PRINCIPLES OF
MECHANICAL VENTILATION
BY:
DR. AHMED S. EL-KOMI, MD
Specialist of Anaesthesia & ICU
Farwaniya Hospital
1
Introduction
• Cornerstone for intensive care medicine
• The ventilators must overcome the pressure
generated by the elastic recoil of the lung at
end inspiration plus the resistance to flow at
the airway.
• Ventilators provide infusion of a blend of air or
oxygen into the circuit.
History
• In 1543, Vesalius demonstrated the ability to
maintain the beating heart in animals with open
chest.
• In 1780, such technique were first applied to humans
• In 1887, fell-o-dwyer apparatus was used for
translaryngeal ventilation via a bellows.
• In 1928, the drinker–Shaw iron lung based on
negative pressure ventilation
• From 1930-1950 – such machines were the mainstay
in ventilation of victims of polio epidemics
Drinker-Shaw iron lung
MECHANICAL VENTILATION
1.
2.
3.
4.
5.
6.
7.
Objectives
Clinical conditions need ventilator support
Physiological effects
Types
Complications
Monitoring
Weaning
Objectives
Overcome mechanical problems:
• Rest fatigued muscles
• Administer anesthesia and NMB
• Prevent or treat atelectasia
Regulate gas exchange:
•
•
•
•
•
Normalize PaCO2
 PaCO2 < Normal   ICT and correct metabolic acidosis
 PaCO2 > Normal  Permissive hypercapnia
Reverse hypoxaemia
 Myocardial O2 consumption
Increase Lung volumes
• During end inspiration  improve V/Q  Treatment of severe
hypoxemic respiratory failure
• During end expiration  improve FRC by PEEP Treatment of
ARDS, postoperative atelectasis and alveolar collapse
• It is the volume remaining after normal
expiration
• Act as an oxygen reserve  maintain
oxygenation of blood passing the lung
during expiration / breath holding.
(PAO2 decreased by 3 mmHg during expiration)
Clinical conditions need ventilator
support
Pulmonary
Extra pulmonary
ARDS
P’ cardiothor. Surg
BA
Head
Trauma
Acute
COPD
Severe
NM def.
Chest
Trauma
Drug
overdose
Severe pneumonia
Sepsis
Physiological effects
• Cardiovascular system
Positive pressure ventilation
results in:
1. Rise in pleural pressure
2. Rise in intra-abdominal
pressure
3. Increased lung volumes
All these produce cardiovascular
changes
Physiological effects
Preload
LV preload reduced by a variety
of mechanisms
Venous return
• In a volume resuscitated
patient venous return does
not fall
• Intrathoracic pressure is positive
rather than negative but
• Intra-abdominal pressure also rises
• Pressure gradient between abdomen
and thorax is maintained
Physiological effects
• In a volume depleted patient
Positive pressure intrathoracic 
Collapse of intra-abdominal veins
and SVC
• Abdomen collapse behind the liver
as a result of positive pleural
pressure transmitted through the
diaphragm and liver
• Superior vena cava results in a fall
in venous return
• RV stroke volume and hence LV
preload
Physiological effects
• Renal
• Renal blood flow falls if cardiac output falls
• Decreased sodium secretion due to fall in cardiac
output and decreased secretion of atrial natriuretic
factor
• Increased water retention due to increased
secretion of ADH, particularly in children.
• CNS
Increased intrathoracic pressure decreases venous
drainage from head and may increase ICP.
MECHANICAL VENTILATION
• Non-invasive ventilation
• Invasive ventilation
Non-invasive ventilation
• Indications
» Decreased ventilation
» Increased work of breathing
» Post-extubation for some COPD patients
• Types
– Face mask / Nasal cannula
– Non-invasive mechanical ventilation
Non-invasive ventilation
Non-invasive mechanical ventilation
– BiPAP
• Bi-level CPAP
• Two pressure levels (PH & PL)
– IPAP (PH)
– EPAP (PL)
• Delivered by nasal or full face mask
Non-invasive ventilation
• Advantages
1. Avoids intubation
2. Preserves lower airways defense mechanisms  decrease
noso-comial infection (Pneumonia or sinusitis)
3. Decreases patient’s discomfort
4. Preserves speech and swallowing
5. Decreases the amount of sedation
6. Allows application of therapy intermittently
7. Decreases the work of breathing 
•  Hypocarbia
•  O2 Consumption
Non-invasive ventilation
• Disadvantages
1.
