provide a less traumatic way of recruiting and stabilising lung volumes. Five mechanisms of gas transport are thought to be important when using HFOV. These are bulk axial flow, interregional gas mixing, axial and radial augmented dispersion (Taylor dispersion), convective dispersion and molecular diffusion. Efficient gas transport seems to involve all five mechanisms and the overall coefficient of gas transport during HFOV is a combined function of Vt² and frequency. HFOV is coupled to a lung volume optimisation protocol, which initially uses high mean airway pressures (‰_AW), to achieve optimal lung volumes via active alveolar recruitment. Optimising lung volume, improves gas exchange and reduces shear stress forces between expanded and collapsed lung units.

HFOV was originally used as a rescue technique in neonates with ARDS when conventional ventilation failed. A number of studies indicate that early application of this ventilatory mode may significantly improve the outcome of this serious lung condition and reduce the development of chronic complications in ARDS survivors.

Key words: High Frequency Oscillator Ventilator, Acute Respiratory Distress Syndrome