PRINCIPLES OF MECHANICAL
VENTILATION and
ABDELMONIEM MOHAMED HAMID
Associate Professor of Paediatrics
Alneelein University
October 2019
Definitions
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Tidal Volume (TV): volume of each breath.
Rate: breaths per minute.
Minute Ventilation (MV): total ventilation per minute. MV
= TV x Rate.
Flow: volume of gas per time.
Compliance: the distensibility of a system. The higher the
compliance, the easier it is to inflate the lungs.
Resistance: impediment to airflow.
SIMV: patient breathes spontaneously between ventilator
breaths. Allows patient-ventilator synchrony, making for a
more comfortable experience.
 PIP: maximum pressure measured by the ventilator
during inspiration.
 PEEP: pressure present in the airways at the end of
expiration.
 CPAP: amount of pressure applied to the airway
during all phases of the respiratory cycle.
 PS: amount of pressure applied to the airway
during spontaneous inspiration by the patient.
 I-time: amount of time delegated to inspiration.
Indications of Mechanical Ventilation
Indications of Mechanical
Ventilation
 Respiratory Failure
◼ Apnea / Respiratory Arrest
◼ inadequate ventilation (acute vs.
chronic)
◼ inadequate oxygenation
◼ chronic respiratory insufficiency with
FTT
Indications
 Cardiac Insufficiency
◼ eliminate work of breathing
◼ reduce oxygen consumption
 Neurologic dysfunction
◼ central hypoventilation/ frequent
apnea
◼ patient comatose, GCS < 8
◼ Increased intracranial pressure
◼ inability to protect airway
Respiratory Failure
◼ Hypoxemic
 Room air PaO2 60 mm Hg (6.7-8 kPa)
 Abnormal PaO2:FIO2 ratio
◼ Hypercapnic
 PaCO2 50 mm Hg (6.7 kPa) with pH <7.36
◼ Mixed (most common)
Respiratory Physiology
PaO2 (mm Hg)
80
60
50
SpO2 (%)
95
90
80
Respiratory Physiology
O2 (%)
21
30
40
50
100
PaO2 (mm Hg)
90
150
200
250
500
PaO2:FIO2 Ratio
− Normal >300
− Severe <200
Examples of Hypoxic Respiratory
Failure
 acute respiratory distress syndrome,
or ARDS
 pneumonia
 congestive heart failure
 pulmonary emboli
 and interstitial lung disease
Case Scenario
◼ 7 years old unwell boy presented
with dyspnea with I/C and S/C
recessions. Pulse 100/min RR
50,Temp 39 C , O2 sat 91%
◼ Arterial blood gas( In 40% Oxygen by
mask):
 pH 7.32, PaCO2 55 mm Hg, PaO2 60 mm Hg
What is his physiological status?
Does he need ventilation? Why?
Examples of Hypercapnic
Respiratory Failure
 Traumatic brain injury
 Sedative drugs
 Neuromuscular disease – such as
myasthenia or Guillain-Barré
syndrome
 Sleep apnea
 Metabolic abnormalities
Examples Of Mixed Respiratory
Failure
 Mixed respiratory failure is common
in critical illness, and its causes are
often multifactorial
 Chronic obstructive pulmonary
disease
 severe congestive heart failure.
TYPES OF VENTILATION
 Noninvasive Ventilation Positive
Pressure Ventilation
 Invasive Ventilation
Noninvasive Positive Pressure
Ventilation vs. Intubation
What are the advantages of noninvasive
positive pressure ventilation over
conventional mechanical ventilation?
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Avoids complications of intubation
Preserves airway reflexes
Improved patient comfort
Less need for sedation
Can improve respiratory rate, heart rate,
work of breathing, and dyspnea score
Copyright 2013 Society of Critical Care
Medicine
Noninvasive Positive Pressure
Ventilation
◼ Bilevel positive airway pressure
◼ Nasal or face mask
◼ Inspiratory positive airway pressure,
expiratory positive airway pressure, FIO2,
rate?
◼ Contraindications
 Altered mental status
 Hemodynamic instability
 High risk for aspiration
 Rapid progression of respiratory failure
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Copyright 2013 Society of Critical Care
Medicine
TYPES OF VENTILATION
 Volume targeted modes
 Pressure targeted modes
Volume targeted modes
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Controlled Mechanical Ventilation (CMV)
Assisted Mechanical Ventilation (AMV)
Assisted Control Ventilation (ACV)
Intermittent Mandatory Ventilation(IMV)
Synchronized Intermittent Mandatory
Ventilation (SIMV)
Controlled Mechanical Ventilation
(CMV)
 The ventilator controls all ventilation
while patient has minimal or no
respiratory effort.
