Basics of Neonatal Ventilation 1

BASICS OF

NEONATAL VENTILATION

Dr Abid Ali Rizvi

Why do we ventilate neonates?

Oxygenation

CO2 elimination

Overwhelming Work of Breathing

Poor respiratory drive

Others: Transport of sick baby, pre-op etc.

Applied Mechanics

Flow of gas Generates the inflating pressure.

PIP minus PEEP Creates a pressure gradient [DP].

Compliance [C] and

Airway Resistance [Raw] Dictate the PIP and PEEP required.

Tidal Volume [TV] Is proportional to the DP size.

TV x Rate = Minute volume Quantifies the CO2 removal.

Mean Airway Pressure [Paw] Quantifies the adequacy of

alveolar recruitment & oxygenation.

Time Constant = [C] x [Raw] Decides optimum Ti and Te

Dead Space [VD]

Right to Left Shunting

Work of Breathing [WOB]

Endotracheal Leak

Start here:

A pressure gradient between the airway opening

(mouth) and the alveoli must be present to drive the

flow of gases during both inspiration and

expiration.

Peak Inspiratory Pressure [PIP]: Opens the alveoli.

Positive End Expiratory Pressure [PEEP]: Prevents the

alveoli from collapsing during exhalation; thereby

maintains adequate Functional Residual Capacity

[FRC].

Components of the inflating pressure

PIP

PEEP

Mean Airway Pressure =

area under the Pressure Time Curve

Pressure

(cm of H2O)

PIP wave form is shaped by the gas flow

rate during inspiration.

Compliance

Compliance describes the elasticity or distensibility

of the respiratory structures (alveoli, chest wall, and

pulmonary parenchyma).

A measure of the ease of expansion of the lungs and

thorax.

Compliance = Δvolume Δpressure

Low Compliance means Stiff lungs [as in Hyaline

Membrane Disease]. It will need higher pressure

gradient for pushing air inside.

Elastance [E] :Recoil Tendency

Elastance is reciprocal of compliance [C].

It measures the ease with which a distended

structure return back to its original size.

E = 1 / C

Alveoli with low compliance are difficult to inflate,

but their elastance is high, so they deflate easily.

Such alveolar units are prone to atelectasis during

expiration.

Compliance

Airway resistance

Airway resistance is the opposition to gas flow.

Ratio of driving pressure to the rate of air flow.

ET is the most important contributor of Raw

Airway resistance depends on:

Radii of the airways (total cross-sectional area)

Lengths of the airways

Flow Type: Laminar or Turbulent

Density and viscosity of gas

Airway resistance

ET resistance increases with flow

Time Constant [Kt] = C x Raw

One time constant of a respiratory system is

defined as the time required by the alveoli to

empty 63% of its tidal volume through the airways

into the mouth/ventilator circuit.

At the end of three [Kt ] 95% of the tidal volume is

emptied.

Airway diameter during inspiration: Raw .

Therefore inspiratory [Kt ] are ~ half of the

expiratory [Kt ].

% filling and emptying of alveoli after

every Time Constant.

Time Constant [Kt] = C x Raw

Stiff alveoli (eg HMD) have very short [Kt ], so small Ti is

sufficient to fill them, and they will empty quickly also.

Conditions with high Raw ( eg MAS, BPD) have long

expiratory time constant, so they will empty adequately

with longer Te, and will be slow to fill too.

It is also dependent on the patient`s size. Every thing

being equal, larger infants have longer time constant

than the extremely premature ones.

Therefore premature neonate will have normal

breathing faster than a term AGA newborn.

Anatomic Dead Space

Anatomic dead space:

The total volume of the conducting airways from the

nose or mouth down to the level of the terminal

bronchioles.

This volume does not participate in the gas exchange.

Extrathoracic : 2-2.5 ml/kg in neonates.

Intrathoracic : 1.03 ml/kg, age independent.

Intrapulmonary RL shunting [ V/Q ]

Alveolar Dead Space [Collapsed alveoli]

Instrumental dead space

In babies <1000 g, the extra

dead space may slightly

increase PaCO2 levels.

The advantages of using flow

sensors for monitoring, volume

targeting and flow triggering,

outweigh the small effect on

PaCO2.

Instrumental dead space

imposes a ventilatory burden

during SIMV weaning in small

preterm infants.

Work of Breathing

Work = Pressure x Volume

Work against Elastic Recoil

Work against Resistance

Airway resistance:

Mainly the narrow ET

Tissue resistance

Viscous forces within tissues

as they slide over each other. Metabolic cost of WOB in spont.

breathing in normal lungs is 1-2%

of total O2 consumption, but can

increase to >30% in ventilated

baby with premature lungs.

2 Components of WOB:

Elastic and Resistive – Resp. Rate Dependency

Imposed work of breathing [WOB]

ET, circuit tubing, ventilator exhalation valve, all

increase the resistance against which the baby must

breathe while on ventilator.

This leads to increased O2 consumption and

exhaustion of respiratory muscles.

Techniques to counter the Imposed WOB:

Avoid narrow ET if possible.

[Poiseuille's equation R . L (Radius)4]

‘Pressure Support’ for the spontaneous breaths.

Adequate PEEP in expiration:

[Maximum WOB is for re-opening a collapsed alveoli]

Optimize the lung volume:

Low lung volume: Airway resistance is high, so WOB .

Over-distended Lungs: Compliance is low, so WOB .

