Measurement of transmural distribution of phosphorus metabolites in the pig heart by 31P magnetic resonance spectroscopy

We used the phase modulated rotating frame imaging technique to measure transmural distribution of phosphorus metabolites in 10 anaesthetised ventilated pigs using a double surface coil placed on the surface of the left ventricle. Anaesthesia was maintained in five animals with halothane, barbiturate and nitrous oxide and in five others with intravenous chloralose. 31Phosphorus spectra were acquired, gated to expiration and systole. From phantom experiments the resolution of the experiment was shown to be approximately 2 mm. The anatomical limits of the myocardium were identified by the appearance of 2,3-diphosphoglycerate peaks from red blood cells. The limits of the epicardium were confirmed by obtaining images after placing a phantom containing fluorophosphate on the surface of the heart. The endocardium was identified by inserting a small balloon catheter through the centre of the coil into the left ventricular cavity, filling it with 0.5 ml of fluorophosphate and pulling it gently against the endocardium. No transmural differences in phosphocreatine to ATP ratio were identified in the normal heart. The animals anaesthetised with chloralose showed a significantly higher phosphocreatine to ATP ratio compared to those anaesthetised with halothane and barbiturate. The chloralose animals tended to have a higher blood pressure and a lower heart rate when compared to the other animals. No transmural differences, however, were identified in either group. When regional ischaemia was produced using a snare to occlude the left coronary artery, phosphocreatine fell and the signal from the inorganic phosphate + 2,3-diphosphoglycerate region increased. The inner wall tended to become more acid compared to the outer wall during ischaemia. These experiments show that the phase modulated rotating frame imaging technique can be used to study the effects of changes in workload, ischaemia, or pharmacological intervention on transmural distribution of metabolites in the heart and thus help elucidate factors responsible for subendocardial vulnerability to stress.