Archives ofDisease in Childhood 1996;75:F135-F140

CURRENT TOPIC

Advances in neonatal conventional ventilation

Sunil K Sinha, Steven M Donn

Despite the implementation of several alterna-tive strategies for management of respiratoryfailure in neonates, such as extracorporealmembrane oxygenation, high frequency venti-lation, and nitric oxide, conventional assistedventilation remains the mainstay of treatment.This has traditionally been accomplished bytime cycled, pressure limited ventilators overthe past three decades primarily because of itsease of application and relative safety. How-ever, several recent advances, both in theconcept and the application of conventionalventilation, have been introduced for use inneonates. The introduction of newer, state ofthe art, microprocessor controlled ventilatorsystems provides clinicians with opportunitiesto apply a number of advanced ventilatorymodalities not previously available for treatingnewborns.Some of these techniques will need further

scientific evaluation in controlled trials, butthis should not preclude their use in clinicalsettings, as their safety has already been provedby "standard setters" for use in neonates.There is a firm physiological rationale for theiruse, and individual centres have alreadyacquired substantial experience in the applica-tion of these modalities.

In this paper scientific information and tech-nological advances leading to the newerconcepts of ventilatory management in neo-nates will be addressed, and clinical perspec-tives based on personal experience will be dis-cussed. Each section includes backgroundinformation, technical details, clinical applica-tion and experience, advantages and limita-tions, and future perspectives.

EquipmentRecent advances in microprocessor based, res-piratory technology have transformed standardmechanical ventilators. These are now highlysophisticated machines which offer the uniqueability to provide rapid and precise control ofgas delivery and ventilatory support in variousdifferent ways, while allowing for the monitor-ing and alarming ofvirtually every aspect of theprocedures involved.

Detailed technical description of these me-chanics is complex and beyond the scope ofthis article, but salient features relating to indi-vidual components of ventilator machinery are

described. As these features may be unique tospecific ventilators, it is imperative that theuser familiarises him or herself with theindividual ventilator specifications, while se-lecting the appropriate device. For this reason,it is always helpful to consider the individualventilator's mechanics in the following se-quence: (a) user interface; (b) gas flowpatterns; and (c) the various approaches usedto monitor and alarm patient-ventilator func-tions. This format allows the various devices tobe compared in a uniform manner.The user interface is the interaction between

the clinician and the ventilator. Unlike the pre-vious machines, which only displayed datarelating to ventilator function as set by the cli-nician, the current generation of microproces-sor controlled ventilators can display outputdata from the patient, thus making the assistedventilation much more precise. The systemthat governs the kinetics of gas flow in the ven-tilator circuit from the point of its entry,through the breathing circuit, and to its exitthrough the exhalation system, is an importantconsideration in terms of ventilator perfor-mance and patient safety.The recent generation of ventilators use a

number ofmethods to regulate inspiratory flowand manipulate the inspiratory flow pattern.Besides automatic adjustment of flow deliveryto meet the patient's need, features available onthese newer ventilators include "educated"inspiratory flow delivery, improved breathtermination criteria, guaranteed tidal volumedelivery, availability of bias flow to maintainstable PEEP, air leak compensation, andbuilt-in system adjustments to prevent pres-sure overshoot. (Product information, VIP-BIRD Product Corp., Palm Springs, Califor-nia, USA)'With the increasing trend toward coordinat-

ing mechanical breaths with the patient's owninspiratory effort (synchronised or patient trig-gered ventilation) (PTV), ventilators mustrespond effectively at minimal effort from thepatient, especially if they are to be effective invery low birthweight babies. Most of the venti-lators are triggered by signals measured withinthe ventilator circuit, but the particular point ofmeasurement varies. Ideally, these sensorsshould be nearer to the patient's airway-at theproximal endotracheal tube, for example- asthe potential for dysynchrony increases the fur-

This is the first of twoarticles in the series.

Neonatal Services,South ClevelandHospital,Middlesbrough,TS4 3BWSK Sinha

Section ofNeonatal-PerinatalMedicine,Holden NeonatalIntensive Care Unit,University ofMichiganMedical CenterAnn Arbor, Michigan,USASM Donn

Correspondence to:Dr Sunil Sinha.

