ObjectiveTo provide a meta-analysis of current literature concerning the validation of thoracic impedance cardiography (TIC) and to explain the variations in the reported results from the differences in the studies.Data SourcesA computer-assisted search of English-language, German, and Dutch literature was performed for the period January 1966 to April 1997. Moreover, references from review articles were obtained.Study SelectionA total of 154 studies comparing measurements of cardiac output or related variables obtained from TIC and a reference method were analyzed.Data ExtractionArticles were classified by differences in TIC methodology, reference method, and subject characteristics. Fisher's Zftransformed correlation coefficients were used to compare results. Data were pooled using the random-effects method.Data SynthesisAn overall pooled r2value of.67 (95% confidence interval, 0.64-0.71) was found. However, the correlation was higher in repeated-measurement designs than in single-measurement designs (r2= .53; 95% confidence interval, 0.43-0.62). Further research using analysis of variance revealed a significant influence of the reference method and the subject characteristics on the correlation coefficient. The correlation was significantly better in animals than in cardiac patients. Subgroup analysis revealed that TIC correlated significantly better to the indirect Fick method than to echocardiography in healthy subjects. No significant influence of the applied TIC methodology was found.DiscussionThe overall r2value of.67 indicates that TIC might be useful for trend analysis of different groups of patients. However, for diagnostic interpretation, a r2value of.53 might not meet the required accuracy of the study. Great care should be taken when TIC is applied to the cardiac patient. However, because the applied reference method was of significant influence, differences between TIC and the reference method are incorrectly attributed to errors in TIC alone. (Crit Care Med 1999; 27:1203-1213)