2.
3.
4.
Limited access for airway management
Mask  discomfort and complications
Leaks  inadequate ventilation
Needs alert and cooperative patient
• Contraindications:
1.
2.
3.
4.
5.
Facial deformity / fracture
Inability to protect airway
Excessive secretion
Decreased mental status
Hemodynamic instability
Invasive Ventilation
 With Intubation
1. Mechanical ventilation
2. T-piece tube
What is a breathing machine (mechanical ventilator)?
• A breathing machine helps the patient breathe.
• It is designed to help patients who cannot breathe adequately on their
own.
• The breathing machine does not fix any problems of the lungs.
• It is a device that simply pushes air and oxygen into the lungs and
withdraws carbon dioxide from the lungs.
• The lungs must function in order for the breathing machine to be
effective
Invasive Mechanical Ventilation
INDICATIONS
Clinical Indices:
• Unconsciousness
• Apnea
• RR > 35 breaths / min
• Respiratory muscle paradox
(agonal breathing)
Invasive Mechanical Ventilation
INDICATIONS
Respiratory Gas Tension
• Direct indices
• PaO2 < 50 mmHg (room air) / < 60 mmHg (FiO2 > 0.5)
• PaCO2 > 50 mmHg (pH < 7.25)
• Derived indices
• PaO2/ FiO2 ratio < 200
• PA-aO2 gradient > 300 (FiO2 1.0)
• Vd / Vt > 0.6 (N = 0.3) *
• Virtual shunt fraction > 20% (N < 5%)
Invasive Mechanical Ventilation
INDICATIONS
Mechanical Indices:
•
•
•
•
Vt < 5 mL/Kg (N = 7-10 mL/Kg)
Vital capacity (VC) < 10-15 mL/Kg (N = 50-55 mL/Kg)
Max inspiratory force > -25 cmH2O (-20 or -15)
Rapid shallow breathing index (RR/Vt) > 200
breath/min/L
• VE < 4 L/min or > 10 L/min
Ventilator cycle
inspiration
pause
pause
expiration
Invasive Mechanical Ventilation
CLASSIFICATIONS OF MECHANICAL
VRNTILATORS
Idea
Indications


Advantages
1.
2.
3.
Disadv.
Pressure Controlled
Volume Controlled
Deliver a volume of air till a
preset pressure is reached
Constant Pinsp & Variable vol.
Deliver a preset volume of air
regardless the oposing pressure
Constant vol. & Variable Pinsp
Neonatal ventilators
Transport ventilators
Compensate for leaks
Protect against barotraumas
Recruit collapsed alveoli
Deliver less Vt in  Paw
e.g. circuit kink, obstruction or
bronchospasm


Adult ICU ventilators
OR ventilators
Deliver enough Vt in  Paw
e.g. circuit kink, obstruction or
bronchospasm
1.
2.
No compensation for leaks
Barotraumas
Components
1.
Bacterial filter
2.
Pneumotachometer, valves & solenoids
3.
Humidifier
4.
Heater/ theremostat
5.
Oxygen analyser
6.
Pressure manometer
7.
Chamber for nebulising drug
Invasive Mechanical Ventilation
INITIAL VENTILATOR SETTINGS
1.
2.
3.
4.
5.
6.
7.
Tidal Volume (Vt)
Respiratory Rate (RR)
Inspiratoty Oxygen Fraction (FiO2)
I:E Ratio
Inspiratory Flow Rate (IFR)
Trigger Sensitivity
PEEP
Invasive Mechanical Ventilation
INITIAL VENTILATOR SETTINGS
1. Tidal Volume (Vt)
preset at 7-10 ml/Kg
Invasive Mechanical Ventilation
INITIAL VENTILATOR SETTINGS
1.
