SDD is based on the observation that critical illness profoundly changes body flora, promoting a shift from:
- Normal (Streptococcus pneumoniae in the throat and Escherichia coli in the gut) towards abnormal carriage (aerobic Gram-negative bacilli and methicillin-resistant Staphylococcus aureus in throat and gut)
- Low to high grade carriage (gut overgrowth) of both normal and abnormal flora.
Gut overgrowth is the crucial event preceding endogenous infections. Primary endogenous infection is caused by ‘normal’ and ‘abnormal’ potential pathogens, present in the patient’s admission flora in overgrowth concentrations. This infection generally develops within a week and is the most frequent type of infection (55%). Secondary endogenous infection is invariably caused by ‘abnormal’ bacteria not present in the admission flora but acquired during treatment on the intensive care unit (ICU) and subsequently carried in overgrowth. This infection generally occurs after one week on ICU (30%). Exogenous infection is caused by ‘abnormal’ bacteria never carried in the patient’s oropharynx and/or gut. This type of infection may occur anytime during ICU-treatment (15%).
Parenteral cefotaxime controls gut overgrowth due to ‘normal' bacteria, enteral polyenes control gut overgrowth due to ‘normal' Candida spp. Enteral polymyxin/tobramycin (with or without vancomycin) eradicate, if already present, and prevent overgrowth with ‘abnormal' bacteria.
These three types of ICU-infection each require different prophylaxis. Primary endogenous can only be controlled by parenteral antimicrobials, secondary endogenous are prevented by enteral antimicrobials and high hygiene standards, exogenous are controlled by topical antimicrobials and hygiene.
These three interventions were first combined by Stoutenbeek who expanded the prophylaxis to include surveillance cultures creating the full four component SDD protocol, the main mechanism of action being gut overgrowth control.
There are 65 randomised controlled trials evaluating SDD and eleven meta-analyses, confirming SDD reduces pneumonia (72%), septicaemia (37%) and mortality (29%) without resistance emerging.