mechanical ventilation

1. Mechanical
Ventilation
by Dr. Islam Ellakany
ER specialist
Alexandria university

2.  Hypoxemia :
 P(A − a)O2 gradient (mm Hg) 25-65
 Palveolar=FIO2(Patm-PH2O)=150
 (0.21x(760-47) = 149.7 at sea level 100% humidity
 Aa gardient=Palv-4/5PCO2-PaO2=5-10
 Increment reflects V/Q mismatch, diffusion problem (ARDS) and right to left shunt
 SaO2 below 90% despite supplemental oxygen
 Hypoxic index below 300 (pneumonia, ARDS or aspiration)
 Hypercarbia
 PaCO2 acute increase than basal line
 Respiratory acidosis PH below 7.2
 Disturbed conscious level due to hypoventilaion
Indications of mechanical ventilation

3.  Oxygen delivery/oxygen consumption imbalance
 Elevated lactate ≥4 mg/dL despite adequate resuscitation
 Decreased mixed venous oxygen saturation <70% despite adequate acute resuscitation
 Increased work of breathing
 Dead space more than 0.5 acute
Vd/VT=PaCo2-PeCo2/PaCo2
 Inspiratory muscle weakness
 Negative inspiratory pressure below 20-30 mm Hg
 Vital capacity below 15 ml/kg
 Acute decompensated heart
 Decompensated heart failure
Jugular venous distension
Pulmonary oedema

4.  Oxygen delivery/oxygen consumption imbalance
 Elevated lactate ≥4 mg/dL despite adequate resuscitation
 Decreased mixed venous oxygen saturation <70% despite adequate acute
resuscitation
 Increased work of breathing
 Dead space more than 0.5 acute
Vd/VT=PaCo2-PeCo2/PaCo2
 Inspiratory muscle weakness
 Negative inspiratory pressure below 20-30 mm Hg
 Vital capacity below 15 ml/kg
 Acute decompensated heart
 Decompensated heart failure
Pulmonary oedema
Jugular venous distension

5.  Inadequate lung expansion
 VT below 4 ml/kgm
 Vital capacity below 10ml/kgm
 Respiratory rate more than 35 per minute

6.  Hippocrates likely gave the first description of endotracheal
intubation “One should introduce a cannula into the trachea
along the jawbone so that air can be drawn into the lungs.”
 The first known mechanical device designed specifically to
provide ventilation for the patient was the foot pump developed
by Fell and O’Dwyer in the 1880s
History

8.  First generations of mechanical ventialators invented and
applied a negative pressure around the body or chest cavity
Two classic devices that provided negative-pressure ventilation
were the iron lung and the chest cuirass widely used during the
poliomyelitis epidemics of the 1930s and 1940s.
 After the polio epidemic of the 1960s, the era of respiratory
intensive care emerged, as positive-pressure ventilation via an
artificial airway became common place

9. Iron lung

10. Iron lung during polio epidemic

11.  Negative pressure ventilation decreased a lot in early 80s then continue for a while
in the 90 lung jacket and cruras.
 Physiologic they may lead to decrease venous return especially in hypovolemia as
they apply negative pressure in abdomen as well as the chest.

13.  Diaphragmatic and intercostal muscle activation during normal inspiration
expands the chest and decreases intrapleural pressure from –5 cm H2O to –8
cm H2O. Alveolar pressure fluctuates from 0 cm H2O during exhalation to –4
cm H2O during inspiration.
 Normal venous admixture is about 2% to 5%. Mechanical ventilation may
increase the venous admixture to approximately 10% in the normal individual.
 Anatomic dead space is the volume of the conducting airways of the lungs,
about 150 mL. Alveolar dead space refers to alveoli that are overventilated
relative to perfusion; it is increased by any condition that reduces pulmonary
blood flow, such as pulmonary embolism (PE) or with overdistention of the
lung. Mechanical dead space refers to the rebreathed volume of the ventilator
circuit; this volume behaves like an extension of the anatomic dead space.
Mechanical ventilation can also increase dead space if it leads to
overdistention.
Basic concepts

15.  Dead space can be calculated by the following equation:
 𝑉𝐷/𝑉𝑇=
𝑃𝑎𝐶𝑜2−𝑃𝑒𝑡𝐶𝑜2
𝑃𝑎𝐶𝑜2
this is equals shunt fraction in the lung
 Causes of increase is
 pulmonary embolism
 lung contusion
 lung consolidation
 An increased dead space fraction requires a greater minute ventilation to maintain alveolar ventilation and Paco2
 Hypoventilation raises Paco2: a modest elevation (50-70 mm Hg) reduces pH and is usually not by itself injurious in the
mechanically ventilated patient.

