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Dr. Ajith Kumar J MD
Dept. of Emergency Medicne
Travancore Medicity, Kollam
India
editor
What is minute ventilation?
MV = Vt * f ; where is tidal volume & f is frequency.
It is reasonable to achieve a tidal volume of 100ml/kg /min in realtion to Ideal body weight.
What is the minimum effective tidal volume & maximum safe Vt?
Minimum advisable TV is 4.4ml/kg.
When tidal volume is decreased to a value close to dead space volume then V'A and CO2 elimination is almost nil.
So the minimum Vt has to double the minimum dead space volume (Vd) (Vd=2.2ml/kg of IBW.)
Max safe TV
A plateau pressure of 25cmH2O is always considered safe.
A pressure of 30cm H20 is probably safe. > 30cmH20 is potentially hazardous.
Max acceptable frequency
Max frequency depeneds on the end expiratory time and respiratory system expiratory constant (Rce).
If Te/Rce ; >3, intinsic PEEP is absent or irrelavant.
If <2, relevant PEEPi is generated.
Rce = resistance * complaince
In severe ARDS compensation of low Vt is by increasing frequency which is safe. Exhalation is much faster due to low compliance combined with nearly normal resistance.
In patients with airway obstruction, frequency should be set low in order to allow a long expiratory time to avoid dynamic pulmonary hyperinflation.
I:E Ratio
The normal I:E ratio is between 1:2 & 1:1.5 corresponding to an inspiratory cycle of 33-40%.
In obstructed patients the a lower I:E ratio contributes with low Fr to prolong the Te and hence minimize intrinsic PEEP.
In restricted patients with ARDS a higher I:E may improve alveolar recruitment and oxygenation by increasing the mean pressure applied to the respiratory system.
The I:E ratio has to be adjusted in accordance to Te/RCe ratio.
Updated on 15/4/2013
Reference
PACT
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emmedonline
Dr. Ajith Kumar J MD
Dept. of Emergency Medicne
Travancore Medicity, Kollam
India
editor