Assessing the Need for Mechanical Ventilation

Principle of Mechanical Ventilation

Disease path physiology.

Respiratory failure mechanisms.

Target the illness Avoid Complications.

Mechanical Ventilation: Device

Breathing

Ventilation

Respiration

Introduction

Breath to Ventilate to Respirate to Oxygenate

Always eat when you are hungry,

Always drink when you are dry,

Always wash when you are dirty,

Don't stop breathing, or you'll die!

Moving air in and out the upper Airway

Moving air in and out the lungs

Gas exchange : Alveolar/Capillary level

Oxygen use by the body cells

Breathing

Ventilation

ExternalRespiration

Internal Respiration

MV limit

Non Respiratory functionOf the Respiratory systemReservoir of blood available for circulatory compensation

Blood filter for thrombi and microagreggates

Activation of Angiotensin I and II

Inactivation of : noradrenalin, Bradykenin, prostaglandins

Immunology: Cytokine reservoir and Ig A secretions

Principle of Mechanical Ventilation

Neuromuscular Disorders

DefinitionsAcute respiratory failure occurs when:

pulmonary system is no longer able to meet the metabolic demands of the body

Hypoxaemic respiratory failure:PaO2 < 60 mmhg when breathing room air

Hypercapnic respiratory failure:PaCO2 >50 mmhg

Circulation or labsHypoxia, hypercapnia

Assessment for the needfor Mechanical ventilation

Need for MV

Ventilation

Work ofBreathing

Gas Exchange

Airway Control

Mechanica

l Ventila

tor challen

ge

Airway secretionsStridorBleeding

Decrease GCSNeuromuscular CVA

COPDAsthma

PneumoniaAtelectasisPulmonary emboliEdema

Gas Excange

Mechanical Ventilation- 1

Mechanical Ventilator challenge

• MV1. Increase PAo22. Improve

Atelectasis3. Improve

ventilation

• Depends on– PAO2– Diffusing capacity/

Diffusion coeffecient

– Surface are of membrane available for gas trnasfusion

– Membrane thickness

– Perfusion– Ventilation-

perfusion matching

Oxygen in

V gas = A* D* (P1-P2)/ T

PAO2= PiO2 – PACo2 ( FiO2+ 1-FiO2 )

For FiO2 less than 0.6

PAO2 = FiO2 (PB- PH2O) – Paco2 * 1.2

PAO2 = FiO2 ( 760- 47) – PaCOco2 * 1.2

Ideal Alveolar Air Equation

MV increase Oxygenation

R

Hypoxia with Low Pao2 Alveolar Ventilation Perfusion mismatch Venous to Artery Shunt Diffusion impairment Low o2 tension in inspired airHypoxia with Normal Pao2 Ischemic Hypoxia Generalized ischemia with low CO, Artery stenosis Anemic Hypoxia Anemia, Met HG, Co Histotoxic Hypoxia Cyanide, ATP insufficiency

Pvo2= 40 Pvco2= 45

PAo2= 104 PAco2= 40 PAo2= 104 PAo2= 40

Pao2= 104 Pvco2= 40

Pio2= 160 Pico2= 0.3 Peo2= 120 Peco2= 27

P02= 40 Pco2= 45 Po2= 40 Pco2= 45

Pao2= 104 Pvco2= 40

Tissue Cells

FIO2

Ventilation without

perfusion(deadspace ventilation)

Diffusion abnormality

Perfusion without

ventilation( shunting)

Hypoventilation

Normal

Most common causes of Hypoxia

Normal V/ Q matching

V/Q mismatch: shunting, Dead Space Perfusion

PAO2=104 mmHg PACO2= 45 mmHg

Shunting QT

QcQs

CcO2

CvO2

Qs/ QT= CcO2- CaO2 / Cco2 – CvO2

MV and Shunting

• Intra-cardiac– Any cause of right to left shunt

• eg Fallot’s, Eisenmenger• Intra-pulmonary

– Pneumonia– Pulmonary oedema– Atelectasis– Collapse– Pulmonary haemorrhage or contusion

Qs/ QT= CcO2- CaO2 / Cco2 – CvO2

Mechanical Ventilation and Shunt

CaO2= HG * 1.34 * SaO2 + PaO2 * 0.003

CcO2= HG * 1.34 * FiO2 + PAO2 * 0.003

FIO2

Ventilation without perfusion

(dead space ventilation)