16. Flow
• Volume per unit
time
• V =
Volume
Flow Rate
Time
 Vt
T

17. Pressure
 Force created by the
flowing gas
 P =
𝐹
𝐴

18. Pressure Vs. Flow Vs. Volume
Volume
Flow Rate
Time

19.  Compliance: change in volume per unit change in pressure. ( inside alveolus)
 (CRS) is ΔV/ΔPalveolar
 equation of motion: Muscle pressure + Ventilator pressure = (Elastance x
Volume)+(resistance x flow)
 Elastance is the inverse of compliance.
 Resistance describes the impedance to airflow through the respiratory
system.
 elastic load is the pressure required to overcome the elastance of the
respiratory system.
 resistive load is the pressure required to overcome flow resistance of the
ventilator circuit, endotracheal tube, and airways.

20.  Muscle power: Elastance X Volume= 1/compliance X Volume = ΔP/ΔV X Volume = ΔP
 Both volumes are tidal volume so muscle power is ∆P
 during mandatory ventilation no muscle contraction and no pressure change due to muscle contraction
N.B bipap where spontaneous breathing is allowed during mandatory breath

21. Lung Compliance
Normal
Emphysema
Fibrosis
FRC
FRC
FRC
Transpulmonary pressure
Volume
In emphysema low pressure needed to attain
VT less than normal lung while fibrosis stiff lung
needs more pressure to attain same VT

22.  Ppeak Maximum pressure in the proximal
airway at the end of inspiration it is the
pressure responsible for deliver tidal
volume
 Pplateau Equilibrium pressure reached if
the expiratory tube is occluded at the
end of inspiration (no flowin the circuit)
 PEEP: Positive end expiratory pressure its
pressure left in the alveoli to prevent its
collapse
 PEEPi auto PEEP its due to trapped air
volume in the lung to get rid of it
prolong expiration or disconnect circuit
then reconnect as it causes hypoxia and
hypoventilation
Airway Pressures

23. Airway pressures
time
pressure
Pres
Pplat
Pres
Scenario # 1
Paw(peak) = (Flow x Resistance) + (Volume x 1/ Compliance)
Paw(peak)
This is a normal
pressure- time waveform
with normal peak
pressures ( Ppeak) ;
plateau pressures (Pplat )
and airway resistance
pressures (Pres)

24.  It represents the area under the curve
 Increase mean airway pressure  improve
oxygenation
 Items increase mean airway pressure
 Increase PEEP
 decrease insp. Time
 Increase tidal volume
 Flow increase
 Spasm (airway obst.)
Mean airway pressure

25.  Resistance: is the impedance of airway and tubing system to flow it increases on mechanical ventilation
 Airway resistance= (PIP-PPlat)/Flow cmH2o.L/sec
 Non intubated its only0.6-2.4 in intubated its 4-6

26.  Boyle marriot law P X V = constant
 Resistance is measured as pressure difference between beginning and end
the tube and the flow of gas volume per unit time.
R= ∆P/flow(mbar/L/second)
Healthy 2-4 higher in children and infant
Intubated 4-6
𝐦𝐚𝐱𝐢𝐦𝐮𝐦 𝐩𝐫𝐞𝐬𝐬𝐮𝐫𝐞 − 𝐩𝐥𝐚𝐭𝐞𝐚𝐮 𝐩𝐫𝐞𝐬𝐬𝐮𝐫𝐞
𝐟𝐥𝐨𝐰
Hangen posseuille’s law:
𝐑 ∝ 𝟏
𝐫𝟒
r is the radius of the airway

27.  It is a compressible tubes designed to deliver air from ventilator to ETT or TT
or mask.
 It is responsible for discrepancy between VT inspired and VTe.
 Its compliance is equal 2-3 cm3/cm H2o
 Part of it is the humidifier which is responsible for humidification and
adjusting temperature of inspired air to reach alveoli near 37oc. And fully
humidified 44 mg/L.
 It is well known that lung uses daily 250 ml water to humidify air delivered
Ventilator circuit

28. An HME, which is placed between the artificial airway
and the ventilator circuit, may be used to replace the
traditional heated humidifier. During exhalation,
moisture and heat from the patient are absorbed into
the honeycomb structure of the exchanger and are
transferred back to the patient during the next
inhalation. Ventilator circuits with bacterial-viral
filtering HMEs cost less to maintain and are less likely
to colonize bacteria than those with heated
humidifiers..
HME(heat moisture exchange)

29. Contraindications for use of an HME:
 Thick or large amounts of secretions
 minute volume exceeding 10 L/minute
 body temperature less than 32° C
 need for aerosolized medications

