ICU nurses or docs or RRTs to answer please ... vent. settings?

Question

One of my intubated patients (dominant-hemisphere meningioma that beld) had been "chugging" really bad on the vent. I'd been suctioning for scant sputum. The nurse giving me shift change repor said the intensivist said his PSV was set at too low a rate. He was in a hurry to go, so I didn't have chance to ask why this helped. Before *his* shift, the previous RN had given him 800 mcg fentanyl, 15 mg Versed, 200 mg of hydralazine, and 60 mg labetalol for high SBP (>180) and high ICP (in the high 20s to low 30s (mmHg)). After the PSV was raised from +6 to +10 (cm H20), the patient's BP was under control and he was settled, needing only 200 mcg fentanyl and 20 mg hydralazine on the shift befor mine, and absolutely nothing on my shift. What is the rationale for increasing the pressure support? Also, what is "autopeeping?" -- his PEEP was raised to +15 to prevent autopeep.

Answer 1

Actually, increasing the pressure support would not fix autopeeping -- as you know, PSV allows the patient to initiate breaths and augments each breath with an additional volume until a defined pressure (e.g 6 cm H2O) above atmospheric pressure is reached. Increasing the pressure support, in itself, cannot prevent autopeep! Only increasing the value of the vent PEEP can do that -- it's an usually an empiric change (i.e. you first suspect autopeeping then raise the PEEP until evidence of continued autopeep disappears).

Based on what we know from what you wrote, I suspect the reason for your patients requirement of fewer sedatives and antihypertensive drugs was simple -- his WOB was decreases by increasing the PS from +6 to +10, making it such that he didn't have to "fight the circuit" to breathe. Even though PS is a "pressure mode," his tidal volumes will be greater on a higher PS, so watch that they don't get too high. Also, he may begin to 'air trap' again if the PEEP is decreased, e.g. in an effort to decrease ICP, one dangerous option would be to turn the peep down too quickly. Of course, during his stay, both his PEEP and PS will have to be weaned before he's extubated.

Being a neurosurgical ICU nurse, I'd be interested to see what his CPPs are now that his BP is under control. Hopefully his ICP readings are consistently below 20 and he hasn't developed any sort of global ischemia; make sure there's a low SBP/MAP/CPP parameter order.

Answer 2

When the girth of the connecting airways is narrowed (due to mucus plugs for example), the gas is trapped - such trapping of the gas is what we call AUTOPEEPING. Such air trapping can exert positive pressure and will ncrease the work in breathing.

If a patient is generating auto PEEP, then the pressure in their airway is positive with respect to the outside atmosphere at the end of expiration.Applying a positive pressure to the airway, level that is higher than the auto-PEEP (waterfall effect) will make a pressure gradient that opens up the airway and will facilitate alveolar gas exchange.

That was the reason of increasing the pressure support to your patient - to apply a positive pressure higher than the AUTOPEEP to make a pressure gradient so that the alveoli can participate in gas exchange.