We evaluated the clinical significance of serum carnitine concentrations in determining the severity of impaired myocardial fatty acid metabolism in idiopathic hypertrophic cardiomyopathy (HCM). We studied 56 asymptomatic or mildly symptomatic patients with HCM. Serum levels of free carnitine and acylcarnitine were measured by the enzymic cycling method. Myocardial scintigraphy with 123I-labelled 15-(p-iodophenyl)-3-R,S-methylpentadecanoic acid (BMIPP) was performed, and the images were analysed quantitatively and semi-quantitatively. Serum free carnitine levels were significantly higher in HCM patients than in normal subjects (52.5±9.5 and 42.3±5.5 nmol/ml respectively; P< 0.0001). On the other hand, serum acylcarnitine levels and acyl/free carnitine ratios were lower in HCM patients than in normal subjects (10.2±4.0 nmol/ml and 0.19±0.08, compared with 13.2±3.9 nmol/ml and 0.32±0.11 respectively; P< 0.0001). Clinical characteristics were not significantly different between the patients showing high and normal free carnitine levels, although female patients with high free carnitine levels were few (P = 0.02). Both quantitative and semi-quantitative analyses revealed that the severity of decreased myocardial BMIPP uptake was significantly correlated with serum free carnitine levels (quantitative analysis: r = -0.422, P< 0.0012; semi-quantitative analysis: r = 0.633, P< 0.0001). In the presence of reduced carnitine uptake into the myocardium in HCM, there may also be reduced transport of acylcarnitines out of the myocardium into the plasma. Although inborn errors of fatty acid metabolism and carnitine deficiencies are reported to provoke secondary HCM and are associated with low serum carnitine concentrations, this study has revealed that the levels of carnitine are, in contrast, increased in idiopathic HCM. Moreover, serum carnitine concentrations are a sensitive indicator of the severity of impaired myocardial fatty acid metabolism even in asymptomatic patients with HCM.

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