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:

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