Neonatal Mechanical Ventilation - Middle East Critical Care Assembly

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Neonatal Modes of Mechanical Ventilation

Mohammed M. Tamim, MDConsultant, NICU, Tawam Hospital

Alain, UAE

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Objectives

Indications of mechanical ventilationBasics of respiratory mechanicsModes of conventional ventilationModes of HFVIndications of HFV

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Introduction

The primary objective of Mechanical Ventilation is to support breathing until patient respiratory efforts are sufficient.First mechanical ventilation for a neonate in 1959.One of the most important breakthroughs in the history of neonatal care.Mortality from RDS decreased markedly after MV.New Morbidity developed CLD (BPD)

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Respiratory Failure

Hypercapnic Respiratory Failure:Inability to remove CO2 by spontaneous breathingCaused by hypoventilation or severe V/Q mismatch

in arterial PCO2 in pH

MV is most commonly needed for treatmentHypoxemia

Usually the result of V/Q mismatch or R L shunt.Diffusion abnormalities & hypoventilation (apnea)

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Respiratory failure can occur because of diseases in the lung, thorax, airway or respiratory muscle.Indication for assisted ventilation:

Respiratory Acidosis pH < 7.2Hypoxemia while on 100% O2

Or CPAP of 60 – 100%Severe apnea

Respiratory Failure

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Respiratory Failure

Clinical Manifestation:Increase or decrease in respiratory rate.Increase or decrease in respiratory effort.Periodic breathing with increase respiratory efforts.Apnea.

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Neonatal Respiratory Physiology

Compliance:-Distensible nature of lungs and chest wall.

Volume (L)= ----------------------

Pressure (cm H2O)

Neonates have greater chest wall compliance.( premature more than FT)Premature infants with RDS have stiffer lungs (poorly compliant lungs).

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Resistance:-Property of airways and lungs to resist gas.

Δ Pressure (cm H2O)= --------------------------

Δ Flow (L/sec)Resistance in infants with normal lungs ranges from 25 to 50 cm H2O/L/sec.It is increased in intubated babies and ranges from 50 to 100 cm H2O/L/sec.


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Resistance

Total respiratory system resistance =

chest wall R (25%)

+ airway R (55%)

+ lung tissue R (20%)

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Time Constant:An index of how rapidly the lungs can empty.Time constant = Compliance X Resistance

In BPD time constant is long because of resistance.In RDS time constant is short because of low compliance.Normal = 0.12-0.15 sec


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Time Constant

Inspiratory time must be 3-5 X time constantOne time conststant = time for alveoli to discharge63% of its volume through the airway.

Two time constant = 84% of the volume leaves Three time constant = 95% of volume leaves.

In RDS: require a longer I time because the lung will empty rapidly but require more time to fill.

In CLD: decrease vent rate, which allows to lengthen the I time and E time.

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Time Constant

Waldemar A. Carlo et. Al. Neoreview Dec 1999

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Conventional Mechanical Ventilation

Mechanical ventilators achieve a pressure gradient between the airway opening and lungs.Ventilator for neonates are usually one of the following types:

Pressure control VentilatorsVolume control Ventilators

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Pressure Controlled Ventilators:A constant flow of gas pass through the ventilator.Pressure is limited to the desired magnitude.When expiration relief valve has been closed for the preset period of time, the valve opens and inspirations ceases.

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Volume Controlled Ventilators:A preset volume of gas is delivered to the system after which inspiration is terminated.When this gas has been delivered by the piston inspiration is terminated.Infants TV (4-8 ml/kg)Volume losses by leaks from tubing system around the endotracheal tube.


