1. lung mechanics

DR/ MAHMOUD EL NAGGAR
EGYPTIAN BOARD OF NEONATOLOGY
HERA NICU 2016
June 1, 2016 Hera NICU 1

 38 W
 Chorioamnionitis
 Retraction
 PH: 7.22
 PCO2: 66
 PO2: 50
 HCO3: 17
Hera NICUJune 1, 2016 2

 42 W
 MSAF
 CS
 PH: 7.1
 PCO2: 70
 PO2: 50
 HCO3: 15

 29 W
 Grunting
 Retraction
 PH: 7.2
 PO2: 40
 PCO2: 50
 HCO3: 16

Respiration
 General function is to obtain O2 for
use by the body’s cells and to
eliminate the CO2 the body cells
produce.
 Including two separate but related
processes:
 A) Internal respiration
 B) External respiration

Physiological function of the
lung (External respiration)
 Ventilation: Movement of air between the atmosphere
and respiratory portion of the lung
 Perfusion: Flow of blood through the lung
 Diffusion: Transfer of gases between the air-filled space in
the lung and blood

Respiration
needs:
1- Effective
ventilation
2- Ventilation-
Perfusion
matching
3- Diffusion

Lung development

Anatomy of respiratory
system

Subdivisions of the airway

Ventilation
movement of air into and out of the lungs

Ventilation

Boyles law
 Air flow from a region of higher pressure to a region of
lower pressure.
 To initiate a breath, airflow into the lungs must be
precipitated by a drop in alveolar pressure.

Normal breath inspiration animation
Diaghram contracts
Chest volume
Pleural pressure
Air moves down
pressure gradient
to fill lungs
-2cm H20
-7cm H20
Alveolar
pressure falls
Normal breath
Lung @ FRC= balance

Normal breath expiration animation
Diaghram relaxes
Pleural /
Chest volume 
Pleural pressure
rises
Normal breath
Alveolar
pressure rises
Air moves down
pressure gradient
out of lungs
-7cm H20
-2cm H20

Spontaneous Inspiration
Hera NICU
Volume Change
Gas Flow
Pressure Difference
June 1, 2016 18

Lung volumes in term
newborns
Static lung volumesVentilatory volumes
10–15 mL/kgRV5–8 mL/kgV T
25–30 mL/kgFRC40–60 b/minF
30–40 mL/kgTVG2–2.5 mL/kgV D
50–90 mL/kgTLC200–480 mL/min/kgMV
35–80 mL/kgVC60–320 mL/min/kgVA

The FRC is four to five times
as large as the Vt

Dead space

Work of breathing
 It is the amount of energy required to
ventilate the lung and overcome all kinds
of resistance.
 There are dissipative and non dissipative
force.

Work of breathing
 Non dissipative force: it is the work needed
to overcome the elastic recoil is stored like
the energy in a coiled spring, and will be
returned to the system upon exhalation.
 Dissipative force: resistive and frictional
force, on the other hand, are lost and
converted to heat.

Non dissipative force

Work of breathing
 The diaphragm does most of the work
 Work of breathing = Pressure x Volume
 In small infants as it may:
overwhelm their metabolic energy
requirement and impede growth

Hera NICU
Understanding airway equation of motion
The respiratory system can be thought of as a mechanical
system consisting of resistive (airways +ET tube) and elastic
(lungs and chest wall) elements in series
Diaphragm
ET Tube
airways
Chest wall
PPL
Pleural pressure
Paw
Airway pressure
Palv
Alveolar pressure
ET tube + Airways
(resistive element)
Resistive pressure varies with airflow
and the diameter of ETT and airways.
Flow resistance
The elastic pressure varies with volume and
stiffness of lungs and chest wall.
Pel = Volume x 1/Compliance
Paw = Flow X Resistance + Volume x 1/ComplianceTHUS
Lungs + Chest wall
(elastic element)
Airways + ET tube
(resistive element)
Lungs + Chest wall
(elastic element)
June 1, 2016 29

Pulmonary Mechanics
 The mechanical properties of the
respiratory system can be described
according to their elastic and resistive
forces.

