Human Toxicol. (1987), 6, 49-55 Treatment of Paraquat Poisoning in Man: Methods to Prevent Absorption T. J. Meredith & J. A. Va1e 1 Department of Medicine, Guy's Hospital, London SEl 9RT, and 1 West Midlands Poisons Unit, Dudley Road Hospital, Birmingham B18 7QH, UK Theoretically, absorption of an ingested dose of paraquat may be reduced by (1) gastric lavage, (2) induced emesis, (3) whole-gut lavage or (4) by the oral administration of adsorbent substances. 1 Animal experiments suggest that paraquat is absorbed poorly from the stomach and absorbed incompletely ( < 5%) from the small intestine over a 1--6-h period. Although gastric lavage would therefore seem a logical way to ameliorate the toxicity of an ingested dose of paraquat, peak plasma concentrations are attained rapidly and evidence for the efficacy of gastric lavage in man is poor. 2 In 1977. a potent emetic (PP796) was added to liquid and solid formulations of paraquat because experiments in primates had demonstrated a fivefold reduction in toxicity. In man, ingestion of formulations containing an emetic is more likely to cause spontaneous vomiting within 30 min than non-emetic preparations. However, definite evidence of benefit, as judged by improved patient prognosis, has yet to be established. 3 Gut lavage has been shown to remove only a small proportion of an ingested dose of paraquat. At the flow rates employed in man (75 ml/min), approximately 0.5-1.0 litres of lavage fluid/h may be absorbed across the intestinal wall. Since there is a theoretical risk of increasing paraquat absorption, the use of whole-gut lavage cannot be recommended. 4 Bipyridilium herbicides are adsorbed by soil and clay minerals, and montmorillonite in particular has been shown to be a strong binding agent in vitro. Accordingly, the use of Fuller's Earth (calcium montmorillonite) and Bentonite (sodium montmorillonite) for the treatment of poisoning has been investigated in animal models. Both agents reduce plasma paraquat concentra- tions and mortality in animals when administered after an oral dose of paraquat. Recently, other adsorbent materials have been found to have high maximum adsorption capacities for paraquat. In particular, activated charcoals and cation-exchange resins have attracted interest. Unfortunately, as yet, there is no evidence that the use of adsorbents in man is of therapeutic value. Introduction Paraquat (1,l-dimethyl-4,4-bipyridilium) is a potent contact herbicide that is potentially lethal to man if ingested. Death due to paraquat poisoning is usually characterized by pulmonary oedema and fibrosis but, if large amounts are ingested, multiple organ failure may develop (Vale et al., 1987). The precise mech- anism of toxicity is uncertain but, once a critical plasma concentration is exceeded, active accumu- lation of paraquat in the lung occurs, with formation of superoxide anion and depletion of NADPH (Smith, 1987). There is no effective antidote for paraquat poisoning (Bateman, 1987) and measures designed to enhance the elimination of paraquat from the body have not proven satisfactory (Bismuth et al., 1987; Proudfoot, 1987). Attention has therefore been directed at the various means by which the absorption of an ingested dose of paraquat may be either prevented or reduced, namely gastric lavage, induced emesis, whole-gut lavage or the oral admini- stration of adsorbent substances. The rationale for the use of each form of treatment is considered below and the evidence for their value in man is reviewed critically. Gastric lavage Paraquat is absorbed incompletely from the gut and, in man, it has been estimated that less than 5% of ©The Macmillan Press Ltd 1987 SYNG-PQ-00059882