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Continuous Positive Airway Pressure

Important tool in management of neonatesIncrease alveolar volumeRedistribution of lung water

Cons ProsRisk of air leak Alveolar Volume & FRC

Over Distension Alveolar Stability

CO2 Retention Redistribution of lung fluids

Cardiovascular impairment Improved V/Q matching

Compliance

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Pressure Controlled Ventilation

Ventilator Components:1. Gas mixer.2. Inspiratory expiratory time adjustment3. Expiratory relief valve to limit the peak inspiratory

pressure4. Pressure gauge to measure applied pressure5. Humidification or nebulization6. Positive end expiratory pressure to maintain functional

residual capacity.7. Exhalation assist to reduce the end expiratory pressure8. Alarms

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Peak Inspiratory Pressure (PIP):Changes in PIP affect both PaO2 & PaCO2 by altering the MAP.Increase in PIP:

Increase in PaO2Decrease in PaCO2

A high PIP should be used cautiously because it may increase the risk of volutrauma air leak and BPDCommon mistake “large babies need higher PIP” requirement is strongly determined by compliance


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Positive End Expiratory Pressure

Adequate PEEP prevents alveolar collapse and maintain lung volume at end of expiration.Improve V/Q matchingVery high PEEP reduce venous return cardiac output decrease oxygen transport increase pulmonary vascular resistance

ConsProsIncreased Risk for air leaks

Alveolar volume & FRC

OverdistentionAlveolar stability

CO2 retentionRedistribution of lung water

Cardiovascular impairment

Improved V/Q matching

Decreased Compliance


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Rate:Change in rate alter alveolar minute ventilationHigh rate low TV is strongly preferredRate change alone with constant I:E ratio do not alter MAPAny change in inspiratory time that accompany change in rate will alter MAP

CONSPROS

Gas trappingAir leak

Generalized atelectasis

Volutrauma

Maldistribution of gas

CVS side effects

resistanceRisk of Pulmonary edema

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Inspiratory Expiratory Ratio:-in I:E ratio lead to in MAP

Long inspiratory time:Pros:

Increased oxygenation

May improve gas distribution

ConsGas trapping

Increased risk of volutrauma and air leak

Impaired venous return

Increased pulmonary vascular resistance


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Inspiratory Expiratory Ratio:Short Inspiratory time:

Pros:Faster weaning

Decreased risk for pneumothorax

Allows use of higher ventilator

Cons:Insufficient tidal volume

May need high flow rate


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Inspiratory & Expiratory Time

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Mean Airway Pressure

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FIO2:Changes alter Alveolar Oxygen Pressure

Flow:Not well studied in infantsMinimal effects on ABGIn general 8-12 LPMHigh Flow is needed with short inspiratory time to achieve adequate TV.


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Oxygenation

Depends largely on the FIO2

Oxygenation increase linearly with increase in MAP.MAP is a measure of the average pressure to which the lungs are exposed.

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Carbon Dioxide Elimination

Depends largely on the amount of gas that passes in and out of the alveoli “Minute Ventilation”.Minute Ventilation = TV X RateAny increase in TV or Rate will eliminate CO2.TV may be increased by in PIP or in PEEP

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Tidal Volume

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Alternative Modalities of MV

Technology advances including improvement in flow delivery systems, breath termination criteria, stability of PEEP, air leak compensation, prevention of pressure overshoot and triggering system led to development of new modalities of mechanical ventilation:

Patient Triggered Ventilation (PTV)Proportional Assist VentilationTracheal Gas Insufflation

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Patient Triggered Ventilation

Modified CMVNeonate is able to initiate ventilatory breath by:

Abdominal motionChest wall impedanceAirway flow

Great degree of synchronacy between patient and ventilator

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Modes:Synchronized Intermttent Mandatory Ventilation (SIMV):

Preset rate that is triggered, other patient breath is not assisted.

Assist Control Mode (A/C):All breath initiated by patient is triggered.Weaning accomplished by reducing PIP.


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Advantages:Reduction in cerebral blood flow variability

(Renie et al 1987)

Shorter time on ventilator(Visveshwara et al 1999)

Improved oxygenation with SIMV(Cleary et al 1995)

No difference between SIMV & A/C in length of weaning.

(chon et al 1994)


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Proportional Assist Ventilation

Synchronize onset & duration of both inspiratory and expiratory support.Ventilatory support is in proportion to the volume and flow of the spontaneous breath.It will reduce ventilatory pressure while maintaining or improving gas exchange.