31. Time constant
 time constant always equals the length of time needed for the lungs to inflate or deflate to a certain
amount (percentage) of their volume.
 One time constant allows 63% of the volume to be exhaled (or inhaled), two time constants allow about
86% of the volume to be exhaled (or inhaled), three time constants allow about 95% to be exhaled (or
inhaled), and four time constants allow 98% of the volume to be exhaled (or inhaled)
 Time constant = C × R
 We need to set expiratory time constant to 3 or 4 to ensure exhalation of most of air and prevent
autopeep

32. Triggering
Inspiration
Cycling
Expiration
Phases of mechanical ventilation

33.  represents the change from expiration to inspiration and
occurs either because of a drop in circuit pressure or diversion
of flow (when patient triggered)
 Sensitivity refers to a preset threshold of pressure or flow
rarely volume. When this threshold is reached, a mechanical
breath is delivered.
 Pressure trigger If sensitivity is set too low, the ventilator will
be triggered by any process that causes the airway pressure
to drop below the set threshold autotrigger.
Triggering

34.  if the threshold is set too high, the work of breathing increases, as
the patient must make a significant effort to overcome the
threshold increase respiratory work.
 In the setting of flow triggering, the patient’s inspiratory effort
induces a disruption of the constant flow in the ventilator
inspiratory circuit. This change in flow signals the expiratory valve
to close and for the ventilator to deliver the next breath.

35.  Ventilator trigger (mandatory ventilation) is by time trigger the ventilator controls the number of breaths
delivered per minute.(CMV or controlled mandatory ventilation)
 Patient trigger by (assisted ventilation)
 Flow if flow is 5 L/min this means patient initiate spontaneous breathing normal flow of return of gas its 2
L/sec
 Volume trigger occurs when the ventilator detects a small drop in volume in the patient circuit during
exhalation. The machine interprets this drop in volume as a patient effort and begins inspiration.
 Pressure usually is set at about −3 cm H2O. The operator must set the sensitivity level to fit the patient’s
needs. If it is set incorrectly, the ventilator may not be sensitive enough to the patient’s effort, and the
patient will have to work too hard to trigger the breath

36. Wave forms
Square or rectangle: rapid rise to maximum
pressure then tidal delivered fast giving way to
expiration which is longer as short inspiratory best
used with COPD leaving longer exp.
Decelerating or exponential: it is pressure target
as the pressure increase in alveoli flow
decrease(flow curve decelerate) longer Tinsp good
for hypoxic condition as ARDS
Sinusoidal: it is the normal breathing of normal
lung it may be good with cause of ventilation is
outside lung as in neuromuscular disease and
guillian barre.

37. Insp.
hold
A plateau pressure measurement can be obtained in assist volume control
mode by the performance of an inspiratory hold to better estimate the
pressure in the lungs.

40.  This phase can be controlled by how one sets flow or pressure in the
ventilator proximal to the open inspiratory valve. For example, volume-assist
control is flow controlled and pressure-assist control is pressure controlled.
 Choice of type of control is clinician decision as both can achieve goals.
 It should be noted, though, that in volume-targeted ventilation, excessive
airway pressures can arise secondary to worsening pulmonary mechanics. In
this situation, the pressure alarm will cause a pressure limit to cycle to
expiration. (overdistension …. Dead space)
Inspiration

41.  In pressure control volume is not guaranteed it
depends on lung compliance and resistance monitoring
depends on PH and Paco2.
 In pressure control flow is naturally decelerating as
pressure in the airway increases flow decreases flow it is
naturally decelerating but it can be altered by
ventilator. Autoflow

42.  During inspiration we set ventilator on either pressure control (PCV) or volume control(VCV) with their
subtypes but we may use the alarm as a limit developing mixed type of ventilation for example volume
control pressure limit
 A limit variable is the maximum value a variable (pressure, volume, flow, or time) can attain. This limits
the variable during inspiration, but it does not end the inspiratory phase.
 As an example, a ventilator is set to deliver a maximum pressure of 25 cm H2O and the inspiratory time
is set at 2 seconds. The maximum pressure that can be attained during inspiration is 25 cm H2O, but
inspiration will end only after 2 seconds has passed.

