Selective decontamination of the digestive tract, SDD: a commentary

Abstract

The paper by Silvestri et al. throws down a challenge to those among us who have evaluated, used and subsequently abandoned selective digestive decontamination (SDD) as a means of reducing infection in the Intensive Care Unit (ICU).1 The rationale of SDD is that conventional infection control measures in the ICU will not succeed in reducing hospital-acquired infection because much infection that is presumed to be hospital-acquired is in fact endogenous in origin. This implies that handwashing and conventional cross-infection prevention measures are of limited value.2 In the original description of SDD lower airway infections were classified as primary endogenous if the causative organisms were colonizing the patient on ICU admission, secondary endogenous if the colonization occurred after ICU admission and exogenous if the infection was not preceded by a period of mucosal colonization.3 In this system SDD is intended to prevent primary and secondary endogenous infections. However, since secondary endogenous colonization is ICU-acquired it should be possible to reduce this by conventional infection control measures. It is only possible to discriminate between primary endogenous and ICU-acquired infection if very robust sampling and typing methods are employed. Routine phenotypic laboratory methods based upon biochemical, cultural and susceptibility observations are inadequate. Consequently data describing the relative prevalence of primary endogenous and ICU-acquired infection must be interpreted with caution. However, it is upon such data that the relative efficiency of the systemic and topical components of the SDD regimen and the ultimate potential of SDD are assessed. The clinical efficacy of SDD has been very extensively investigated although study protocols, SDD regimens and definitions of infection have varied widely. Nonetheless, the overwhelming majority of studies have shown a worthwhile reduction in infection, particularly in trauma and some surgical patients, and this has now been confirmed for unselected ICU patient populations by meta analyses.3–6 The most striking conclusion from the analysis of D’Amico et al., which was both large and methodologically robust, is that SDD significantly reduces mortality in unselected ICU patients provided both systemic and topical components of the regimen are applied. Why, then has SDD not been more widely advocated by microbiologists and adopted by intensivists? There is continuing concern regarding the microbiological safety of SDD. The authors suggest that the only fully effective regimen is that of Stoutenbeek et al.,3 which comprises systemic cefotaxime for the first few days in ICU with topical oropharyngeal and nasogastric polymyxin, tobramycin and amphotericin (PTA) throughout ICU stay. This gives rapid and effective decontamination of oropharynx and stomach provided that the mucosal flora does not contain micro-organisms resistant to the regimen and the patient is not receiving sucrafate.7,8 Tracheal decontamination is less reliable, possibly because of subcidal drug concentrations.9 Gut decontamination is dependent upon peristalsis and rectal decontamination may take seven days.7 This is longer than many patients spend in ICU. Consequently the majority of patients may have subcidal concentrations of the topical agents in the gut for most or all of their ICU stay. The gaps in the SDD antimicrobial Received 28 April 2000; revised manuscript accepted 4 May 2000. Author for correspondence: Dr C. H. Webb, Department of Clinical Bacteriology, Royal Hospitals Trust, Belfast BT12 6BA, UK. Fax: 44 (0) 28 90 311416; E-mail: hugh.webb@bll.n-i.nhs.uk