High-frequency ventilation


High-frequency ventilation is a type of mechanical ventilation which utilizes a respiratory rate greater than four times the normal value and very small tidal volumes. High frequency ventilation is thought to reduce ventilator-associated lung injury, especially in the context of Acute respiratory distress syndrome and acute lung injury. This is commonly referred to as lung protective ventilation. There are different types 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.

Physiology

With conventional ventilation where tidal volumes exceed dead space, gas exchange is largely related to bulk flow of gas to the alveoli. With high-frequency ventilation, the tidal volumes used are smaller than anatomical and equipment dead space and therefore alternative mechanisms of gas exchange occur.

Procedure

  • Supraglottic Approach—The supraglottic approach is advantageous as it allows a completely tubeless surgical field.
  • Subglottic Approach
  • Transtracheal Approach

    High-frequency jet ventilation (passive)

High-frequency jet ventilation minimizes movement of the thorax and abdomen and facilitates surgical procedures where even slight motion from spontaneous or intermittent positive pressure ventilation may significantly affect the duration and success of the procedure. HFJV does NOT allow: setting specific tidal volume, sampling ETCO2. In HFJV a jet is applied with a set driving pressure, followed by passive exhalation for a very short period before the next jet is delivered, creating "auto-PEEP". The risk of excessive breath-stacking leading to barotrauma and pneumothorax is low but not zero.
In HFJV exhalation is passive whereas in HFOV gas movement is caused by in-and-out movement of the “loudspeaker” oscillator membrane. Thus in HFOV both inspiration and expiration are actively caused by the oscillator, and passive exhalation is not allowed.
In the UK, the Mistral or Monsoon jet ventilator is most commonly used. In the United States the Bunnell LifePulse jet ventilator is most commonly used.

Bunnell LifePulse jet ventilator

HFJV is provided by the Bunnell Life Pulse High-Frequency Ventilator. HFJV 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 creates 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%. Usage of high-frequency jet ventilation is recommended in neonates and adults with severe lung injury.

Indications for use

The Bunnell Life Pulse High-Frequency Ventilator is indicated for use in ventilating critically ill infants with pulmonary interstitial emphysema. Infants studied ranged in birth weight from 750 to 3529 grams and in gestation age from 24 to 41 weeks.
The Bunnell Life Pulse High-Frequency Ventilator is also indicated for use in ventilating
critically ill infants with respiratory distress syndrome complicated by pulmonary air leaks who are, in the opinion of their physicians, failing on conventional ventilation. Infants of this description studied ranged in birth weight from 600 to 3660 grams and in gestational age from 24 to 38 weeks.

Adverse effects

The adverse side effects noted during the use of high-frequency ventilation include those
commonly found during the use of conventional positive pressure ventilators. These adverse effects include:
High-frequency jet ventilation is contraindicated in patients requiring tracheal tubes smaller than 2.5 mm ID.

Settings and parameters

Settings that can be adjusted in HFJV include 1) inspiratory time, 2) driving pressure, 3) frequency, 4) FiO2, and 5) humidity. Increases in FiO2, inspiratory time, and frequency improve oxygenation, while an increase in driving pressure and a decrease in frequency improve ventilation.
Peak inspiratory pressure (PIP)
The peak inspiratory pressure window displays the average PIP. During startup a PIP sample is taken with every inhalation cycle and is averaged with all other samples taken over the most recent ten-second period. After regular operation begins, samples are averaged over the most recent twenty-second period.
ΔP (Delta P)
The value displayed in the ΔP window represents the difference between the PIP value and the PEEP value.
Servo pressure
The servo pressure display indicates the amount of pressure the machine must generate
internally in order to achieve the PIP appearing in the servo-display. Its value can range from 0—20 psi. If the PIP sensed or approximated at the distal tip of the tracheal tube deviates from the desired PIP, the machine automatically generates more or less internal pressure in an attempt to compensate for the change. The servo-pressure display keeps the operator informed.
The servo display is a general clinical indicator of changes in the compliance or resistance of the patient's lungs, as well as loss of lung volume due to tension pneumothorax.

High-frequency oscillatory ventilation

In High-frequency oscillatory ventilation the airway is pressurized to a set mean airway pressure through an adjustable expiratory valve. Small pressure oscillations delivered at a very high rate are superimposed by the action of a “loudspeaker” oscillator membrane. HFOV is often used in premature neonates with respiratory distress syndrome who fail to oxygenate appropriately with lung-protective settings of conventional ventilation. It has also been used in ARDS in adults, but two studies showed negative results for this indication.
Parameters that can be set in HFOV includes the continuous lung-distending pressure, oscillation amplitude and frequency, I:E ratio, fresh gas flow, and FiO2. Increases in continuous lung-distending pressure and FiO2 will improve oxygenation. Increases in amplitude or fresh gas flow and decreases in frequency will improve ventilation.

High-frequency percussive ventilation

HFPV — High-frequency percussive ventilation combines HFV plus time cycled, pressure-limited controlled mechanical ventilation.

High-frequency positive pressure ventilation

HFPPV — High-frequency positive pressure ventilation is rarely used anymore, having been replaced by high-frequency jet, oscillatory and percussive types of ventilation. HFPPV is delivered through the endotracheal tube using a conventional ventilator whose frequency is set near its upper limits. HFPV began to be used in selected centres in the 1980s. It is a hybrid of conventional mechanical ventilation and high-frequency oscillatory ventilation. It has been used to salvage patients with persistent hypoxemia when on conventional mechanical ventilation or, in some cases, used as a primary modality of ventilatory support from the start.

High-frequency flow interruption

HFFI — High Frequency Flow Interruption is similar to high-frequency jet ventilation 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 ventilation (active)

High-frequency ventilation — HFV-A is notable for the active exhalation mechanic included. Active exhalation means a negative pressure is applied to force volume out of the lungs. The CareFusion 3100A and 3100B are similar in all aspects except the target patient size. The 3100A is designed for use on patients up to 35 kilograms and the 3100B is designed for use on patients larger than 35 kilograms.

CareFusion 3100A and 3100B

High-frequency oscillatory ventilation was first described in 1972 and is used in neonates and adult patient populations to reduce lung injury, or to prevent further lung injury. HFOV is characterized by high respiratory rates between 3.5 and 15 hertz and having both inhalation and exhalation maintained by active pressures. The rates used vary widely depending upon patient size, age, and disease process. In HFOV the pressure oscillates around the constant distending pressure which in effect is the same as positive end-expiratory pressure. 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 are believed to come into play in HFOV compared to normal mechanical ventilation. It is often used in patients who have refractory hypoxemia that cannot be corrected by normal mechanical ventilation such as is the case in the following disease processes: severe ARDS, ALI and other oxygenation diffusion issues. 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.