43. • Male patient 60 years old developed ARDS with compliance of 10 ml/cmH2o patient body weight is
100 Kgms clinician decided to set parameter as follows
 VCV with VT=5 ml/Kgm
 F 20
 Tinsp=1 sec
 flow wave is square
 PEEP 10
 FiO2 100%
 RAMP 0 second
 ALARMS set Pressure limit 40 High RR 20 calculate actual minute volume provided patient had no spontaneous breathing

44. VT given is equal 5X100=500
Actual is pressure limit (PIP)-PEEP=40-10=30
Then VT=PXC=30X10= 300
MV=300X20=6000
Provided flow 60 and plateau 30 and compliance became 20 does this patient had bronchospasm or not

45. Resistance= PIP-Pplat/flow (l/sec)=5
Then patient had no bronhspasm
Does this patient had autoPEEP
Time constant= CXR=0.02X5=0.1sec then expiration is 3-4X0.1sec=0.4 seconds

46.  Cycling is the transition from inspiration to expiration
(closing inspiratory valve and opening expiratory valve).
 Flow cycled: in PS breaths it is to set ceasation of inspiration at certain
limit of decrease of flow typically it is set at 25%of initial flow we may
increase it in certain condition as bronchopleural fistula.
 Time cycled: inspiration is terminated after a preset interval, regardless of
whether preset pressure has been achieved or a desired tidal volume has
been delivered. yielding a square pressure waveform. Delivered VT
depends on Tinsp as VT=Flow X Tinsp
Flow: 60 L/sec Tinsp 1sec VT=1L.
cycling

47. Volume cycled: in volume control ventilation Volume is delivered until that volume is reached
and may stop early if a preset pressure limit or exceeds pressure alarm limit of ventilation. if
there is inspiratory pause ventilation will continue till end on inspiratory pause and still its
volume cycled not time cycled
Flow: 60 L/sec
VT: 500
Inspiratory pause 0.5 second
Therefore Tinsp will be 1second

48.  All ventilators have a maximum pressure limit control, which is used to prevent excessive pressure from
reaching a patient’s lungs. This maximum safety pressure is typically set by the operator to a value of 10
cm H2O above the average peak inspiratory pressure.
 Reaching the maximum high-pressure limit ends the inspiratory phase. The machine is therefore
pressure cycled for that breath.
 Ventilators also have an internal maximum safety pressure. By design, the machine cannot exceed that
limit, regardless of the value set by the operator. Ventilator manufacturers usually set internal maximum
safety pressure at 120 cm H2o

49.  The expiratory phase encompasses the period between inspirations.
During mechanical ventilation, expiration begins when inspiration
ends, the expiratory valve opens, and expiratory flow begins. As
already mentioned, opening of the expiratory valve may be delayed if
an inspiration hold (pause) is used to prolong inspiration.
 The pressure level from which a ventilator breath begins is called the
baseline pressure (PEEP or zero if PEEP is zero)
Expiration

50.  Control Variables Control variables are the main variables the ventilator adjusts to produce inspiration.
The two primary control variables are pressure and volume.
 Phase Variables Phase variables control the four phases of a breath (i.e., beginning inspiration,
inspiration, end inspiration, and expiration). Types of phase variables include :
 Trigger variable (begins inspiration)
 Limit variable (restricts the magnitude of a variable during inspiration)
 Cycle variable (ends inspiration)
 Baseline variable (the parameter controlled during exhalation)

51.  Types of Breaths :
 Mandatory breaths: The ventilator determines the start time for breaths (time triggered)
or the tidal volume (volume cycled).
 Spontaneous breaths: Breaths are started by the patient (patient triggered), and tidal
volume delivery is determined by the patient (patient cycled)
 Assist ventilation: Patient trigger ventilation either (Volume, Pressure or flow) and
ventilator support the ventilation.

52. Volume control ventilation
 Advantage of volume control modes to be sure of preset tidal volume delivery with target
PaCo2.
 Disadvantage is if lung mechanics change (compliance or sort of airway obstruction) tidal
volume will delivered with higher peak pressure and plateau pressure overdistension of
alveoli worsen lung condition and may cause barotrauma and pneumothorax.

53. Factors That Affect Pressures During Volume-
Controlled Ventilation
• Patient Lung Characteristics
 Reductions in lung or chest wall compliance produce higher peak and plateau pressures;
increased compliance produces lower peak and plateau pressures.
 Increased airway resistance produces a higher peak pressure; reductions in airways resistance
produce lower peak pressures.
• Inspiratory Flow Pattern
 Peak pressure is higher with a constant flow and lower with a decelerating flow pattern.
Decelerating flow pattern has a higher mean airway pressure; constant flow generates the
lowest mean airway pressure
 High inspiratory gas flow creates a higher peak pressure.

54. • Volume Setting
 High volumes produce higher peak and plateau pressures; low volumes produce lower peak
and plateau pressures.
• Positive End-Expiratory Pressure (PEEP)
 Increasing PEEP increases the peak and mean pressures. Auto-PEEP
 Increases in auto-PEEP increase the peak inspiratory pressure.

