BLOOD PRESSTJEE 



363 



copper cannula was inserted in the artery in the groin of a horse which 

 in turn was connected with a vertical tube of glass, measuring nine 

 feet in height and one-sixth of an inch in diameter. On removing the 

 ligature from the artery, the blood was seen to enter the tube to a 

 height of eight feet and three inches above the level of the left ventricle. 

 However, it did not rise to this height at once, but gradually, and 

 finally exhibited small oscillatory fluctuations. 



This single vertical tube was displaced later on by a U-shaped tube, a further 

 reduction in its length being made possible by filling it with mercury, because this 

 element possesses a specific gravity 13.5 greater than that of water. Ludwig finally 

 equipped the distal limb of the mercury column, of these manometers with a float 

 and slender vertical rod to which he attached a writing point. This arrangement 

 enabled him to record the excursions of the mercury upon the paper of a kymograph 

 (Fig. 149). In recent years use has also been made of various types of membrane- 

 manometers, in which the intravascular pressure is counter-balanced by the elastic 

 force of a rubber membrane. The displacements of this membrane can be accu- 

 rately recorded by permitting it to act 

 against a writing lever, or by permitting it to 

 reflect a beam of light from a delicate mirror 

 fastened to its surface. 



For obvious reasons the direct method 

 can only be applied to arteries and veins 

 which are sufficiently large to allow the in- 

 troduction of a cannula. On the arterial 

 side, the pressure is measured most con- 

 veniently in the carotid and femoral arteries, 

 the former blood-vessel being used most 

 frequently, because it is more accessible and 

 in closer proximity to the center of the cir- 

 culatory system. In either case, it should 

 be remembered that we are not determining 

 the pressure in this particular vessel, but in 

 the one situated centrally to it. To illustrate, 

 the carotid artery leaves the aorta almost at 

 right angles and plays, therefore, the same 

 role as the free end of a T-tube, i.e., it per- 

 mits the pressure which is exerted in a radial 

 direction upon the internal surface of the wall 

 of the aorta to be propagated directly out- 

 ward into the manometer (Fig. 189). It 



must be clear, therefore, that the pressure prevailing in the carotid artery itself can 

 only be ascertained if this vessel is connected with the recording instrument 

 either by means of a T-tube, or by means of a straight cannula inserted into one 

 of its branches. This purpose may be served by the arteria thyroidea, because 

 the lateral carotid pressure is propagated through this blood-vessel directly into 

 the manometer (Fig. 189, II). In this connection attention should also be called 

 to the fact that the distal stump of an artery is not necessarily without pressure, 

 because in most cases anastomoses are present which permit at least a slight 

 quantity of blood to enter this channel in an indirect way. 



In order to ascertain the venous pressure, it is necessary to insert a T-tube, 

 the free end of which is connected either with a U-shaped manometer filled with 

 normal saline solution, or with a membrane manometer possessing the least possible 

 resistance. The oscillations of the column of saline solution may be registered 

 by placing a bell-shaped float and writing needle upon its distal limb. Thie modi- 

 fication in the method of registration is made necessary by the fact that the 



Aorta. 



FIG. 189. DIAGRAM TO SHOW 

 THAT A MANOMETER CONNECTED WITH 

 THE CAROTID ARTERY MEASURES THE 

 LATERAL PRESSURE IN THE AORTA. 



