Intramyocardial pressure measurements in the isolated perfused papillary muscle of rat heart.

The intramyocardial pressure (IMP) plays a role in the interaction between heart contraction and coronary flow. In order to measure IMP we developed the isolated perfused papillary muscle. The papillary muscle was suspended in a muscle bath with oxygenated Tyrode's solution. Perfusion with Tyrode's solution took place via the septal artery. Diastolic intramyocardial pressure was measured with micropipettes with a tipdiameter of 3 to 4 microns 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 cmH2O and stimulated with 0.2 Hz the IMP measurements started. Pressure changes resulting from changes in perfusion pressure and injection of ink via the pipette made it possible to distinguish between interstitial and vascular localization. No leakage along the shaft of the pipette to the outside was found. Perfusion caused interstitial edema in the muscle so that the cross-sectional area (CSA) of myocytes relative to total muscle CSA decreased from 71% in non-perfused muscle to 51% in perfused muscle. Interstitial edema increased approximately from 10% to 30%. The amount of edema was reduced by rapid pacing (3.3 Hz) as judged from decreased muscle diameter and a lower IMP. It is concluded that reliable IMP measurements can be made in the isolated papillary muscle and that increased filling of the interstitium increases its pressure.