High Frequency Oscillatory Ventilation (HFOV) in Pediatrics

Review Article

INTRODUCTION

It is known that conventional ventilation is not withoutpotential harm to the lungs. The large volumes andpressures to which the lung is subjected during prolongedand difficult ventilation can damage the lung especiallythe one which is already compromised or diseased. Thereis a constant search for modalities of ventilation which arelung protective or less invasive.

High frequency ventilation refers to modes ofventilation in which supra physiologic rates (>60/min) areused for ventilation [1]. The advantages this confers to thepatient are manifold. The pressure swings in these modesof ventilation are much less than with conventionalventilation and this reduces the barotraumas to which thelung is subjected. The tidal volume for ventilation at theserates in less than or equal to the dead space in conventionalventilation. In conventional ventilation the tidal volumesare approximately 6-8 mL/kg and maybe as high as 10-12mL/kg. In the diseased lung and classically the lung withARDS the reduction in lung compliance leads to “stifflungs’’ and the delivery of this volume of air to the lungmay require very high ventilation pressures.

As with any other therapeutic modality in medicine

HIGH FREQUENCY OSCILLATORY VENTILATION ( HFOV) IN PEDIATRICS

Anita S BakshiSenior Consultant, Pediatric Intensive Care, Indraprastha Apollo Hospitals, Sarita Vihar, New Delhi 110 076, India.

e-mail: as_bakshi@yahoo.com

Conventional ventilation is not without its own set of hazards for the lungs. The concept of ventilator inducedlung injury (VILI) was defined in the 1960s and modes of ventilation which were likely to be less injurious to thelung were explored. HFOV is one such modality. In high frequency ventilation, as the term states, the lungs areventilated at very high frequencies (rates) with the aim to provide adequate minute ventilation through smallvolumes at high rates. The main clinical conditions in which HFOV has demonstrated benefits are situations inwhich there is refractory hypoxemia as in severe adult respiratory distress syndrome (ARDS) and those inwhich very high pressures are required for ventilation in lungs having air leaks as in patients withpneumothoraces or bronchopleural fistulae. Settings for this form of ventilation are different from those forconventional ventilation (CV). Improvement in oxygenation is early, consistent and sustained with HFOV. Themost pertinent decision remains as to whether it should be initiated or not, and, that whether it may actually bebeneficial to use it as an initial mode of ventilation. More studies are needed to give clear guidelines orprotocols. The basics of HFOV have been reviewed, where it helps, how it works, the initiation, management,weaning advocated, and, the likely complications.

Key words : Conventional Ventilation (CV), Fraction of inspired oxygen (FiO2), High Frequency OscillatoryVentilation (HFOV), Arterial Carbon Dioxide Levels (PaCO2), Peak End Expiratory Pressure (PEEP), ArterialOxygen Levels (PaO2), Adult Respiratory Distress Syndrome (ARDS), Mean Airway Pressure (MAP),Ventilator Induced Lung Injury (VILI).

mechanical ventilation is not without its own set ofhazards. Ventilation with high pressures can lead to VILIor ventilation induced lung injury.The mechanisms for theoccurrence of VILI are multiple and complicated. Highpressures in the alveoli leads to “barotrauma’’ whileoverdistension of the alveoli with large tidal volumes leadsto “ volutrauma’’ which can cause epithelial damage at thelevel of the alveoli. As a result of repeated distension andcollapse in each respiratory cycle the alveoli are subjectedto shear forces resulting in what is described as“atelectrauma”. Continued and repetitive cycles ofcollapse and inflation lead to injury with conventionalmechanical ventilation that can lead to the release ofinflammatory mediators which may worsen the lunginjury and also lead to multi- organ failure on a systemiclevel [2,3].