 This is the mode used at the
intitiation of mechanical ventilation
Assisted Mechanical Ventilation
(AMV)
 All breaths are triggered when the
patients inspiratory effort exceeds the
preset sensivity threshold of negative
pressure.
 Patients with central hyperventilation
syndromes and tachypnea generate
an extremely high minute ventilation.
 Oversedated patients or those too
weak to trigger the breath, may
receive slow ventilator rates.
Assisted Control Ventilation
(ACV)
 ACV is a combination of AMV and
CMV.
 The patient initiates the breathing as
in AMV.
 However, if the patients fails to
initiate the breathing within a
prescribed time the ventilator triggers
the breathing , thus ensuring a
gauranteed minute ventilation
Intermittent Mandatory
Ventilation (IMV)
 It is a combination of spontaneous
breathing and CMV.
 A modified circuit provides a
continuous gas flow that allows the
patient to breathe spontaneously with
minimal work of breathing.
 At a predetermined frequency, the
ventilator provides a positive
pressure breath to the patient.
Synchronized Intermittent
Mandatory Ventilation (SIMV)
 In conventional IMV, the controlled
breaths may conflict with the patient’s
own respiratory effort.
 SIMV allows the patient to trigger the
mandatory breath in the assist mode
thereby synchronizing it with the
patients respiratory effort.
 If the patient does not trigger a breath
within an alloted time; the ventilator
delivers a conventional breath.
Pressure targeted modes
 Pressure Support Ventilation (PSV)
 Pressure Control and Pressure Assist
Control Ventilation (PCV and PACV)
Pressure Support Ventilation
(PSV)
 The patient triggers the breath as in assisted
ventilation. Therefore, this mode is applicable
only to spontaneous breathing patients.
 Once intiated the ventilator delivers air and gas
mixture at a preset positive pressure in the
ventilator circuit.
 Patients determine their own inspiratory time
and tidal volume.
 It is mainly used as a weaning mode and may
be tolerated better than SIMV by some
patients.
Pressure Control and Pressure Assist
Control Ventilation (PCV and PACV)
 This is a time initiated, pressurelimited and time-cycled mode
intended for patients requiring total
mechanical ventilator support.
 Most ventilators also allow patient
triggering of these breaths; producing
pressure assisted breaths (pressure
assist control ventilation).
Conventional Modes Of Ventilation
Controlled mechanical ventilation,
or CMV.
 Assist-control ventilation, or AC.
 Synchronized intermittent.
mandatory ventilation, or SIMV.
 Pressure Support ventilation.
 CPAP is not a mode of ventilation!
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Triggering the Ventilator
CPAP-Pressure Support
 No mandatory breaths
 Patient sets the rate, I-time, and respiratory
effort.
 CPAP performs the same function as PEEP,
except that it is constant throughout the
inspiratory and expiratory cycle.
 Pressure Support (PS) helps to overcome
airway resistance and inadequate pulmonary
effort and is added on top of the CPAP during
inspiration.
 The ventilator increases the flow during
inspiration to reach the target pressure and
make it easier for the patient to take a breath.
SIMV + PS
Initial Ventilator Settings
 Rate: 20-24 for infants and preschoolers
16-20 for grade school kids 2-16 for
adolescents
 TV: 6-8ml/kg
 PEEP: 3-5cm H2O
 FiO2: 100%
 I-time: 0.7 sec for higher rates, 1sec for
lower rates
 PIP (for pressure control): about 24cm
H2O.
 Pressure Support: 5-10cm H2O.
Mechanical Ventilation Setting
 Tidal volume (6-8 ml/kg) should be
adjusted to achieve visible chest
excursion and audible air entry while
maintaining a SpO2 >90% and PaO2
>60 mm Hg.
 FiO2 should be decreased to <0.5 as
soon as possible to prevent oxygen
toxicity and atelectasis.
 In chronically ventilated patients
(more than 2-4 weeks), it is better to
maintain the airway through a
tracheostomy tube
Table IV - Initial Ventilator Settings in Common Diseases
Disease
PIP (cm water)
PEEP (cm water)
Rate (per
minute)
I:E ratio
Bronchopneumoni
a
15-25
0-3
20-30
1:2 (Ti 0.3-0.4)
Bronchial asthma
30-40
0-4
8-12
1:4
Meconium
aspiration
syndrome
25-35
0-3
40-60
1:3 (Ti 0.2-0.3)
Respiratory
Failure (Post
operative, Guillain
Barre syndrome)
15-20
4-6
20-30
1:1.5
PIP: Peak inspiratory pressure; PEEP: Positive end expiratory pressure;
I:E = Inspiration to expiration ratio; Ti: Inspiration time (seconds).