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ther away the sensor is from the patient.2 Thetrigger signal can be impedance (thoracic orabdominal), pressure, or flow. The two impor-tant ingredients of successful triggering are"magnitude" (the amount of patient effortrequired to trigger) and "delay" (the time inbetween the initiation of patient effort and therise in pressure at the proximal airway). Flowtriggering seems to be more efficient than pres-sure triggering.2 Flow derived signals also allowfor complete synchronisation in both inspira-tory and expiratory phases ofbreathing. Newermachines, such as the VIP BIRD ventilator,have a device called "termination sensitivity"which stops the inspiratory phase of mechani-cal breath when inspiratory flow decreases to5-25% of peak flow, thus triggering the expira-tory phase. When this mode is used, the babycustomises his/her own inspiratory time. Thisis particularly helpful in smaller babies whoexhibit relatively short inspiratory times whilebreathing at a fast rate. A fixed longer inspira-tory time in such cases is likely to cause inver-sion of the inspiratory:expiratory ratio, andhence increase the risk of air trapping. One ofthe problems with measuring flow at the proxi-mal endotracheal airway is the risk of false trig-gering (autocycling). This is caused by theaccumulation of water in ventilatory circuit, ora leak around the endotracheal tube which isincorrectly "sensed" as patient effort. To elimi-nate this, a number of newer ventilators use"automatic purge" or "leak compensation"systems (VIP BIRD Corp).Important applications of microprocessor

technology to current ventilators are theprocessing of pressure, flow, and volumesignals; calculation of lung mechanics; andpresentation of the results in numerical,graphical, analog, or digital displays. Severalcommercially available ventilators have op-tional monitoring packages which displaywaveforms and pulmonary mechanics (loops).However, those devices with integrated, on-line capabilities for breath-to-breath continu-ous analysis and display have the advantage.3These measurements, also available as trendsover a longer period of time, facilitate ventila-tory management and will be described in aseparate paper.Thus the latest generation of ventilators

offers a new breed of sophisticated machineswhich have the flexibility of multiple modeselection, more effective methods of allowingthe patient to work in synchrony with themachines, and improved safety compared withtraditional pressure ventilators. This trend

Table 1 Summary ofimprovements in design ofnewerventilators

Flxibility ofmode selection on a single venlaor:Pressure limited, time cycled (PTV or SIMV/CPAP)Volume cycled (PTV or SIMV/CPAP)Pressure support ventilationPressure limited, time cycled, and high frequency

oscillationImprowed pneumatics and triggering:

Sensitive triggering devices for both inspiration andexpiration

Leak compensation system and auto-purging to eliminateauto-cycling"Educated" inspiratory flow deliveryImproved breath termination criterisAvailability ofdemand and bias gas flowGuaranteed tidal volume deliveryMaintenance of stable PEEP/CPAP

Safety features:Built-in algorithms to self-correct "inappropriate"

settingsIntegrated alarm and safety valve systemSystem accuracy by built-in automatic zero calibration

functionOn-line pulmonary function monitoring:

Inspiratory and expiratory tidal volume monitoringGraphic displays ofvolume, pressure, and flow

waveformsPressure volume and flow volume loops

toward increasing sophistication is likely tocontinue and clinicians would be well advisedto familiarise themselves with these changes.The salient features of these newer devices aresummarised in table 1 , and a brief comparisonof the commonly used neonatal ventilators isgiven in table 2.

Strategies for mechanical ventilation innewbornsTIME CYCLED PRESSURE LIMITED VERSUS VOLUMECYCLED VENTILATIONBecause of newer designs and concepts, it isuseful to understand some specific nomencla-ture currently used to present and interpret thedata relating to ventilatory management. Posi-tive pressure breaths delivered by a mechanicalventilator are described by three variables:trigger variable (pressure, flow), which initiatesinspiration;limit variable, which governs gas delivery (pres-sure, flow, or volume target that cannot beexceeded during inspiration);cycle variable (pressure, flow, volume, or time)which terminates the breath.Thus in pressure limited (targeted) modes, a

peak inspiratory pressure is set, and duringinspiration gas flow is delivered to achieve thattarget pressure. After the target is reached flowdecelerates in an exponentially reducing man-ner to maintain pressure at the target until theinspiratory phase is complete. If the patient

Table 2 Comparison ofsome commonly used neonatallinfant ventilators

Ventiaor Manufacturer Modes Signal Comments

Babylog 8000 Driger (Lubeck, Germany) PTV Airway flow Hot wire anemometerSIMV Graphic displays, combined HFOV

Sechrist/SAVI Sechrist Industries (Anaheim, CA) PTV Thoracic Real time analogue outputimpedance

SLE HV2000 Specialised Laboratory Equipment PTV Airway Combined HFOV(Surrey, UK) SIMV pressure Graphic display

