Mechanical Ventilation — Basics

v

Negative pressure v

• Compliance: ΔV/ΔP
• Elastance: ΔP /ΔV
• Dead Space: Portion of each breath not involved in gas exchange
• Anatomic – conducting airways (volume)
• Physiologic – anatomic + alveolar dead space
• Functional residual capacity (FRC): Volume air remaining after normal expiration, equilibrium point between lung elastic recoil & expansion of chest wall

• Oxygenation – O2 exchange
• How we adjust (simplified)?
• FiO2
• PEEP
• Ventilation – CO2 exchange
• How we adjust (simplified)?
• TV
• RR
• Aka Minute Ventilation (Ve) = TV x RR
• Target 5-10 L/min

• Volume Modes
• Set volume of air delivered (constant tidal volume)
• Pressure variable
• Pressure Modes
• Set pressure delivered
• Volume variable

Changes patient to patient, breath to breath:

• Anatomy/habitus
• Pulmonary factors ( i. e. resistance/compliance)
• Position
• Some advanced modes a hybrid of both

• Tidal Volume (TV)
• How obtained depends on mode
• 6-8ml/kg in normal lungs (IBW)
• Respiratory rate (RR) – frequency, breaths per minute
• Oxygen Concentration (FiO2)
• Positive End Expiratory Pressure (PEEP)
• Constant pressure applied during expiration
• Increases FRC
• I:E
• Typically 1:2
• Inspiratory flow

• Trigger – initiator of breath
• Machine (time)
• Person (effort based)
• Target – Variable that regulates gas delivery during inspiration
• flow for volume
• Inspiratory pressure for pressure
• Cycle goal - What terminates inspiration
• Volume
• Time
• Flow

• Can be constant through inhalation = square waveform

OR

• Decelerating as breath delivered = ramp waveform
• Square waveform: faster delivery of inspiration, decreasing inspiratory time, increasing expiratory time
• Useful when longer expiration needed:
• Asthma, COPD
• High RR (encourage full exhalation)
• Ramp waveform: Decrease the flow as delivered volume increases
• More comfortable
• Better volume distribution and equalization heterogeneous lungs (i. e.

• Square waveform
• Ramp waveform
• Back to Ventilator Modes

• Assist Control Volume Control (ACVC)
• Synchronized Intermittent Mandatory

Ventilation (SIMV)

• What do you set?
• TV
• RR
• FiO2
• PEEP
• More advanced
• I:E
• Flow

• Assist Control?
• Adjusts trigger (breath initiator)
• NO spontaneous breath – time initiated breath
• Spontaneous breathing – senses negative inspiratory effort & gives breath
• Example: RR set to 15
• No spontaneous breathing – RR=15
• Spontaneous breathing – RR=15 plus patient breaths above this rate
• Gives full preset TV regardless of trigger

• Gives full preset TV every breath
• Full control of minute ventilation
• Useful to manage respiratory/metabolic acidosis
• Barotrauma risk
• Uses whatever pressure needed to get volume
• Non-compliant (“stiff”) lungs
• Tachypneic patients
• Reduced expiration time – breath stacking, auto-PEEP
• > hyperinflation, increased pulmonary pressures
• Hyperventilation - respiratory alkalosis
• Recruitment may be poorer in poor compliance
• Discomfort – vent synchrony

ACVC

• Combines mandatory breaths w/ patient initiated breaths
• What do you set?
• TV
• RR
• FiO2
• PEEP
• +/- PS (assist with self initiated breaths)

• How is this different from ACVC?
• Set mandatory breaths (RR) synchronized to patient effort
• Mandatory breaths give full preset TV (same as

• Spontaneous breaths patient determined TV (different than ACVC)
• Can add PS
• If NO spontaneous breathing same as ACVC

• Guarantees minimum RR with set TV
• Patient can trigger additional breaths (can adjust degree of support)
• Can increase work of breathing
• Avoid respiratory alkalosis (tachypneic patient)
• Improved patient comfort (synchronization)?
• Weaning mode?

Patient trigger Machine (time trigger) Machine breath Spontaneous breath

• Pressure Control (PC)
• Pressure Support (PS)
• Airway Pressure Release Ventilation (APRV)

• What do you set?
• Pressure
• Set to target goal TV range (i. e. 6-8ml/kg)
• RR
• FiO2
• PEEP
• More advanced
• I:E

• Volume is variable – TV can change with airway resistance (aka peak airway pressure) & compliance (aka plateau)
• Needs close monitoring
• Risk of hypoventilation (low TV)
• Risk of volutrauma (high TV)
• Barotrauma protection (non-compliant lungs)
• Constant airway pressure
• Improved alveolar distention/recruitment –>
• xygenation & dead space ventilation
• Improved patient tolerance/vent synchrony
• Flow rate variable to patient needs

PC

• Spontaneous breathing
• Does have a back up RR for apnea
• What do you set?
• Pressure (inspiratory pressure)
• Set to target goal TV range
• FiO2
• PEEP
• Pressure delivered until inspiratory flow drops below a determined threshold (i. e. 25% of peak flow)

• High continuous airway pressure (oxygenation) with brief pressure releases (ventilation)
• Unrestricted spontaneous breathing throughout
• What you set?
• Pressure high (25-30)
• Time high
• Pressure low (0)
• Time low
• FiO2
• (I:E usually 7:1 or 10:1)

• Improved oxygenation
• Improved mean airway pressures
• Alveolar recruitment
• Spontaneous respirations
• Facilitates gas exchange – CO2 clearance
• Maximizes continued recruitment
• Ventilation of dependent areas
• Reduced sedation

• Requires spontaneous breathing
• i. e. neuro intact, not on chemical paralysis
• Hypotension with initiation (usually transient)
• Reduced venous return (Phigh)
• Patient needs to be fully resuscitated prior
• Avoid in hemodynamically unstable
• Risk volutrauma
• Can increase work of breathing

APRV

• Pressure Regulated Volume Control (PRVC)
• Lowest inspiratory pressures to achieve set TV
• Adjusts pressure to achieve TV target
• Feedback method breath-to-breath
• What you set?
• TV
• Uses plateau pressure from previous breath to calculate inspiratory pressure need to reach set TV for following breath
• RR (same as VC)
• FiO2
• PEEP

• Best of both worlds???
• Change in resistance or compliance, pressure will adjust to deliver the desired TV
• PC benefit - improved mean airway pressure
• > better alveolar distention/recruitment –>
• xygenation & dead space ventilation
• VC benefit - guaranteed minute ventilation
• Can increase work of breathing in patients with high respiratory drive

• Peak Inspiratory Pressure: Highest pressure recorded at peak inspiration
• Plateau p plus additional pressure applied to overcome flow resistance & elastic recoil of the lungs/chest wall
• Plateau pressure: Alveolar pressure applied during the majority of the breath cycle
• Measure via inspiratory hold
• Goal ≤30 cm H2O
• Driving pressure: Plateau p – PEEP
• Lung strain
• 14-18cmH20
• Auto/intrinsic-PEEP: Residual pressure in the circuit at end expiration = Total PEEP (measured with expiratory pause) in the circuit – set PEEP