Fat emulsion infusion is routinely used as a source of calories and essential fatty acids for critically ill patients who may be at risk for acquired ventricular repolarization alterations due either to drugs or electrolyte disturbances. The aim of this study was to evaluate whether acute elevations of plasma free fatty acid concentrations influence the corrected Q-T interval (Q-Tc), Q-Tc dispersion and sympathetic nervous system activity in patients requiring parenteral nutrition. Thirty hospitalized patients (mean +/- SD: 62 +/- 17 yr of age) requiring total parenteral nutrition received an infusion of 10% (500 ml) triacylglycerol emulsion as a source of calories (450 Kcal); on another occasion, and in random order, the same patients received an infusion of 20% (500 ml) triacylglycerol emulsion (900 Kcal). The infusion lasted 8 h and was preceded by a sc injection of heparin (5,000 U). Infusions of both 10% and 20% triacylglycerol emulsion increased plasma free fatty acid (p<0.00 1) and triacylglycerol (p<0.01) concentrations, and was associated with no significant change in mean BP, heart rate, and plasma catecholamines. At baseline, Q-Tc and Q-Tc dispersion were within the normal range (<440 milliseconds for QTc and <40 ms for QTc-d) and did not show any significant change at any time during infusion of triacylglycerol emulsion at both concentrations. In the setting of a balanced parenteral nutrition, acute elevation of plasma free fatty acid concentrations in critically ill patients do not modify ventricular repolarization.