HFOV

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To adequately ventilate a patient, it must be understood that a mechanical tidal volume breath must exceed the physiologic dead space in the lungs. Conventional ventilators utilize this principle by inflating the patient’s lungs with a known capacity of air.  Expiration then occurs by the passive recoil of the thorax and lung.

High-frequency ventilation (HFV) is a technique of ventilation that delivers small tidal volumes at respiratory rates greater than 150 per minute. The breaths given are expressed in hertz (Hz), where one Hz is equal to 60 cycles per minute or one cycle per second. The term amplitude is associated with High-frequency Ventilation. Amplitude is described as the difference between inspiration and expiration during the mechanical breath. It is the end-inspiratory breath or also known as the peak inspiratory pressure (PIP) and the pressure that remains in the lungs at end-expiratory. The pressure that remains in your lungs at end-expiratory is also known as the positive end-expiratory pressure (PEEP).

With the use of conventional ventilation, high pressures and volumes are needed to achieve adequate ventilation and oxygenation to lungs with low compliance (stiff lungs). These high pressures and volumes can contribute to barotrauma (lung injury) and the development of Bronchopulmonary Dysplasia (BPD) (Links to an external site.) in neonates. These new forms of mechanical ventilators were developed to serve this population of patients better.

The use of high-frequency ventilation is unique. This type of ventilation promotes adequate ventilation with tidal volumes lower than the dead space in the lung along with higher respiratory rates. The primary advantage of delivering small tidal volumes is that it can be done at relatively low pressures, significantly reducing the risk of barotrauma.

The indications for the necessity of this type of HFV in the neonatal population is the respiratory failure that does not respond to previous methods of mechanical ventilation. Infants with respiratory failure usually have other existing complications such as pulmonary air leaks or persistent pulmonary hypertension that would be exacerbated by traditional positive pressure ventilation. 

Hazards

Problems associated with HFV such as: 

  • Air trapping in the lung caused by insufficient time to exhale
  • Hyperinflation of the lung caused by the air trapping
  • Obstruction of the airway with secretions
  • Hypotension caused by the positive pressure
  • Necrotizing tracheobronchitis caused by inflammation that can lead to airway obstruction

Chest assessment of a patient receiving HFV is often difficult to observe. With the use of HFV, adequate ventilation is based on chest wall vibration rather than chest rise and fall that is noted with traditional ventilation; therefore additional training may be needed to detect patient complications such as:

  • A decrease in chest wall vibration, with an increased PaCO2 and a normal PaO2. May indicate airway obstruction or malposition of the endotracheal tube. The patient should be suctioned and the tube repositioned.
  • A decrease in chest wall vibration with an increase in PaCO2, or a reduction in the PaO2 with a decreased in lung compliance would indicate a strong possibility that a pneumothorax may be present. An assessment of the infant should be conducted immediately for signs of pallor, cyanosis, bradycardia, hypotension, and increased respiratory effort, all of which indicate a worsening of status.

Types of High-Frequency Ventilators

High-frequency ventilators deliver rates between 150 and 3000 breaths per minute (bpm). The major types of HFV are categorized by the frequency of ventilation and the method with which the tidal volume is delivered. The four categories of ventilators are high-frequency positive pressure ventilation (HFPPV); high flow jet ventilation (HFJV), high-frequency flow interruption (HFFI), and high-frequency oscillatory ventilation (HFOV).

  • HFPPV – HFPPV is merely conventional ventilatory breaths delivered at rates between 60 and 150 breaths per minutes. The delivery of low tidal volumes during HFPPV occurs via convective air movement, in which tidal volume exceeds dead space and results in lower PaCO2 blood levels with the use of lower FiO2 percentages.
  • HFFI – HFFI can deliver frequencies as high as 15 Hz. In this type of HFV, a controlled mechanism, usually a rotating ball with a gas pathway, interrupts a high-pressure gas source to deliver rapid rates. As with HFPPV and HFJV, exhalation occurs passively.
  • HFJV – HFJV operate in the range of 4 to 11 Hz. The high-frequency jet ventilator delivers a high-pressure pulse of gas to the patient’s airway. This is done through a special adaptor attached to the end of the endotracheal tube or through a specially designed endotracheal tube (ETT) that allows the pulsed gas to exit inside the endotracheal tube. HFJV is used in tandem with a conventional ventilator. The purpose of the conventional ventilator is threefold.
    • First, it provides occasional sighs, which help stimulate the production of surfactant and prevent microatelectasis.
    • Second, the conventional ventilator provides PEEP to the patients’ airway.
    • Third, it provides a continuous flow of gas through the ETT for entrainment by the jet ventilator.
  • HFOV– HFOV utilizes the highest rates, usually in the range of 8-30 Hz. These oscillatory waves produce a vibration or shaking motion of the patient’s chest that should be observed to verify sufficient amplitude and thus gas delivery. HFOV is the most widely used method of HFV. It does not require a specialized ETT or conventional ventilation in tandem for operation.

