High-frequency ventilation in neonates

n Neon

Department

of Anesihesioiogy,

Study Objective: ventilation

by computer

MEWLINE,

Study

Studies

application

complications,

ventilation

high-frequency

the group frequent suggested

Address reprint requests to Dr. Tsuzaki at :he Department of Anesthesia, Massachusets General Hospital, 32 Fruit St., Boston, MA 02114, USA.

Library of identified

to physiologic

background

the physiologic

reports

effectiveness and

tria,i were evaluated

tests of highs-frequency

was applied

of chronic

ventilation,

lower than that used in convenofp~~mo~a~

to infants

pulmonary

seve-ral reports supported

multiple

the prior

basis, clinical randomized

might reduce the frequency

ventilation

Although

occurrence

ar;r,d clinical

individual&.

that airway pressure

the development

treated

and 1990 were idendata base, arl:cLes.

of MedicineS

a curfe-ent concept.

benefkial

trial involving

Fellow in Anaesthesia, Harvard Medical School acd Massachusetts General Hospital, Boston, MA; Research and Clinical Instructor, Department of Anesthesiolspy, Meio University, Tokyo; Japan

bibliographies

related

Data concerning

it was considered

to decrease.

status o~~~~g~~~requen~y

between i980

the Nationat

were selected

tional mechanical syndrome,

*Research

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of Data Synt esis: In early clinical

es&s

Japan.

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and used to develop

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Tokyo,

care.

Publications

and bJ searching

Data Extraction:

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respiratory

searches

Selection:

of neonatal

University,

To prouzde a brrefreview

in neonatal

Data Identification: tified

Keio

barotrauma.

with respiratv/y

distress

~o~~~icati~~s aLso was expected this h~pot~es~s, a recent controlled

clinical

centers did not find sign~fi~a~t improvement Rather, they recognized with high-f re q uency ventilation. of ~ompl~ca,t~ons associated use of conventional

this study design requires further

with high-freq~e?~~~l ventilation

ventilation.

studies to elucidate

However,

a possible

in the and

defect of

the source of thefe conflict&g

results.

Conclusions:

As a mode of mechanical

useful for maintaining pulmonary

fistula

as an alternative

ventilation

or during

ventilation,

in patients

bronchoscopic

to conventional

ventilation

high~freq~e~,~y ventilation 0 syrndrcme or bronchowith air le examination. ut in general, its role still remains

~o~tr~~l~~sia~.

ronc~Q~u~m~~ary clysplasia; b~gb-~re~~~~cy ventilation; yWC9dS: trachea; respiratory distress syndrome; neonatal diseases and abnormalities.

Received for pubhcation April 12: 1990; revised manuscript accepted for publication Quiy 20, 1890. 0 ;99(! Bu:terworth-Heinemartn

n the past two e%ecac!es, there have been physiologic basis of the new category of m

ny efforts to jnvestigate the rmlcal ventilation generically

Orgird

Contributions

termed high-frequency ventilation (HFV). As early as the mid- 19 lOs, it was suggested that tidal volume (V,) less than dead space volume (V,) could result in gas exchange, but its mechanism was unknown and there was no attempt to apply this principle in mechanical ventilation until 1959, when Emerson’s patented device was invented for vibrating portions of a patient’s airway. The next application was introduced in 1967 by Sjjiistrand and coworkers, who developed high-frequency positive pressure ventilation (HFPPV) by means of endotracheal insufflation and found that FPPV was able to provide adequate ventilation as well as oxygenation in spite of lower airway pressure.’ Since &en, with the support of subsequent studies, it has been generally accepted that, as long as respiratory frequency is high enough, normal gas exchange can be well maintained even with a very small V,. In the field of neonatal respiratory care, a marked improvement has been observed in the survival rate of infants suffering from pulmonary disorders. Although the widespread use of mechanical ventilation contributed largely to this progress, it also was accompanied by the increased frequency of severe pulmosuch as barotrauma and nary complications, bronchopulmonary dysplasia (BPD). In both cases, exposure to higher airway pressure, required to expand diseased lungs, has been considered one of the main predisposing factors. To reduce such risk, it is strongly recommended that airway pressure be maintained at the lowest possible level. HFV has been used as an alternative to conventional ventilation (GV), which does not necessarily satisfy the above requirement. The first success of HFV in neonates with res(RDS), reported by piratory distress syndrome Heijman and Sjiistrand,’ promoted clinical application of various modalities of HFV, which grew into one of the major issues of mechanical ventilation for neonates.

