ZSFG Pearls: Procalcitonin, PRVC, and some Tree in Bud ddx

The ZSFG ICU team presented a fascinating case of a patient with the elusive diagnosis of bilateral pneumonia, ultimately requiring mechanical ventilation (PRVC) for hypoxemic respiratory failure.

First, a few quick/random pearls from the discussion with Chris Berger, pulmonary fellow extraordinaire:

1) Remember to look at the trend, not just snapshot, in use of HFNC: a pt with ILD who is stable for a few days on 15L (100% FiO2) HFNC is much different than someone who is escalating from 10 to 20 to 30 Liters high flow during the course of the day. Latter=impending decompensation!
2) Fellow-perspective-pearl: To do a bronch safely in the ICU, the patient needs to be on minimal respiratory support–like extubation-level of support. Even when a patient is on 6L NC, it can be tough to get a good, thorough look safely given the “scope in airway” issue.
3) Yay for the existence of pressure regulated volume control (or PRVC or Auto-flow or PVC-VG) for lung protective ventilation! You deliver a constant volume, with decelerating inspiratory flow, while adjusting for changes in patient’s lung characteristics, AND decreasing peak inspiratory pressures! See below for more diagrams and explanations…

Procalcitonin (PC) was whispered in report today!

What is it? PC is a peptide precursor of calcitonin that is released by parenchymal cells in response to bacterial toxins, leading to elevated serum levels in patients with bacterial infections; in contrast, procalcitonin is down-regulated in patients with viral infections.

Where has it been useful? PC has been studied prospectively to decide whether to give abx in pt’s w/ PNA or when to stop abx.

  • Recent data suggests that physicians consider 1) NOT initiating abx in patients with a PC level <0.1 mcg/L and 2) definitely initiating abx for pt’s whose PC levels >0.25 mcg/L
  • Using PC to guide abx demonstrated lower rates of antibiotic exposure (intuitive).
    • A Cochrane meta-analysis of 14 trials with 4221 patients with acute respiratory infections (half with CAP) showed a reduction in antibiotic exposure (8–>4 days) without an increase in mortality or treatment failure when using PC to guide abx decisions as above
  • Limited evidence about PC levels predicting severity based on magnitude of lab result

During a recent commute, you may have been thinking about that Tree-in-bud CT pattern*!
tree-in-bud
What is it: pattern referring to centrilobular nodules with a linear branching pattern, bronchiolitis Ddx includes:
-aspiration
-infection: bronchioles/thickening of bronchiolar walls with pus or inflammatory exudate (TB is classic)
-talc or cellulose granulomatosis in patients who use IV drugs (or occasionally, oral opiates crushed and injected)
-tumor embolization
-pan-bronchiolitis
*If the CT has this pattern, ask yourself: are there clues in the location, history, additional imaging findings that can point you to the diagnosis (i.e. lower lobe predominant finding, consider aspiration)

So what is Pressure-regulated volume control (PRVC)?
A mode of ventilation that combines the advantages of pressure control (the decelerating inspiratory flow pattern) with the best of volume control (ease of use, set tidal volume). It simultaneously avoids the downsides of both traditional modes: high peak airway pressures of VC and the varying tidal volume of PC.

How does it work? By using tidal volume as a feedback control for adjusting the pressure limit

The ventilator monitors each breath (as the airway resistance and system compliance changes) and compares the delivered tidal volume with the set tidal volume. If the delivered volume is too low, it increases the inspiratory pressure on the next breath by no more than 3 cmH2O from one breath to the next. If it is too high, it decreases the pressure. Or as an image:
prvc-flow-chart
As the breaths advance from A to C, the pressure is automatically increased to achieve the set tidal volume.
prvc-loops

What is set in PRVC? Minimum respiratory rate, target tidal volume, upper pressure limit (maximal delivered pressure is 5cm H2O below the pressure alarm limit), FiO2, I:E ratio

Advantages:

  • Decelerating inspiratory flow pattern
  • Pressure automatically adjusted for changes in compliance and resistance within a set range (Vt guaranteed, limits barotrauma, prevents hypoventilation)
  • Recent studies have determined that PRVC consistently lowers peak inspiratory pressure compared to VC

Disadvantage:

  • Pressure delivered is dependent on tidal volume achieved on last breath (intermittent patient effort–> somewhat variable tidal volumes)

References:
https://radiopaedia.org/articles/tree-in-bud-sign-lung
Schuetz et al. Procalcitonin to initiate or discontinue antibiotics in acute respiratory tract infections. Cochrane Database Syst Rev. 2012 Sep 12;(9):CD007498. doi: 10.1002/14651858.CD007498.pub2. http://www.respiratoryupdate.com/members/PRVC_Pressure_Regulated_Volume_Control.cfm Guldager H, Nielsen SL, Carl P, Soerensen MB. A comparison of volume control and pressure-regulated volume control ventilation in acute respiratory failure. Critical Care. 1997;1(2):75-77. https://www.aic.cuhk.edu.hk/web8/prvc.htm
Low SU, Nicol A. Talc induced pulmonary granulomatosis. Journal of Clinical Pathology. 2006;59(2):223.

 

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