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%)
pH<7.25
PaCO2 > 50 mm Hg
Definitions:
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.