Sunday, July 9, 2017

Settings for mechanical ventilation

These are my quick and dirty notes to help ventilator settings related questions on the USMLE.

Normal values:
pH: 7.35-7.45
PaCO2: 35-45 mmHg
PaO2: > 80 mm Hg
SpO2: 92-97%
FiO2: 21% (room air)

PaO2: Partial pressure of oxygen
FiO2: Fraction of inspired oxygen
SpO2: Oxygen saturation
PEEP: Positive end expiratory pressure

PS: PaO2 goals may vary according to the patient you are dealing with!

PaO2 < 60 mm  Hg (or SpO2 < 90%) reflects inadequate oxygenation.

You can make two adjustments:
1. Increase FiO2
2. Increase PEEP

1. PEEP is usually between 5-20 cm H2O.
2. Immediately following intubation, a high FiO2 is provided ( > 60% ) - ventilator settings are adjusted subsequently.
3. FiO2 is usually weaned to < 60% because prolonged high FiO2 increases the risk of oxygen toxicity.

PaO2 > 80 mm Hg (or SpO2 > 95%) indicates more than adequate oxygenation.

You can make two adjustments:
1. Reduce FiO2 to < 60%
2. Reduce PEEP to 5 cm H2O

PaCO2: Partial pressure of carbon dioxide
EtCO2: End Tidal CO2
RR: Respiratory rate
VT: Tidal volume
PIP: Peak inspiratory pressure
Pplateau: Plateau pressure

Respiratory rate is used for titrating ventilation, PaCO2 is usually adjusted to pH range. 

Change RR first, then adjust VT.

PS: PaCO2 goals may vary according to the patient you are dealing with!

For a pH < 7.30 (or PaCO2 > 45 mm Hg or EtCO2 > 50 mm Hg)
1. Evaluate to ensure the cause is respiratory.
2 . If appropriate, increase rate to a maximum of 24 breaths/min until pH is > 7.30.
3. If further adjustment is needed increase VT (until PIP > 40 cm H2O or Pplateau > 30 cm H2O.)
4. If unable to maintain these parameters, consider allowing permissive hypercapnia

For a pH > 7.45 (or PaCO2 < 35 mm Hg or EtCO2 < 30 mm Hg)
1. Evaluate to ensure the cause is respiratory.
2. If appropriate, reduce rate to a minimum of 8 breaths/minute or until pH is < 7.45.
3. After rate is decreased to 8 breaths/minute, if pH is still > 7.45, reduce volume to a minimum of 4 mL/Kg (IBW).

1. VT is usually 4-12 mL/kg of Ideal Body Weight.
2. RR is usually 8-26 breaths / minute.
3. For a healthy person, use the "rule of 12" (12 ml/kg delivered 12 times per minute.)
Mnemonic: The word, "TIDAL VOLUME" has ~12 letters.
4. Lower tidal volume of 5 ml/kg is preferred in ARDS.
Mnemonic: The word, "ARDS" has ~5 alphabets.
5. Lower rate ( < 10 breath per minute) is preferred in COPD and asthma.

That's all!

Sources of information for these notes:

1 comment:

  1. I really appreciate your efforts in explaining this topic. While skimming through the topic, I noticed that "weaning off the ventilator' was not given, only weaning from fi02 was mentioned. Kindly throw some light on when to wean off the ventilator. Also, what would a physician do when paO2 is high, but paCO2 also is high? increase TV or stop ventilation? It would be more precise if the entire topic is explained based on scenarios. Again these are only my opinion and I hope that these topics are addressed very soon. TIA


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