High frequency ventilation

# Overview
High frequency ventilation is a type of mechanical ventilation that employs very high respiratory rates (>150 breaths per minute) and very small tidal volumes (usually below anatomical dead space). The primary goal of HFV is to achieve ventilation and oxygenation at a reduced risk of ventilator induced lung injury (VILI). This is commonly referred to as lung protective ventilation.

## Ventilator induced lung injury
Ventilator induced lung injury is caused by multiple factors. The human lung is designed to ventilate effectively by using negative pressure in the thorax. Once positive pressure is applied, some degree of VILI is likely to occur.
One major causative factor is the over stretching of the airways and alveoli. During mechanical ventilation, the flow of gas into the lung will take the path of least resistance. Areas of the lung that are collapsed (atelectasis) or filled with secretions will be under inflated, while those areas that are relatively normal will be over inflated. These areas will become over distended and injured. This may be reduced by using smaller tidal volumes.
During positive pressure ventilation, atelectatic regions will inflate, however the alveoli will be unstable and will collapse during the expiratory phase of the breath. This repeated alveolar collapse and expansion (RACE) will cause VILI. By opening the lung and keeping the lung open RACE (and VILI) is reduced.

# Types of High Frequency Ventilation
There are different flavors of High frequency ventilation. Each type has its own unique advantages and disadvantages. The types of HFV are characterized by the delivery system and the type of exhalation phase.
High Frequency Ventilation may be used alone, or in combination with conventional mechanical ventilation. In general, those devices that need conventional mechanical ventilation do not produce the same lung protective effects as those that can operate without tidal breathing. Specifications and capabilities will vary depending on the device manufacturer.

## High Frequency Oscillatory Ventilation (HFOV)
High Frequency Oscillatory Ventilation is characterized by high respiratory rates generally between 3-7 hertz (180-420 breaths per minute). In HFOV the pressure oscillates (60-90 cmH20)around the constant distending pressure (equivalent to the PEEP - usually around 20-30 cmH20).  Thus gas is pushed into the lung during inspiration, and then pulled out during expiration. HFOV generates tidal volumes of 1-3 mL/kg; less than the dead space of the lung.  Tidal volume is dependent on endotrachial tube size, power and hertz. Different mechanisms of gas transfer come into play in HFOV compared to normal mechanical ventilation. It is used in patients who have hypoxia refractory to normal mechanical ventilation severe ARDS, ALI and other other Oxygenation issues that cannot be corrected with normal means of ventilation.

## High Frequency Jet Ventilation (HFJV)
High Frequency Jet Ventilation employs a small cannula placed in the airway at the opening of the endotracheal tube (ett). A high pressure jet of gas flows out of the cannula and into the airway. This jet of gas occurs for a very brief duration, about 0.02 seconds, and at a very high frequency: 10-20 hertz. Conventional mechanical breaths are frequently required to aid in ventilation.

## High Frequency Flow Interruption (HFFI)
High Frequency Flow Interruption is similar to HFJV but the gas control mechanism is different. Frequently a rotating bar or ball with a small opening is placed in the path of a high pressure gas. As the bar or ball rotates and the opening lines-up with the gas flow, a small, brief pulse of gas is allowed to enter the airway. Frequencies for HFFI are typically limited to maximum of about 15 hertz.

## High Frequency Positive Pressure Ventilation (HFPPV)
High Frequency Positive Pressure Ventilation is typically utilized by using a conventional ventilator at the upper frequency range of the device (typically 90-100 breaths per minute). A conventional breath type is used and tidal volumes are usually higher than (HFOV, HFJV and HFFI). With newer and specifically designed devices becoming popular, HFPPV is rarely used clinically any more.