Elastic recoil
It is a tendency of a stretched object to
return to its original shape.
 This happens to chest wall, diaphragm
and lung during expiration, which are
stretched during inspiration, recoil to
their original shape.
 These elastic elements behave like
springs.

 Elastic recoil is tendency of chest wall and lung that are stretched during
inspiration to recoil (arrows) to original state at the end of expiration.
 At this point FRC , the springs are relaxed and the structure of rib cage
allows no further collapse.

Elastic recoil
 Surface tension is the main contributor
of elastic recoil of the lung in neonate.
 The surface tension forces at the air
liquid interface in the distal bronchioles
and terminal airways tend to decrease
the surface area of the air-liquid
interface.
 Laplace law: pressure to counteract ST
P=2ST/RJune 1, 2016 Hera NICU 33

Elastic recoil
 Resting state of the respiratory system
(FRC) : reached when forces tending to
collapse (elastic recoil and surface tension)
= forces resisting collapse (surfactant, chest
recoil)
Elastic recoil + Surfactant + chest recoil
Surface tension

Transmural pressure across the lung and the
tendencies of the lung and chest wall to
approach their resting states.

Elastic recoil
 Preterm baby has:
a) very compliant chest wall: no
opposition to collapse and low FRC
b) Deficiency of surfactant: lung
volume achieved during inspiration
rapidly lost in expiration that needs
high opening pressure and high
energy expenditure with each breath

Compliance
 This term is used to describe the elastic
properties of a system (the lung and chest
wall); it is the measure of distensibility of
respiratory system and specifies the ease with
which the lung and chest wall can be stretched
or distorted.
 Compliance is the inverse of elastic recoil,
compliance=1/ elastance
Change in volume (ml)
Compliance (ml/cmH2O) =
Change in pressure (cmH2O)

Extended compliance curve with flat tented areas (A and C) at both ends.
Area A represents the situation in diseases state leading to atelectasis or
lung collapse RDS. Area C represents the situation in an over expanded
lung as in MAS.June 1, 2016 Hera NICU 39

Open Lung Ventilation Strategy
Volume
Pressure
Zone of Overdistention
Safe
window
Zone of
Derecruitment
and atelectasis
Our goal is to avoid injury zones
and operate in the safe window

 Therefore, the higher the compliance,
the larger the delivered volume per unit
changes in pressure.
 Normal compliance = 0.03-0.06
L/cmH2O.
 Compliance is decreased with:
a) surfactant deficiency (0.005-0.01
L/cmH2O)
b)excess lung water
c) lung fibrosis.

 In these cases, PIP would have to be
increased to maintain Vt.
 If compliance improves after
surfactant therapy, PIP must be
lowered, otherwise over-inflation
and air leak develops.

Resistance
 This term is used to describe the property of the
lungs that resists the airflow.
 The pressure is required to overcome the
resistance of the respiratory system, to force gas
through the airways (airway resistance), and to
exceed the viscous resistance of the lung tissue
(tissue resistance).
Change in pressure (cmH2O)
Resistance (cmH2O/L/sec) =
Change in flow (L/sec)

Airway resistance
 Generated by friction between gas
molecules and those of the conducting
airways.
 It depends on:
1. Flow rate
2. Length of conducting airways
3. Diameter of the tubes.
4. Properties of gas inhaled

Airway resistance
 Poisseuille’s law (Resistance =
Length/radius4)
 Reduction of radius by ½ results in a 16
fold increase of resistance.
 Resistance can change rapidly if, for
example, secretions partially occlude
the endotracheal tube.

Tissue resistance
 It is the resistance within the lung tissue
during inflation and deflation(viscus
resistance).
 Tissue resistance is high in neonates
(40%) due to:
1. The low ratio of lung volume to lung
weight
2. Relative pulmonary interstitial fluid.June 1, 2016 Hera NICU 51

 Resistance in healthy infants = 30
cmH2O/L/sec.
 Resistance during inspiration is less
than during expiration ?
 Resistance is high in diseases
characterized by airway obstruction,
such as meconium aspiration and
BPD?
 Lung compliance and airway
resistance are related to lung size.
 The smaller the lung the low
compliance and high resistance.

