Respiratory variations of the heart rate - I—The reflex mechanism of the respiratory arrhythmia

Abstract The fact that under certain conditions in some animals and in man inspiration is accompanied by an acceleration of the heart rate has been demonstrated by many observers and can be considered as well established. Agreement has not yet been reached, however, about the mechanism underlying this respiratory cardiac arrhythmia. E. Hering (1871), H. E. Hering (1895), Nikiforovsky (1910), Blumenfeldt and Putzig (1914) regard the arrhythmia as caused by a reflex the stimulus for which originates within the lungs. Traube (1865), Frédéricq (1882), Foa (1913), Snyder (1915), and Heymans (1929) attribute it to a purely central influence; Bainbridge and Hilton (1918-19), and Bainbridge (1920) deny any direct influence of the respiratory act upon the heart rate and ascribe the arrhythmia to the rise in the intra-auricular blood pressure taking place during inspiration. Thus Bainbridge regards the respiratory changes of the heart rate as a reflex which accompanies respiration but is not directly caused by it. Recently H. E. Hering accepted this point of view (1933). The question whether inspiration provides the stimulus for the cardiac acceleration or whether the expiration provides the stimulus for the retardation or both is not yet settled. Neither is there an agreement about the efferent path through which the changes in the heart rate are produced, the effect being attributed by some authors to variations of the vagus tone and by others to variations of the accelerator tone. The present investigation was undertaken in order to obtain further information upon the subject and, if possible, to find the cause of the difference between the observations of the previous authors. The research was begun in 1929 in the Physiological Laboratory of Cambridge and continued up to the present in Cairo. Our preliminary experiments convinced us that there were at least two distinct mechanisms underlying the respiratory cardiac arrhythmia, a reflex and a central mechanism. It was also soon found that the result of action of these two mechanisms may be greatly changed or even reversed under different conditions. The reflex and the central mechanism may overlap or interfere with each other so that the study of one of them in the presence of the other is confusing and unreliable. Our investigation thus became naturally divided into three sections: (a) the study of the reflex mechanism in absence of the central mechanism, (b) the study of the central mechanism in absence of the reflex mechanism, and (c) the study of the interaction between the reflex and the central mechanisms. Only reaction (a) is considered in Part I; sections (b) and (c) are discussed in Part II.