BASICS OF NEONATAL VENTILATION
Dr Abid Ali Rizvi
Why do we ventilate neonates?
Oxygenation CO2 elimination Overwhelming Work of Breathing Poor respiratory respiratory drive dri ve Others: Transport of sick baby, pre-op etc.
Applied Appli ed Mechanics Mec hanics
Flow of gas
Generates the inflating pressure.
PIP minus PEEP
Creates a pressure gradient [ DP].
Compliance [C]
Airway Resist Resistance ance [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]
Quantif ies the adequacy of Quantifies alveolar recruitment & oxygenation.
Decides optimum Ti and Te
Time Constant = [C] x [Raw] Dead Space [VD]
Right to Left Shunting
Work of Breathing [WOB]
Endotracheal Leak
and
Start here:
A pressure gradient between the airway opening (mouth) and the alveoli must be present to drive the flow 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 Residual Capacity [FRC].
Components of the inflating pressure Mean Airway Pressure = area under the Pressure Time Curve
Pressure (cm of H2O)
P E E P
PIP
PIP wave wave form is shaped by the gas flow rate during inspiration.
Compliance
Compliance describes the elasticity or distensibility of the respiratory structures (alveoli, (alveoli, chest c hest wall, wall, and pulmonary parenchyma) parenc hyma).. A measure of the ease of expansion of the lungs and thorax. Compliance = Δvolume Δpressure Low Low Compliance means Stiff lungs [as in Hyaline Hyaline Membrane Disease]. It will need higher pressure gradient for pushing air inside.
Elastance Elastanc e [E] :R :Recoil Tenden endency cy
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 Suc h alveolar units are prone to atelectasis during expiration.
Compliance
Airway Airw ay resistance 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 R aw Airway resistance depends on: Radii of the airways airways (total cross-sectional area) airways Lengths of the airways Flow Type: Laminar or Turbulent Density and viscosity of gas
Airway Airw ay resistance resistance
ET resistance increases with flo flow w
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 mouth/ventilator circuit. At the end of three [K t ] 95% of the tidal volume is emptied. Airway Airway diameter diameter during inspiration: Raw . Therefore inspiratory [K t ] are ~ half 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 premature neonate will have normal breathing faster than a term ter m AGA newborn. newborn.
Anatomic Dead Space
Anatomic dead space: The total volume of the conducting airways airways from the nose or mouth down to the level of the t he terminal bronchioles. par ticipate in the gas exc exc hange. hange. This volume does not participate Extrathoracic : 2-2.5 ml/kg in neonates. 1.03 ml/kg, age independent. Intrathoracic :
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
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.
Work ork = Pressure Pressure x Volume 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.
2 Components Components of WOB WOB:: Elastic and Resistive – Resp. Rate Dependency
Imposed work of breathing [WOB]
ET, circuit tubing, ventilator exhalation ET, exhalation valve, valve, all increase the resistance against against which whic h the baby must breathe while on ventilator. This leads to increased O2 consumption and exhaustion 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: 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. cycling. Good nutrition. Earl y extubation extuba tion ASAP. ASAP. Early
Mean Airw Airway ay Pressure Pressure [MAP/P [MAP/Paw] aw] Contributing parameters {PIP, PEEP, Ti, Flow, Rate} Important for the:
MAP= (PIP-PEEP) x [Ti (Ti+T (Ti+Te)] e)] + PEEP
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.
Mean Airw Airway ay Pressure Pressure [MAP/P [MAP/Paw] aw]
e m u l o V g n u L
Safety & Efficiency of ventilation is best in this Lung Volume & Paw range.
Mean Airway Pressure
Importance of of PEEP
Presence of ET in the glottis disables the braking action of the vocal cords during expiration, 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 provides a means of oxygenat oxygenation ion 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 cycled (matching (matc hing with the patient`s pat ient`s own Ti) Flow cycled
Hybrid modes mix multiple techniques from above.
CMV & IMV: by definition…
Continuous Mandatory Mandator y Ventilation: 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 Manda tory Ventilation: 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' patient'ss effort. Synchronization is not intended in both.
Poor Sync Synchroni hronization zation causes:
Baby fighting with the ventilator. Increased WOB Abnormally high intra-thoracic intra-thoracic and intra-pulmonary pressure surges. Decreased venous return. Increased intracranial pressure. Barotrauma Sub-optimal training training of muscles in weaning. weaning.
Synchr Sync hronized onized ventilation ventilation modes
1. 2.
Nomenclature is a mess. Heart of synchronized ventilation is the breath sensor attached between the ventilator tubing & ET. Pressure sensor Flow sensor 1. 2.
3.
Pneumotachograph Hot wire anemometer
Hybrid
Limitations of flo flow w 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 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, space, may increase the work of breathing in very 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 Ventilato entilatorr rate therefore th erefore belongs be longs to bab baby y. 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 flo flow w curve cur ve is the patient`s patient`s effort
Sync hronized Synchroni zed Intermittent Inter mittent 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 allowing the patient to breathe spontaneously in between ventilator breaths. Each Eac h ventilator breath is delivered delivered in synchron sync hrony y with the patient’s breaths, yet the patient is allowed to
completely control the spontaneous breaths. Work of breathing and respiratory muscle fatigue increase increase with low parameter SIMV S IMV..
SIMV breaths: Green spontaneous;
Blue ventilator
Volume Targeted Ventila entilation tion [VTV] [V TV] Targeted Tidal Volume [TTV] Ventilation Volume Guarantee [VG]
Physician selects a desired tidal volume (app. 5-6 mL/kg) mL/k g) for the baby. baby. The ventilator then delivers the desired tidal volume at the lowest feasible PIP and Ti according to c hanges in Raw, aw, C and bab ba by`s effort. effor t. Main benefits of TTV: TTV: Reduction in volutrauma and barotrauma. A stable Tidal Volume avoiding swings in pCO 2. Ventilation is at the lowest possible parameters. wean. Ability to self wean.
Pressure Support Suppor t Ventilation Ventilation [PSV]
Peak Expiratory Flow
PSV with SIMV
Effect of vario various us parameters on oxyg xygenation enation and ventilation. ventilation.
In brief: Always Check:
Chest movement, air entry, presence of retractions, hyper-inflated hyper-inflated chest, c hest, 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. posturing. Abnormal Abnor mal heart hear t rate, BP, BP, temperature. temperature. Signs of excessive excessive sedation.
