icu intro resp

Absolute Basics of Mechanical Ventilation

Dr David Howell

Consultant in Intensive Care, Respiratory and Acute Medicine

Aims and Objectives

• Define Positive Pressure Mechanical Ventilation

• Explain Continuous Mandatory Ventilation (CMV)

• Explain Synchronised Mandatory Ventilation (SIMV)

• Explain Pressure Support Ventilation (PSV)

• Explain Basic Ventilator Settings

• Not a Talk on Physiology of Mechanical Ventilation

What you Encounter

Positive Pressure Mechanical Ventilator

Lots of Monitors and Knobs to Turn

Some are More Complicated than Others

Wake, Warm & Wean

Weaning Screen/standard

protocol

Long Term Weaning/Individual

planWeaning

Non-Invasive Ventilation

Oxygen Therapy

Mask CPAP

Non-invasive support

Tracheostomy

Intubation

ExtubationDecannulation

Standard Ventilation

AdvancedVentilation

Invasive support

Optimising the Pt for weaning

Prone Position

Nitric Oxide

Suctioning

Humidification

• NIV is defined as ventilatory support provided via a tight fitting mask or similar interface as opposed to invasive support, which is provided via a laryngeal mask, endotracheal tube or tracheostomy tube.

• Tight fitting masks deliver can CPAP, BIPAP or NIV via the mechanical ventilator.

NIV vs. Invasive Mechanical Ventilation

• The work of breathing usually accounts for 5% of oxygen consumption (V02).

• In the critically ill patient this may rise to 30%.

• Invasive mechanical ventilation eliminates the metabolic cost of breathing.

Indications for Mechanical Ventilation

• Inadequate oxygenation (not corrected by supplemental O2 by mask).

• Inadequate ventilation (increased PaCO2).

• Retention of pulmonary secretions (bronchial toilet).

• Airway protection (obtunded patient, depressed gag reflex).

Indications for Mechanical Ventilation

Intubation

1 Airway: oral Guedel airway to lift tongue off posterior pharynx to facilitate mask ventilation during pre-intubation phase.

2 Liquids: stop feed and aspirate ng tube.

3 Suction: extremely important to avoid pulmonary aspiration.

4 Oxygen: preoxygenate patient and ensure a source of O2 with a delivery mechanism (ambu-bag and mask) is available.

Bare Essentials for IntubationALSOBLEED

5 Bougie: to facilitate tube insertion in more difficult airway.

6 Laryngoscope: have a long and short blade available.

7 Endotracheal tube: for average adult, cuffed oral endotracheal tube 7.0 for women and 8.0 for men.

8 End tidal CO2: to confirm correct position of tube.

9 Drugs: an induction agent, muscle relaxant, sedative are usually required.

Bare Essentials for IntubationALSOBLEED

Principles of Mechanical Ventilation

ET tubeVentilator Tubing

Major Airways

Alveoli

PEEP

• Positive pressure ventilation involves delivering a mechanically generated ‘breath’ to get O2 in and CO2 out.

• Gas is pumped in during inspiration (Ti) and the patient passively expires during expiration (Te).

• The sum of Ti and Te is the respiratory cycle or ‘breath’.


Flo

wP

ress

ure


Ti TeTiTe

• In the fully ventilated patient, positive pressure breaths are delivered either as preset volume or pressure continuous mandatory breaths (CMV) breaths.

• The mechanical ventilator triggers the breath and switches from inspiration to expiration when the preset volume, pressure (or time) is achieved/delivered.

• During CMV the patient takes no spontaneous breaths.

• CMV is usually used in theatre and in very unwell ICU patients.



Volume control

• Tidal volume is preset

• Usually 500 mls

• Airway Pressure is Variable

Pressure control

• Inspiratory Pressure is preset

• Usually 15-20 cm H20

• Tidal Volume is Variable

• Mandatory breaths are delivered during inspiration, to generate a tidal volume (Vt), at a set rate (f), the quotient of which is the minute volume (MV).

• Minute Volume = Tidal Volume x frequency

• In volume control ventilation, an inspiratory flow rate is also set.

• The ratio of the time spent in inspiration:expiration (I:E ratio) is usually 1:2.


Flo

wP

ress

ure

Ti TeTiTe


Volume Control Breath Pressure Control Breath

• Mechanically ventilated patients usually receive positive end-expiratory pressure (PEEP), to overcome the loss of physiological PEEP provided by the larynx and vocal cords.

