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Since the actions of physical and chemical influences upon myocardial contractility are not necessarily independent, study of interactions between the two may give clues to the mechanisms of action of both. A method is described for study of the interrelations of temperature, the interval between contractions, distending pressure, volume, amplitude of contraction against constant pressure, and pressure generated by contraction at constant volume in isolated thin-walled cardiac chambers. Unusual features are the use of gas as a distending medium and a volume recorder which measures the conductance of the fluid displaced by the chamber. As illustrations of use of the method, observations on some fundamental properties of rabbit atria are presented. Hysteresis and slow yielding to stress are prominent. The passive pressure-volume relationship is not affected by contractions against constant pressure, but is changed slightly by contractions at constant volume. The Starling curve of the isolated rabbit atrium always has a descending limb at high atrial volumes. However, the distending pressures required to produce a prominent descending limb are so high that their application causes irreversible changes in the muscle.

Method for Study of Contraction of Isolated Heart Muscle Under Various Physical Conditions