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INDICATIONS

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					HFOV Guidelines, PICU,QMC. Reviewed January 2007 Dr Sharmila Nair Dr Megan Smith

HIGH FREQUENCY OSCILLATION VENTILATION High frequency oscillatory ventilation is a method of mechanical ventilation that uses fast respiratory rates and tidal volumes less than the anatomic dead space. HFOV differs from conventional ventilation by delivering an almost constant distending pressure with active inspiratory and expiratory phases and maximising CO2 removal primarily through facilitated diffusion, allowing the use of lower tidal volume and lower pressures. Three mechanisms summarises the principles behind gas exchange in HFOV. These are the pendulluft effect, convection streaming and molecular diffusion. Because of novel gas exchange mechanisms, HFOV can provide adequate gas exchange using extremely small tidal volumes and maintain high end-expiratory lung volume without inducing over-distension which should result in minimising ventilator induced lung injury. INDICATIONS  Severe parenchymal lung disease with high oxygenation index unresponsive to conventional ventilation. Consider HFOV when OI >15. OI= MAP  FiO2 100 PaO2 (kPa)  7.6 Failure of conventional ventilation in CO2 removal. Consider HFOV when peak inspiratory pressure(PIP> 30) To minimise Ventilator induced lung injury (VILI) in ARDS. Pulmonary air leaks do better in oscillatory mode of ventilation

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CAUTION    In shocked patients, ensure they are “well filled” ( CVP>12).Optimise perfusion and blood pressure. Patients with gas trapping (bronchiolitis, asthma) require closer monitoring. Patients with raised ICP- the ICP can go up as high mean airway pressure can impede jugular venous flow

PARAMETERS Frequency(Hertz)    Breathing rate or frequency. 1 Hz = 1 breath per second (60 bpm) Frequency depends on the size of the child and can vary between 6-15 Hz. Increasing the frequency decreases the piston displacement which in turn decreases both P (vT) and inspiratory/expiratory time. The end result is decreased CO2 clearance.

Mean Airway Pressures  Mean airway pressure recruits and maintains lung volume. Small adjustments can result in significant changes in lung volume.

Amplitude (P or power)   The volume of gas per breath observed by the chest vibration or wobble Increasing the amplitude is akin to increasing the tidal volume and will result in increased CO2 removal. A small increase will cause a large effect on vT.

Inspiratory time  Bias Flow  % of time spent on active inspiration

Important determinant of MAP and CO2 washout. A small increase will significantly increase MAP.

SETTING UP THE OSCILLATOR  Refer to the educational package on setting up the circuit and checking the oscillator Step 1. Choose the right ventilator. Although Sensormedics A can ventilate up to 35 Kg , for patients >20 kg use Sensormedics B as this can give more power. Then Plug into electrical sockets and connect gas hoses to supply outlets. Step 2. Calibrate system prior to initiating HFOV. See the side of the ventilator for instructions. Step 3. Initial settings.   FiO2 MAP 100% +3-5 cm of H20 above the MAP on conventional ventilator.Turn the MAP limit and MAP adjust knob to the maximum. Then turn the MAP limit knob anticlock wise to 5 cm

above the treatment setting. Then rotate the MAP adjust knob anticlockwise to the desired MAP. There is no MAP limit knob in SensorMedics B.  Frequency Up to 10 kg weight 10-20 kg weight >20 kg 20 LPM Depending on the size of the child aiming to get a “mid thigh wobble”. In a neonate start at 30-35 and go up to achieve the a “ mid thigh Wobble”. In a 70 kg child start at 60-70. set to 33% Set alarm limits +/- 3 cm H2O around set MAP Start at 37 C (main dial) and 0C on the little dial. 10-15 Hz 8-10 Hz 6-10 Hz

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Bias Flow Delta P

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I time Alarm limits

Humidifier

Step 4

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Ensure patent airway; since only proximal airway pressure is monitored, no alarm will go off in the event of an obstruction or restriction. Optimise blood pressure and perfusion. If there is a significant leak around ETT it may need to replaced with a larger ETT. A larger ETT results in greater distal pressure waveforms and a greater reduction in arterial pCO2. However, a leak around the ETT allows for escape of CO2 into the larynx, and therefore less dead space. Does he/she need neuromuscular blockade? Optimise sedation. Although it is possible to oscillate when the child is spontaneously breathing neuromuscular blockage and optimising sedation prior to initiating oscillatory mode of ventilation is necessary in the initial stage. Position the oscillator at the top end of the bed and make sure the tubing is sloping down from the patient towards the oscillator. If the tube is sloping towards the patient water may run to the patient end. If the tube is kinked it may impede the oscillations

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Ensure the mean pressure required is achieved via the oscillator prior to commencing the actual oscillations (do this by depressing the “reset” button and holding until the required MAP is achieved, only then commence oscillating by pressing start button). Centre piston using the piston-centring knob. Prior to institution of HFOV suction patient well and then via the T piece, give one 10 second recruitment inflation of about 5cm of H20 above current PIP. Closed in line suction should be put into the circuit prior to connecting the patient to the oscillator. In-line suctioning allows one to maintain ET tube patency without letting the MAP fall with the subsequent loss in lung recruitment.