VIP-BIRD Bird Products Corporate Airway flow Variable orifice differential pressure(Palm Springs, CA) PTV Airway transducer

SIMV pressure Volume cycled or time cycled ventilationPSV Partner volume monitor and on-line

graphic display monitor

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Table 3 Suggested ventilatory management protocol

Volume cycled Time cycled pressure limited

Start in PTV mode Start in PTV modeProvide proximal inspiratory tidal volume of 5-8 ml/kg to Adjust PIP to provide 5-8 mi/kg of inspiratory tidal volume to

maintain "target" ABGs maintain "target" ABGsWean by reducing rate but continue to deliver tidal volume of Wean by reducing PIP as tolerated, but continue in PTV with

5-8 ml/kg. When rate < 40 bpm, switch to SIMV with PS control rate of at least 20-40 bpm(minimum 10 cm H2O)

Use flow to adjust inspiratory time of 0.3-0.5 seconds Inspiratory time 0.3-0.5 seconds use breath termination"criteris" to achieve full synchronisation in inspiratory andexpiratory phase

should inspire more forcefully, flow acceleratesto maintain target pressure. This "adjustable"flow feature of pressure limited breaths mayseem advantageous in terms of improvedpatient ventilator synchrony, but because ofthis gas delivery format, tidal volume isvariable on a breath-to-breath basis.

In contrast, with volume limited modes, theprimary gas delivery target is tidal volume, andpeak inspiratory pressure may vary from breathto breath.4 This has been a source of concernto clinicians in the past, but it should berealised that peak pressure measurements are areflection of airway flow (and resistance) andnot necessarily the alveolar pressure, which isprimarily determined by compliance andalveolar volume.5 Thus, in volume modes ofventilation, peak inspiratory pressure may"wean"5 automatically as the lung complianceimproves either spontaneously or in responseto surfactant treatment.

Moreover, the presumption that pressurelimited ventilation causes less barotrauma thanvolume ventilation because of the lower peakinspiratory pressure, may be an oversimplifica-tion and is not supported by scientific evi-dence. Ifventilator induced lung injury does, infact, result from "volutrauma,"' it may beargued that control of tidal volume deliverymay have an advantage in neonatal ventilationbecause of rapidly changing pulmonary com-pliance. Interestingly, the first ventilator de-signed and built exclusively for neonates was amodified version of an adult volume cycledventilator, which was eventually discardedbecause of technological limitations. However,the development of microprocessor technologyand availability of precise flow and pressuretransducers has produced substantial improve-ments in ventilator design and performance,allowing its safe application in babies weighingas little as 1200 g. This is based on our ownexperiences with over 300 neonates treatedwith volume cycled ventilation in our twoneonatal units, using the VIP-BIRD infant/paediatric ventilator (Bird Product Corpora-tion, Palm Springs, California, USA). Forthose babies receiving time cycled pressurelimited ventilation, we adjust and monitorthe ventilatory parameters using tidal volumeand minute ventilation as two of the indexvariables. Table 3 describes our unit protocols.Unfortunately, many of the neonatal unitsstill do not use tidal volume monitoring as apart of their ventilatory management proto-col despite the fact that it is now widelyavailable.

SYNCHRONISED VENTILATIONSynchronised ventilatory modes are character-ised by delivery of mechanical breaths inresponse to a signal derived from the patient'sspontaneous inspiratory effort (patient trig-gered), and are available in both pressurelimited and volume cycled ventilators. Collec-tively, these modes of ventilation are a form ofpartial ventilation where the patient has to con-tribute to the work of breathing.The modes of synchronised ventilation

include:

Assistlcontrol ventilation (A/C),also known asPTV, is a combination mode in which the ven-tilator delivers a positive pressure breath inresponse to the patient's inspiratory effort(assist) provided it exceeds a preset thresholdcriteria. This mode also provides the safety ofguaranteed mechanical breath rate set by theoperator (backup rate) if no patient effort isdetected (control). The backup control rateensures a miniimum mandatory minute ventila-tion in case the patient stops making inspira-tory efforts. It is probably the best mode to usein the premature infant in the acute phase ofillness because it requires the least amount ofpatient effort, and produces improved oxygen-ation at the same mean airway pressure, asshown in recent studies.`9 Earlier clinical expe-rience, however, using different ventilators(SLE Newborn 250, Drager Babylog 8000,and SLE HV 2000), indicated that prolongedsupport of very low birthweight infants ( < 28weeks) might not be feasible because of patientfatigue. "" Factors adversely affecting patienttriggered ventilation in these studies includeddevelopment of expiratory asynchrony, longtrigger delays, very short inspiratory times, lowgestational age and use of PTV early in thecourse of disease. The difference in resultsfrom the above studies may be machinespecific, as despite improvement in the soft-ware, some of the ventilators seem to performless well in one mode than another. The Baby-log 8000, for example, was more likely to trig-ger in SIMV (synchronised intermittent man-datory ventilation) rather than in assist/control." The overall evidence, however,suggests that patient triggered ventilationreduces the duration of time from weaning toextubation in mechanically ventilated neo-nates, and may have a beneficial effect on sec-ondary outcome measures, such as chroniclung disease and intraventricular haemor-rhage."91' This, however, requires verificationin larger studies.

