Relationship between lactate arteriovenous gradient and tissue lactate content in subendocardial and subepicardial layers of the ischaemic in situ heart

Myocardial lactate content and simultaneous evolution of a lactate arteriovenous gradient associated with incomplete obstruction of the left coronary bed were studied in dog hearts. Samples of blood were taken from a peripheral artery and the coronary sinus; transmural samples of myocardial tissue were obtained from the left ventricular wall by drill biopsy in animals under total cardiopulmonary by-pass. Lactate content was assessed in subendocardial and subepicardial layers separately. A 40 to 70% reduction in coronary flow induced a quick reduction and even an inversion of the positive lactate gradient while the tissue content, which was similar to the arterial content, rose considerably, chiefly in the subendocardial layer. Lactate accumulating in tissue was released into the blood approximately in proportion to the tissue concentration. The value of employing the lactate concentration difference between arterial blood and coronary sinus blood as an index of the severity and duration of myocardial ischaemia is discussed.