High Frequency Oscillatory Ventilation

Dr. Tarek SayedNICU Registrar MCH Buraydah

HFOV - What?

Background

• High frequency oscillatory ventilation (HFOV) is a type of mechanical ventilation that uses a constant distending pressure (mean airway pressure [MAP]) with pressure variations oscillating around the MAP at very high rates (up to 900 cycles per minute). This creates small tidal volumes, often less than the dead space.

• In conventional ventilation large pressure changes (the difference between PEEP and PIP) create physiological tidal volumes.

• HFOV is the only mode of ventilation where both Inspiratory and Expiratory is ACTIVE. Meaning the ventilator pushes and pulls air in and out of the lung due to the forward and backward action of the piston.

Indications :-

1 Failure of conventional ventilation in the term infant (Persistent Pulmonary Hypertension of the Newborn [PPHN], Meconium Aspiration Syndrome [MAS]).NB: The evidence for HFOV in term infants with severe pulmonary dysfunction is not strong.

2 Air leak syndromes (pneumothorax, pulmonary interstitial emphysema [PIE])

3 Failure of conventional ventilation in the preterm infant (severe RDS, PIE, pulmonary hypoplasia) or to reduce barotrauma when conventional ventilator settings are high.

Indications :-

• Types of Patients HFOV is used on: • Early Intervention• Pro-active• Rescue

• Early Intervention :A term used to describe the application of HFOV toan infant within the first FOUR hours of life, or onethat has not been conventionally ventilated.Proactive

• ProactiveIs a term applied to the Infant on ConventionalVentilation that reaches a specific thresholds and isthen transferred to the Oscillator prior to the onset ofbarotrauma or airleak.

• Rescue : Is a term used to describe the Infant that has failed allMechanical Ventilation strategies, and gas exchangecontinues to deteriorate; or develops airleak and isthen transitioned to the oscillator.

Indications :-

HFOV is not as yet proven to be of benefit in the elective or rescue treatment of preterm infants with respiratory dysfunction and may be associated with an increase in intraventricular haemorrhage.8Furthermore, caution is needed when HFOV is used as high airway pressures may result in impaired cardiac output causing hypotension requiring inotropic support or volume expansion. Some infants poorly tolerate the extra handling involved in switching ventilators or may not respond to HFOV. If there is no improvement with HFOV, consider reverting to conventional ventilation.

Terminology

Frequency •High frequency ventilation rate (Hz = cycles per second, i.e. 10Hz = 10 cycles/sec = 600 cycles/min)

MAP •Mean airway pressure (cmH2O)

Amplitude •delta P or power is the variation around the MAP

Oxygenation •Oxygenation is dependent on MAP and FiO2. MAP provides a constant distending pressure equivalent to CPAP. This inflates the lung to a constant and optimal lung volume maximising the area for gas exchange and preventing alveolar collapse in the expiratory phase. 

Ventilation •In HFOV oxygenation can be separated from ventilation as they are not dependent on each other as is the case with conventional ventilation. Ventilation or CO2 elimination is dependent on amplitude and to a lesser degree frequency.

Initial settings on HFOV

• It Depends Upon UR Strategy

Initial settings on HFOV

Optimal lung volume strategy(aim to maximise recruitment of alveoli).

•Set MAP 2-3 cmH2O above the MAP on conventional ventilation•­MAP in 1-2 cmH2O steps until oxygenation improves•Set frequency to 10 HzConsider recruitment manoeuvres after discussion with consultant

Low volume strategy(aim to minimise lung trauma)

•Set MAP equal to the MAP on conventional ventilation•Set frequency to 10 Hz•Adjust amplitude to get an adequate chest wall vibration.

What after ……………… ?

• Obtain an early blood gas and adjust settings as appropriate.

• Obtain chest radiograph to assess inflation

HFOV Controls

PaO2PaCO2

Primary control of CO2 is by the stroke volume produced by the Power Setting

Making adjustments once established on HFOV

Poor Oxygenation

Over Oxygenation

Under Ventilation

Over Ventilation

Increase FiO2 Decrease FiO2Increase

AmplitudeDecrease Amplitude

Increase MAP*(1-2cmH2O)

Decrease MAP(1-2cmH2O)

Decrease Frequency**

 (1-2Hz)if Amplitude

Maximal

Increase Frequency**

 (1-2Hz)if Amplitude

Minimal

Chest Radiograph

• Initial chest radiograph at 1-2 hrs. to determine the baseline lung volume on HFOV (aim for 8 ribs).

• A follow-up chest radiograph in 4-6 hours is recommended to assess the expansion.

• Thereafter repeat chest radiography with acute changes in patient condition.

