Physiology of Mechanical Ventilation

Dr Baswaraj T

To know the basics of pulmonary mechanics and initiation of breathing

Application of the knowledge of physiology to clinical situations and planning ventilatory strategies

Lung Mechanics : A Balance between two forces

Compliance : Distensibility (stretchability):

Ease with which the lungs can expand

Responsible for inspiration

Elasticity :Tendency to return to initial size after distension

Recoil ability

Elasticity : Stroma of lungs and respiratory muscles

Lung Compliance

Volume change per unit change in pressure

Low or High – Inefficient gas exchange

Volume Change (V)Compliance (C) = Pressure Change (∆ P)

Compliance curve is sigmoidal

Low Compliance (High elastance) Stiff / Non-compliant Work of breathing – ↑ ↓FRC ↓LV, ↓ MV, Restrictive lung defect Refractory hypoxemia Eg:

Atelectasis, ARDS, Tension pneumothorax, Retained secretions

Bronchospsam, Kinked ETT, Airway obst

High Compliance

Incomplete exhalation & CO2 elimination

↑FRC, Obst lung defect Eg:

Emphysema

Compliance & Work of Breathing

Compliance is inversely related to pressure change (Work of breathing)

Hypoventilation: Unable to compensate for ↓ compliance by ↑ and maintaining a higher level of work of breathing

Surface Tension and Surfactant…

Force exerted by fluid in alveoli to resist distension

Lungs secrete and absorb fluid, leaving a very thin film of fluid which causes surface tension

Surfactant lowers surface tension by reducing attractive forces of hydrogen bonding by becoming interspersed between H20 molecules

Law of Laplace…

Pressure in alveoli is directly proportional to surface tension; and inversely proportional to radius of alveoli

Higher Pressures required to keep open the smaller alveoli of preterm neonates with high surface tension

Pressure : Volume Loops…

Volume

Pressure

RAD

Slope = ComplianceNormal lungs

Airway Resistance

Airflow obstruction in the airways Length, Size & Patency of Airways,

ETT & Vent circuits Factors affecting:

Inside - Secretions Wall – Neoplasm Outside - Lymphnodes

Airway Resistance

Simplified form of Poiseuille’s Law:

Normal Raw : 0.6 & 2.4 cm H20/L/sec at a flow rate of 30 L/min

Varies directly with length & inversely with the diameter of the ETT

Vent Circuit – Condensed water

∆ P = V r4

Increased airway resistance

TYPE CLINICAL CONDITION

COPD Emphysema

Chronic bronchitis

Asthma

Bronchiectasis

Mechanical Obstruction Post intubation obst

FB aspiration

ET Tube

Condensatation in vent circuit

Infection LTB

Epiglottitis

Bronchiolitis

Raw & Work of Breathing

Pressure change (Peak – Pleatu) = Work of breathing

Ventilatory & Oxygenation failure

Pressure (∆ P)Raw = Flow (V)

Time Constant

Kt = CL X R (sec)

Time taken for transthoracic pressure change to be transmitted as volume change in the lungs

Measure of how quickly the lungs can inflate or deflate or how long it takes for alveolar & proximal air way pressures to equilibrate

Time Constant

1 TC : The it takes the alveoli to discharge 63% of its VT through the airway to mouth or vent circuit

3 to 5 time constants : requires for near-total inflation or deflation of the lungs

Ch

an

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in p

ress

ure

(%

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ha

ng

e in

pre

ssu

re (

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Time ConstantsTime Constants

100100

8080

6060

4040

2020

00

63%63%

86%86%95%95% 98%98% 99%99%

00 11 22 33 44 55 TimeTime

Time Constant

3 to 5 Kt = 0.36 to 0.6 sec – should be the inspiratory / expiratory phase

Very short Ti : Incomplete delivery of tidal volume

Very short Te : Inadvertent PEEP, Gas trapping

Time Constant

Restrictive diseases (eg., Atelactesis): Low compliance

Shorter time constant

Inflation/deflation faster than normal lungs

Tolerate rapid rates without compromising minute ventilation

Obstructive diseases (eg., Asthma) : High resistance

longer time constants –

To avoid gas trapping

Shorter Ti

Longer Te & Low PEEP

What is Mechanical Ventilator

Complex system consisting:

Power supply, Compressed air and oxygen, A drive mechanism to

provide motive force to push oxygen into the patient’s lungs and

A control mechanism to manage the gas flow, volume, pressure and timing

Simply, any machine to push or pull gas mixer ( air & oxygen) into the lungs. By applying positive pressure at the airway

either invasively or non invasively ( positive pressure ventilation)

Negative pressure around the chest ( negative pressure ventilation)

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What is Mechanical Ventilator

Anatomy of ventilator

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O2

Air

To Patient

Inspiratory limb Expiratory limb

Expiratory valve

Blender

Ventilator : TerminologiesVentilator : Terminologies

Tidal Volume : Volume of gas that flow in and out of the chest during quiet breathing (7-10 ml/Kg).

Respiratory Rate (RR): The frequency of breaths delivered by the ventilator

Peak Inspiratory Pressure (PIP): Highest pressure that is met during inspiration

Positive End Expiratory Pressure (PEEP) : Pressure at the end of expiration to prevent alveolar atelectasis

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Ventilator : TerminologiesVentilator : Terminologies

Inspiratory Time : Time of inspiration : 0.45 – 0.60

I:E Ratio: Relationship between inspiratory time (I) and expiration time (E). The normal ratio 1:1.5 to 1:2.

Sensitivity: Used to determine the patient’s effort to initiate an assisted breath (inspiration)

Minute Volume (MV): Determines alveolar ventilation (RR x VT = MV)

Typical vent settings

FIO2 Rate Volume PIP and PEEP Flowrate, I-time, I:E Ratio Mode ???

Different brands of ventilators have differentcontrol layouts, but they all accomplish

essentially the same functions.

Different brands of ventilators have differentcontrol layouts, but they all accomplish

essentially the same functions.

Guidelines for Initiating Positive Pressure Ventilation

Select Rate – physiologic norm for age.

Select tidal volume: 10 – 15 ml/kg (volume controlled)

Select PIP:15 – 25 cm H2o (pressure controlled).

Select inspiratory time: 0.40 to 0.50 Secs.

I:E Ratio: 1:1.2 or 1:1.3

FiO2: 60 to 100%

PEEP: 3 cm or higher as needed

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Summary

Compliance is reduced at both high and low lung volumes : Do not overstretch a spring

Resistance is higher in narrower and longer airways (Poiseuille’s law ) : Select appropriate ETT Size & Cut short any extra lengths of ET

Normal time constant is 0.12 sec : Min Ti or Te should be 0.36 sec

Restrictive diseases : Shorter Ti

Obstructive diseases : Longer Te