Mechanical Ventilation

Epidemiology

• 28 day international study– 361 ICUs in 20 countries– All consecutive adult patients who received

MV for > 12 hours

– 33% Patient admitted to those ICUs received mechanical ventilation

• Mean age 59• M > F (61 v. 39%)

Esteban et al. JAMA 2002

• Indication for mechanical ventilation– Acute respiratory failure 68%

• Post-op (21%)• Pneumonia 14%• CHF 10%• Sepsis 9%• Trauma 8%• ARDS 4.5%• Aspiration 2.5%• Cardiac arrest 1.9%

– Acute on chronuic respiratory failure• COPD 10%• Asthma 1.5%• Chronic respiratory disease (non_COPD) 1.8%

– Coma 16.7%– Neuromuscular disease 1.8%

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Esteban, A. et al. JAMA 2002;287:345-355.

Ventilator Modes Used Each Day During the Course of Mechanical Ventilation

• Duration of mechanical ventilation– Overall 5.9days– COPD pts 5.1 days– ARDS pts 8.8 days

• ICU LOS: 11.2 days

• Hospital LOS: 22.5 days

• Mortality:– ICU mortality 30.7%– Hospital mortlaity 39.2%

Copyright restrictions may apply.

Esteban, A. et al. JAMA 2002;287:345-355.

Kaplan-Meier Curves of the Probability of Survival Over Time of Mechanical Ventilation

Mechanical ventilation

• Physiology:– Positive pressure ventilation versus naturanl negative

pressure ventilation

• Effects:– Heterogeneous ventilation

• Preferential ventilation of the non-dependent regions

– Increased physiologic dead space– Improvement of physiologic shunt causes by

atelectasis and/or alveolar filling– Rapid disuse atrophy of the diaphragm– Impairment of mucociliary clearance

• Cardiovascular effects:– Decreased venous return

• Exacerbated by:– Auto-PEEP– Applied PEEP– Intravascular volume depletion– Cardiac tamponnade

– Increased right ventricular afterload:• Compression of the pulmonary vascular bed Increased

PVR

– May decrease left ventricular afterload• Lung exansion decreased extramural pressure

Mechanical ventilation

• Benefits– Improves gas exchange by improved V/Q

matching predominantly be decreasing shunt– Decreased work of breathing

Mechanical ventilation: Complications

• Barotrauma– Incidence ~3%– To Avoid: Keep plateau pressure < 35 cm

• VILI– Over stretch– Atelectotrauma

• Auto-PEEP

• Asyncrhony

Mechanical ventilation: Modes

• Choices:– Mandatory v. non-mandatory

• Mandatory– Volume v. pressure limited ventilation– Mandatory rate– Modes:

» SIMV» Assist Control» PCV» Hybrid Modes: PRVC, SIMV/PRVC

• Non-mandatory or assisted breaths– PSV

Variables: some default values

• Trigger sensitivity: -1 to -3 cm

• Tidal volume: 6-8mg/kg/IBW

• Rate: 10 to 14

• PEEP: 5 cm H2O

• Flow rate: 60 L/min

• I to E ratio

Volume limited v. pressure limited

• Volume limited– Physician sets:

• Tidal volume• Rate

– Guaranteed constant tidal volume

– Guaranteed minute ventilation

– High peak pressures

• Pressure limited– Physician sets:

• Peak airway pressure• Inspiratory time

– Tidal volume and minute ventilation depends entirely on patient factors: compliance and airway resistance

– Associated with lower peak airway pressure

– Associated with more homogenous gas distribution

No difference in mortality, oxygenation, or work of breathing

P = Vt/CR + Vt/Ti * R + PEEPtotal

Where CR = compliance of the respiratory system, Ti = inspiratory time and VT/Ti = Flow, RR = resistance of the respiratory system and PEEP total = the alveolar pressure at the end of expiration = external PEEP + auto (or intrinsic) PEEP, if any. Auto PEEP = PEEP total – P extrinsic (PEEP dialed in the ventilator) adds to the inspiratory

pressure one needs to generate a tidal breath.

• Peak pressure• Plateau pressure

– Surrogate for peak alveolar distending pressure

• Peak – Plateau – Resistive pressure

• Mean airway pressure– Pressure applied acorss the lung and chest

wall averaged throughout the ventilary cycle

• Patient factors:– Airway resistance– Compliance of the respiratory system

• Chest wall recoil• Lung recoil

Assist-Control

• Set variables– Tidal volume– Flow rate or Ti– PEEP FiO2– Mandatory rate

• Spontaneous breaths– Additional cycles can be triggered; they are

identical to the mandatory breath

SIMV

• Set variables– Targeted volume– Flow rate– Manatory frequency– PEEP– FiO2– PS augmentation for spontaneous breaths

• Spontaneous breaths– Unrestricted and aided by the selected level of

pressure support

PCV

• Set variables:– Peak pressure– Inspiratory time– Frequency of mandatory breaths

• Spontaneous breaths– PCV (AC): same as mandatory breaths– PCV/SIMV: unsupported or pressure

supported

Tidal volume during PCV

• Changes in mechanics– Increased airway resistance– Decreased respiratory system compliance

• Increased auto-PEEP

• Decreased inspiratory time

Waveforms

Waveforms

Waveforms

Waveforms

Ventilator changeFlow (lpm)

Pressure (cm H2O)

Volume (mL)

Waveforms

40

30

20

10

0

Airway pressure

Time

Pause

Waveforms

Pause

• What changes on the ventilator should you make for hypoxemia?

• What changes for hypercapnia and respiratory acidosis?

• Hypotension on the ventilator?