• PEEP is delivered throughout the respiratory cycle and is synonymous to CPAP, but in the intubated patient.

• Standard PEEP setting is 5 cm H20.

• Sedation is often required to prevent ventilator-patient asynchrony.


Basic Settings on the Ventilator

• Tidal Volume

Pressure controlled breath (15-20 cm H20)Volume controlled breath (500 mls)

• Rate (frequency) (10-12 breaths/minute)

• Positive end expiratory pressure (PEEP) (5 cm H20)

• FiO2 (0.21-1)

• Peak airway pressure (PAP)


• Why is the peak airway pressure (PAP) important?

• Ventilator Induced Lung Injury (VILI).

• Mechanical ventilation is injurious to the lung.

• Aim PAP< 35 cm H20.

Flo

wP

ress

ure

Ti TeTiTe

Principles of Mechanical VentilationVolume Breath Pressure Breath

35 cm H20

Pneumothorax

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Don’t forget that the peak airway pressure will also include the PEEP that is added

• Once stabilised on CMV, the level of ventilatory support may be reduced (weaning).

• This can be done by providing a mixture of synchronised intermittent mandatory breaths (SIMV) and spontaneously triggered pressure supported breaths (PSV).


• Ventilator assisted breaths are synchronized with the patient’s breathing to prevent the possibility of a mechanical breath on top of a spontaneous breath.

• However, the patient’s attempt at a breath would not be enough to generate an adequate tidal volume on its own, hence the term ‘pressure support’.


• Pressure support is only delivered during inspiration and the patient’s attempt at breathing triggers the breath rather than the ventilator.

• A standard level of pressure support delivered in inspiration is 20 cm H20


SIMV and Pressure Support Ventilation

Ventilator Patient

• As patients improve, mandatory breaths are withdrawn and receive pressure-supported breaths alone.

• Finally, as tidal volumes improve, the level of pressure support is reduced and then withdrawn so patients breathe spontaneously with PEEP alone.

• Extubation can now be contemplated.

• Spontaneous modes of breathing should always be encouraged as respiratory muscle function is maintained


Pressure Support Ventilation

Patient Patient

• To succeed, the initiating cause of respiratory failure, sepsis, fluid and electrolyte imbalance and nutritional status should all be treated or optimised.

• Failure to wean is associated with:

• Ongoing high V02.

• Muscle fatigue.

• Inadequate drive.

• Inadequate cardiac reserve.

Successful Weaning and Extubation

• Weaning screens exist to help select patients for extubation.

• In the unsupported patient, if f/Vt is >100, extubation is likely to be unsuccessful.

• There is some evidence to support extubation to NIV, particularly in patients with COPD.

Successful Weaning and Extubation

• Continuous Mandatory Ventilation (CMV)Pressure control Volume controlNo spontaneous breathingVentilator triggers breath

• Synchronised intermittent mandatory ventilation (SIMV)/Pressure Support Ventilation (PSV) Pressure control (SIMV)

Volume control (SIMV)Some spontaneous breathing is allowed (PSV)Mixture of ventilator and patient triggered

breaths

Basic Ventilatory Modes: Summary

• Pressure Support Ventilation (PSV)Spontaneous breathing with inspiratory support All patient triggered breaths

• PEEP/CPAP (5 cm H20)Entirely spontaneous breathingConsider extubation


PSVPEEP

SIMVPSV

CMV

Mandatory SpontaneousOverlap

PSVPEEP

SIMVPSV

CMV PSVPEEP

SIMVPSV

CMV

Mandatory SpontaneousOverlap


Mode

O2

Respiratory Rate

Inspiratory Action

Inspiratory Time

Expiratory Action

Standard Ventilator SettingsMORITE

Mode CMV, Volume Control

O2 0.5 (50% 02)

Respiratory Rate 12/minute

Inspiratory Action Set Vt at 500 mls

Inspiratory Time Set I:E ratio 1:2

Expiratory Action Set PEEP at 5 cm H20

Be Aware PAP ≤35 cm H2O

Standard Ventilator SettingsMORITE

Patient Requiring Basic Invasive Mechanical Ventilation

Spontaneously Ventilating Patient Failing Conventional Therapy

Escalation

BIPAP

OptimiseConsider

Patient Position Humidification

CMV (VCV or PCV) PSV PEEP/CPAP

NIV on ICU

BIPAP on Ward

IMV (VCV or PCV)

De-escalation

CPAP on Ward

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