Serum carnitine concentrations in patients with idiopathic hypertrophic cardiomyopathy: relationship with impaired myocardial fatty acid metabolism

We evaluated the clinical significance of serum carnitine concentrations in determining the severity of impaired myocardial fatty acid metabolism in i diopathic hypertrophic cardiomyopathy (HCM). We studied 56 asymptomatic or mildly symptomatic patients with HCM. Serum levels of free carnitine and ac ylcarnitine were measured by the enzymic cycling method. Myocardial scintig raphy with (123)l-labelled 15-(rho-iodophenyl)-3-R,S-methylpentadecanoic ac id (BMIPP) was performed, and the images were analysed quantitatively and s emi-quantitatively. Serum free carnitine levels were significantly higher i n HCM patients than in normal subjects (52.5+/-9.5 and 42.3+/-5.5 nmol/ml r espectively; P <0.0001). On the other hand, serum acylcarnitine levels and acyl/free carnitine ratios were lower in HCM patients than in normal subjec ts (10.2+/-4.0 nmol/ml and 0.19+/-0.08, compared with 13.2+/-3.9 nmol/ml an d 0.32+/-0.11 respectively; P < 0.0001). Clinical characteristics were not significantly different between the patients showing high and normal free c arnitine levels, although female patients with high free carnitine levels w ere few (P = 0.02). Both quantitative and semi-quantitative analyses reveal ed 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 th e presence of reduced carnitine uptake into the myocardium in HCM, there ma y also be reduced transport of acylcarnitines out of the myocardium into th e plasma. Although inborn errors of fatty acid metabolism and carnitine def iciencies 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 c arnitine concentrations are a sensitive indicator of the severity of impair ed myocardial fatty acid metabolism even in asymptomatic patients with HCM.