INTRAMYOCARDIAL PRESSURE MEASUREMENTS IN THE ISOLATED-PERFUSED PAPILLARY-MUSCLE OF RAT-HEART

The intramyocardial pressure (IMP) plays a role in the interaction bet ween heart contraction and coronary flow. In order to measure IMP we d eveloped the isolated perfused papillary muscle. The papillary muscle was suspended in a muscle bath with oxygenated Tyrode's solution. Perf usion with Tyrode's solution took place via the septal artery. Diastol ic intramyocardial pressure was measured with micropipettes with a tip diameter of 3 to 4 mu m in combination with the servo-null technique. After an equilibration period of at least a half hour where the muscle was perfused with a perfusion pressure of 40 cmH(2)O and stimulated 0 .2 Hz the IMP measurements started. Pressure changes resulting from ch anges in perfusion pressure and injection of ink via the pipette made it possible to distinguish between interstitial and vascular localizat ion. No leakage along the shaft of the pipette to the outside was foun d. Perfusion caused interstitial edema in the muscle so that the cross -sectional area (CSA) of myocytes relative to total muscle CSA decreas ed from 71% in non-perfused muscle to 51% in perfused muscle. Intersti tial edema increased approximately from 10% to 30%. The amount of edem a was reduced by rapid pacing (3.3 Hz) as judged from decreased muscle diameter and a lower IMP. It is concluded that reliable IMP measureme nts can be made in the isolated papillary muscle and that increased fi lling of the interstitium increases its pressure.