THE PULSATORY VARIATIONS IN BLOOD PRESSURE 389 



The Speed and Character of the Physiological Venous Pulse. —In 



agreement with the low tension prevailing in the venous system, the 

 physiological venous pulse does not attain a considerable velocity. 

 Morrow^ states that it is only 1-3 m. in a second. A study of its 

 general outline shows that it consists of three undulations (Fig. 203). 

 In accordance with Fredericq,^ the initial elevation (A) is caused by the 

 contraction of the auricle, the wave of high intra-auricular pressure 

 being propagated into the veins. The second positive wave (C) 

 is due to ventricular systole, because the auriculoventricular valves 

 are forced upward and thus encroach upon the space of the auricles. 

 The third rise (F) is dependent upon a reflection caused by the rapid 

 influx of venous blood into the passive auricles. If this explanation 

 is accepted, and it seems to be the most feasible one, the physiological 

 venous pulse is to be regarded as the counterpart of the curve of intra- 

 auricular pressure, the latter being propagated outward into the central 



Fig. 203, — Diagrammatic Representation of the Physiol. Venous Pulse from the 

 Central End of the Ext. Jugular Vein. 

 A, a- wave; C, c-wave; V, ■y-wave. 



venous charxnels through the incompetent caval and pulmonary 

 orifices. The a-wave is generally the largest, but if it should prove 

 difficult at any time to differentiate these summits from one another, 

 it is advisable to identify the c-wave first of all. This is a simple 

 matter, because it merely involves the determination by auscultation 

 or palpation of the onset of ventricular systole. For this reason, it is 

 always safest to record the venous pulse in conjunction with the arterial 

 pulse or the apex-beat. 



In accordance with the view presented by Mackenzie,^ the changes 

 in intra-auricular pressure should not be regarded as the sole cause 

 of the venous pulse, because its real character is more directly deter- 

 mined by the pulsations occurring in the blood current of the neighbor- 

 ing carotid artery. If we follow the usual custom of designating 

 the three elevations of the venous pulse as the a, c and v waves, it 

 becomes evident that: 



1. The a-wave is dependent upon the outward propagation of the principal 

 elevation of the intra-auricular pressure and is caused, therefore, by the contrac- 

 tion of the auricle. 



2. The c-wave is not identical with the second rise in the intra-auricular pressure 

 caused by the systolic elevation of the auriculoventricular system, but is occa- 

 sioned by the transfer of the pulse from the neighboring carotid artery. 



1 Pfliiger's Archiv, Ixxix, 1900, 442. 



2 Centralbl. fiir Physiol., xxii, 1908. 



^ Study of the Pulse, London, 1912; also see: Lev/is, Mechanism of the Heart 

 Beat, London, 1911. 



