Basics of pediatric ventilation

Basics Of Pediatric Ventilation

Soumya Ranjan Parida

Basic B.Sc. Nursing 4th year

Sum Nursing College

Topics

Anatomy Physiology Pathophysiology Biophysics

Anatomy

Peculiarities of Pediatric respiratory system Airway of the infant or child is much smaller in diameter and

shorter in length. This markedly increases the resistance and therefore the work of breathing.

Pediatric airway is placed anteriorly and superiorly. The glottic opening lying higher, at approximately the level of C-2 or C-3, as opposed to C-6 in the adult.

Infant’s tongue is relatively larger to the oropharynx. Posterior displacement of tongue causes severe airway obstruction.

Epiglottis in children is long, floppy, narrow and angled away from the axis of trachea.

In children the narrowest portion is below the vocal cord at the level of non distensible cricoid cartilage and larynx is funnel shaped. Whereas in adults the narrowest portion is at glottic inlet and larynx is cylindrical in shape.

In children the subglottic airway is smaller, more compliant and less supported by cartilage.

Physiology

In children, the chest is cylindrical in shape. Respiration are shallow and rapid due to predominant diaphragmatic breathing and inadequate costovertebral bucket handle movement.

Pediatric patient metabolizes oxygen twice as quickly as the adult (6 mL/kg versus 3 mL/kg). So tolerance to hypoxia is less.

At the end of inspirationAt the end of expiration

Intrapleural sapce

Alveolar-arterial Gradient

Oxygen and Carbon dioxide exchange takes place at the at respiratory membrane because these two gases move along their respective concentration gradient to achieve equilibrium between the blood phase in the capillary and the gas phase in the alveoli.

Difference between the alveolar oxygen concentration i.e. PAO2 and arterial oxygen concentration i.e. PaO2 is called alveolar-arterial oxygen gradient. (A-a gradient

Inspired atmospheric room air has PO2 of approx 159 mm Hg and alveolar air PAO2 is 104 mm Hg.

Alveolar air has lower PAO2 because of humidification and because oxygen in the alveoli is constantly being absorbed in the pulmonary blood

Alveolar Gas Equation

PAO2 is calculated by the alveolar gas equation:

PAO2= [FiO2(Pb-PH2O) ]-(Paco2/R)

FiO2 = Fraction of the inspired oxygen

e.g. Breathing oxygen at room air with conc 21% FiO2=0.21

Pb is the barometric pressure ( assumed to be 760 mm Hg at sea level.

PH2O= Water vapor pressure which dilutes dry oxygen content of the atmosphere= 47 mm Hg.

R= respiratory quotient assumed to be 0.8 Thus PAO2 depends on FiO2 i.e. fraction of the inspired oxygen.

More the FiO2 ,higher will be the PAO2.

A-a gradient:

Normal oxygen A-a gradient is < 10 mm Hg because no lung pathology exists to prevent the equlibrium.

An elevated A-a gradient indicates underlying pulmonary pathology and is used to assess severity of impairment of gas exchange.

TLC

RV

VC

TV

FRC

ICIRV

ERV

RV

Can UseSpiromenter

Can’t Use a Spirometer

Closing Volume

Because of the lack of elastic lung recoil of the pediatric patient’s chest, the child also possesses a proportionally smaller FRC. This reduces the time that an apnea can be allowed in a child.

Closing Volume

Volume of the gas present in the lung at which small conducting airways begin to collapse.

In children older than 6 years, FRC exceed the closing volume so that small airways and alveoli remain open.

However, in children less than 6 years closing volume exceeds FRC. This explains the tendency to atelectasis in infants and children.

Compliance:

Compliance is the measure of stiffness of the chest and is determined by the elastic forces of the lung, chest wall and surface tension.

It is defined as the change in volume per unit change in pressure and is expressed as litres/ cm of H2O i.e. ∆V/∆P.

Normal lungs are highly compliant while the diseased lung have reduced compliance.

Pressure-Volume curve:

Time Constant

It is the product of airway resistance and compliance and is measured in seconds.

It is the measure of how quickly the lungs can inhale or exhale. One time constant fills in 63% of an alveolus and three fill 95%

of the alveolus. In normal child one time constant equals 0.15 seconds and

three time constants equals 0.45 sec. Thus minimum inspiratory and expiratory time should be about 0.5 seconds.

If sufficient time is not allowed in expiration air trapping will occur.

An understanding of this concept aids greatly in selecting the safest and most effective ventilator settings.

In normal lung the difference between inspiratory and expiratory time constant is minimal.

Various diseases alter the time constant giving rise to inspiratory and expiratory difference.

E.g. in asthma expiratory time constant increases because of airway resistance while in stiff lung time constant decrease causing faster filling and emptying of alveoli.

V/Q ratio:

Ventilation (v) is the amount of gas delivered to and exhaled through the lungs.

Perfusion (Q) is the amount of mixed venous blood brought to the pulmonary capillary bed.

Relation between them is called V/Q ratio. Normal V/Q ratio is 1.

Pathophysiology

Dead Space Ventilation

Dead Space ventilation occurs when inspired air is delivered to area without perfusion.

Anatomic Dead Space: It exits in the area of nose, nasopharynx, trachea and large conducting airway.

