THE QUESTION OF CARDIAC TONUS

Abstract

Tonus (as used in cardiac physiology) is defined as a condition of sustained diastolic contraction, independent of the systolic contractions, by virtue of which the muscle fibers resist distention more than they would because of their mere physical properties. In testing for cardiac tonus in this sense it is necessary to find spontaneous changes in diastolic relaxation or changes due to variations within normal limits of some physiological condition. Relaxation due to slower rates, modification in the course of the systolic contraction, or pressures outside the normal must be excluded since they are not primary but secondary in their action. It must be kept always in mind that the diastolic size of the heart in itself offers no evidence of tonus unless all other factors are controlled. By definition contraction remainders, contractures, elasticity and variations in the contractile power are not regarded as tonus. The action of drugs can not be expected to furnish evidence for the existence of cardiac tonus. There is general agreement that tonus waves occur in the tortoise auricle, although recent workers attribute them to the rhythmical contraction of a smooth muscle sheath. In the ventricles of all forms tonus waves have been reported only occasionally. The view, still widely current, that the vagus may induce greater ventricular diastolic relaxation is not consistent with the classical work of Frank or Hoffmann and is unsupported by any convincing recent experimentation. The existence of supraventricular tone centers has been alleged by a number of recent workers, but the evidence for these does not meet the criteria laid down by such careful workers on cardio-dynamics as Frank, Hoffmann, Straub or Wiggers. Gaskell''s old observation that acid perfusion led to diastolic relaxation may have real physiological significance if it can be shown that the result is not secondary to variations induced in the phases of the regular systoles. Contraction remainder may well play an important part in diastolic filling as Henderson believes, but we do not feel it should be regarded as an example of tonus. Neither, in our opinion, though its existence remains unproved, should the old physiological conception of the word tonus be usurped by such a definition as that proposed by Starling, one having to do not with a sustained contraction but with the available contractile power. Contractures, while offering possible points of attack on the nature of muscular contraction, have little or nothing to do with tonus. Whatever may be disclosed in the future, there is as yet no convincing experimental evidence that any kind of a sustained contraction persists throughout diastole which is subject to physiological modification.