Physiological hypotheses--intramyocardial pressure. A new concept, suggestions for measurement.

Intramyocardial pressure is supposed to play a major role in systolic coronary flow impediment. Via its assumed relation with radial wall stress it is supposed to be similar to ventricular pressure at the endocardium and decreases linearly to negligible values epicardially. Many attempts to measure intramyocardial pressure have been reported in the literature with rather different results. For instance, with most of the various methods, intramyocardial pressures both higher and lower than left ventricular pressure have been obtained and intramyocardial pressures of more than 125 mm Hg have been found in low-loaded isobaric beats (negligible pressure development in systole). In this "physiological hypotheses paper" I suggest left ventricular pressure and intramyocardial pressure both to result from the varying stiffness of cardiac muscle over the heart cycle. For any intramuscular cavity a time varying pressure-volume (P-V) relation results from the changes in muscle stiffness, the so-called time varying elastance defined as E(t) = P(t)/V(t), and with maximal or systolic elastance called Emax. For a constant contractile state the time varying elastance (E(t)) is suggested to be almost independent of preload and afterload. This concept has been well established for the ventricular cavities, but is here proposed to hold for the interstitial space as well. If a cavity is subject to isovolumic conditions the pressure will be high, but when volume in systole decreases (ventricular ejection or squeezing out of interstitial fluid) pressures will be lower. Thus for constant load on the interstitial cavities, but different loads on the ventricle, left ventricular pressure will vary while intramyocardial pressure remains the same. For low-loaded isobaric beats where left ventricular pressure is minimal intramyocardial pressure will remain the same as during normal ventricular loads and isovolumic beats. Augmented contractility will increase Emax and this will increase left ventricular and intramyocardial pressure only by the same amount if loading conditions of both cavities remain the same. Both ventricular pressure and intramyocardial pressure arise from varying stiffness of cardiac muscle and intramyocardial pressure does not result from left ventricular pressure. A proportionality of left ventricular and intramyocardial pressure is therefore not to be expected. The results on intramyocardial pressure obtained by the different methods used in the literature should be re-interpreted taking this concept into account.