Tidal Volume (Vt)
2. Respiratory Rate (RR)
– If the patient is clinically stable  8-14 breath/min
– Restrictive lung disease 
RR
– Chronic respiratory acidosis 
RR
– Too high RR  Respiratory alkalosis - barotraumas
– Too low RR  hypoventilation -  work of breathing
Invasive Mechanical Ventilation
INITIAL VENTILATOR SETTINGS
1.
2.
Tidal Volume (Vt)
Respiratory Rate (RR)
3. Inspiratoty Oxygen Fraction (FiO2)
Adjusted according to ABG within 20-30 min to keep SaO2
> 90% and PaO2 > 60mmHg
4. I:E Ratio
Normally 1:2 -
in ARDS: IRV 2:1
Invasive Mechanical Ventilation
INITIAL VENTILATOR SETTINGS
1.
2.
3.
4.
Tidal Volume (Vt)
Respiratory Rate (RR)
Inspiratoty Oxygen Fraction (FiO2)
I:E Ratio
5.
Inspiratory Flow Rate (IFR)
= tidal volume/inspiratory time Vt/Ti
set at 40-90 L/min
• High IFR ≥ 90 L/min
– Used in COPD to increase Te  AutoPEEP
– Disadvantages: PIP  risk of barotrauma
• Low IFR ≤ 40 L/min
– Used in ARDS to avoid PIP and risk of barotraumas
– AutoPEEP
Invasive Mechanical Ventilation
INITIAL VENTILATOR SETTINGS
1.
2.
3.
4.
5.
Tidal Volume (Vt)
Respiratory Rate (RR)
Inspiratoty Oxygen Fraction (FiO2)
I:E Ratio
Inspiratory Flow Rate (IFR)
6. Trigger Sensitivity
Determines start of inspiration
Pressure set at -0.5 to 2.0
– -0.5  too sensitive  self cycling
– -2.0  too insensitive   work of breathing
Invasive Mechanical Ventilation
INITIAL VENTILATOR SETTINGS
1.
2.
3.
4.
5.
6.
Tidal Volume (Vt)
Respiratory Rate (RR)
Inspiratoty Oxygen Fraction (FiO2)
I:E Ratio
Inspiratory Flow Rate (IFR)
Trigger Sensitivity
7. PEEP


Positive end-expiratory pressure
Holds alveolar sacs open and recruits more alveoli

Preset (physiological) = 3-5 cmH2O
Invasive Mechanical Ventilation
Invasive Mechanical Ventilation
Your patient acutely develops inadequate alveolar
ventilation despite correct alveolar settings
What causes must you rapidly rule out?
Invasive Mechanical Ventilation
Invasive Mechanical Ventilation
MODES OF MV
i.
ii.
iii.
A/C
SIMV
PSV
Assisted Control
Synchronized IMV
Pressure Support Vent.
Invasive Mechanical Ventilation
MODES OF MV
i.
A/C
•
•
•
•
Assisted Control
Volume controlled mode (preset Vt + RR)
The ventilator senses any patient-generated resp. effort and
intermediately follows with a full machine-powered breath
Little patient’s work of breathing
Respiratory alkalosis is common
Invasive Mechanical Ventilation
MODES OF MV
iii.
•
•
SIMV
Synchronized IMV
The ventilator senses the patient’s spontaneous
breath and times the machine-delivered breaths
with the patient’s breathing cycle
SIMV prevents breaths stacking
Which is the best ventilation mode for
each of the following clinical scenarios ?
 A trauma patient who is
totally apneic
SIMV
 ARDS patient with very
low lung compliance
A/C
 24HRS P’ laporotomy
with spont. breaths
PCV
Which is the best ventilation mode for
each of the following clinical scenarios ?
