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Welcome back to the tasty morsels of critical care podcast.
In yet another departure from the stone tablets of Oh’s manual, today we’ll talk a little about one of favourite gram +ve cocci: staphylococcus aureus. Diagnosis and management of infections with this bug are a common occurence in the ICU and it behoves us to have a working knowledge of some of the complexities of its investigation and treatment that often fall to our micro or ID colleagues. So accept this for what it is – an intensivists summary of someone else’s expertise.
About 30% of us are colonized with staph aureus. Much of this is simple MSSA but there are increasing cohorts of MRSA in the mix there too.
When causing an actual infection we see staph aureus implicated in lots of different types of infection such as
- skin and soft tissue infections from cellulitis to foliculitis to abscesses to impetigo
- a toxin mediated GI illness from eating food contaminated with staph aureus
- bone and joint infections
- a sepsis with toxic shock syndrome
- pneumonias especially in the hospitalised population
- cerebral infections in the context of trauma, neurosurgery and drains
- blood stream infections from IE to device infection
Many of the list above will have nothing to do with critical care but staph really comes into its own when it gets into the blood. Getting a phone call from micro that your septic patient in the ICU has staph aureus in the blood is a fairly common phenomenon and should trigger a specific sort of response. A staph bacteraemia carries with it a mortality of roughly 20% so it’s something to take very seriously.
This is a disease which has significant metastatic complications so once you realise there’s staph in the blood you should be hunting out the other sites it could have spread to. It’s often difficult to tease out if the infected site is a primary site of infeciton or has seeded form somewhere else but a good list of places to look at would include
- heart valves
- cardiac devices
- the spine
- bones and joints
- embolic abscesses in the lungs or even a primary pneumonia
- septic emboli in the brain
- skin and soft tissue infections
In ICU level staph bacteraemia I find it’s the heart, the spine and the lungs are the main sites of infection. Needless to say that if there’s a collection of pus somewhere in staph bacteraemia strong consideration should be given to draining it acknowledging the complexities of decompressing a spine etc…
The brain primarily becomes an issue in someone with IE and a valve that is falling to pieces. In these patients surgery is typically considered but there is some understandable reluctance to give 40000 units of heparin to anticoagulate someone for a bypass run when there may or not be a lump of staph aureus in the cerebrum. As a result they often end up getting MRI to ensure the brain is clean.
Interestingly the PET-CT has become a key tool in the diagnosis and management of staph bacteraemia. At least it seems to have become key in centres with timely access to PET-CT. I can say that I have never managed to get an ICU patient through a PET-CT and they inevitably get it later on when all the excitement of the ICU stay has died down.
Once we know there’s a staph aureus in the blood what should we be reaching for treatment wise? Typically the patient will already be on some antimicrobials before you even get the result. So usually it’s a question of rationalising the bug juice. Vancomycin is typically our go to until we get an ID as the MRSA status is often unknown at this point. If it turns out to be MSSA then the swap is typically to a high does flucloxacillin, say 2g every 4-6 hrs. This swap is important as an anti stpahylococcal penicillin like fluclox is a better drug for killing staph aureus than vancomycin, so if it’s an option it should be used.
There are some nuances once you get into the weeds (and beyond the scope of an ICU exam) about addition of aminoglycosides (not routinely recommended) or addition of rifampicin (generally only in the context of prosthetic joints or hardware).
The next job is to document clearance from the blood. once on the bug juice repeat the cultures. If you’re killing the bug it should be gone within 48 hrs. failure to do so suggests an issue with source control so go back and have a think about what you need to image further – eg the heart valves, the spine etc..
Duration is split into uncomplicated and complicated. an example of uncomplicated might be someone with a PICC line staph bacteraemia that clears their cultures very quickly, gets the lines out and as a good quality negative TTE. They have uncomplicated staph aureus bacteraemia and typically 2 weeks is sufficient. Complicated might be someone with a spinal osteomyelitis not needing surgery and maybe it took a week before the cultures cleared. They likely need 6 weeks.
Reading:
largely a summary of the UTD article on staph bacteraemia.