Watch the video below to enhance your understanding of the history, indication, and function of HFOV. (Total video time 3:32 minutes)

https://www.youtube.com/watch?v=NZDzOndckUU (Links to an external site.)

Once the type of ventilator and mode is determined, it is time for the physician to prescribe the initial settings. The initial settings are based on clinical protocols at each institution according to the patient’s diagnosis. In most cases, only personnel with advanced training in neonatal and pediatric care are allowed to administer care to neonatal patients. Before being allowed to work in the neonatal care or intensive care units proficiency training is required. The training methods usually include:

  • Proper gas temperature and humidity procedures to assure proper secretion mobility
  • Proper breathing circuit position for a smooth transition and movement to prevent airway trauma
  • Appropriate initial settings and interventions

The patient’s age, weight, and diagnosis will also play a major role in determining initial settings. See the table below indicating the recommended initial settings for the initiation of mechanical ventilation.        

Initial Settings of Ventilation 

Premature Infant

Infant 

Toddler 

Small Child 

Child 

Adolescent 

Respiratory Rate (BPM)

40-60

25-40

20-35

20-30

18-25

12-20

VT (ml/kg)

4-6

5-8

5-8

6-9

7-10

7-10

Inspiratory Time (s)

0.25-0.4

0.3-0.5

0.6-0.7

0.7-0.8

0.8-1

1.1.2

PEEP (cm H20)

5

5

5

5

5

5

FI02 

10% higher than pre-intubation 

10% higher than pre-intubation 

10% higher than pre-intubation 

10% higher than pre-intubation 

10% higher than pre-intubation 

10% higher than pre-intubation 

Manipulating Oxygenation During Mechanical Ventilation

Improving oxygenation can be easily achieved by adjusting the oxygen control knob (Fi02). A simple formula using the arterial blood gas results can determine how much to adjust the Fi02 control to achieve a desired arterial blood oxygen level (Pa02). The formula is written as: Fi02 desired= Pa02 desired X (set) Fi02 /Pa02 known. Extreme caution is to be given to oxygenating premature infants; the lowest acceptable levels should be used to avoid potential complications such as blindness and lung injuries.

Increasing mean airway pressure can reexpand collapsed alveoli resulting in improved oxygenation (PaO2). However, this method can cause shunting and pulmonary volume trauma. The controls that affect mean airway pressure are:

  • Inspiratory hold
  • Peak pressure
  • Rate (frequency)
  • Flow rate
  • Positive end-expiratory pressure (PEEP) level.

Manipulating Alveolar Ventilation During Mechanical Ventilation

To maintain normal ventilation (PaC02), the minute volume should be adequately maintained. The tidal volume and respiratory rate control will determine the minute volume. Therefore, making changes to these pentameters based on the arterial blood gases (ABG) results will maintain appropriate ventilation.

Specialty Medical Gases

In addition to oxygen, the administration of other medical gases such as nitric oxide and helium is becoming popular in respiratory care. Nitric oxide has shown excellent results in the treatment of neonates with pulmonary hypertension. Helium has proven its value in the treatment of severe airway obstructions.

Inhaled Nitric Oxide (I-NO)

Nitric oxide is a colorless gas that is produced in the endothelial cells in the body. Through the years, it was discovered that NO diffuses from the endothelium into smooth muscle cells that form the vascular walls and is responsible for vascular dilation. Inhaled NO mimics the effect of naturally released NO and selectively produces pulmonary vasodilation. The pharmacological effect is an increase in oxygenation tension due to dilation of pulmonary vessels resulting in improved ventilated areas of the lung. The recommended dose of I-NO is 20 ppm in conjunction with oxygen. I-NO must be delivered through a system, such as the INOvent Delivery System, which provides consistent concentrations of NO throughout the respiratory cycle with continuous monitoring of NO, NO2, and oxygen concentrations. Watch the video below describing the indications, use, and hazards of I-NO therapy. (Total video time 3:03 minutes)

https://www.youtube.com/watch?v=13ixpPbyKYE (Links to an external site.)

Heliox Therapy

Helium (He) and oxygen (O2) mixtures have been used therapeutically since as early as the 1930s. Because of helium’s known low density, it makes it an ideal gas to mix with oxygen to reduce both turbulence in airflow and resistive pressure in the airways. The therapeutic result is a reduction in airway resistance and the work of breathing. Delivery of oxygen, as well as medicated aerosols, can, therefore, be delivered deep into the lungs when helium is used in place of nitrogen.

Prompt

Choose one type of high-frequency ventilation listed above and answer the following questions. 

1. Describe the type of ventilation you selected and the type of patient indicated for use

2. Discuss how initial settings are determined including alarms 

3. Describe potential hazards and or complications that may occur with the use of the ventilation you selected

4. Discuss a long-term injury that may occur as a result of prolonging ventilation and high levels of oxygen. Indicate how the injury you discussed could have been prevented. 

Provide your answer in 500 words or more. You must submit the assignment in IWG format including two cited references.

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