 A trauma patient who is
totally apneic
SIMV
 ARDS patient with very
low lung compliance
A/C
 24HRS P’ laporotomy
with spont. breaths
PCV
Invasive Mechanical Ventilation
COMPLICATIONS
1) RESPIRATORY:
– Hypo or hyperventilation
– Complications of excessive pressure and flow rates
• Pulmonary barotraumas
• Pulmonary volutrauma
–
–
–
–
Patient/ventilator asynchrony
Ventilator malfunction
Ventilator associated pneumonia
Complications of tracheal intubation
Invasive Mechanical Ventilation
COMPLICATIONS
2) CVS:
Effects of  ITP and PEEP
 Cardiac output
 Hepatic blood flow
 RBF
 ICP
3) OXYGEN
TOXICITY:
4)
GIT:
GIT bleeding
GIT colonization
Invasive Mechanical Ventilation
COMPLICATIONS
5)
6)
RENAL:
NUTRITIONAL:
 UOP
–  Caloric requirements
–  PaCO2
7)
PSYCHOLOGICAL TRAUMA:
8)
ADJUVANT DRUGS SIDE EFFECTS:
– Morphine
– Benzodiazepines
– NMBs
Monitoring
• Clinical
• Radiological
• Biochemical
• Bacteriological
• others
Clinical monitoring
General Appearance
• Level of activity
• Response to stimulus
• Eye opening
• Posture
• Perfusion
• Color
• Edema
Clinical monitoring
Adequacy of mechanical breath
• Movement of chest
• Retractions
• Synchronization
• Air entry
Clinical monitoring
Monitoring of O2 & CO2 status
• Pulse oximetry
• EtCO2 monitoring
• ABG analysis
• Capillary gas determination
• Transcutaneous monitoring
• Oxygenation indices
Clinical monitoring
Ventilator Parameters
• PIP
• PEEP
• MAP
• RR
• Ti & I:E Ratio
• FiO2
• VT
• Trends of Ventilator Parameters
• Pulmonary Graphics
Hemodynamic Stability
• Oxygenation
• Adequacy of Circulation
Radiological Monitoring
When to do CXR ?
• At the start of ventilation
• After ET tube change
• Sudden deterioration
• Prior to extubation
• Post extubation
Biochemical Monitoring
• Blood Gases
• Blood Sugar
• Serum calcium
• Serum electrolytes
Bacteriological Monitoring
Blood culture
ET tube culture
Other Monitoring
• Humidification & warming of ventilator circuit gases
• Position of patient
• Skin
• Fluid & electrolytes
• Nutrition status
• Sensorium
• Infection control
Invasive Mechanical Ventilation
WEANING
Definition:
Process by which ventilator-dependent patient
is removed from ventilator
Invasive Mechanical Ventilation
 VC > 15 ml/kg
Standard criteria for initiating Max inspiratory pressure >
-25 cm H2O
weaning:
 Tobin Index of rapid and
 Patient awake and
shallow breathing RR/Vt
cooperative
 Clinically and radiologically – RR/Vt>105 95% wean
attempts unsuccessful
resolving lung disease
– RR/Vt<105 80% successful
 PaO2>60 mmHg on FiO2
<0.4 and < 10 cm PEEP
 PaCO2 <50 mmHg
 RR < 30/min
 Minute volume >10 L/min
WEANING
Approaches To Weaning
•
•
•
•
SIMV – low rate
Spontaneous breathing trials (SBT)
Pressure support ventilation (PSV)
New weaning modes - ASV
Mechanical Ventilation
Rest 24 hrs
PaO2/FiO2 ≥ 200 mm Hg
PEEP ≤ 5 cm H2O
Intact airway reflexes
No need for continuous infusions of vasopressors or inotrops
> 100
RSBI
<100
Stable Support Strategy
Assisted/PSV
24 hours
Daily SBT
Low level CPAP (5 cm H2O),
Low levels of pressure support (5 to 7 cm H2O)
“T-piece” breathing
30-120 min
Yes
RR > 35/min
Spo2 < 90%
HR > 140/min
Sustained 20% increase in HR
SBP > 180 mm Hg, DBP > 90 mm Hg
Anxiety
Diaphoresis
No
Extubation
Invasive Mechanical Ventilation
WEANING
Correction of factors
complicates weaning:
a) Bronchospasm
b) Malnutrition
c) Anemia
d) Infection
e) Metabolic alkalosis
f) Sleep deprivation
g) Increased CO2
production
h) Hypothermia or
hyperthermia