When attempts are made to understand the physiologic background of HFV or to evaluate its clinical efficacy, confusion arises from the too broad concept of a diversified spectrum of ventilatory modalities. Aside from this confusion, however, HFV is defined as a mode of mechanical ventilation used with a combination of supraphysiologic frequency Ii) and V.,.less than V,. As a consequence, in neonates, respiratory frequency above 1 Hz (60 breaths/minute) can be considered to fall in this category, though it seems reasonable to use this term at f 2 4 Hz to clearly distinguish j. CPin. Anesth., vol. 2, November/December 1990

it from Cd. Iw. contrast, it cannot be argued &at smalk V, appears in the above definition because technical difficulties prevent its precise measurement in the pediatric population. Based on this simplified definition, FV can be classified further into either sigh-frequency positive pressure ventilation (HFPPV), high-frequency jet ventilation (HFJV), high-frequency flow interr (MFFI), or high-frequency oscillation ( classifications are determined primarily of the ventilator mechanism to deliver gas to the patient, with each exhibiting special flow characteristics. Frequency range also is determined by such intrinsic mechanical properties, and considerable overlap exists among them. For exa ) frequencies ranging from 60 to 150 (HFPPV), 60 to 600 breaths/minute 900 breaths/minute (HFFI), or 300 to 3,000 breaths/ minute (HFO) may be used during each ventilatory -mode indicated in parentheses (Fig-ure 1).

by Sjj8strand, 3ut sevMFPPV was originally design ave been mad.e since, eral technical modifkations including open systems for endoscopic procedm-es and closed systems for volume-controlled ventilation. In general, V, is delivered by the insufflation technique with or witbout a closed valve on the expiratory limb,

1. ,Frequency range used

Figure

of neonates.

(Hz)

for mechanical

is plotted

positive

pressure

ventilation;

WFFI

ventilation;

HFJV

=

high-frequency

= high-frequency

oscillation.)

ventilation

in logarithmic

conventional ventilation; HFPPV

/CV =

HFO

Frequency

scak

= high-frequency

= high-frequency flow

jet

interruption;

which determines whether 1 e system is open or closed. assively. The frequency range is red with other ventilatory modes, and higher frequency affects the pressure waveform are shape. Conventional ventilators deliver volume-controlled HFPPV, but one should pay careful attention to the mechanical limitatioas of these ventilators, as listed in Table 1.

n IIFJV (figure 2), gas is delivered to the end of the endotracheal tube via a small-diameter ‘ector catheter. Inspiration begins with injected jet ow, which may cause gas entrainment from side flow ffecc. The volume delivered strongly ventilator settings and downstream as airway resistance. The presence of a large back pressure may impede both gas entrainment and jet flow. As with IIFPPV, expiration depends only on elastic recoil of the respiratory system. A major drawback of this ventilatory mode is the inability to provide appropriate humidification, which may be related t.o the reported higher frequency of the fatal complication necrotizing tracheobronchitis

ventilation by interruptmg gas ssure source with a ball or solenoid valve; this e can be considered a hybrid of The difference between HFFI and is the absence of gas entrainment. There is also humidification in IIFFI. Expiration occurs pas-

os~tn~ ~XSSCX Ventilation ;XFPPV)

Using Conventiona!

Ventilators

Mechanical limitations High internal compression

Stroke volume varies with fr~-equency Less efficient valve system Upper limit of frequency .4t most 2.5 Hz Expirdtory valve acts as resistance Inadvertent positive end-expiratory

/ PRESSURE

SOURCE (A:R 8 02)

SIDE FLOW

PEEP VAL\IE

Figure 2. Schematic diagram of a high-frequency jet ventilator. A high-pressure source supphes both injected gas and side flow. Humidifkation is given by an infusion pump li,r injected gas and by a humidifier for side flow. The expiratory limb connects to the PEEP va endotracheai lube; PEEP valve = positive end-expiratory pressure valve.) I---

