Transcript Slide 1
MECHANICAL
VENTILATION
• Phunsup Wongsurakiat, MD, FCCP
• Division of Respiratory Disease and TB
• Department of Medicine, Siriraj Hospital
Mechanical Ventilation
• Invasive (intubation)
• Non-invasive (other interface such as face mask):
- negative pressure
- positive pressure
Inspiratory Hold: PIP and
Plateau pressures (VC)
Peak Pressure
Plateau
Proximal
airway
pressure
Inspiratory Hold
DISTENDING PRESSURES
Plat P - Base P (PEEP) = pressure to distend resp system
(lung + chest wall)
Plat P = peak alveolar pressure
Transpulmonary pressure = Plat P – Pleural pressure
Influence of chest wall
stiffness
35 cm H2O
35 cm H2O
5 cm H2O
30 cm H2O
10 cm H2O
25 cm H2O
FLOW Resistance (R)
Peak P - PlatP = pressure for flow
R = Flow/(PeakP - Plat P)
Physiology of Respiration
O2 consumption (VO2) ≈ 250 mL/ min
CO2 production (VCO2) ≈ 200 mL/ min
Cardiac output (CO) ≈ 5 L/min
Minute ventilation = RR X VT ≈ 5 - 8 L/min
VT ≈ 5 - 8 mL / kg
RR ≈ 12 - 20 bpm
PaCO2 = 35-45 mmHg
pH = 7.35-7.45
Physiology of Respiration
PAO2 = PIO2 – PaCO2 / R
[PIO2 = FIO2 x (barometric pressure – 47)]
PaCO2 = k x VCO2 / VA
Mechanical Ventilation
Variables
Mode
Objectives
Clinical settings
Complications
Mechanical Ventilation
Variables
Tidal volume
Rate
Total time (inversely related to rate):
- inspiratory time (adjustable)
- expiratory time (total time - inspiratory
time)
Flow (tidal volume / inspiratory time)
Minute ventilation
Minute ventilation
VE = VT * f
end of inspiration-beginning of expiration
total breath duration
VT
tidal volume
f
tidal volume
VT = 500 ml
breaths/ minute
beginning of inspiration
f = 12 breaths/minute
VE = 500 ml * 12 = 6 L
Mechanical Ventilation
Variables
Trigger sensitivity
FiO2
Pressure:
- peak pressure
- plateau pressure
Compliance
Mechanical Ventilation
Modes
Limit
variables rise no higher than some preset
value and increase to preset value before
inspiration ends = limit variable
Cycle
Variable that terminate inspiration = cycle
variable
Breath characteristics
Gas Delivery
Mechanical Ventilation
Bird
Limit: pressure, flow
Cycle: pressure
Bennett 7200 (volume assist-control)
Limit: volume, flow
Cycle: volume
Pressure support
Limit: pressure
Cycle: flow
Mechanical Ventilation
Modes
Volume-targeted: Pre-set tidal volume
Pressure-targeted: Pre-set inspiratory
pressure
Mandatory breaths: Breaths that the
ventilator delivers to the patient at a set
frequency, volume/pressure, flow/time
Spontaneous breaths: Patient initiated breath
Mechanical Ventilation
Modes
Volume-targeted
Controlled mechanical ventilation (CMV)
Assist/control (A/C) mechanical
ventilation
Synchronized intermittent mandatory
ventilation (SIMV)
Modes of Ventilation
(CMV)
Background:
• Full preset tidal volume at a fixed preset rate
• Rate and minute ventilation cannot by patient effort
• Trigger sensitivity is locked out, need sedated or paralyzed
Modes of Ventilation
(CMV)
Advantages:
Near complete resting of ventilatory muscles
Respiratory muscle rest, secured minute ventilation
Disadvantages:
“Stack" breaths (air trapping) and develop barotrauma
“AutoPEEP" with barotrauma or hypotension
Need sedated or paralyzed
Respiratory muscle atrophy
Uses:
apnea, little breathing effort, unstable
Modes of Ventilation
(Assist/Control)
Background:
• Full preset tidal volume at a minimum preset rate