55. Other disadvantages of volume-controlled breaths are related to flow and sensitivity
settings. Specifically, the delivery of flow is fixed on some ventilators and may not
match patient demand. Similarly, if the sensitivity level is not set appropriately for the
patient, it can make it more difficult for the patient to trigger inspiration. Both
situations can lead to patient-ventilator asynchrony and patient discomfort.

56. Pressure control ventilation
 When choosing pressure as the variable
 Advantages of pressure control is :
 It limits pressure as the peak pressure is limited to preset pressure preventing overdistension
 It uses decelerating flow making peak pressure more close to normal
 Disadvantages
 Volume delivery varies
 Change in lung mechanics may lead to decrease in minute ventilation to compromise ventilation

57. Factors That Affect Volume Delivery During
Pressure-Controlled Ventilation
 Pressure Setting Higher pressure settings produce larger volumes, whereas lower pressure settings
produce lower volumes. In other words, increasing the peak inspiratory pressure (PIP) while maintaining
a constant end-expiratory pressure (EEP) increases volume delivery (and vice versa).
 Pressure Gradient • Increasing EEP (PEEP+ auto-PEEP) while keeping PIP constant reduces the pressure
gradient (PIP − EEP) and lowers volume delivery (and vice versa).

58.  Patient Lung Characteristics
 Reduced compliance results in lower volume; increased compliance results in increased volume for a given
inspiratory pressure.
 Increased airway resistance (Raw) results in lower volume delivery if active flow is present; reductions in airway
resistance results in higher volume delivery if active flow is present.
 With an increased Raw, after flow drops to zero during inspiration, resistance no longer affects tidal volume (VT)
(i.e., no flow, no resistance).

59.  Inspiratory Time : When the inspiratory time (TI) is extended, volume delivery increases. This
is true as long as flow is present during inspiration (i.e., the flow-time curve shows flow
above zero when inspiration ends). However, if flow returns to zero before inspiration ends,
further increases in TI can decrease volume delivery if adequate time is not provided for
exhalation.
 Patient Effort: Active inspiration by the patient can increase volume delivery.

60.  Any type of ventilation for every patient but its better to choose pressure type in
lung protective strategy as ARDS and volume to control PaCo2

61.  It is the first positive pressure ventilation mode
 Locking out a patient by making the ventilator totally insensitive to patient effort
 Patient must be sedated and on neuromuscular blockade
 Data set:
 Tidal volume
 Time cycled or volume cycled
 Respiratory rate
 Control flow or pressure
Controlled mechanical ventilation
continuous mandatory ventilation
CMV

64.  Advantages:
 Rests muscles of respiration
 Disadvantages:
 Requires use of heavy sedation/neuromuscular
blockade

65.  2 types volume and pressure
 Volume type it differs from CMV that ventilator feels
the patient needs if patient try to trigger ventilator
assist patient and let him had a full breath another
thing we set peak flow if ventilator feels peak
pressure increased it increase peak flow.
Assist control mandatory ventilation
A/C

66.  Trigger : patient or time
 Cycling: time
 Wave form: Square, decelerating
 Control: flow

67.  To start a patient on assist-control one must select a PEEP (as
determined by lung compliance), a minute volume (MV
100ml/kg), a tidal volume (TV 6ml/kg), and a peak flow. The
respiratory rate is the MV/TV. The peak flow is usually four times
the minute ventilation. The trigger is either set as “flow-by” or a
negative pressure of -2cmH2O. Refinement of the settings is
determined by the patient’s plateau pressure (should be less than
30cmH2O), the patients inspiratory flow demand, and blood gas
targets.

69.  Trigger: patient or time
 Control: Pressure
 Cycling: time
 Wave: decelerating
Pressure control/assist control (PC/AC)

71.  Advantages:
 Reduced work of breathing
 Guarantees delivery of set tidal volume (unless peak pressure
limit alarm is exceeded)
 Disadvantages:
 Potential adverse hemodynamic effects
 May lead to inappropriate hyperventilation and excessive
inspiration pressures

72.  Volume control assist control lead to increase patient effort by 33-50% of flow is inadequate known by
concave pressure wave form
 PC CMV is used in early 90s to ventilate ARDS before developing of SIMV and BIPAP patient was put on
this mode and limit pressure above pressure control by 10 cmH2o.
 Pressure control also develop from it PCIRV which is pressure control inversed ratio ventilation to
prolong inspiration more than expiration to ventilate very stiff lung.(in expense of autoPEEP and high
peak airway pressure)

73. Thank you