Time constant
 The concept of time constants is the
key to understanding the interactions
between the elastic and resistive
forces, and haw the mechanical
properties of the respiratory system
work together to modulate the volume
and distribution of ventilation.
 A working knowledge of time constant
is essential for choosing the safest and
most effective ventilator setting.

Time constant
The time constant is a measure of the
time (expressed in seconds) necessary
for the alveolar pressure (or volume)
to reach 63% of a change in airway
pressure (or volume).
Is a measure of how quickly the lung
can inhale or exhale (how quickly a
change in pressure occurs)
 Time constant = Resistance ×
ComplianceJune 1, 2016 Hera NICU 54

 1 Kt = 0.15 sec in a normal newborn!
 By the end of 3 Kt 95% of TV is
discharged.
 In less than half a second from the
onset of expiration 95% of TV is
exhaled in the normal newborn.
 Inspiratory time constant is shorter
than expiratory time constant?

 A duration of inspiration or expiration
equivalent to 3–5 time constants is required
for a relatively complete inspiration or
expiration, respectively.
 The time constant will be shorter if
compliance is decreased (e.g., in patients
with RDS) or if resistance is decreased.
 The time constant will be longer if
compliance is high (e.g., big infants with
normal lungs) or if resistance is high (e.g.,
infants with chronic lung disease).

 Patients with a short time constant
ventilate well with:
short inspiratory and expiratory
times and high ventilatory frequency.
 whereas patients with a long time
constant require:
longer inspiratory and expiratory
times and slower rates.

If inspiratory time is too short
Incomplete inspiration
Tidal volume Mean airway pressure
Hypercapnia Hypoxia

If expiratory time is too short
Incomplete expiration
Gas trapping
Complianc Tidal volume Mean airway pressure
Tidal volume Cardiac output
Hypercapnia Hyperoxemia

 The mechanical properties vary with
changes in the lung volume, even within a
breath.
 Furthermore, the mechanical
characteristics of the respiratory system
change somewhat between inspiration and
expiration.
 In addition, lung disease can be
heterogeneous, and thus, different areas of
the lungs can have varying mechanical
characteristics.

Control of Respiration

Peripheral Chemoreceptors
 Carotid bodies are located in the carotid sinus
 Aortic bodies are located in the aortic arch

Influence of Chemical Factors on Respiration

Perfusion

Diffusion

 It is diagnosed when the patient’s
respiratory system loses the ability to
provide sufficient oxygen to the blood, and
hypoxemia develops, or when the patient is
unable to adequately ventilate, and
hypercarbia and hypoxemia develop
Respiratory failure

Respiratory failure of neonate

Neonatal respiratory differences
A) EXTRATHORACIC CAUSES:
 obligate nasal breathe
 large tongue
 cephalic larynx
 The epiglottis is larger and more horizontal to
the pharyngeal wall
 narrow subglottic area
 congenital anatomic abnormalities

The airway is short and narrow

Upper airway differences

Shape of the chest & size of
occiput

Adult and infant tracheas showing
different angles of main stem
bifurcation

B) INTRA-THORACIC:
 Fewer alveoli
 Surfactant deficiency in preterm
 The alveolus is small
 Collateral ventilation is not fully developed
 Smaller intrathoracic airways
 Relatively little cartilaginous support of the airways
 Residual alveolar damage

Diaphragm & thoracic cage

C) RESPIRATORY PUMP :
 The respiratory center is immature
 More rapid eye movement sleep
 The ribs are horizontally oriented
 The ribs of the neonate are relatively elastic
 The small surface area for the interaction between the
diaphragm and thorax
 The musculature is not fully developed, The
diaphragm of preterm babies contains approximately
10% of type I ﬁbers (slow twitch) which rises to 25% at
term
 The soft very compliant chest wall

1. lung mechanics

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