Theoretically HFOV can minimize thesemechanisms of injury

HFOV has basically emerged as an option for patientswith severe ARDS and refractory hypoxemia .It has beenshown to be safe and efficacious in improvingoxygenation. This effect has however not translated intobetter outcomes in terms of improved mortality in thesepatients.In the pediatric patients too, studies of elective

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Review Article

HFOV versus CV in preterm infants with RDS have beencontradictory and there is no clear evidence that HFOVoffers any clear advantages [4].

HOW DOES HFOV WORK?

The exact mechanism of gas transport in highfrequency ventilation in presently not clear. HFOVmaintains lung inflation at a constant elevated meanairway pressures (Paw ) while using a piston to cycle theventilation rate at several hundred times per minute. If gasflow to the lung is increased to more than 200 times theminute ventilation the lung parenchyma can be made tooscillate [4,5]. When the gas in the airways is oscillated athigh frequency, the airways begin to undergo spatialoscillation inside the chest. These oscillations arecomposed of periodic changes in the length and width,movements of the cureved or angular bronchi, and wavemotions of the bronchi.These result in shaking andsqueezing of the neighbouring parenchyma, resulting inintraparenchymal and interparenchymal gas mixing [6].This results in tidal volumes which are often smaller thanthe anatomical dead space of the lungs. Recent studies,most prominent being by the ARDS network group, haveshown that lover tidal volumes ( less than 6mL/kg ) help toreduce volutrauma to the lungs [7]. The smaller tidalvolumes of HFOV are consistent with this concept. Inaddition HFOV minimizes the cycle of alveolar distensionand collapse with each breath by maintaining airwaypressures throughout ventilation. The rapid pressurechanges created by the oscillating piston are attenuated atthe alveolar level thereby minimizing atelctrauma andimproving alveolar recruitment [3].

Other mechanisms involved in gas transport duringhigh frequency ventilation include accelerated axialdispersion, increased collateral flow through pores ofKohn, intersegmental gas mixing or pendulluft pheno-menon, Taylor dispersion ,asymmetrical gas flow profiles,and gas mixing within the airway due to nonlinearpressure-diameter relationship of the bronchi [1].

CLINICAL SETTINGS IN WHICH HFOV ISUSEFUL

1. HFOV should be considered in children withoxygenation failure refractory to conventionmechanical ventilation ( FiO2 ≥0.7, PEEP ≥15 cmH2O). Classically refractory hypoxemia is seen insevere ARDS (more often in Viral pneumonias thanwith bacterial especially with H1N1 infections,Dengue ARDS and children with febrile neutropenicsepsis to name the commonest settings in our clinicalpractice).

2. HFOV has been shown to support adequate gas

exchange with severe pulmonary interstitialemphysema (PIE ) in neonates [8,9].

3. Management of Bronchopleural Fistula. There is aconsistent improvement in the arterial blood gaseswhen conventional ventilation has previously failed[10,11].

4. Recent studies have suggested a role for HFV inchildren after cardiac surgery and with ARDS [12-16].

THE VENTILATOR

Many high frequency ventilators are presentlyavailable to us some being especially targeted for the useof the neonatal population, In most institutions the SensorMedics 3100A and 3100B ventilators are used for HFOV.The 3100A is meant for use in children under 35 kg weightwhile the 3100B is for adults and children over 35 kgweight (Fig.1).

PATIENT SELECTION

HFOV is not the initial or primary modality ofventilation in most situations. It is used when conventionalventilation is failing. Hence patients who are put onHFOV are likely to be patients on whom the conventionalventilation strategies to improve oxygenation have alreadybeen applied ie high PEEP, longer inspiratory times, proneventilation, etc.

These patients are likely being ventilated on high meanairway pressures and the “lung protective’’ ventilationrequirements e.g. p Plat <30 cm of H2O are not being met.

Fig.1 High frequency oscillatory ventilator

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39 Apollo Medicine, Vol. 8, No. 1, March 2011

So a reasonable threshold to consider changing to HFOVwould be a Pmaw (mean airway pressure, not peak airwaypressure) >24 cm of H2O [17]. Early institution of HFOVfor patients deteriorating on conventional ventilation maybe important for better survival rates with HFOV thoughthis has not been validated.