Synchronization of ventilator and baby`s cycling.

Good nutrition.

Early extubation ASAP.

Mean Airway Pressure [MAP/Paw]

Contributing parameters {PIP, PEEP, Ti, Flow, Rate}

Important for the:

Recruitment of alveolar units:

Oxygenation is directly proportional to MAP.

Surfactant preservation.

Optimization of Lung volume:

Airway resistance is high at low lung volumes.

Compliance is poor at high (over-distended) lung volume.

Pulmonary vascular resistance is high at low lung volume

Venous return and Cardiac output is compromised when MAP is abnormally high.

MAP= (PIP-PEEP) x [Ti (Ti+Te)] + PEEP

Mean Airway Pressure [MAP/Paw]

Mean Airway Pressure

Lung V

olu

me

Safety & Efficiency of

ventilation is best in

this Lung Volume &

Paw range.

Importance of PEEP

Presence of ET in the glottis disables the braking action of the vocal cords during expiration, which would normally prevent the collapse of alveoli.

It is easy to expand an already open alveoli, rather than opening a fully collapsed one.

FRV provides a means of oxygenation of pulmonary blood flow during expiration.

PEEP split opens the floppy airways of preterm neonate, thereby preventing their collapse during expiration; so helps in reducing the airway resistance in expiration.

Modes of Neonatal Ventilation -

Classified by three factors:

Breath initiation:

Controlled or

Synchronized with the patient`s effort.

Gas flow control during the breath delivery:

Pressure limited or

Volume limited

Breath is termination:

Time cycled (fixed inspiratory time) or

Flow cycled (matching with the patient`s own Ti)

Hybrid modes mix multiple techniques from above.

CMV & IMV: by definition…

Continuous Mandatory Ventilation: Used most often in the paralyzed or apneic patients. The ventilator rate is set faster than the patient's own breathing rate.

Intermittent Mandatory Ventilation: The ventilator rate is lower (less than 30 bpm), therefore the patient gets chance to breathe spontaneously between two controlled breaths.

In both CMV and IMV, breaths are delivered regardless of the patient's effort.

Synchronization is not intended in both.

Poor Synchronization causes:

Baby fighting with the ventilator.

Increased WOB

Abnormally high intra-thoracic and intra-pulmonary

pressure surges.

Decreased venous return.

Increased intracranial pressure.

Barotrauma

Sub-optimal training of muscles in weaning.

Synchronized ventilation modes

Nomenclature is a mess.

Heart of synchronized ventilation is the breath

sensor attached between the ventilator tubing & ET.

1. Pressure sensor

2. Flow sensor

1. Pneumotachograph

2. Hot wire anemometer

3. Hybrid

Limitations of flow sensors

ET leak: expiratory TV may be underestimated.

Less than the expected expiratory tidal volume due

to ET leak is registered as a negative flow ( same

as baby`s breath initiation).

This artifact falsely triggers a ventilator breath in

the middle of the baby`s expiration:

[AUTOCYCLING], ventilator can end up with very

high auto triggered rate.

Imposing 1 mL of dead space, may increase the

work of breathing in very tiny preterm.

Assist Control [A/C]

Patient Triggered Ventilation [PTV]

Every breath of baby that the flow sensor detects is supported with PIP/PEEP

Ventilator rate therefore belongs to baby.

Ti is fixed by the physician.

Backup rate [20-30/min] is set by physician in case of apnea or flow sensor failure.

Weaning is done by decreasing the PIP.

If baby is excessively tachypneic, the A/C mode may deliver abnormally high ventilator breaths, causing hypocapnea.

A/C: Green parts at beginning of flow

curve is the patient`s effort

Synchronized Intermittent Mandatory Vent.

[SIMV]

SIMV was developed as a result of the problem of high respiratory rates associated with PTV.

SIMV delivers the preset pressure and rate while allowing the patient to breathe spontaneously in between ventilator breaths.

Each ventilator breath is delivered in synchrony with the patient’s breaths, yet the patient is allowed to completely control the spontaneous breaths.

Work of breathing and respiratory muscle fatigue increase with low parameter SIMV.

SIMV breaths:

Green spontaneous; Blue ventilator

Volume Targeted Ventilation [VTV]

Targeted Tidal Volume [TTV] Ventilation

Volume Guarantee [VG]

Physician selects a desired tidal volume (app. 5-6 mL/kg) for the baby.

The ventilator then delivers the desired tidal volume at the lowest feasible PIP and Ti according to changes in Raw, C and baby`s effort.

Main benefits of TTV:

Reduction in volutrauma and barotrauma.

A stable Tidal Volume avoiding swings in pCO2.

Ventilation is at the lowest possible parameters.

Ability to self wean.

Pressure Support Ventilation [PSV]

Peak Expiratory Flow

PSV with SIMV

Effect of various parameters on

oxygenation and ventilation.

In brief: Always Check:

Chest movement, air entry, presence of retractions, hyper-inflated chest, wheezing etc.

Level of ET at lips, visible secretions in ET, any kinking or disconnection, any warning alarms on the ventilator.

Assess baby`s own respiratory drive: depth & rate.

Signs of baby fighting the ventilator: air hunger, asynchrony, gross difference between ventilator and baby`s breathing rate.

Signs of pain, agitation, abnormal posturing.

Abnormal heart rate, BP, temperature.

Signs of excessive sedation.