Ventilation basics

Normal PaO2/FiO2 100/0.21 = 500

FiO2 – a fraction of oxygen in the gas 

PaO2 – arterial oxygen pressure in mm Hg Fi02 – a fraction of inspired oxygen: in normal atmosphere = 0.21

signs of respiratory failure:

RR> 30

Not able to sustain arterial oxygen saturation > 90% with FIO2 >0.6

(ABG oxygen 80-100 mmHg = Sat 95% to 100%)


PaCO2 > 50 mm Hg


Peak airway pressure – pressure needed to move a fixed volume of gas into the lungs

PAP = PEEP+resistive pressure+elastic pressure

resistive pressure = resistance from the diameter of tubes < 10 cm of H20 if higher tube may be kinked

elastic pressure = recoil of lung tissues

PEEP = end-expiratory pressure; pressure in alveoli = typically same as atmospheric pressure

No-invasive positive pressure ventilation

No ET tube but tightly fitted mask needed

CPAP – continues positive airway pressure – constant pressure maintained during the respiratory cycle – patient breathes on his own

BiPAP – inspiratory airway pressure (IPAP) and expiratory airway pressure (EPAP) are adjusted individually; patients triggers respirations

Methods of mechanical ventilation: 

Main methods:

A. volume-controlled cycle = constant volume delivered

B. pressure controlled cycle = constant pressure delivered – volume may vary

C. mixed of above

Assist-control means ventilator will deliver a minimum respiratory rate regardless if the patient initiates

Knobology – what can be adjusted

Respiratory rate

Tidal volume: the amount of gas delivered with each respiration cycle

Trigger sensitivity – the amount of negative pressure to trigger the ventilation cycle, start at -2 cm H2O

Flow rate: inspiratory flow rate 60 l/min – not all modes allow to change 

Inspiratory/expiratory ratio: time spend in each phase typically 1:3 may increase to 1:4 in COPD

FiO2: oxygen concentration in gas, start at 100% and then reduce to keep saturation goals

PEEP: start at 5 cm of H2O – avoids atelectasis

ARDS vent settings

ARDS definition: 

PaO2/FiO2 <=300, Peep >5 cm H2O

Bilateral capacities on chest Xray

Not explained by heart failure 

Use predicted body weight in calculations (PBW) of settings for the ventilator.

Tidal volume: start with 6 ml/kg of PBW For 70 kg = 420 ml

plateau pressure max at 30 cm H20

rate <35

PEEP > 5 cm H20

high peep in severe ARDS – watch hemodynamics

high-frequency ventilation not recommended

In patients presenting with severe ARDS – neuromuscular blocking agents start early for 48 h and then reevaluate. PaO2/FiO2 < 150 mmHg – indicates severe ARDS

Spontaneous ventilation: pressure mode, keep tidal volumes at 6ml/PBW, and less than 8ml/PBW.

Prone position 16 h/24h in severe ARDS

May consider inhaled nitric oxide if the above measures fail to achieve oxygenation goals.