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Tracheal Gas Insufflation

Reduce anatomical dead space in alveolar minute ventilationGas is delivered to the distal part of the endotracheal tubeResult in decrease in PCO2 and PIPStill under study

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Preparation for MV

Establishment of artificial airwayTracheal intubation oral vs. nasalExamine and continue assessing your patientUse a manometer when baggingFollow HgB O2 saturation continually & ABGsInsure that nebulization is adequateUnderstand the effect of every ventilator knobSelect ventilator sitting that is appropriate for your patient

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Suggested CMV Sittings

HMDNormal Lungs

18-2512-16PIP

4-52-3PEEP

20-4020Rate / min

0.60.5Inspiratory Time

0.4 – 1.00.21 – 0.3FIO2

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High Frequency Ventilation

Definition:Ventilation at a high rate at least 2 – 4 times the natural breathing rate, using a small TV that is less than anatomic dead space:

Types:High Frequency Jet Ventilator (HFJV)

Up to 600 breath / minHigh Frequency Flow Interrupter (HFFI)

Up to 1200 breath / minHigh Frequency Oscillatory Ventilator (HFOV)

Up to 3000 / min

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IntroductionThe respiratory insufficiency remains one of the major causes of neonatal mortality. Intensification of conventional ventilation with higher rates and airway pressures leads to an increased incidence of barotrauma. Either ECMO or high-frequency oscillatory ventilation mightresolve such desperate situations.Since HFOV was first described by Lunkenheimer in the earlyseventies this method of ventilation has been further developedand is now applied the world over.

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Definition

There are three distinguishing characteristics of high-frequency oscillatory ventilation:

The frequency range from 5 to 50 Hz (300 to 3000 bpm)

active inspiration and active expirationTidal volumes: about the size of the deadspace volume

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Commercial ventilators

Various technical principles are used to generate oscillating ventilation patterns.

The so-called "true" oscillators provide active inspiration and active expiration with sinusoidal waveforms:

Piston oscillators move a column of gas rapidly back and forth inthe breathing circuit with a piston pump. Its size determines the stroke volume, which is therefore fairly constant. A bias flow system supplies fresh gas

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The "flow-interrupters" chop up the gas flow into the patient circuitat a high rate, thus causing pressure oscillations. Their power,

however, depends also on the respiratory mechanics of the patient

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Commercial ventilators

Other devices (e.g. Sensormedics 3100A) generate oscillations with a large loudspeaker membrane and are suitable also beyond the neonatal period. As with the piston oscillators, a bias flow system supplies fresh gas.However, this device cannot combine conventional and HFO ventilation.

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The InfantStar interrupts the inspiratory gas flow with a valve bank. Some authors regard this device as a jet ventilator because of its principle of operation.

The Babylog 8000 delivers a high inspiratory continuous flow (max 30 l/min) and generates oscillations by rapidly switching the expiratory valve. Active expiration is provided with a jet Venturi system.

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Characteristic parameters andcontrol variables of HFV

Three parameters determine oscillatory ventilation:Firstly, there is the mean airway pressure (MAP): around which the pressure oscillates. Secondly, the oscillatory volume: which resultsfrom the pressure swings and essentially determines the effectiveness of this type of mechanical ventilation. Thirdly, the oscillatory frequency: the number of cycles per unit of time.

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Mean Airway Pressure (MAP)

The Babylog 8000 uses a PEEP/CPAP-servo-control system toadjust MAP. In the CPAP ventilation mode, MAP equals the set PEEP/CPAP level. When conventional IMV ventilation cycles are superimposed, MAP also depends on both the peak inspiratory pressure (PIP) and the frequency.MAP in HFV should be about the same as in thepreceding conventional ventilation, depending on the underlyingdisease, and should be higher than pulmonary opening pressure.In prematures with RDS this opening threshold is approximately12 mbar .

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Mean Airway Pressure (MAP)The crucial physiologic effect of such continuously applied (inflation) pressure is the opening of atelectatic lung areas, resulting in marked recruitment of lung volume.

Intermittent application of additional sigh manoeuvres can further enhance this effect.

Opening of atelectases reduces ventilation-perfusion mismatch and thus intrapulmonary right-to-left shunting.

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Mean Airway Pressure (MAP)

Therefore MAP is the crucial parameter to control oxygenation.

By way of the PEEP/CPAP-servo-control system the mean airway pressure with the Babylog 8000 can be set in the range from 3 to 25 mbar.