Patients with longer durations of ventilation or patientswith likely fibro proliferative phase of ARDS are unlikelyto do well with HFOV.

PREPARING THE PATIENT

1. Prior to starting the patient on HFOV it is imperativethat the patients airway be properly suctioned andknown to be patent.

2. If Bronchoscopy is required it should be performedprior to putting on the HF ventilator.

3. The patients volume status and haemodynamicsshould be assessed and evaluated given that thehigher Pmaw may lead to hypotension. Echocardio-graphy if needed for evaluation should also be doneprior to putting on high frequency ventilation as is itnot possible to do after patient is on HFOV.

4. Adequate sedation, analgesia and neuromuscularblockade must be ensured .

SETTING UP THE VENTILATOR – GUIDELINESFOR INITIAL HFOV SETTINGS

1. Prior to starting HFOV, it is advocated to perform arecruitment maneover on the oscillator by incrasingthe Paw to about 35 to 40 cm H2O for about 30-40seconds.

If there is any haemodynamic compromise abortimmediately.

Note: the oscillator should be OFF during thismaneover.

2. Set the initial Paw at 5 cm of H2O higher than whatthe patient was being ventilated on the conventionalventilator.

3. Set the power to achieve vibration from chest to mid-thigh (delta P about 35-45 ) In the adult patients thissetting is higher at about 65.

4. Set Hz at 6-7

5. I time is set at 33%.

6. The initial FiO2 is at 1.0. As soon as the oxygenationimproves the FiO2 may be reduced to lower values toachieve saturations in the 90s.

7. Bias flow is set at approximately 40 lpm. Childrenwith large broncho-pleural fistulae or endotrachealtube cuff leaks may not achieve the desired Pawwithout increases in bias flow. In such patients withsevere air leak syndromes very high bias flow (uptothe maximum of 60 lpm )may be set.

8. After putting the patient on HFOV a chest XRayshould be done to see the lung inflation – ideally 8-9ribs should be visualized on the chest film.

CARE OF THE PATIENT ON HFOV

Once the child is on HF ventilation there are severalimportant issues different from conventional ventilationwhich must be paid attention to. For most part they arerelated to nursing but the physician must closely supervisethem too. As is obvious chest rise as on conventionalventilation does not occur. One has to note the “ChestWiggle” – this chest wiggle is the continuous chestvibration which occurs when the child is on HFOV andmust extend from the chest to mid thigh. These must becarefully assessed periodically and especially afterrepositioning the patient.

The “Chest Wiggle” should be bilaterally symmetrical.Any change in the symmetry maybe indirect evidence of apneumothorax which is often difficult to pick up onHFOV. There is no role for auscultation while on HFOV.

Suctioning must be done with the “closed suctionsystem” without disconnecting the patient from theventilator. Ideally the child should be suctioned imme-diately prior to putting on high frequency ventilation andshould not need suction for the first 24 hours.Disconnecting for suction can lead to de-recruitment anddesaturation which often takes hours to improve. Whensuctioning with the closed system care must be taken towithdraw the catheter all the way out of the ET tube oncompletion of suction.

The sight of a child on the Sensor Medics ventilatorbeing “oscillated’’ can be extremely disturbing for thefamily to witness. Hence the family must be extensivelycounseled regarding the fact that the child will be deeplysedated and muscle relaxed while on this modality ofmanagement such as to prepare them.

VENTILATING WITH THE HFOV VENTILATOR

The HFOV ventilator uncouples oxygenation andventilation. What this means in practical terms is thathypoxia and hypercarbia can be managed independentlyof each other.

