the-ventilator-book-3rd-edition-0985296569-9780985296568_compress

5. FiO 2 of 100% to start; lower this to 60% as long as the SpO 2 remains
88% or better.
Sometimes, patients with COPD or asthma will remain tachypneic despite
adequate sedation. In assist-control, every patient-triggered breath delivers a
full tidal volume, and this can lead to air trapping or severe respiratory
alkalosis. If this is the case, switching the mode to SIMV may help.
Severe Metabolic Acidosis
Examples: Salicylate poisoning, septic shock, toxic exposures, acute renal
failure, diabetic ketoacidosis
The normal response of the respiratory system in the setting of metabolic
acidosis is to hyperventilate. CO 2 is a volatile acid, and the lungs can rapidly
eliminate this acid from the body in an attempt to bring the pH closer to
normal. In a patient with a HCO 3 of 4 mEq/L, for example, the PaCO 2 will be
14-15 mm Hg if there’s appropriate respiratory compensation. This requires a
very high minute ventilation to accomplish.
It is very difficult to set the ventilator to provide a high minute ventilation,
even if you set the rate to be 30-35 and the tidal volume to be 800-1000 mL.
Patients with severe metabolic acidosis will often breathe in when the vent is
trying to breathe out, and vice versa—this leads to significant patientventilator
dyssynchrony and alarming of the machine. More consequentially,
the volume and pressure alarms that are normally helpful will actually work
against the patient by limiting the minute ventilation that can occur.
Consider the aforementioned example—a patient who has a pH of 6.88 and a
HCO 3 of 4 needs a PaCO 2 of 14-15. If he’s intubated and sedated, and the
vent settings are put in the “usual” range, his PaCO 2 may rise to 25-30. In the
setting of severe acidemia, this increase in CO 2 will cause his pH to fall to
6.6 or so, which will most likely lead to a cardiac arrest.
The best way to deal with this situation is to let the patient’s naturally high
respiratory drive work in his favor.
1. Use the bare minimum of sedation to intubate and avoid neuromuscular