High Frequency Oscillatory Ventilation

It is worthwhile considering HFOV as one of the modalities for managing our ARDS patients

"In 1970s ,we used to ventilate our patients with high tidal volumes, but in 2000 we prefer lower tidal volume as a lung protection strategy" - Dr Prasad Rajhans Chief Intensivist Deenanath Mangeshkar Hospital Vice President ISCCM

ARDS Definition

The patient should fulfill the following criteria:

• Pao2/Fio2 < or = 200mmHg ARDS.
• Pao2/Fio2 < or = 300 mm Hg ALI.
• Onset- Acute. lBilateral Or Interstitial infiltrates. lPCWP< or =18 or no clinical evidence of increased left atrial pressure.

ARDS Mortality

The intensivists all over the world are trying to develop various strategies to treat Acute Respiratory Distress Syndrome (ARDS) patients. Even in the best of the ICU with the best of the strategies, the mortality of ARDS is still between 41 to 65 per cent.

A large scale, randomised controlled trial sponsored by National Institute Of Health and conducted by the ARDS Network compared low tidal volume 6ml/kg to conventional 12ml/kg. Mortality was significantly lower in low tidal volume patients (31 per cent) compared to conventional (40 per cent)- an absolute reduction in mortality by 9 per cent . The ARDS net trial was one of the landmark trials which showed significant decrease in mortality of ARDS patients with lung protective strategies.

ARDS Net Protocol

• Calculate Predicted Body Weight in Kg.
• Men=50+ 0.9 [Height cm-152.4].
• Women= 45.5+ 0.91[Height cm-152.4].
• Ventilator Mode Volume Cycle, Assist Control.
• Tidal Volume- Initial 8ml/kg PBW.
• Reduce to 6ml/kg over 2-4 hr if ventilation adequate.
• Goal inspiratory plateau pressures<30 cm H2O; reduce Vt to as low as 4ml/kg.
• Oxygenation; Pao2 goal=55-80 or pulse oximetry saturation 88-95% with use of minimal amount of PEEP to keep Fio2, 0.6.

ARDS Ventilation

The various strategies that are used for ARDS are prone position ventilation, inverse ratio ventilation, recruitment maneuvers, nitric oxide therapy, extra- corporeal membrane oxygenation, Partial Liquid ventilation and High Frequency Oscillatory Ventilation (HFOV). Nitric oxide therapy has been proved to be more useful in neonates. Extra corporeal membrane oxygenation is not very easy to implement. Recruitment Maneuvers although useful have their own limitations.

We use ARDS net protocol, recruitment maneuvers, prone position and/or HFOV for our ARDS patients in our ICU at Deenanath Mangeshkar Hospital, Pune. HFOV has been initially tried in neonates and paediatric patients with good success. HFOV is now been proved to be safe in adult patients as well. We have been using HFOV for adults in our ICU since 2004.

Why HFOV?

Volutrauma, shear stress and biotrauma cause the ventilator induced lung injury. In 1970s ,we used to ventilate our patients with high tidal volumes, but in 2000 we prefer lower tidal volume as a lung protection strategy. In 1970 we were reluctant to use high PEEP but in 2000 we are comfortable with the PEEP of 14 cm of H2O and even up to 20 cm of H2O. HFOV would be a natural progression of these developments over the years as we use very low tidal volumes and high mean airway pressures in HFOV.

On the pressure volume curves the area of overdistension causes e dema fluid accumulation, surfactant degradation, high oxygen exposure and mechanical disruption. The area of derecruitment causes atelectasis, repeated closure / re-expansion, stimulation of inflammatory response, inhibition surfactant, local hypoxemia and sompensatory overexpansion. HFOV is in the safe window.

How HFOV Works?

We all have seen dogs panting and the tidal volumes in panting are very low. Similarly in HFOV, the volumes are very low and yet the gas exchange takes place. There are various theories that have been put forth to explain the gas exchange, but the truth is HFOV works with improved oxygenation and wash out of carbon dioxide in ARDS patients. The various theories are Taylor Dispersion, Convection Ventilation, Asymmetrical Velocity Profile, Pedeluft, Cardiogenic Mixing and Molecular Diffusion.

HFOV Jargon

High Frequency Oscillatory Ventilator is very easy to use. It increases the Functional Residual Capacity with a super CPAP system. HFOV has only a few parameters that need to be set up. It has a blender to set up FIO2. Frequency determines the oscillations per minute. Frequency is between 3 to 15 Hz. That is 180 to 900 oscillations per minute. Amplitude to determine the movement of the diaphragm in the machine. Bias flow to determine the gas flow in the circuit. Mean Airway Pressure (MAP) which determines the oxygenation. A knob to set up inspiratory time. There are standard protocols to initiate and wean off HFOV. There are alarms that also need to be set. The settings are modified as per the consecutive ABG reports

Which Patients?

Initially HFOV was used as a rescue therapy for ARDS when nothing else worked. Now it is recommended to use HFOV as a lung protective strategy. If the PEEP is more than 12 cm H2O, Plateau pressures are more than 30 cm of H2O and FIO2 is more than 0.6 on a conventional ventilator, HFOV should be considered.

Initial Settings with HFOV

• Use sedation & muscle relaxation lGive Attention to volaemic status lFiO2 = 1.0 lFrequency 6Hz lMAP = CMV MAP + 3-5cmH2O lPower = 60 to 90 cm
• Inspiratory time = 33.

Next Steps

• Adjust power until wobble factor seems right.
• Recruit lung by increasing MAP.
• Stepwise construction of MAP v oxygenation plot.
• Recruitment manoeuvre and set MAP at best PO2.
• Set optimal MAP.
• Adjust power and frequency according to PaCO2.
• Aims are FiO2<0.6, Sats 88-90%, pH 7.2.

Issues in HFOV

• Position lSedation/paralysis lHumidification
• Suctioning/physiotherapy lMonitoring lAirway management lFeeding lUnfamiliarity lNoise

Weaning Criteria

• Initial insult ameliorated lFiO2 < 0.5 lPO2/FiO2 >200
• MAP < 18cm lPermissive CO2

How to Wean?

• Thorough Airway suction. lBronchoscopy or Change ET /TT.
• Off sedation, Spontaneous breathing lPressure Support Ventilation with PEEP~ 7-8 cm.

Indian Experience

We have ventilated 31 adult ARDS patients with HFOV at Deenanath Mangeshkar Hospital since 2004. We had classified them as responders and non-responders to HFOV. Responders were defined as patients who were successfully weaned from HFOV as well as from any ventilation to a state which required no ventilatory support for more than 12 hrs.

Non Responders were defined as patients who could not be weaned from ventilatory assistance due to severe hypoxia. We had 17 Responders and 14 Non-responders. In the Responder group, seven patients recovered completely and were discharged and others died due to organ failure. For these seven patients, who survived HFOV made a difference between life and death.

Conclusion

With the ARDS mortality so high and very few options to offer to our patients, it is worthwhile considering HFOV as one of the modalities for managing our ARDS patients.

prajhans@gmail.com