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Step 5  Connect the oscillator to the patient. Press and hold the reset buttonto pressurize the system before then pressing the start/stop button to commence oscillation. Assess the „wobble‟. Increase delta P till you see a mid thigh „wobble‟. If the O2 saturations do not come up to the desired level (usually 88-92% in a patient with bad lungs) increase the MAP by increments of 1 to achieve the desired saturation bearing in mind other factors which can affect ventilation like over-dissention and reduced cardiac output.

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MONITORING  Airway Since only proximal airway pressure is monitored, no alarm will go off in the event of an obstruction or `restriction. ET tube position should be checked regularly Check for changes in pitch/rhythm of delivered breaths. Check chest wobble and changes in chest wobble frequently. To assess lung expansion. Optimal inflation is 9 posterior ribs. CXR should be performed within half hour to one hour after initiation of HFOV. As lungs heal they become more inflated; hyperinflation may lead to pneumothorax. Regular chest films must be taken to determine improvement or decompensation as guided by clinical situations. With in half an hour after initiation of HFOV. As dictated by clinical situation and then ½ hour after each setting change.

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CXR

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ABG

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Haemodynamic status Increased thoracic pressures impede venous return and cardiac output. Further fluid volume and inotropic support may be needed. May need to turn down the MAP to achieve

cardiovascular stability. Arterial lines are necessary when a patient is placed on HFOV.

TROUBLE SHOOTING

Problem PaO2

Think? ?Pneumothorax

Action -Asymmetrical chest vibration/ -CXRChest drain

?Overinflation

Decrease MAP

?Underinflation

Increase MAP

?Underperfusion

-Give fluids, -consider inotropic support

PaCO2

? Mucous plugging/ Blocked ETT

Suction /physiotherapy

? Overdistention

Decrease MAP

?Pneumothorax

CXR chest drain -Increase P by 2-3 to optimise wobble -Consider reducing the frequency by 0.5 Hz increments to min of 5 Hz after discussing with the Consultant. -Consider the need for SensormedicsB if P>50 -Increase %inspiratory time ( rarely done)

Above excluded

PaCO2

Decrease P by 2-3 Increase frequency

Disconnection from the Disconnection can ventilatortheoretically cause accidental/following alveolar collapse and loss physio and suctioning of lung volume. This is a controversial point as there is no evidence behind this

It may be necessary to „bag‟ ventilate, give sustained inflation for 10 seconds and then recommence oscillation. Re recruit by putting the MAP by 2-3 cm and come down quickly once Sa02 are in the desirable range.

ONGOING CARE  Closed suction: When the child is first placed on the oscillator , he/she is usually very sick, and it should not be necessary to perform in-line suctioning during the first 12 hours. Thereafter, when the patient is more stable, suctioning should be performed ideally twice a shift, unless otherwise instructed by the medical staff. A good indication that secretions are accumulating is that one may see the PaCO2 begin to rise. The patient should ideally never be disconnected from the oscillator. If the patient is disconnected for some reason procedures for commencing oscillation should be employed. In-line suctioning does not completely maintain MAP with in the circuit during suction. It is also not a substitute for proper physiotherapy and suctioning. If necessary the patient will need to be bagged, with the appropriate amount of PEEP, and re-recruitment manoeuvres used on returning to HFOV.  Humidification: The oscillator uses a lot of water from the humidifier, be aware that the humidifier dome will probably need filling at least hourly. Set humidifier main dial at 37C (regulates the temperature of the inspired gas), and the little dial set at 0C (regulates temperature at the patient).One should see „misting‟ on the expiratory limb of the tubing. If there is condensation or „rain-out‟ on the inspiratory limb it means that the system is losing humidity and the temperature on the main dial should be increased to 38C and reduce the little dial to -1C and so forth (39C-2, 40C –3).  Water trap: Ensure that the trap is not emptied below the level marked. The water acts as a seal in the circuit

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Keep a spare ventilator set up and checked, available by the bedside. If the child has been changed from conventional ventilation, you can use that vent, but do not leave a wet, dirty circuit on (encouraging bacterial growth etc). Either have the vent stripped and a new circuit put on (and labelled as such), or just put a new circuit but ensure this is labelled as for that patient only. Ensure the vent is checked and ready to use if needed. Spare circuit: As a precaution have a spare oscillation circuit at the bedside. Pressure area care: It may be difficult to turn the patient, the patient may not tolerate turning. Tilting is probably a better option. It is possible to oscillate prone. Paralytic agents: These may cause fluid retention. Although these agents need to be used initially, there is no indication for its use for extended period of time. If tolerated by the patient it is possible to oscillate a patient who is spontaneously breathing.

WEANING  FiO2 Wean FiO2 aiming for SaO2 between 88-92%. Once it comes down from 1.0(100%) to 0.4-0.6 (40-60%) one can reduce the MAP by 2 cm at a time. HFOV can be discontinued by changing to conventional ventilation, CPAP or removing ventilatory support completely. In practise most patients are placed back on conventional ventilation when the MAP is 14-16 cm H2O. FAILURE OF OSCILLATION Before condemning HFOV as a failure, assess the state of inflation, circulation carefully. Assess the need for Nitric oxide. Discuss with Consultant on-call earlier than later about rising OI or no improvement in OI. HFOV does not always rescue the situation. One may need to go back on conventional ventilation and try different ventilator strategies.

References