Introduction

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] This article is presented for historical purposes as it is no longer used in adults, only in neonates.

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 the 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]

Anatomy and Physiology

During conventional mechanical ventilation, the gas transfer happens by bulk transport of gas molecules from large central airways to smaller peripheral airways. This needs tidal volume to be more than dead space ventilation. During high-frequency ventilation (HFV), this is not possible as tidal ventilation delivered is little. One well-known mechanism for gas transfer in HFV is the bulk transfer by convection, which may contribute to gas exchange in proximal airways though it plays only a minor role in peripheral gas exchange.

Turbulence is another method of gas transfer, especially in larger airways. In this theory, the different velocity profiles of various particles that are asymmetric will lead to net convective transport. This mode of gas exchange is seen most often in the bifurcation of airways. Taylor dispersion and molecular diffusion are one of the most critical mechanisms of gas exchange during HFOV. The other mechanisms described include pendelluft, cardiogenic mixing, and collateral ventilation.[5]

Indications

Indications for high-frequency ventilation (HFV) in adults and neonates include:

Adults [6]

• It is mainly used in severe ARDS  to prevent VILI. 
• Large air leak syndromes with the inability to keep lungs open  like bronchopleural fistula, pneumothorax, pulmonary interstitial emphysema
• Failure of conventional mechanical ventilation 
• Refractory hypoxemia  as a rescue therapy

Neonates

• Persistent pulmonary hypertension 
• Acute respiratory distress syndrome 
• Pulmonary interstitial emphysema 
• Meconium aspiration 
• Pulmonary hypoplasia 

There is no classic recommendation for HFOV in adults. It is mainly used as rescue therapy in severe ARDS. It is used as one of the methods to prevent VILI. It is also used in patients with bronchopleural fistula. It is considered beneficial when ventilation is difficult and lungs cannot be kept open, like pneumothorax and pulmonary interstitial emphysema. It is also used in ARDS due to inhalational injury, blunt trauma chest, and even in patients with traumatic brain injury with high intracranial pressure.[7]

Contraindications

There are no absolute contraindications for high-frequency oscillatory ventilation (HFOV). However, HFOV can be less effective in certain diseases with high intrathoracic pressure and increased airway resistance, like bronchial asthma. It can lead to air trapping and hyperinflation. It can also lead to complications like barotrauma and air leak syndromes like pneumothorax, pulmonary interstitial emphysema, and pneumomediastinum. It can also be avoided in patients with intracranial hemorrhage and severe sepsis with multiorgan failure.[8]

Equipment

There are various different brands of high-frequency ventilation devices.[8][9][10]

Personnel

Skilled personnel to set up the machine and troubleshoot alarms is necessary.

Preparation

• The machine has to be checked for proper functioning.
• Alarms need to be set.
• MAP of the patient on a conventional ventilator has to be noted.

Technique or Treatment

SETTINGS IN HFOV [6]

The set variables are:

1. Respiratory rate/Frequency
2. Amplitude/Power/Delta P 
3. Mean airway pressure (Paw)
4. Bias flow
5. Inspiratory time
6. FiO2

Frequency 

The rate at which the machine oscillates is set by setting the frequency. A 1 Hz frequency is equal to 60 oscillations per minute. As the I: E ratio is fixed, as the frequency increases, tidal volume/amplitude decreases. Higher frequencies are useful to decrease barotrauma as the pressure change transmitted to alveoli is less at high frequencies. This also increases the zone of safety for ventilation and leads to homogenous aeration of alveoli. The frequency set most often is between 3 to 6 Hz on initiation; it can be as high as 10 to 15 Hz.

Amplitude/Delta P

This is the primary determinant of tidal volume. It occurs due to the piston movement towards or away from the airway of a patient. The voltage across the piston, in turn, determines this. Thus increase in polarity increases amplitude and piston movement and, therefore, tidal volumes above the mean airway pressure. In HFOV, inspiration and expiration are both active processes. The amplitude settings are initiated and subsequently adjusted according to PaCO2 levels. 

Mean Airway Pressure (Paw)

This can be adjusted directly or indirectly. Mean airway pressure is the main determinant of oxygenation, and an increase in Paw leads to the improvement of oxygenation. The settings are adjusted as per oxygenation requirements. Most often, the value initiated will be 5 cm of H2O above plateau pressure in conventional ventilation. Mean airway pressure is adjusted slowly based on saturation in increments of 2 cm H2O. However, the maximum value is 35 cm H2O, and the usual targets are less than 30cm of H2O.

Bias Flow

This is the rate of the gas flow in the ventilator and the circuit apparatus. In HFV, the bias flow is usually around 40 L/min to a maximum of 60 L/min. An increase in bias flow is an indirect method of increasing MAP and, thereby, oxygenation. In patients with air leak syndromes, the bias flow needed will be high.

Inspiratory Time

The inspiratory time set is always less than 50% of the total respiratory cycle, and most often, it is set at 33%. An increase in inspiratory time increases mean airway pressure and thus improves oxygenation. However, it can increase the risks of barotrauma; thus, it is mostly kept constant.  

Initial Setting for HFOV [8]

The initial settings are a primary guide and have to be adjusted according to subsequent arterial blood gas and clinical assessment. 