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Synchronised intermittent mandatory ventiation(SIMV) refers to a ventilatory mode where themechanically delivered breaths are delivered ata fixed rate but are synchronised to the onset ofthe patient's own breath, resulting in fullinspiratory synchrony. However, unless theinspiratory times are identical, the patient mayterminate his/her own effort and begin exhala-tion while the ventilator is still in theinspiratory phase, resulting in partial asyn-chrony. Unlike A/C, in SIMV the ventilatorysupport can be varied according to the ratefixed by the clinician from full (higher rate) topartial (lower rate) assisted ventilation. Theflexibility of SIMV in providing a range ofven-tilatory support levels makes it useful, both as aprimary means of ventilatory support or as amethod for weaning.'4 However, a low setSIMV rate is undesirable when the patient'sventilatory demand is high. Similarly, reducingthe SIMV to a very slow rate (less than20/minute) may be unwise when discontinua-tion of mechanical ventilation is imminent, asthis may impose significant breathing work foran intubated baby, contributing to weaningfailure.'5 This disadvantage of slow rate SIMVcan be compensated by some further means ofbreath support such as pressure support venti-lation.

Pressure support ventilation (PSV) can bedefined as patient initiated, pressure targeted,and patient controlled ventilation which is gen-erally flow cycled.'6 This is designed to assistthe patient's spontaneous breathing with aninspiratory pressure "boost." Although mostlyused as a weaning mode, this can be used as aprimary modality in patients with acute orchronic ventilatory failure if they have suffi-cient respiratory drive. In such patients, PSVreduces the imposed work of breathing,created by the resistive forces of endotrachealtubes, the ventilatory circuit, and demand valvesystems. In PSV, once the breath is triggered bypatient inspiratory effort, a preset system pres-sure is rapidly achieved and maintainedthroughout inspiration by adjustment of ma-chine inspiratory flow. The inspiration endswhen the inspiratory flow falls below a presetvalue, usually determined as a percentage ofdelivered volume or flow. Although PSVclosely resembles A/C ventilation, it seems thatbecause of its unique design, PSV is better cus-tomised to support and synchronise with

patient effort because the patient has thecontrol of both the inspiratory flow rate andinspiratory time.'7 The main role of pressuresupport ventilation is to assist respiratorymuscle activity and so reduce workload. Thereduction in the respiratory muscle load is pro-portional to the level of pressure given. As inany pressure targeted ventilation, tidal volumein PSV depends on respiratory mechanics andmay vary. To overcome this disadvantage, someof the newer ventilator designs have combinedpressure support with a guaranteed minimumtidal volume-"volume assured pressure sup-port" or "volume support."'8 The use of PSVin neonates and infants is poorly researched,but information from adult studies can beapplied. In most cases, PSV is used in conjunc-tion with volume cycled SIMV during the pro-cess of weaning. By boosting the spontaneousbreaths according to the set pressure level, PSVnegates the disadvantages of SIMV at low rate.PSV, when combined with SIMV, lowersoxygen consumption and shortens the dura-tion of weaning.'920 We have reported a seriesof successful cases of neonatal volume cycledventilation followed by PSV (abstract pre-sented at Joint meeting of the Irish PerinatalSociety and the British Association of PerinatalMedicine, Dublin, October 1994; Sinha et at).In these babies volume cycled ventilation wasused as a rescue modality, because conven-tional time cycled ventilation was proving inef-fective in providing optimal oxygenation. Wewere able to wean each of these babies success-fully using PSV in conjunction with SIMVwithout any air leaks or radiological findings ofchronic lung disease.Another application for it may be in infants

who are chronically ventilatory dependent, andin those with bronchopulmonary dysplasia.2'The rationale to use any form of assisted

ventilation (A/C, SIMV, or PSV) should be tooptimise patient/ventilator synchronisation,improve patient comfort, facilitate or reducethe duration of ventilation, and avoid undesir-able respiratory/cardiovascular consequences.At present, strict comparisons of the usefulnessof the above modalities remain untested. Table4 compares the features of the newer ventila-tory modes.