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Amplitude – oscillatory volume

The term amplitude has stood for pressure amplitude. In the end, however, ventilation does not depend on the pressure amplitude but on the oscillatory volume. as a setting parameter the amplitude is one of the determinants of oscillatory volume.The oscillatory volume exponentially influences CO2 eliminationDuring HFV volumes similar to the deadspacevolume (about 2 to 2.5 ml/kg) should be the target.

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In any HF ventilator, the oscillatory volume depends characteristically on the oscillatory frequency.

Normally, lower frequencies permit higher volumes.

Even small changes in resistance and/or compliance of the respiratory system, e.g. by secretion in the airways, or through the use of a different breathing circuit or ET tube, can change the oscillatory volume and thus the effectiveness of HFV.

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Amplitude – oscillatory volumeThe amplitudes and oscillatory volumes vary also with MAP. Especially at MAP below 8 mbar oscillatory volumesare markedly reduced.

The oscillation amplitude is adjustable as a percentage from 0 to100%, where 100% means the highest possible amplitude underthe given circumstances of MAP and frequency settings as well as the characteristics of the respiratory system (breathing circuit,connectors, ET tube and airways)

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Oscillation amplitude and flow as functions of MAPand frequency with the Babylog 8000:

a) Start: FHFO = 10 Hz, MAP 6 mbar, VTHFO = 4,6 ml

b) Increase in MAP: FHFO = 10 Hz, MAP 12 mbar, VTHFO = 5,8 ml

c) Decrease in frequency: FHFO = 7 Hz, MAP 12 mbar, VTHFO = 8,5 ml

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Oscillatory frequency

The oscillatory frequency, measured in units of Hertz influences the oscillatory volume and the amplitude depending on the ventilator type used.

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The choice of an optimal oscillatory frequency is currently subject of controversial discussion.

With the Babylog 8000 frequencies of 10 Hz and below have been found to be favourable because then the internal programming permits high flow rates and in consequence high oscillatoryvolumes.

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The gas transport coefficient DCO2

In conventional ventilation the product of tidal volume and frequency, known as minute volume or minute ventilation.Different study groups have found that CO2 elimination in HFO correlates well with VT2 x fVT and f stand for oscillatory volume and frequency, respectively.This parameter is called ‘gas transport coefficient’,DCO2 is measured and displayed by the Babylog 8000. An increase in DCO2 will decrease pCO2.

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the clinical relevance of the gas transportcoefficient

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HFV: Indications 1

When conventional ventilation fails– reduced compliance– RDS/ARDS– airleak– meconium aspiration– BPD– pneumonia– atelectases– lung hypoplasiaOther:– PPHN

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Indications 2

When conventional ventilation failsPrematures

relative: PIP > 22 mbarabsolute: PIP > 25 mbar

Newbornsrelative: PIP > 25 mbarabsolute: PIP > 28 mbar

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Combining HFV and IMV, and‘sustained inflation’

Oscillatory ventilation on its own can be used in the CPAP mode, or with superimposed IMV strokes, usually at a rate of 3 to 5 strokes per minute.

The benefit of the IMV breaths is probably due to the opening of uninflated lung units to achieve further ‘volume recruitment’.

Sometimes very long inspiratory times (15 to 30 s) are suggested for these sustained inflations (SI).

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Combining HFV and IMV, and‘sustained inflation’

By applying them about every 20 minutes compliance and oxygenation have been improved and atelectases prevented. Especially after volume loss by deflation during suctioningPrevention of atelectases, which might occur under HFV with insufficient MAP is the primary benefit of combining HFV and IMV.HFV superimposed to a normal IMV can markedly improve CO2 washout (‘flushing the deadspace’ by HFV) at lower peak pressures than in conventional ventilation.

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Effect of a sigh manoeuvres through sustained inflation (SI):prior to the SI the intrapulmonary volume equals V1 at the MAP level (pointa); the SI manoeuvres temporarily increases pressure and lung volumeaccording to the pressure-volume curve; when the pressure has returnedto the previous MAP level, pulmonary volume remains on a higher level, V2(point b), because the decrease in pressure occurred on the expiratorylimb of the PV loop.

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HFV: Start

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Strategies for various lung diseases

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Neonatal Mechanical Ventilation - Middle East Critical Care Assembly