• Additional full tidal volumes given if the patient initiates extra breaths
Modes of Ventilation
(Assist/Control)
Advantages:
• Near complete resting of ventilatory muscles
• Comfortable, respiratory muscle rest, secured minute ventilation
• Effectively used in awake, sedated, or paralyzed patients
Disadvantages:
• Hyperventilate and become alkalotic
• “Stack" breaths (air trapping) and develop barotrauma
• “AutoPEEP" with barotrauma or hypotension
Uses:
• apnea, little breathing effort, unstable
Modes of Ventilation
(SIMV + Pressure support)
Background:
•Preset tidal volume at a fixed preset rate
•Ventilator waits a predetermined trigger period
•Patient can take additional breaths but tidal volume of these
extra breaths is dependent on the patient's inspiratory effort
Modes of Ventilation
(SIMV + Pressure support)
Advantages:
• Improved venous return: intermittent negative pressure
(spontaneous) breaths
• More comfortable: more control over their ventilatory pattern
and minute ventilation
Disadvantages:
• Can result in chronic respiratory fatigue if set rate is too low;
Uses:
• Weaning, bronchopleural fistula
Mechanical Ventilation
Modes
Pressure-targeted
Pressure support ventilation (PSV)
Pressure-control ventilation (PCV)
Pressure-targeted assist-control (A/C-PC)
Pressure-targeted SIMV (SIMV-PC)
Pressure support Ventilation
Background:
• Patient triggers, a preset pressure support is delivered
• Terminate inspiration by flow rate
• Tidal volume and minute ventilation are dependent on the preset pressure
and patient’s lung-thorax compliance
Pressure support Ventilation
Advantages:
• Avoids patient-ventilator asynchrony
• More comfortable: full control over ventilatory pattern and minute
ventilation
• Avoids breath stacking and autoPEEP (especially in patients with COPD)
Disadvantages:
• Required patient’s triggering, cannot be used in heavily sedated, paralyzed,
or comatose patients
• Respiratory muscle fatigue if pressure support is set too low
Pressure Control Ventilation
Background:
• Ventilator control predetermined pressure in a
fixed predetermined time and rate
Pressure Control Ventilation
Advantages:
• Pressure and time are controlled
• High flow rate
Disadvantages:
• Tidal volume variable
• Produced autoPEEP if inspiratory time too long
• Uncomfortable mode for most patients
Pressure-targeted assist-control
(A/C-PC)
All breaths machine-delivered at a preset
inflation pressure
Patients can rate by triggering additional
machine breaths if desired
Same as assist/control volume targeted mode
Pressure-targeted SIMV
(SIMV-PC)
Fixed rate of machine-delivered breaths at a
preset inflation pressure
Patients can breathe spontaneously between
machine-delivered breaths if desired
Same as SIMV volume targeted mode
Mechanical Ventilation
Objectives:
• Physiology
• Comfortable
• Least complications
Mechanical Ventilation
Usual settings
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Minute Ventilation = RR X VT ( 5 - 8 L/min)
VT = 5 - 8 mL / kg
RR = 12 - 20 bpm
PaCO2 = 35-45 mmHg
pH = 7.35-7.45
Mechanical Ventilation
Triggering: sensitivity of ventilator to patient’s
respiratory effort.
• Flow or pressure setting that allows ventilator to detect
patient’s inspiratory effort
• Allows ventilator synchronize with patient’s
spontaneous respiratory efforts
• Improving patient’s comfort during mechanical
ventilation.