Continuous positive ainray pressure (CPAP) hassome major advantages in babies with suffi-cient drive in that it can improve oxygenation

Table 4 Comparison of newer ventilaory modes

Time cycled Vohme cycled

Modes PTV SIMV PTV SIMV PSV

*ParametersRate P C P C PPIP C C P P CPEEP C C C C CTi P/C P/C P/C P/C P/CFlow C C C C PSpontaneous breath Triggers mechanical Between mechanical Triggers mechanical Between mechanical Triggers mechanical

breath breaths breath breaths breathFlow Continuous Continuous Demand Demand VariableBest patients Weight < 1300 g, Weaning Weight < 1300 g, Weaning Weaning, chronic lung

acutely ill severe respiratory disease, highfailure inspiratory resistance

* describes whether parameter is controlled by patient (P), clinician (C), or both (P/C).

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and reduce breathing work. The first is achieveby improving the functional residual capacity(FRC), which determines the amount of freshoxygenated gas available to the alveoli duringthe resting phase of respiration. This is accom-plished either by increasing the volume ofpatent alveoli within their limits of distensibil-ity or by inflation of the previously collapsedalveoli (alveolar recruitment). CPAP also helpsto redistribute interstitial water, thus improv-ing the oxygen diffusion across the alveolar-capillary membrane, and is thought to contrib-ute towards maintaining the integrity ofpulmonary surfactant. The increase in FRCpermits more tidal volume per unit of pressure,which helps to maintain an adequate minuteventilation and reduce the breathing work.22 Insome respects the function of CPAP is similarto the action of surfactant, and physiologicallyefficient application of CPAP makes sense butrequires the appropriate machinery to main-tain effective and constant pressure control.Design characteristics of a device must, there-fore, be carefully considered in terms of theireffect on airway anatomy and function, airwayresistance, and the work of breathing beforedeciding to use them clinically.Although almost all mechanical ventilators

designed to provide conventional ventilationprovide the option of CPAP in both pressurelimited and volume cycled modes, its applica-tion through an endotracheal tube may, in fact,be counterproductive in terms of increasedwork of breathing, unless supported with apressure "boost" (as in PSV), which is not yetavailable in pressure cycled ventilators. Newerdevices dedicated to CPAP, such as the InfantFlow Nasal CPAP System (EME Ltd, UK),seem to be designed to ensure stable airwaypressure and reduced work of breathing whichshould avoid respiratory fatigue in very smallbabies. Specially designed nasal prongs seem toreduce irritation and ensure a constant seal;intranasal pressure monitoring provides accu-rate assessment of airway pressure measure-ments (product information, EME Ltd,UK). 23

Most studies of CPAP have focused on itsphysiological effect, but only a few deal with itseffect on clinical outcome when used either intherapeutic or supportive roles. Recent ran-domised trials assessing the role of nasal andnasopharyngeal CPAP in neonates as a wean-ing mode, by using predetermined criteria,have produced conflicting results. Larger con-trolled trials are needed to investigate the safetyand efficacy of this modality (product informa-tion, EME Ltd).2325

Future directionsMicroprocessor based technology has started anew era of mechanical ventilation that allowsfor the provision of ventilatory support in sev-eral ways not previously available for neonates.The trend toward increasing sophistication andgreater versatility is likely to continue, and cli-nicians involved in the care of sick infants mustkeep abreast of these developments. As for thenewer methods of ventilation, their clinicaloutcomes will require scientific validation

despite inherent difficulties in conducting suchcontrolled trials in critically ill neonates,because the optimal pattern of ventilation var-ies even in a single infant, presumably reflect-ing the change in disease process. Moreover,although respiratory failure in neonates repre-sents severe lung injury, it is often associatedwith a systemic process which may be associ-ated with other organ and tissue injuries. Anynew respiratory treatment per se is unlikely tohave a total impact on mortality or morbidity.It is also important for the clinicians to realisethat despite improvements, conventional venti-lation will still fail in certain specific situationswhere alternative treatments such as ECMO,high frequency ventilation, or nitric oxideadministration may have potential benefit.While mechanical ventilation ofthe newborn

is more complex than it has been before, it hasnever been safer.

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