Post on 24-Feb-2016
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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).