Setting: lowest but not self cycling
Less work in flow triggering
Mechanical Ventilation
Flow rate:
• adjust to patient’s comfort
• Usually > 60 L/min
Pressure:
• Plateau pressure < 30 cmH2O
Positive End Expiratory Pressure
(PEEP)
• Functional residual capacity
• Move fluid from alveoli into interstitial space
• Improve oxygenation
PEEP
In COPD : To offset auto-PEEP
To improve oxygenation in acute lung injury
To improve oxygenation, preload, afterload
in cardiogenic pulmonary edema
PEEP Titration
- PaO2
- FiO2 (< 0.5)
- No adverse effect:
cardiac output
lung compliance from
overdistension
( plateau pressure)
Goal:
• Obtained baseline respiratory & hemodynamic
• Change (PEEP) only , keep other parameter
constant
• PEEP 5 cmH2O q 15 – 20 min
O2 delivery
O2 delivery
SaO2
SaO2
No adverse effect
Compliance
SaO2
SaO2
BP or co
Volume +
vasopressor
Indequate
Adequate
Use current PEEP
PEEP more
FiO2
Tidal
volume
PEEP to
previous level
Contraindication
Absolute: none
Relative: unilateral lung disease,
bronchopleural fistulae, intracranial
pressure, high plateau pressure, pulmonary
embolism
Ventilator Alarms
Setting
Alarm
Setting
High minute ventilation
10-15% > set or target minute volume
Low minute ventilation
10-15% < set or target minute volume
High Vt
10-15% > set or target Vt
Low Vt
10-15% < set or target Vt
High-system pressure
10 cmH20 > average peak airway
pressure
Low-system pressure
5-10 cmH20 < average peak airway
pressure
Alarms & Troubleshooting
High Peak Inspiratory Pressure:
• Secretions
• Patient biting ETT
• Patient coughing
• Changing patient’s clinical status
Low Pressure Alarm or low PEEP alarm:
• Disconnect (check all connections)
• Apnea
Mechanical Ventilation
Complications
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Disconnection
Malfunction
Patient-ventilator asynchrony
hemodynamic effects:
Barotrauma
Ventilator-induced lung injury
Oxygen toxicity
Infection
- ventilator-associated pneumonia
- sinusitis
Mechanical Ventilation
Complications
Hemodynamic effects:
Impaired venous return, increased
pulmonary vascular resistance
cardiac output
AutoPEEP
Mean lung volume or mean alveolar
pressure correlate best with
hemodynamic effects
AutoPEEP
Exhalation is not complete by the time the next
breath is given:
- expiratory airflow obstruction
- high minute ventilation (> 15-20 l/min)
Alveolar pressure & volume remain increased
at end-expiration
AutoPEEP
Adverse Effect
Same effect as externally applied PEEP:
- Hyperinflation
- cardiac output
Difficult to trigger ventilator → ↑ work of
breathing
AUTOPEEP
Auscultation: persistent exhalation (specific
but not sensitive)
Untriggering
Flow-time graphs
End-expiratory hold
Steps for Reducing
Dynamic Hyperinflation & AutoPEEP
Eliminate unnecessary ventilation:
Weaning
Minute ventilation
Permissive hypercapnia
Expiratory airflow obstruction:
Rx bronchospasm
Keep airways free of secretions
Maximize expiratory time:
peak inspiratory flow rate to 70 – 100 l/min
Mechanical Ventilation
Complications
Oxygen toxicity
• High FiO2 is potentially injurious
• Tissue injury depends on FiO2 and duration of
exposure and some diseases/conditions
• No evidence that sustained exposure to FiO2 < 0.5
causes tissue injury
• Lowest FiO2 with adequate tissue oxygenation
• Measurements to keep FiO2 < 0.5
Hemoglobin and 02 Transport
280 million
hemoglobin/RBC.
Each hemoglobin
has 4 polypeptide
chains and 4
hemes.
In the center of
each heme group
is 1 atom of iron
that can combine
with 1 molecule
02.
Insert fig. 16.32
Oxyhemoglobin Dissociation Curve
Insert fig.16.34
Mechanical Ventilation
Respiratory Distress
• Patient
• Ventilator (malfunction)
• Patient-ventilator asynchrony
- flow rate
- trigger
- autoPEEP
Ventilator-Induced
Lung Injury (VILI)
• Barotrauma : extraalveolar air,
pneumothorax
• Diffuse alveolar damage, pulmonary edema
Overdistention
Overdistention may be regional
Even a “normal” VT can create regional
overdistention
Ventilator Management of ARDS
INITIAL VENTILATOR TIDAL VOLUME AND
RATE ADJUSTMENTS
Calculate predicted body weight (PBW)
• Male= 50 + 2.3 [height (inches) - 60]
• Female= 45.5 + 2.3 [height (inches) - 60]
Mode: Volume Assist-Control