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OXYGENATION

The main parameter which determines theoxygenation is the Pmaw or the mean airway pressure. Atthe onset this is set at 5 cm of H2O higher than the numberon which the patient was being ventilated on theconventional ventilator. Patients who are haemodyna-micallv labile maybe put on pressure 2-3 cm higher andinitial hypotension managed with fluid boluses till thepatient stabilizes.The initial FiO2 is 1.0 which is thenreduced in increments of 0.05 when the saturations touch90% or more. If after initiating HFOV the saturations havenot improved within 1-2 hours the Pmaw is increased by 2-3 cm of H2O every 30-60 minutes to improve recruitment.Some patients may improve oxygenation slowly butgenerally oxygenation is a parameter which showsremarkable improvement on starting HFOV. Initialmanagement requires patience and close watch of vitalsigns. The highest pressures which can be achieved withthe 3100 B ventilator (which is used for children morethan 35 kg and adults) are in the range of 45-55 cm andthese are rarely required. Presuming that we start or switchto high frequency at MAPs touching 24 cm of H2O and usepressures 5 more at the onset the Pmaw is around 30 andincreases will generally be 2-3 cms, maybe once or twiceand hence Pmaw of 40 cm of H2O are rarely required.Hence management of oxygenation is:

1. If oxygenation worsens, the Pmaw may be increasedin increments of 2-3 cm of H2O slowly every 30 minstill the saturations touch 90%. The maximumpressure recommended in th adult patients are about50 cm of water. There are no clear recommendationsfor the pediatric patients .

2. Oxygenation may also be improved by increasing theFiO2.

2. VENTILATION

The main determinants of the CO2 elimnation are theAmplitude of Oscillation or the Delta P and the Frequencyor Hertz. The frequency (Hz) is in multiples of 60 andhence 3Hz means a rate to the tune of 180/min. Increasingthe δ P or the amplitude brings down the CO2. The rate isdealt with in a manner opposite to that in conventionalventilation. In CV increases in the rate lead to a washout ofCO2 but this is not true of HFOV. Decreasing thefrequency in high frequency ventilation will increase thetidal volume delivered to the lungs and will lower thecarbon dioxide levels. Increases in frequency increase thePaCO2.

For most part the there is no fixed number which canbe followed for setting the initial amplitude for ventilation

as we use in setting the Pmaw. The amplitude is setaccording to the “chest wiggle’’ that is the vibrationswhich are the hallmark of high frequency ventilation. Theamplitude should be such that vibrations are visible allover the chest and the abdomen and also the upper thigh.The initial setting is generally in the range of 35-50 cm ofH2O The initial frequency is set between 6-8 in thepediatric patient and at 10 in the newborn. in the pediatricpatient.

Initial frequency is usually set at 5 Hz.

Patients who show a rising level of CO2 should bemanaged with aggressive increases in the amplitude,which maybe increased by 10 at one go. At the onsetABGs maybe done at 20-30 minute intervals to achievethe desired PaCO2 and pH and subsequently may be doneat longer intervals. Changes in PaCO2 do not occur asquickly after changing settings in high frequencyventilation as they do in conventional ventilation so risingcarbon dioxide levels must be managed aggressively, nth

lowest value achievable (3 Hz on the SensorMedics3100B).

Theoretically worsening CO2 may be managed bydisconnecting the patient from the ventilator and manualbagging but this is not advocated as it leads to

1. Derecruitment of the alveoli and desaturation whichoften takes a long time to improve and

2. Manual bagging leads to hyperinflation of the lungsand barotrauma.

Hence if the Pa CO2 worsens

1. PaCO2 increases but the pH is more than 7.2amplitude may be increased to improve the washoutof carbon dioxide and

2. Reducing the Hz reduces the CO2. The lowest Hzsetting on the Sensor Medics 3100A and 3100Bventilators is 3 Hz.

WEANING FROM HFOV

Once the oxygenation improves bring down the FiO2gradually upto 0.4.

