Intramyocardial pressure in the canine heart. An experimental study

During 11 acute open-chest experiments with dogs, intramyocardial pressure was measured in the anterior wall of the left ventricle with a miniature pressure transducer mounted on a 1.6 mm diameter needle. Pressures were measured at the subendocardium (+/- 10 mm), midwall (+/- 7.5 mm), and subepicardium (+/- 5 mm). Simultaneous recordings of left ventricular pressure and two measures of intramyocardial pressure were made during control, acute volume overload, and after administration of verapamil. Maximal amplitude of the subendocardial pressure was higher and the maximal amplitude of the subepicardial pressure was lower than maximal left ventricular pressure for all interventions (p less than 0.001 and p less than 0.01). During volume overload left ventricular pressure increased more than intramyocardial pressure (left ventricular pressure 34%, subendocardial pressure 6%, midwall pressure 14%, and subepicardial pressure 14%). After the administration of verapamil intramyocardial pressure decreased more than left ventricular pressure (left ventricular pressure 16%, subendocardial pressure 26%, midwall pressure 13%, subepicardial pressure 32%). Positive and negative first derivatives of subendocardial pressure were higher than those of left ventricular pressure during control and after verapamil (between p less than 0.01 and p less than 0.001). Positive and negative first derivatives of subepicardial pressure were lower than those of left ventricular pressure during all interventions (p less than 0.001). The timing of the C-point (onset of mechanical contraction) and the positive first derivative of all tracings was synchronous within 8 msec in all interventions. The 0-point (crosspoint of the tangent to the diastolic plateau and the tangent to the relaxation slope; early diastole) of intramyocardial pressure came later than the 0-point of left ventricular pressure, indicating longer relaxation times in the myocardium (subendocardial pressure: control, p less than 0.001, volume, p less than 0.05, verapamil, no significance; midwall pressure: between p less than 0.05 and p less than 0.001; subepicardial pressure: between p less than 0.01 and p less than 0.001).