BALL 0, SOLENOID VALVE

0 circuit consists of an oscillator and a fresh gas source, as shown in Figure 4. V, is delivered by an oscillator while fresh gas is continuously supplied as bias flow. In contrast to the other ventilatory modes, expiration is not passive in IIFO. Therefore, expiratory time can be shortened to empty the lung, and the rapping gas in the periphery may be less sean in the other modes. The pressure waveform is genedly sinusoidal, so the inspiratory-toexpiratory time ratio is equal to unity. The frequency can be increased to nearly 50 Hz, though such a high frequency seldom is used.

pressure (PEEP)

/

EXPIRATORY LIMB (highsi Impeddence,

ADJUSTABLE

VALVE

Figure 3. Schematic diagram of a high-frequency flow interrupter. Humidified gas is supplied via a ball or solenoid valve, which chops inspiratory fkrw. The expiratory limb las a higher impedance than the patient. An adjustable valve controls the distending pressure. /ET tube = endo:racheal tube.)

PRESSURE SOURCE (AIR 8 02)

FRESH GAS FLOW OSCILLATOR (PISTON or SPEAKER)

I!t

LOW-PASS FILTER

Figrare 4. Schematic diagram of a high-frequency oscillatory ventilator. Tidal volume is delivered by means of a piston or loudspeaker, while fresh gas is supplied as bias ilow through a humidifier. Expiration is active. The expi-atory limb works as the low-pass filter, which exhibits high rmpedance with increasing frequency. (ET tube = endotracheal tube.)

Although these ventilator ferent ventilatory conditions,

modes may create it is necessary

dif-

to un-

their common mechanical background, which governs overall ventilator performance. A recent report4 testing neonatal high-frequency ventilators of various designs, including jets, flow interrupters, and oscillators, showed similar results. It was found that, at a given resistance and compliance load, delivered V, increased with endotracheai tube size and decreased with frequency regardless of a given elastic load. This finding suggests that the mechanical performance of these ventilators might be strongly affected by downstream impedance-i.e., the patient’s condition. Such common findings provide a frame of reference when comparing the results of one study with those of others.

how gas exchange rakes place. any &borate models have been developed to interpret the underlying mechanism that maintains adequate alv lion in spite of a smaller V,. As a result, port mechanism responsible has b considered as (1) direct alveolar ve vective streaming due to directiona Pendelluft resulting from the lack of homogeneipj of peripheral impedance, or (4) augmented dispersion caused by a combination of axial convection and radial mixing.5 First, some aiveolar units located in proximity to the airway opening may receive fresh gas even if V, is smaller than V,, thereby contributing to overall gas exchange. Second, the to-and-fro movement of gas creates skewed velocity profiles, which promote the dispersion beyond the initial boundary. Secondary flows, flow separation, and turbulence further affect this mechanism in a complicated manner rd, nonhomogeneous local lung mechanisms It in an asynchronous flow distribution, causing interregional gas mixing. Consequently, it is believed that all these mechanisms are at work in various degrees during HFV, though the relative importance of each mechanism is not yet fully understood.

derstand

ic

Classic

respiratory

asi

physiology

based on the bulk flow concept,

explains

gas transport

as alveolar

ventilation

(di,) equals the product of respiratory frequency the arithmetic difference between V, and V,:

an

lJ* = ,f-. (V, - V,) HFV, where V, approaches or becomes less than V,, this equation is no longer valid in explaining

During

390

$. Ciin. Anesth., vol. 2, November/December

1990

In GV, lung expansion can Se estimated from zmeasurement of the airway pressure. uring HFV, however, airway ssure does not always convey reliable information. ot only is the site of pressure measurement critical, as in the case of CV, but also misinterpretations

sometimes

arise

from

ignoring

the

frequency response of the total pressure measusement system, which depends on the lung mechanics. fn addition, large pressure discrepancies may exist between the proximal airway and the aiveoirrs,” re‘nflation and subsequent sulting in a peripheral hy ssure. As a further comincreases of intrathoracic plication, the pressure dis ution differs among the peripheral regions, thought to result from unequal flow distribution in asymmetrical branching systems.’ ip Setween regional gas transport and lung expansion, distributions in healthy and unilaterally lung-lavaged dogs were measured by a two-dimensional positron camera with a newly developed imag’ capnic FIFO, frequencythe distribution of gas t were found, reflecting transport mechanism. Unilateral lung Eavage, by decreasing regional compliance, caused the changes in

e regiomi V,FJ, ratio, thereby strongly affecting the regional efficiency of gas transport. These findings might be important in the ventilatory management of the lungs with nonuniform regional compliance, fucb as in S or in diaphragmatic hernia.

