Blood clearances of lignocaine and indocyanine green together with indocyanine green half-lives were measured in 17 post-myocardial infarct patients (one patient was studied twice) between 8 h and 36 h after starting intravenous lignocaine infusions for the treatment of cardiac arrhythmias. 2. Mean +/- s.d. values of lignocaine clearance (ml min-1 kg-1) were higher in patients without heart failure (11.8 +/- 2.6, n = 9) than in those with heart failure (7.2 +/- 1.9, n = 9) (P < 0002). 3. Clearances of lignocaine and indocyanine green were not correlated but lignocaine clearance was directly related to the reciprocal of indocyanine green half-life (rs = 0.67, P < 0.01). 4. In eight patients who received both lignocaine and indocyanine green and in a further five patients received only lignocaine and whose lignocaine infusions lasted 24h or more, a 25% rise in lignocaine concentrations was observed between 8-12h and 24-28h. 5. The mean +/- s.d. post-infusion terminal half-life of lignocaine in four patients whose lignocaine infusions lasted 30h or longer was 7.2 +/- 2.1 h. 6. Heart failure was associated with greater changes in lignocaine kinetics than in indocyanine green kinetics. 72% of the variance between observed and predicted lignocaine clearances could be accounted for by multiple linear regression analysis incorporating indocyanine green half-life and the presence or absence of heart failure. Indocyanine green half-life contributed only 17% of the variance indicating that by itself it is of limited value in predicting lignocaine requirements. 7. Lignocaine kinetics during and after prolonged intravenous infusion were not predicted by data obtained after intravenous bolus injection. 8. A lowering of lignocaine dosage may be clinically desirable in the presence of heart failure and if an infusion lasts longer than 24 h.
