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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