Tag Archives: antibiotics

Vancomycin Dosing

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When to use vancomycin

The most common use for vancomycin is in invasive Gram positive infections

You need to consider

  • Infection site
  • Patient weight
  • Kidney function
  • Pathogen susceptibility

Pharmacokinetics

  • Vancomycin has bad oral bioavailability so it’s almost never used as a pill
    • Occasionally it is orally to supplement C. diff infections (because that’s going on in the GI tract)
  • Volume of distribution: IV serum 0.4-1 L / kg
  • Normally vancomycin doesn’t cross the blood brain barrier very well, but in the setting of meningitis the inflamed meninges increases permeability

Adverse effects

  • Redman syndrome: A histamine-like flushing during or immediately after dose. Occurs mostly on the face and neck. This is NOT life threatening
    • Treatment: anti-histamine, pause infusion, then restart at a slower rate
    • If the reaction is severe, stop the infusion, give antihistamines, wait until symptoms resolve before restarting. When you restart, give the infusion reaaaalllllly slooooooowly (over more than 4 hours)
  • Nephrotoxicity

Dosing

This is where vancomycin can get tricky, because you are aiming for a target trough (between dose) serum concentrations.

  • Generally the target is 10 mcg/ml, but this may need to be higher for treating MRSA or osteomyelitis
  • Trough concentrations should be measured 30 minutes before the 4th dose any time a course of vanco is started or the dose is changed
  • Monitor creatinine at least once a week (remember that whole nephrotoxicity bit)

Starting dose should be 15-20 mg/kg (based on actual not ideal body weight) every 12 hours. This usually works out to 1-2 g IV Q12H. If the kidneys are not working well, reduce the dose.

References

  • UpToDate.com “Vancomycin: Parenteral dosing, monitoring, and adverse effects in adults”

Signs of Meningitis

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Meningitis is very literally inflammation of the meninges. Something swollen in a closed space is never good, so it’s important to not miss meningitis when it presents.

Classic triad of meningitis

  1. Fever
  2. Neck stiffness
    • Brudzinski
    • Kernig
  3. Mental status change – in babies this can be an increase in somnolence or irritability (unconsolably crying)

Important organisms:

  • Bacterial
    • Listeria*
    • E. coli*
    • GBS (Group B strep)*
    • Neisseria meningitidis
    • Strep pneumoniae
    • Staph aureus
    • Gram neg bacilli
    • Haemophilus influenza
  • Viral (“aseptic”)
    • HSV
    • Enterovirus
    • HIV
    • Mumps

* These are the common ones in the neonatal period

Bacterial:

  • Positive Gram stain
  • CSF white blood cell (WBC) count >1000/uL with a predominance of neutrophils
  • Low CSF glucose concentration <40 mg/dL (2.2 mmol/L)
  • Empiric treatment: high doses of a 3rd generation cephalosporin (cefotaxime, ceftriaxone) and vancomycin (this covers antibiotic-resistant S. pneumoniae, N. meningitidis, and Hib)

Cephalosporins (generations and spectrum of activity)

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Cephalosporins work much like penicillins, inhibiting peptidoglycan cell wall synthesis in bacteria (remember those sites of action and mechanisms?)

Of course the issue is that they just keep making new cephalosporins and each generation is a little bit different in terms of its spectrum and whether it’s better at fighting Gram positive or Gram negative bacteria. Generally the newer the generation, the more broad spectrum and less Gram positive coverage. To add another layer to the confusion, there are separate oral and IV cephalosporins for each generation and all of the cephalosporins are usually recognizable by starting with “CEF-” or “KEF-” (except for Suprax and Ancef, who ever came up with those brand names didn’t get the memo)

 

Antibiotic classes and mechanism

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These are the different classes of medications and their mechanism of action. I’ve sorted them by which ones are exclusively Gram + or Gram – and which ones do both.

Here’s a handy mnemonic for remembering the antibiotics that act on the ribosomes (I find they’re the most confusing to remember):

Buy AT 30
Aminoglycosides
Tetracyclines

CELL for 50
Clindamycin
Erythromycin
Linezolid

Sites of antibiotic actions

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Antibiotics are the current bane of my existence. Most medical conditions there are a couple of different classes of drugs, but overall things are straight forward. Not so with antibiotics, not only are there a bunch of different classes, they all have different indications!

This is part 1 of my antibiotics doodles. This one outlines where the different commonly used antibiotics act on the cell, the class they belong to, and whether they are used mostly for Gram + (Staph, Strep), Gram -, or a little bit of both. Keep in mind that the bacterial cell in the drawing is a Gram + coccus.

Part 2 will go into a little more detail about the different classes.