Weaning

•Reduce FiO2 to <40% before weaning MAP (except when over-inflation is evident).•Reduce MAP when chest radiograph shows evidence of over-inflation (>9 ribs).•Reduce MAP in 1-2cm H2O increments to 8-10 cm H2O.•In air leak syndromes (low volume strategy), reducing MAP takes priority over weaning the FiO2.•Wean the amplitude in 2-4cm H2O increments.•Do not wean the frequency•Discontinue weaning when MAP 8-10 cm H2O and Amplitude 20-25•If infant is stable, oxygenating well and blood gases are satisfactory then infant could be extubated to CPAP or switched to conventional ventilation. Discuss with consultant.

Suctioning

•Suction is indicated for diminished chest wall movement (chest wobble), elevated CO2 and/or worsening oxygenation suggesting airway or ET tube obstruction, or if there are visible/audible secretions in the airway.•Avoid in the first 24 hours of HFOV, unless clinically indicated.•In-line suctioning must be used (see Suction Protocol for full procedure)•Press the STOP button briefly while quickly inserting and withdrawing suction catheter (PEEP is maintained)

 Nursing Management – Monitoring and Assessment and Documentation

Step Action

VIBRTIOn

Visibly assess the chest vibration and note changes. Unlike conventional mechanical ventilation (CMV), you cannot assess a rise and fall of the chest. You need to assess the amount of vibration being produced. Vibration mainly in the neck could indicate a dislodged ET tube and asymmetry vibration could indicate pneumothorax. The vibration produced depends on the amount of amplitude and lung compliance. Use a visual assessment of the depth of bounce ranging from the umbilicus to the clavicle.

ABG

An ABG needs to be done 10-15 minutes after going onto oscillation. In that first hour another 2-3 ABG’s will probably be required as oscillation can produce significant changes in oxygenation and ventilation (CO2). After the 1st hour ABG;s should be done after any change in oscillation settings, or any clinical reason that deems an ABG to be done (e.g. falling saturations, increased saturations). Otherwise 6 hourly if stable and minimal changes occurring with the oscillator settings.Frequent blood gas monitoring is required at first to assess effectiveness of HFOV.

CXREnsure CXR taken within ½ hour after commencement of oscillation, to assess the degree of lung distension, to ensure adequate alveolar expansion and to check that hyperinflation has not occurred.  This will determine MAP setting. NOTE: X-rays may be performed through mattress.

DOCUMENTATION

Amplitude, Hz, FiO2 and MAP settings must be clearly documented by NS-ANP/Medical staff on the level 3 chart.

Nursing Management – Monitoring and Assessment and Documentation

HR

Monitoring of infant’s heart rate may be problematic via ECG electrodes. Heart rate can be monitored as a ‘pulse’ through the UAC . Evaluation for heart murmurs may require a temporary pause in HFOV therapy.

CNSAssess infant’s neurological and behavioural state on HFOV. Analgesia and sedation may be required for comfort and avoidance of ET tube dislodgment.

BPBlood Pressure. Be prepared for a potential blood pressure drop; this is due to the increased intra-thoracic pressure that oscillation can cause, resulting in decreased venous return. Have volume and / or an inotrope (usually dopamine) ready.

AUSCULTATION

Auscultation:Listening to breath sounds in infants ventilated on HFOV may be helpful, as the sounds (friction sounds) become reduced in the affected side when the endotracheal tube is low and ventilates only 1 lung or when a pneumothorax is present. These changes may occur before the infant becomes symptomatic. Thus auscultation should be performed at the time of routine assessment or if there is clinical deterioration.

Nursing Management of the Baby Requiring HFOV

Step Action1 •PositioningThe brakes on the oscillator and incubator / heat table

must always be on.•Position the oscillator and incubator/heat table diagonally across the bed space.•Careful positioning of the oscillator is required to avoid pulling on ET tube. Know and check hourly your ETT landmark.•Ensure the incubator or heat table is slightly higher than the oscillator to promote circuit drainage of rainout from the humidification.•Position the infant's body in alignment with the oscillator so that only the head is being moved when it is time for a position change.

2 •RepositioningShould be individually assessed on condition of skin integrity and infant physiological status. You will need a minimum of two people.•Gel mattress must always be used.•Do not disconnect tubing during repositioning.

3 •DisconnectionDisconnection is discouraged as it can cause alveolar collapse and loss of lung volume.•Use of Neopuff is discouraged unless mechanical failure or severe deterioration of infant's condition.

Nursing Management of the Baby Requiring HFOV

4 •Suction In-line suction must be used.•Press Stop button briefly on SensorMedics while briefly inserting and withdrawing catheter. PAW is maintained throughout.

• Rationale for pausing – The oscillator causes a pressure pulse in the airways. When suctioning if the sensormedics isn’t turned off the secretions get pushed back down because of this pulse pressure. So you are having ineffective clearance of secretions. There is also the potential of air trapping with active piston movement. 

5 •WeighingInfants should not be weighed on HFOV routinely.•Only weigh an infant if specifically ordered and discussed on ward round.•Always use a warm weigh scale.

6 •X-ray SensorMedics onlyTurn oscillations off at start/stop knob briefly while X-ray is taken•Remember if MAP is lost, the reset button will need to be held down to restore MAP and then you can press start again.

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