HFV is considered to have nating from its key featureto keep airway pressure lower than during CV. First, lower airway pressure is expected to decrease the frery barotrauma, sue FV may minimize the monary disorders o sing factors have been postulated n toxicity, or barotrauma due to sustained high airway pressure. Third, in air leak syndromes (e.g., pneumothorax, pneumomediastinum, and pulmonary interstitial emphysema) and in broncbopulmonary fist&, iower airway pressure may help decrease gas leaks and thus increase effective ventilation. These potential advantages over CV have attracted neonatal pulmonologists’ attention to the clinical trial of IIFV in various pathophysiologic conitions, including IX, pulmonary air leak syndromes, and congenital diaphragmatic hernia.

ositivi?

ressure Ventilation

y lower ventilatory rate (60 to brea~hsimi~~t~)~ an initial report of IIFPPV in treat ent of neonates with RDS showed succes results of adequate ventilation.’ Later HFPPV studies, some of which were done with conventional ventilators at frequencies beyond 60 breaths/minute and did not use the term HFP V, demonstrated a reduction in airway pressure and the decreased frequency of neumothorax.lO~ll Since these frequencies were not far from those of CV, bowever, it is not easy to identify ifference between them.

IIFJV, originally designe for ventilation during surgical procedures relating to the airway, was first introduced in neonates with respiratory failure by Pokora et al. in 1983.!* They observed a marked improvement in gas exchange, such as a reduction of the alveolar-

wteria; partial pressure of oxygen (P ifference improved carbon di some cases, the success o be development of the fatal complication NTB and subsecguent tracheal obstruction.‘” oth inadequate humidification of the delivered gas d the extremely high velocity of jet flow have been attribut to the possible patbogenesis. NT served in other HFV modes, as well as in rapid rate mechanical ventilation, suggesting a ~~~ti~acto~~al rather ban a single etiology.14

HFFI was first introduced as a hybrid of HF0.15 The application of HF and pulmonary interstitial emphy Frovement in gas exchan at a 1 anging mean pressure. S and decreased emphysema could st cases. Even though better humidification could be supplied, a rece report demonstrate a higher frequency of N eban of CV or XF8.‘6

!n 198 1) Marchak et al. l7 reported a controlle :,f short-term HE0 with RDS infants. They found an improvement in oxygenation, but the data suggesting he efficacy of HFO were insufficient. A more recent controlled trial involving ten chical centers showed that there were no particular imp ts in overall aspects when HFO was deliver-e S infants.18 Contrary to expectations, the frequency of air leaks was significantly higher in WFO (45%) than in CV 138%). Other serious complications such as intravenricular hemorrhage (IVH) an periventricular leuVC) also were re gnized to be more HFQ (49%) than in CV (42%). During be study, these increased complications served in he HFO group were clear1 onstr d by the higher rate of crossover from to CV (26%) than (17%). Alt~o~~b these resul

! ~re~ti~atio~ is critical,

where bigher opening

orer alveolar recruitment ssure is a crucial factor.

3riganaL Contributions

In premature baboon experiments,‘Y immediate application of HFQ at a high mean airway pressure showed significant improvement in oxygenation, reduction of mortality, and reduction in the development of BPD. These results were, however, accompanied by the adverse effects of high airway pressure. Overdistension of the lung was recognized -more frequently, and increased intrathoracic pressure caused the reduction of venous return and the resultant decrease of cardiac output. The increased frequency of both neurologic complications and hepatic cellular damage could be explained by increased venous pressure. Therefore, in the clinical application of HFO, careful monitoring and prompt treatment are required to reduce these complications.

5. Chang HK: Mechanisms of gas transport aa’uringven-

tilation by high-frequency oscillation.J~~~lP~~~s~ol:Respk Environ Exercise PhysZol1984;56:553-63. 6. Fredberg JJ, Keefe DH, G!ass GM, Castiie KG, Frantz ID III: Alveolar pressure nonhomogeneity during smallamplitude oscillation. J A@ Physiol: Respir Environ Exercise Physiol 1984;57:788-800.

