High frequency ventilation

High frequency ventilation is a type of mechanical ventilation that employs very high respiratory rates (&gt;60 breaths per minute) and very small tidal volumes (usually below anatomical dead space).&lt;ref name=krishan&gt; Free Full Text.&lt;/ref&gt;

High frequency ventilation is thought to reduce ventilator-associated lung injury (VALI), especially in the context of ARDS and acute lung injury.&lt;ref name=krishan/&gt; This is commonly referred to as lung protective ventilation.

Types of High Frequency Ventilation
There are different flavors of High frequency ventilation.&lt;ref name=krishan/&gt; 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 up to 15 hertz (900 breaths per minute). The rates used vary widely depending upon patient type and disease condition. In HFOV the pressure oscillates around the constant distending pressure (equivalent to the Positive End Expiratory Pressure [PEEP]). Thus gas is pushed into the lung during inspiration, and then pulled out during expiration. HFOV generates very low tidal volumes that are generally less than the dead space of the lung. Tidal volume is dependent on endotracheal tube size, power and frequency. Different mechanisms of gas transfer come into play in HFOV compared to normal mechanical ventilation. It is often used in patients who have hypoxia refractory to normal mechanical ventilation: severe ARDS, ALI and other oxygenation issues that cannot be corrected with conventional ventilation. In some neonatal patients HFOV may be used as the first-line ventilator due to the high susceptibility of the premature infant to lung injury from conventional ventilation.

High Frequency Jet Ventilation (HFJV)
High Frequency Jet Ventilation employs an endotracheal tube adaptor in place for the normal 15 mm ET tube adaptor. A high pressure ‘’jet’’ of gas flows out of the adaptor and into the airway. This jet of gas occurs for a very brief duration, about 0.02 seconds, and at high frequency: 4-11 hertz. Tidal volumes ≤ 1 ml/Kg are used during HFJV. This combination of small tidal volumes delivered for very short periods of time create the lowest possible distal airway and alveolar pressures produced by a mechanical ventilator. Exhalation is passive. Jet ventilators utilize various I:E ratios--between 1:1.1 and 1:12-- to help achieve optimal exhalation. Conventional mechanical breaths are sometimes used to aid in reinflating the lung. Optimal PEEP is used to maintain alveolar inflation and promote ventilation-to-perfusion matching. Jet ventilation has been shown to reduce ventilator induced lung injury by as much as 20%.

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.