Pathologic dead space occurs when alveoli are ventilated but not perfused.

e.g. pulmonary embolism. V/Q ratio tends to infinity as Q 0

Intrapulmonary Shunt

Reverse of dead space ventilation. Here alveoli are perfused but not ventilated with non

oxygenated blood shunting into the arterial circulation decreasing the PaO2

It occurs in conditions like ARDS, pneumonia, pulmonary hemorrhage, atelectsis and pulmonary edema.

It is often an indication for mechanical ventilation. V/Q ratio tends to zero.

Obstructive Vs Restrictive

Obstructive Restrictive

Difficult to get air out of lung Difficult to get air inside the lung.

Obstruct expiration Restrict expiration

Decreased VCIncreased TLC, RV, FRC

Decreased VCDecreased TLC, RV, FRC

emphysemachronic bronchitisasthma

intersitial fibrosissarcoidosismuscular diseaseschestwall deformities

Normal

RV

ERV

TV

IRV

FRC

VC

Restrictive

RV

ERV

TV

IRV

FRC

VC

Obstructive

RV

ERV

TV

IRV

FRC

VC

125

100

75

50

25

0

% N

orm

al T

LC

Obstructive Disease

Restrictive Disease

FEV1.0 Decreased Decreased

FVC Decreased Decreased

FEV1.0/FVC Decreased Unchanged or Increased

Peak Flow Decreased Decreased or Unchanged

RV/TLC Increased Unchanged

Auto-Peep

Auto-PEEP is gas trapped in alveoli at end expiration, due to inadequate time for expiration, bronchoconstriction or mucus plugging. It increases the work of breathing.

If auto-PEEP occurs during mechanical ventilation, the amount of time given over to expiration needs to be lengthened: either by reducing the respiratory rate or the inspiratory time, or both.

History of ventilators

Negative-pressure ventilators (“iron lungs”) Non-invasive ventilation first used in Boston

Children’s Hospital in 1928 Used extensively during polio outbreaks in

1940s – 1950s

Positive-pressure ventilators Invasive ventilation first used at

Massachusetts General Hospital in 1955

The iron lung negative pressure ventilator: decreases C.O.

Iron lung polio ward in 1953.

Advance Ventilator with ECMO

Basic Terminology

Tidal Volume: Volume of the gas that flows in and out of the chest during breathing. In children tidal volume is 6-8 ml/kg.

Ventilator rate: Rate of mechanical breath per minute on ventilator.

Minute ventilation: T.V.x V.R. Inspiratory Time and I:E ratio: Adjustment of the inspiratory time

is primary method by which I:E ratio is altered.Normal setting for inspiratory time range from 0.4 to 1.5 seconds depending on the ventilatory rate and underlying lung condition. Expiratory time should not decreased to <0.5 seconds except in condition associated with reduced compliance and shortened time constant. Normal I:E ratio is usually 1:2 to 1:3.

FiO2 is the fractional inspired oxygen concentration.

PIP: Highest pressure during inspiratory period. It depends upon T.V., inspiratory time, gas flow, compliance of chest and lungs. PIP is usually kept as low as possible as it can cause barotrauma, pnemothorax, air leak.

But PIP should be kept sufficient so that patient will get adequate tidal volume and the minute ventilation.

PEEP PEEP: PEEP is the baseline positive pressure in the airway during expiration. It

is designed to keep alveoli from collapsing at the end of expiration. It is very useful when the lugs are non-compliant and have tendency to atelectsis. Normal PEEP is 2-3 cm H2O.

PEEP prevents decrecruitment of alveoli and it has protective effect in prevention of ventilator associated lung injury. But the excessive PEEP can cause following problems:

1.Alveolar overdistention: alveoli in non-dependent zones are less likely to collapse at end expiration. Further expansion of the alveoli may cause damage (barotrauma).

2. Excessively high alveolar pressures may squash the blood vessels which surround the airspaces, causing an increase in dead space (wasted ventilation) and an unnecessary increase in the work of breathing.

3. Increased intrathoracic pressure as a result of PEEP/CPAP will reduce the pressure gradient along which blood returns to the heart. This reduces right ventricular preload, right ventricular output and ultimately cardiac output. This may lead to a reduction in blood pressure and pooling of blood in the abdomen and peripheries. Conversely, in severe heart failure this may be beneficial.

Mean Airway Pressure (MAP)

MAP is the measure of average positive pressure generated in the lung throughout the respiratory cycle. It is not a ventilator setting but a result of ventilator setting.

It is determined by several factors including PIP, PEEP, inspiratory time and flow rate.

MAP is critical in determining both oxygenation and barotrauma.

Many ventilators have the ability to continuosly monitor and display MAP allowing clinicians to see effects of ventilatory adjustments.

Trigger/Sensitivity:

This refers to ease with which a ventilator can sense the patient’s demand for a breath. It is usually expressed as a amount of negative pressure or change in flow that a patient must create through spontaneous breathing effort to switch on the ventilator to deliver a mechanical breath.

Setting the sensitivity too high may increase the work of breathing as the patient must create a higher intrathoracic negative pressure in order to get assistance from ventilator.

Setting sensitivity too low may lead to over triggering and patient being over ventilated.

Physiologic range of ventilation:

Modes of Ventilation

Negative pressure vs Positive pressure Positive Pressure: Assisted vs Controlled Pressure targetted vs volume targetted.Specific Modes:Pressure regulated volume controlSIMVAssist controlPressure support.

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