7. Ailen JL, Frantz ID III, Fredberg JJ: Heterogeneity of mean alveolar pressure during high-frequency oscillations. J Appl Physiol: Respir Environ Exekse Physiol 1987;62:223-8.

8

9

:0.

FV in the neonatal population have provided many intriguing results, ranging from the early success of HFPPV to the recent disappointment of HFO. It may be too early to judge the superiority of one result over the other. Closer matching of experimental and clinical data are indispensable for evaluating the eventual usefulness of HFV. Furthermore, one should pay attention to a combined strategy of HFQ and CV, which may yield a better outcome. Considering the other fields of application the value of HFV as an alternative to CV should not be underestimated. For example, HFJV may be the best ventilatory technique when the patient receives endoscopic examination of the airway, and it is also useful in some cases of cardiopulmonary resuscitation. Gonsequently, the final grading of HFV should be left until all ambiguities are clarified.

JonLon A, aberg PA, Sedin G, Sjostrand iJ: igh-r’requency positive-pressure ventilation by endotracheal insufflation. Acta Anaesthesio2 Stand 197 1;43(Suppl): I-

43. IIeijman K, Sjostrand U: Treatment of the respiratory distress syndrome: preliminary report. @pusc Med 1.974;19:235--44. Boros 8J, Mammel MC, Goleman JM, et al: Neonatal high-frequency jet ventilation: four years’ experience. Pediatrics 1985;75:657-63. Fredberg JJ, Glass GM, Boynton BR, Frantz ID III: Factors influencing mechanical performance of neonatal high-frequency ventilators. J Appl Physiol: Respir Environ Exercise Physiol 1987;62:2485-90.

392

J. Clin. Anesth., vol. 2, November/December

1990

Fox BJ, Tsuzaki K, Simon BA, Males CA, Venegas JG:

Relationship between regional lung expansion and gas transport during high-frequency oscillatory ventilation (HFOV) [Abstract] FASXFEBJ 1990;4:k422. Tsuzaki K, Hales GA, Venegas JG: Regional effects of high frequency oscillatory ventilation (IIFOV) in unilaterally lavaged canine lungs [Abstract]. fA%rZB 9 1990;4:A422. Heicher DA, Kasting DS, Marrod JR: Prospective ciinical comparison of two methods for mechanical ventilation of neonates: rapid rate and short inspiratory time versus slow rate and long inspiratory time. i Pediat-f !981;98:957-61.

11. Eyal FG: At-ad ID, Godder igh-frequency positive-pressure ventilation in neonates. Cril Care Med 1984;12:793-7. 12. Pokora T, Bing D, Mammel M, Boros S: Neonatal higbfrequency jet ventilation. Pediattics 1983;72:27-32. 13. Boros SJ, Mammel MC, Lewallen PM, Goleman _JM? Gordon MJ, Ophoven J: Necrotizing tracheobronchitis: a complication of high-frequency ventilation. g Pediatr 1986;109:95-100. 14. Mammel MC, Ophoven JP, Lewallen P Sutton MG, Boros 8J: High-frequency ventilation tracheal injuries. Pediatrics 1986;77:608-13. 15. Frantz ID III, Werthammer J, Stark AR: quency ventilation in premature infants with lung diseases: adequate gas exchange at low tracheal pressures. Pediatrics 1983;71:483-8. 36. Wiswell TE, Clark RH, Null DM, Kuehi TJ, deLemos RA, Goalson JJ: Tracheal and bronchiai injury in bighfrequency oscillatory ventilation and high-frequency flow interruption compared with conventional positivepressure ventilation. J Pediatr 1988; 112:249-56. 17. Marchak BE, Thompson WK, Duffy P, et al: Treatment of RDS, by high-frequency oscillatory ventilation: a preliminary report. J Pediatr 1981;99:287-92. oscillatory 18. The HIFI Study Group: high-frequency ventilation compared with conventional mechanical ventilation in the treatment of respiratory failure in pretrrm infants. N En&J Med 1989;320:88-93. 19. deLemos RA, Coalson JJ, Gerstmann DR, et al: Ventilatory management of infant baboons with hyaline membrane disease: the use of high-frequency ventilation. Pediatr Res 1987;21:594-602.