1. lung mechanics

May 2, 2023 Hera NICU 1

1. Understanding lung physiology& mechanics

DR/ MAHMOUD EL NAGGAR

EGYPTIAN BOARD OF NEONATOLOGY

HERA NICU 2016


38 WChorioamnionitisRetractionPH: 7.22PCO2: 66PO2: 50HCO3: 17


42 WMSAFCSPH: 7.1PCO2: 70PO2: 50HCO3: 15


29 WGruntingRetractionPH: 7.2PO2: 40PCO2: 50HCO3: 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 respirationB) 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 lawAir 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 gradientto fill lungs

-2cm H20

-7cm H20

Alveolarpressure falls

Normal breathLung @ FRC= balance


Normal breath expiration animation

Diaghram relaxes

Pleural / Chest volume

Pleural pressure rises

Normal breath

Alveolarpressure rises

Air moves down pressure gradientout of lungs

-7cm H20

-2cm H20


Spontaneous Inspiration

Volume Change

Gas Flow

Pressure Difference




Lung volumes in term newborns

Static lung volumes

Ventilatory volumes

10–15 mL/kg RV 5–8 mL/kg V T

25–30 mL/kg FRC 40–60 b/min F

30–40 mL/kg TVG 2–2.5 mL/kg V D

50–90 mL/kg TLC 200–480 mL/min/kg MV

35–80 mL/kg VC 60–320 mL/min/kg VA


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


Dead space


Work of breathingIt 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 breathingNon 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 breathingThe 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



Understanding airway equation of motionThe 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

PawAirway pressure

PalvAlveolar 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)


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 recoilSurface 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/R



Elastic recoilResting 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 recoilPreterm 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


ComplianceThis 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.



May 2, 2023 42Hera NICU


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.


ResistanceThis 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 resistanceGenerated by friction between gas molecules and those of the conducting airways.

It depends on:1. Flow rate2. Length of conducting

airways3. 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 resistanceIt 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.


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 constantThe 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 constantThe 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 × Compliance



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 ChemoreceptorsCarotid bodies are located in the carotid

sinusAortic 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 differencesA) EXTRATHORACIC CAUSES:obligate nasal breathelarge tonguecephalic larynxThe epiglottis is larger and more

horizontal to the pharyngeal wallnarrow subglottic areacongenital 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


Neonatal respiratory differencesB) 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


Neonatal respiratory differencesC) RESPIRATORY PUMP :The respiratory center is immature More rapid eye movement sleep

The ribs are horizontally orientedThe 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

May 2, 2023 80Hera NICU

Health & Medicine

1. lung mechanics