High Frequency Ventilation

High-frequency ventilation (HFV) is a type of ventilation that is utilized when conventional ventilation fails. It is a technique where the set respiratory rate greatly exceeds the normal breathing rate. In this rescue strategy, the tidal volume delivered is significantly less and can also be less than dead space ventilation.[1] A few stated advantages of this technique are: It reduces the risk of volutrauma and thus helps prevent ventilator-induced lung injury. It also maintains constant alveolar inflation and thus prevents the inflate - deflate cycle and improves oxygenation. There are mainly four types of HFV.[2]: 1. High-frequency oscillatory ventilation (HFOV). 2. High-frequency positive pressure ventilation (HPPV). 3. High-frequency jet ventilation (HJV). 4. High-frequency percussive ventilation (HFPV). HFOV (High-Frequency Oscillatory Ventilation) This is one of the most common methods of HFV. It is most often used as a rescue strategy when conventional ventilation fails in severe ARDS. In this technique, the tidal volume set is less than dead space ventilation, and respiratory rates are very high, ranging from 300 to 900 /minute. The technique uses a reciprocating diaphragm to deliver very high respiratory rates and is connected to a standard endotracheal tube. The primary setting is mean airway pressure (MAP) as the flow oscillates around a constant MAP due to high respiratory rates (frequency). The settings involved are respiratory rate (or frequency), which is set directly, and MAP, which most often is set by adjusting inspiratory flow rates and expiratory valve (PEEP). In some machines, the MAP is set directly. The tidal volume delivered is very low and is less than anatomical dead space. The tidal volume is also known as amplitude and is determined by various factors like the size of the endotracheal tube used and respiratory rate/ frequency set.[3] The mechanism of maintaining constant mean airway pressure helps in alveolar recruitment and improvement of oxygenation. The low tidal volumes prevent volutrauma and ventilator-induced lung injury (VILI). It is used as one of the rescue methods in patients with severe ARDS when conventional ventilation has failed. In neonatal patients, HFOV can be used in premature lungs as the first line to prevent lung injury by conventional ventilation.  HJV (High-Frequency Jet Ventilation) This method is mainly used in neonates. In this technique, a jet of gas is delivered via a 14 -16 gauge cannula inserted in the endotracheal tube. It delivers a respiratory rate of about 100 to 150 per minute. It provides very low tidal volumes of less than 1ml per kg. Exhalation is passive. It is often combined with conventional ventilation for the reinflation of lungs. Taylor dispersion is the most common method of gas exchange in HFJV.[4]  HFPPV (High-Frequency Positive Pressure Ventilation) It is delivered using a conventional ventilator in which the respiratory rates are set at maximum limits. This technique is obsolete and is rarely used.  HFPV (High-Frequency Percussive Ventilation) This involves a combination of high-frequency ventilation and conventional ventilation (pressure control mode). It can be described as HFOV oscillating between two different pressure levels. It is presumed to have lesser risks of barotrauma and also improve oxygenation when compared to conventional ventilation alone. The general requirements of sedation and paralysis are lesser in this mode compared to other methods of HFV. It is also more efficient in clearing secretions.[4]
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