The initial parameters are usually set as below:

1. Bias flow is usually set from lower values, starting from 20 L/min to 35 L/min, and is adjusted subsequently as per the patient’s needs.
2. Inspiratory time is most often constant and is set at 33%, giving an I: E ratio of 1 to 2.
3. Respiratory rate/frequency are the main determinants of PaCO2 and are set between 3 to 15 Hz. It is most often initiated at 5 to 6 Hz giving a respiratory rate of about 300 per minute. Subsequent adjustments are made by serial ABG monitoring.
4. The amplitude (ΔP) is the primary determinant of tidal volume and is initiated & titrated by looking for chest wiggles/vibrations. In infants, a chest wiggle up to the umbilicus is targetted, whereas, in children and adults, chest wiggles up to the pelvic region and the mid-thigh are targetted, respectively. It is usually set at a value of about 20 to 30 cm H2O higher than patients' PaCO2 on conventional ventilation.
5. Oxygenation is mainly determined by FiO2 and mean airway pressure settings. The mPaw is set at 5 cm H2O above mPaw seen on a conventional ventilator when initiating HFOV. FiO2 is initially set at 1.0 and subsequently tapered as per oxygenation requirements of the patient titrated to a target SpO2 of 88 to 92%. Some patients may need a recruitment maneuver before initiating HFOV.

Weaning from HFOV [8] 

Weaning from HFOV is not based on fixed protocols, and various methods are followed. The process involves weaning to an extent where the patient will be able to tolerate conventional ventilation. The parameters that determine weaning readiness are FiO2 and mean airway pressures. Once oxygenation goals are met, two steps can be followed. The first step is to wean FiO2  to less than 60%, considering a saturation of more than 88%, and the next step is to decrease MAP in increments of 2 cm H2O to achieve a MAP of 30 cm H2O.  

1. Once the above is done, the next thing is to wean FiO2 to 0.4 and the MAP slowly in increments of 2 cms to reach a final goal of 20 to 25 cm H2O.
2. After the above is achieved, the next approach is to transition the patient to conventional lung-protective ventilation and target saturation of more than 88% with mPaw of 20 to 25 cms for the next 24 hours. 
3. Weaning failure is considered when the patient is weaned to conventional low tidal volume ventilation and subsequently fails to maintain saturation above 88% in the initial 48 hours.

Complications

 High-frequency ventilation (HFV) is not without complications. The following are the challenges faced in HFV.

• Its efficacy is questionable in patients with high airway resistance and can lead to air trapping and barotrauma like pneumothorax, pneumomediastinum, pneumopericardium, and pulmonary interstitial emphysema.[11]

• It can also cause harmful heart-lung interactions and can, lead to high intrathoracic pressures and thus can cause decreased venous return and cardiac output.[12]

• As it is an unorthodox method of ventilation, it leads to decreased clearance of secretions and higher risks of secondary sepsis.[11]

• Other limitations include transport difficulties, noisy machines causing problems in clinical examinations, and delayed identification of complications.[12]

Clinical Significance

Mechanical ventilation, despite the unavoidable need, tends to cause ventilator-induced lung injury. In positive pressure ventilation, the most common mechanisms that lead to lung injury are barotrauma, volutrauma, and atelectrauma (due to repeated opening and closing of alveoli). Thus high tidal volumes and high pressures (plateau and peak air pressures) cause and aggravate ventilator-induced lung injury. The most proven preventive method is to use low tidal volume ventilation, as proposed by the ARDS network.[13] 

In HFV, this is avoided as the entire cycle of aeration operates in the safe zone, thus preventing overdistention, which causes VILI. It also prevents derecruitment and avoids atelectrauma and barotrauma. It not only prevents barotrauma but also helps in improving the ventilation/perfusion(V/Q) ratio by allowing uniform aeration of the lungs.[6] The tidal volume used in HFOV is less than dead space volume, preventing cyclical opening and closing of alveoli. By this, it delivers a constant MAP, thereby reducing VILI. It also helps in recruitment by using high PEEP.

Few trials have been done to study the risks and clinical benefits of HFOV  in ARDS. In the OSCILLATE trial, adult patients with moderate to severe ARDS were randomized to receive conventional ventilation with ARDS  protocol or HFOV. This study noted significantly higher mortality in the HFOV group (41%) than in conventional ventilation (35%). The trial concluded that early institution of HFOV  in moderate to severe ARDS was not beneficial and could be harmful.[14] The OSCAR randomized control trial was a multicenter study in the UK in patients with severe ARDS. It proved no difference in mortality between the study groups, and HFO may not be beneficial.[15] 

The RESTORE study done in pediatric ARDS patients found that the use of HFOV in ARDS  led to increased sedation requirements and length of ICU stay.[16] A meta-analysis published in 2017 analyzed that HFOV was not useful but could be potentially harmful when used in mild to moderate ARDS with higher PO2/FiO2 ratios. However, its use as a potential modality, mainly as rescue therapy in refractory hypoxemia, is still considered beneficial. Thus in patients with severe ARDS with refractory hypoxemia, the use of HFOV may help, especially when proning and ECMO are impossible or unavailable.[17]

Enhancing Healthcare Team Outcomes

Successful execution of high-frequency ventilation (HFV) highlights the role of the interprofessional team in evaluating and improving care for patients who need close coordination and cooperation between various health care providers & specialty teams. High-frequency ventilation is currently accepted only as a rescue strategy in refractory hypoxemia, where other conventional or supportive options are impossible or unavailable.

The decision to utilize HFV needs to be taken after close interaction and coordination between the intensive care, pulmonology, and respiratory therapy team. Biomedical or technical assistance may be required at the bedside due to a lack of experience and familiarity with this ventilatory mode with many providers. Indisputably, close interaction & coordination between various healthcare professionals will bring out the best outcome in the given patient.