High Frequency Ventilation

Excerpt

High-frequency ventilation (HFV) is a type of ventilation utilized when conventional ventilation fails. In this technique, 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. This topic is presented for historical purposes as it is no longer used in adults, only in neonates.

A few stated advantages of this technique are:

1. It reduces the risk of volutrauma and thus helps prevent ventilator-induced lung injury (VILI).

2. It also maintains constant alveolar inflation, thus preventing the inflate-deflate cycle and improving oxygenation.

There are mainly 4 types of HFV:

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

This is 1 of the most common methods of HFV. It is often used as a rescue strategy when conventional ventilation fails in severe acute respiratory distress syndrome (ARDS). In this technique, the tidal volume set is less than dead space ventilation, and respiratory rates range 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), set directly, and MAP, often set by adjusting inspiratory flow rates and expiratory valve (positive end-expiratory pressure). 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 and the respiratory rate/ frequency set. The mechanism of maintaining constant mean airway pressure helps in alveolar recruitment and improvement of oxygenation. The low tidal volumes prevent volutrauma and VILI. It is used as 1 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

This method is mainly used in neonates. This technique delivers a gas jet 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 the lungs. Taylor dispersion is the most common method of gas exchange in HFJV.

HFPPV

It is delivered using a conventional ventilator where respiratory rates are set at maximum limits. This technique is obsolete and is rarely used.

HFPV

This involves a combination of HFV and conventional ventilation (pressure control mode). It can be described as HFOV oscillating between 2 different pressure levels. It is presumed to have lesser risks of barotrauma and improve oxygenation compared to conventional ventilation alone. The general requirements for sedation and paralysis are lower in this mode than in other methods of HFV. It is also more efficient in clearing secretions.