BLOOD PRESSURE 171 



The animal is placed in a convenient position with respect to the mano- 

 meter and the cannula connected to the latter. The pressure bottle is raised 

 until it is about five feet above the level of the mercury in the manometer, 

 and with the tap s turned on fluid is run from the pressure bottle through 

 the manometer along the tubing to the cannula, until all the air is displaced 

 from these tubes. The flow from the pressure bottle is then stopped. The 

 writing point of the upper chronograph is brought to the level of that of 

 the float, and consequently draws an abscissa line of zero pressure. It 

 should be noticed that the level of the mercury in the two limbs is not the 

 same, that in the open limb being a little higher than that in the other, 

 because this mercury has to balance the pressure of a column of salt solution 

 equal in height to the difference in level between the mercury and orifice of 

 the cannula. The clip on the tubing on the free orifice of the cannula is next 

 screwed tight. The clip on the tubing from the pressure bottle is once more 

 opened, but now no fluid can escape. The mercury in the open limb of the 

 manometer therefore rises, and carries the float with it, until the difference 

 in level of the mercury in the two limbs balances the pressure of the fluid in 

 the pressure bottle acting on the closed manometer limb. This will be at 

 a level of about 70 to 80 mm. above the zero abscissa line. The increase of 

 pressure inside the manometer is measured by the increase in distance of the 

 two mercury surfaces, but the height recorded by the float is only the move- 

 ment of the mercury surface in the open limb. If the bore of the tubing 

 forming the two limbs be equal, the rise in the one limb is equal to the fall in 

 the other. Consequently the increase of pressure exerted by the fluid is 

 measured by a mercury column 140 to 160 mm. in height. Therefore in 

 a tracing obtained by the mercury manometer to determine the pressure 

 recorded at any instant, we must measure its height above the zero abscissa 

 line and multiply this by 2, which gives the equivalent height of a mercury 

 column which just balances the pressure. 



The bull-dog forceps are now removed from the artery, when the mercury 

 will begin to oscillate up and down, and at about the same level as that it has 

 been raised to by the pressure bottle. The object of the pressure bottle is in 

 the first place to give a convenient means of filling the apparatus with a fluid 

 which when mixed with blood tends to prevent its coagulation. Sodium 

 carbonate or magnesium sulphate solutions are also employed for this purpose ; 

 but, on the whole, sodium sulphate is perhaps the best salt to employ. The 

 pressure bottle further performs another very important service in raising 

 the level of the mercury to a height which will approximately represent that 

 of the blood pressure. If this be not done, on connection with the artery the 

 blood itself would have to raise the mercury thus entering the cannula in 

 some volume, not only causing a loss of blood to the animal, but leading 

 sooner or later to coagulation and blocking of the cannula. Placing the 

 pressure bottle too high is also to be avoided, for then some of the fluid will 

 leave the manometer and cannula, and pass into the circulating blood. 



In taking the blood pressure by a cannula placed in the carotid we must 

 remember that we are not measuring the blood pressure in the carotid as it 

 exists in life, because we have completely stopped its flow along the vessel. 

 We are really measuring the pressure on the side wall of the artery from 

 which the carotid springs, i.e. either the subclavian or aorta, as the case may 

 be, along which the blood is still flowing in a normal manner. 



We may now carry out the experiment in the following manner : 

 1. Record of the normal tracing. — Two or three short pieces of 

 tracing are taken with varying speeds of recording surface. Fig. 135 

 gives a typical piece of tracing from the rabbit, but the student should 



