In hyperpolarized 13C MRI, the achievable base SNR depends on the T1 relaxation of pyruvate in blood. Heart rate dependency on the resulting SNR is examined using simulations and exemplarily shown on human data of the heart. Measured T1 and T2 values at 1.5T and 3T of hyperpolarized 13C pyruvate in blood are reported. The results indicate that for a range of pyruvate concentrations, T2 remains >6s and hence T2* is primarily determined by field inhomogeneities. T1 is reduced at lower concentrations, and therefore heart rate plays a decisive role for the achievable SNR. Exemplary in-vivo data illustrate these findings.