Diffusion abnormality

Perfusion without

ventilation (shunting)

Hypoventilation

Normal

PAO2=104mmHg PACO2=40 mmHg

DeadSpace Perfusion

MV and Dead Space Perfusion

Pulmonary emboliLow blood pressureBlood lossCardiac pump failureHigh intra alveolar pressureInappropriate massive vasodilatation

PAO2=104 PACO2=45

75% 92%

Diffusion abnormalities

Pulmonary Fibrosis Pulmonary edemaThick Alveolar secretions

Brainstem

Spinal cordNerve rootAirway

Nerve

Neuromuscular junction

Respiratory muscle

Lung

Pleura

Chest wall

Sites at which disease may cause ventilatory disturbance

Hypoventilation

Co2 wash-out

Mechanical Ventilation Challenge-2

)V-(V xRR nventilatio Alveolar DT

Carbon Dioxide Elimination

CO2 Production200 ml/min

Plasma PaCO2= 40

CO2 elimination= 200 ml/L

VA= 4 L/minPACO2= 40 mmHg

VA= 2 L/minPACO2= 80 mmHg

CO2 elimination= 200 ml/L

CO2 (1.2 mmol) + H2O ==== H2Co3 (1 molecule) === Hco3 + HCO2 (2.4 mmol) + H2O ==== H2Co3 (2 molecule) === Hco3 + H

Plasma PaCO2= 80

PACO2= 80mmHg

Hypoventilation

Fever, Seizures, Hyper metabolic stateOverfeedingHyperthyroidism

Work of Breathing

MV challenge-3Resistance and Elastance

length

Forc

e

ParenchymaAirways

Resistance to Flow Resistance to Expansion

Flow

Pres

sure

R Raw

Pres

sure

E

Elastics

Volume

Pmus = Resistance x flow + Elastance x volume

Elastance

Resistance

WOB: Mechanical Ventilation

Asthma, COPDSecretions

Croup, Epiglottitis

Pleural Effusion, PneumothoraxAlveolar Edema

AscitesObesity

summery

Restore breathing and protect airways• Optimize Minute Ventilation Improve ventilation/perfusion relationshipDecrease work of breathingImprove oxygenation

summeryDoes not correct every path physiology of

respiratory failure.Corrects only certain types of hypoxiaDoes not replace the non respiratory

function of the lungs.Using Mechanical Ventilation without

knowing the mechanism of a particular respiratory failure could be detrimental.

THANK YOUThank You

Mechanical Ventilation: Indications

Ventilation abnormalitiesRespiratory muscle dysfunction

Respiratory muscle fatigueChest wall abnormalitiesNeuromuscular disease

Decreased ventilatory driveIncreased airway resistance and/or

obstruction

Mechanical Ventilator challenge

Volume

Flow

Pmus = Elastance x volume + Resistance x flow

Mechanical Ventilation: Indications

Oxygenation abnormalitiesRefractory hypoxemiaNeed for positive end-expiratory pressure

(PEEP)Excessive work of breathing

Demands

Alveolar ventilation

Carbon dioxide

Water vapour

Oxygen

Nitrogen

2A2A2A2A NPOHPCOPOPpressure Alveolar

Oxygen inDepends on

PAO2FIO2PACO2Alveolar pressureVentilation

Diffusing capacityPerfusionVentilation-perfusion matching

Pmus + Pvent = elastance x volume + resistance x flow

Unassisted spontaneous inspiration P mus= elastance x volume +

resistance x flow

Assisted ventilation of a paralyzed patient

P vent = elastance x volume + resistance x flow

OxygenationThe primary goal of oxygenation is to maximize O2 delivery to blood (PaO2)

• Alveolar-arterial O2 gradient (PAO2 – PaO2)

• Equilibrium between oxygen in blood and oxygen in alveoli

• A-a gradient measures efficiency of oxygenation

• PaO2 partially depends on ventilation but more on V/Q matching

• Oxygenation in context of ICU

• V/Q mismatching• Patient position (supine)• Airway pressure, pulmonary

parenchymal disease, small-airway disease

• Adjustments: FiO2 and PEEP

V/Q Matching. Zone 1 demonstrates dead-space ventilation (ventilation without perfusion). Zone 2 demonstrates normal perfusion. Zone 3 demonstrates shunting (perfusion without ventilation).