Mechanically ventilating the morbidly obese patient has many considerations
Pulmonary Mechanics
- Reduced FRC, supine & trendelenburg positions can cause ventilation perfusion mismatch.
- At rest Vt 20% reduced of IDBW.
- Excess adipose tissue on the CX wall & abdominal cavity result in restrictive pulmonary physiology, decreases compliance and lung volumes.
- Peak inspiratory pressure may rise by 50-81%
- Obese patients produce excessive carbon dioxide (VCO2) & have increased oxygen consumption (VO2).
- The excessive chest wall adiposity results in atelectasis.
How Can One Setup ASV for Effectively Ventilating the Morbidly Obese Patient?
1. Settings for the negative impact of pulmonary mechanics
From the attached video one can see that due to higher peak inspiratory pressures that ASV will automatically decrease the tidal volume and increase the respiratory rate to maintain the target minute ventilation. The reduction of tidal volume may increase atelectasis.
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| Image 1: Due to excessive chest wall adiposity and higher PIP, ASV will target settings similar to an ARDS patient resulting in lower target tidal volumes. |
One does not want to simply increase the Ideal Body weight setting to trick the ventilator in administering a higher tidal volume. This is unsafe and not necessary.
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| Image 2: Patient Height setting that effects the Ideal Body weight target. This should remain unchanged. |
If there is additional atelectasis and/or oxygenation problems then one can perform a recruitment maneuver and set the PEEP appropriately.
2. Settings for the increased oxygen consumption
One can set PEEP appropriately and titrate the FIO2.
3. Settings for the increased CO2
One can adjust the %MinVol setting to find the "ASV Target Point".
Even though the patient may not have ALI or ARDS these lower tidal volumes should not have an impact on the obese patient as long as the PEEP and %MinVol setting is set appropriately.
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REFERENCE
Alvarez, A. et. al. (2010). Morbid Obesity Peri-Operative Management. 2nd ED. Cambridge University Press.
McConachie, I. (2009). Anesthesia for the High-Risk Patient. 2nd Ed. Cambridge University Press.
Pelosi et. al. (2013). Is Protective Lung Ventilation Safe in Morbidly Obese Patients? Controversies in the Anesthetic Management of the Obese Surgical Patient. Part 3. 179-198.