Blood Gas Interpretation
NORMAL VALUES
Arterial
Venous
pH
7.4 (7.38-7.42) 7.36 (7.31-7.41) 7.35-7.40
pO2
80-100 mm Hg
35-40 mm Hg
45-60
mm Hg
pCO2 35-45 mm Hg
41-52 mm Hg
40-45
mm Hg
Sat >95% on RA 60-80% on RA
>70%
HCO3 22-26 mEq/L
22-26mEq/L
2226mEq/L
BE -2 to +2
-2 to +2
-2 to +2
Adjusting The Ventilator
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pCO2 too high
pCO2 too low
pO2 too high
pO2 too low
PIP too high
pCO2 Too High
pCO2 Too High
Patient’s minute ventilation is too low.
Increase rate or TV or both.
If using PC ventilation, increase PIP.
If PIP too high, increase the rate instead.
If air-trapping is occurring, decrease the rate
and the I-time and increase the TV to allow
complete exhalation.
 Sometimes, you have to live with the high
pCO2, so use THAM or bicarbonate to
increase the pH to >7.20.
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pCO2 Too Low
pCO2 Too Low
 Minute ventilation is too high.
 Lower either the rate or TV.
 Don’t need to lower the TV if the PIP
is <20.
 PIP <24 is fine unless delivered TV is
still >15ml/kg.
 TV needs to be 8ml/kg to prevent
progressive atelectasis
 If patient is spontaneously
breathing, consider lowering the
pressure support if spontaneous TV
>7ml/kg.
pO2 Too High
pO2 Too High
 Decrease the FiO2.
 When FiO2 is less than 40%,
decrease the PEEP to 3-5 cm H2O.
 Wean the PEEP no faster than about
1 every 8-12 hours.
 While patient is on ventilator, don’t
wean FiO2 to <25% to give the
patient a margin of safety in case
the ventilator quits.
pO2 Too Low
pO2 Too Low
 Increase either the FiO2 or the mean airway
pressure (MAP).
 Try to avoid FiO2 >70%.
 Increasing the PEEP is the most efficient way
of increasing the MAP in the PICU.
 Can also increase the I-time to increase the
MAP (PC).
 Can increase the PIP in Pressure Control to
increase the MAP, but this generally doesn’t
add much at rates <30bpm.
 May need to increase the PEEP to over 10,
but try to stay <15 if possible.
PIP Too High
PIP Too High
 Decrease the PIP (PC) or the TV
(VC).
 Change to another mode of
ventilation. Generally, pressure
control achieves the same TV at a
lower PIP than volume control.
 If the high PIP is due to high airway
resistance, generally the lung is
protected from barotrauma unless
air-trapping occurs.
Weaning Priorities
Wean PIP to <35cm H2O
Wean FiO2 to <60%
Wean I-time to <50%
Wean PEEP to <8cm H2O
Wean FiO2 to <40%
Wean PEEP, PIP, I-time, and rate
towards extubation settings.
 Can consider changing to volume control
ventilation when PIP <35cm H2O.
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Weaning-Pulmonary
 Patient should have a patent airway.
 Pulmonary compliance and resistance should
be near normal.
 Patient should have normal blood gas and
work-of-breathing on the following settings:
◼ FiO2 <40%
◼ PEEP 3-5cm H2O
◼ Rate: 6bpm for infants, 2bpm for toddlers,
CPAP/PS for 1hr for older children and
adolescents
◼ PS 5-8cm H2O
◼ Spontaneous TV of 5-7ml/kg
Complications
 Pulmonary
◼ Barotrauma
◼ Ventilator-induced
lung injury
◼ Nosocomial
pneumonia
◼ Tracheal stenosis
◼ Tracheomalacia
◼ Pneumothorax
 Cardiac
◼ Myocardial ischemia
◼ Reduced cardiac
output
 Gastrointestinal
◼ Ileus
◼ Hemorrhage
◼ Pneumoperiteneu
m
 Renal
◼ Fluid retention
 Nutritional
◼ Malnutrition
◼ Overfeeding
Acute Deterioration
 DIFFERENTIAL DIAGNOSES
◼ D
◼ O
◼ P
◼ E
◼ S
◼ Air leak