The next parameter to be reduced is the Pmaw which isreduced in increments of 2-3 over a period in which it istolerated and monitored by the blood gases.

Weaning should ideally be done every 4-6 hours.

Once the Pmaw is in the range of 22-25 with the FiO2at 0.4 the child can be considered for transition to

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conventional ventilation .

Can shifted to PCV at a rate of 20-25 , with a PEEP of12 and a p Plat < 30-35 cm of H20 to achieve tidal volumesto the tune of 6-7 mL/kg IBW, I:E 1:1, and the Pmawshould be 20 cm of H2O (+/- 2 cm of H2O)

The algorithm for HFOV for adult is given in Fig. 2.

THINGS TO WATCH OUT FOR

1. Endotracheal tube obstruction – As the frequency ofsuctioning is much less than on CV the ET can getblocked with secretions. Any reduction in the

Fig.2 High frequency oscillatory ventilation algorithm.

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vibrations and subsequent hypoxia must imme-diately be attended to.

2. Pneumothorax – The mean airway pressures duringHFOV are sustained and high. Hypercarbia withhypoxia must alert the physician to the likelihood ofa airleak. As auscultation is non – contributory thesymmetry of the chest vibration should be observedand a Chest XRay called for immediately.

3. Hypotension – Changes and deterioration inhaemodynamics are not uncommon when startingthe patient on HFOV. This is due to the fact that thehigh Paw interfere the venous return leading toreduced preload and resultant fall in blood pressure.Fluid boluses of saline are enough to correct this.

4. A number of studies have linked HFOV to trachealinflammation and a condition called NecrotisingTracheobronchitis. Adequate humidification ofinspired gases likely reduces the incidence of thiscomplication.

CONCLUSION

The conceptual advantages of using HFOV are smallertidal volumes, a constant, less variable airway pressureand the fact that different gas flow mechanisms mayimprove V/Q matching. Overdistension of the lung andcontinuing atelectasis on conventional ventilationcontribute to the progressive lung injury which arises notonly from the disease process but also from the impact ofthe ventilation used to support gas exchange during thecourse of the illness.

Several prospective randomized trials have comparedHFOV as the initial mode of ventilation with CV inpreterm infants who have HMD (Hyaline MembraneDisease) [21-24]. The results of these have beencontradictory and there is no clear evidence that HFOV,compared with CV, offers important advantages. Moststudies evaluating HFOV in term neonates have includedneonates with unresponsive respiratory failure who arecandidates for ECMO [25]. These studies show that 46-83% of these patients responded and avoided ECMO.HFOV is now frequently used as rescue treatment forhypoxemic pediatric patients. Some of the studies ofHFOV in patients failing CV strategies have demonstratedimproved oxygenation and gas exchange in pediatricrespiratory failure. Only one prospective, randomized trialof HFOV in pediatric patients who had respiratory failurehas been performed [26]. This showed that HFOV, usingan optimal lung volume strategy, results in significantimprovement in oxygenation compared with a conventio-nal ventilatory strategy designed to limit increases in peak

airway pressures. In addition HFOV was associated with alower frequency of barotraumas and improved outcome.In a cohort of pediatric patients submitted to HFOV afterfailure of CV, HFOV caused significant improvements ofoxygenation variables but children with pre-existing lungdisease had a higher rate of ECMO or death than patientswith no pre-existing pulmonary disease. (64% vs 38%)[26]. In a more recent retrospective study, survival in 19pediatric ARDS patients treated with HFOV wassignificantly better than predicted by the PediatricRespiratory Failure Score but could not be predicted usingseveral outcome scores or the oxygenation index in thefirst 24 hours [28].

So at the end of the day we know that in pediatricpatients with severe acute respiratory failure and diffuselung disease HFOV improves oxygenation parameters in asustained and rapid fashion.However more trials arerequired to prove its benefits over conventional ventilationand especially the timing of initiating high freqeuencyventilation.

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