An apparatus for monitoring the effects of cardioplegia administered to a heart, comprising: a near infrared spectrophotometric transducer configured to be applied onto a region of the heart; and a processor in communication with the transducer and adapted to receive data signals from the transducer; characterised in that : the processor is adapted to determine the oxygen saturation level within heart tissue using the data signals received from the transducer ; the processor is adapted to determine the oxygen saturation level within the heart tissue using first data signals produced by the spectrophotometric transducer sensing the heart tissue before an administration of a cardioplegia to the heart, and produce a pre-administration sensed heart tissue oxygenation level value; and the processor is adapted to determine the oxygen saturation level within the heart tissue using second data signals produced by the spectrophotometric transducer sensing the heart tissue after the administration of the cardioplegia to the heart, and produce a post-administration sensed heart tissue oxygenation level value; and wherein the processor is further adapted to determine a time under threshold (TUT) value or an area under threshold (AUT) value, based on a comparison of the post-administration sensed heart tissue oxygenation level value and a heart oxygenation level threshold. a near infrared spectrophotometric transducer configured to be applied onto a region of the heart; and a processor in communication with the transducer and adapted to receive data signals from the transducer; characterised in that : the processor is adapted to determine the oxygen saturation level within heart tissue using the data signals received from the transducer ; the processor is adapted to determine the oxygen saturation level within the heart tissue using first data signals produced by the spectrophotometric transducer sensing the heart tissue before an administration of a cardioplegia to the heart, and produce a pre-administration sensed heart tissue oxygenation level value; and the processor is adapted to determine the oxygen saturation level within the heart tissue using second data signals produced by the spectrophotometric transducer sensing the heart tissue after the administration of the cardioplegia to the heart, and produce a post-administration sensed heart tissue oxygenation level value; and wherein the processor is further adapted to determine a time under threshold (TUT) value or an area under threshold (AUT) value, based on a comparison of the post-administration sensed heart tissue oxygenation level value and a heart oxygenation level threshold. the processor is adapted to determine the oxygen saturation level within heart tissue using the data signals received from the transducer ; the processor is adapted to determine the oxygen saturation level within the heart tissue using first data signals produced by the spectrophotometric transducer sensing the heart tissue before an administration of a cardioplegia to the heart, and produce a pre-administration sensed heart tissue oxygenation level value; and the processor is adapted to determine the oxygen saturation level within the heart tissue using second data signals produced by the spectrophotometric transducer sensing the heart tissue after the administration of the cardioplegia to the heart, and produce a post-administration sensed heart tissue oxygenation level value; and wherein the processor is further adapted to determine a time under threshold (TUT) value or an area under threshold (AUT) value, based on a comparison of the post-administration sensed heart tissue oxygenation level value and a heart oxygenation level threshold. <EOS>