High-frequency ventilation

In the high-frequency ventilation (High Frequency Oscillation Ventilation, HFOV ) high continuous alveolar Distentionsdruck (similar to the CPAP ) using a high gas flow in the ventilation system (not the patient ) established. The mechanism of gas transport to the patient is fundamentally different from other forms of ventilation.

Technical structure and mode of action

An oscillator integrated into the system ( speakers or flask ) was added the flow of gas into oscillating oscillations typically having a frequency of 5-15 Hz ( up to 900 " breaths " per minute). Here there is at any time a positive pressure in the breathing system, similar to the CPAP ( Continuous Positive Airway Pressure) increases the diffusion area of the lung; the alveoli ( air sacs ) are minimal overinflated. Every single ( very low ) pressure fluctuation, thereby moving a minimal volume of gas that is many times smaller than the anatomical dead space. Thus, the ventilation is based upon HFOV method not in the motion of the gas volumes of the airways, but rather to the continuous mixing of the respiratory gas in the respiratory system at any level. Various physical effects come of it probably to the fore:

• Turbulence in the large airways leading to mixing with fresh gas.
• Proximal ( located near the large airways ) alveoli are ventilated directly.
• Asymmetric profiles of the forwarded pressure wave: During the " inhalation ", the air flows on one side faster than in the other bronchus while the profile on exhalation is quite symmetrical. Over many pressure cycles migrates a fresh air front down the respiratory tract.
• Diffusion: In distal ( situated towards the end of the branches of the air-conducting system ) alveoli, the pressure wave probably does not matter much more, here oxygen diffuses probably primarily along the concentration difference.

Practical Application

Prerequisite for an optimal application of HFOV is a previous Recruitment ( Alveoleneröffnung ) of the lung.

Two parameters affect the gas transport:

• The mean airway pressure
• The shape, height and frequency of the oscillation

The level of mean airway pressure determines the extent of " flatulence " and opening of the alveoli and thus the supply of oxygen ( oxygenation ). Low mean airway pressures lead to lower opening and thus to a small gas exchange surface. High mean airway pressures lead to hyperinflation and a compression of pulmonary vessels with the following right heart failure. It is therefore necessary to find the "right" pressure and respiration in this safe window.

The shape of the amplitude is determined from the amount and the relative duration of the positive and negative phase, and from the slope of the rise and fall of the wave. Frequently a rectangular shaft with 1/3 positive duration is used. An adjustment of amplitude, frequency and waveform affects primarily the removal of carbon dioxide.

Clinical applications and evidence

HFOV was first applied in children and neonates for the treatment of respiratory distress syndrome of the newborn ( IRDS, Infant respiratory distress syndrome ), and is doing compared to CPAP in improved gas exchange, but not to reduced long-term consequences or reduced mortality.

In adults, HFOV is sometimes used in acute lung injury ( ARDS). This also shows early improved oxygen saturation compared to the standard therapy, but no significant decrease in total mortality. This was confirmed by the British OSCAR study, in which the high frequency ventilation brought no advantage. The Canadian Oscillate study with planned 1200 patients had to be stopped because of an increased mortality in high frequency oscillatory ventilation after 548 patients. In contrast to newborns, it seems for adults so no advantage, perhaps even to give a disadvantage by the high- frequency ventilation.

However, today's HFOV protocols use slightly different settings (lower tidal volume ), may be effective. Current studies are missing it. HFOV remains in clinical practice, an experimental treatment for ARDS patients who do not get enough oxygen despite optimal conventional treatment.