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ing, which occurs only at the expense of capillary bloodsupply.
The application of cold by reducing the bath temperature by 10° or 15° C. diminishes capillary pressure; with hot baths the results may or may not be the reverse.
This author logically suggests that the condition of the arterioles is far more important in determining capillary blood-pressure than is the arterial pressure; for, if the arterioles are contracted, the capillaries receive a scant supply of blood and the resulting pressure naturally is lowered, from which we may conclude that the most important factor affecting capillary blood-pressure is the condition of the arterioles.
Federn takes the same stand, and states that since 1894 he has been endeavoring to perfect a method which would be easily applied, and which would be accurate in determining the systolic pressure in the smaller vessels, which would be a more accurate means of determining changes in the arteriolar and capillary pressures, than the usual method of compressing large trunks. In his “Optic Estimation of Blood-pressure"l he describes a method of measuring blood-pressure in a small artery over the tibia, and in the superior anterior intercostal artery by means of a von Basch compressor connected to a sphygmomanometer, the effect of compression being shown by a straw index fastened to a disk of paper or cork (5.6 mm. in size) and pasted to the skin at the point where the pulse is felt. He states that this method eliminates alterations in pressure due to compression ischemia of a large area, gives accurate readings and is the best means of estimating the functional capacity of the heart as it gives an indication of the pressure in the capillary system.
i Berl. klin. Wochen., Mar. 30, 1914, 21, 13.
General Considerations. Despite the close relation which must exist between the blood-pressure in the large veins and the proper filling of the auricular chamber, and through it the capacity of the ventricular output, which is the direct source of arterial pressure, there have been but few observations and experiments which are sufficiently reliable to be depended upon for clinical deductions.
Methods of Measuring Venous Pressure.—The direct canular method of Moritz and von Taboral is undoubtedly most accurate and, at the hands of the originators, has not been followed by any untoward results. This method could be used experimentally, though it is hardly available in the consulting room.
Other methods have appeared from time to time since 1900, but have failed of general adoption because of fundamental errors in the apparatus.
Two recent instruments (Hooker and Eyster, and Howell) employing air as the medium of pressure, and recording the findings in centimeters or millimeters of water, are probably, at this writing, the best instrumental means for the measurement of venous pressure.
The instrument of Hooker and Eysteris a modification and improvement on that of von Recklinghausen. It is composed of an aluminum form, shaped to fit the forearm;
1F. Moritz and D. v. Tabora, Deutsch. Arch. f. klin. Med., xcviii, No. 4, p. 475.
Johns Hopkins Hos. Rep., 1909, xix.
partly closed on the arm side by a thin sheet of rubberdam having a central rectangular opening, and hermetically closed above by a glass window. When applied, the opening in the rubber-dam is placed over a large vein, when by suitable means the pressure within the chamber is increased until the vein collapses. The amount of air required to accomplish this is measured in centimeters of water by a manometer connected with it.
The apparatus of Howell' operates upon a different principle. It employs two cups, each connected to a separate water manometer. One cup is applied to the arm and the other to the forearm, the forearm cup being of very thin rubber. The latter is inflated until it fits snugly without exerting pressure (not more than 1 to 3 cm. of water). The upper cuff is then slowly inflated until the manometer connected with the lower cuff shows a rise. This rise shows compression of the veins in the arms and is incident to an increase in volume in the forearm. At this moment the venous pressure reading is made. It will be noted that the degree of pressure originally placed in the lower cuff will to some extent affect the pressure reading, since the higher the pressure over the forearm, the higher will be the venous pressure recorded.
A simple method, ascribed to Gartner, is described in detail and highly recommended by Oliver’ and by Brunton,3 for measuring variations in venous pressure without the aid of a sphygmomanometer. This method requires nothing but a foot rule or measuring tape, the veins of the subject being utilized as an indicator or manometer. If the veins on the dorsum of the hand are sufficiently visible, we find that when the hand is held in a vertical position with the fingers extended, and is slowly raised, the veins at a certain height above the level of the apex of the heart can be seen to collapse rather suddenly.
1 A. A. Howell, Arch. Int. Med., February, 1912, ix, No. 2, p. 148. 2 G. Oliver, Quart. Jour. of Med. Oxford, October, 1907.
* L. Brunton, "Therapeutics of the Circulation," P. Blakiston's Sons, 1908, p. 84.
Oliver states that at the moment of collapse of the veins the blood-pressure within them is practically nil, being balanced, as it were, by the force of gravity. We may, therefore, express this force, annulling the hydrostatic rise of the blood, in millimeters of mercury. This may be done by making a very simple calculation. If we take the average specific gravity of the blood as 1.060 and that of mercury as 13,570, the 25.5 mm. of blood contained in 1 in., will represent 1.985 mm. Hg. (or approximately 2 mm. Hg.); therefore, if we multiply by 2 the number of inches above the level of the apex of the heart when the veins collapse, we ascertain in millimeters of mercury the venous pressure. According to Oliver, this simple method of measuring the venous pressure affords uniform results and is definite and delicate as it enables one to discriminate between differences of 1 mm. It is important to see that the pressure is not artificially raised. This may occur through nervous perturbation or by obstruction of the venous flow by tight clothing.
In making any observations upon the veins, the arm should be maintained in a position such that the point of pressure is about at the level of the heart, irrespective of the position of the patient. If this detail is not regarded, the force of gravity will enter in and invalidate the findings.
Venous pressure readings are also influenced by the surrounding temperature and by the thickness of the skin over the vein which is being tested, and also by the prominence of the superficial veins and the presence or absence of phlebosclerosis (said to be of frequent occurrence by Hoobler and Eyster) and edema.
OUR PRESENT KNOWLEDGE OF THE PHYSICS AND PHYSIOLOGY
OF VENOUS BLOOD-PRESSURES
Regarding the variations of pressure in different parts of the venous system very little is definitely known, some authorities believing that there exists a negative pressure in the great veins near the heart while others contend that a positive pressure within the venæ cave is essential to normal auricular function.
Until quite recently we have assumed that venous pressure passively responds to changes in the peripheral resistance and that it rises and falls inversely with the pressure within the arterial tree. There is now a growing belief supported in part by experimental work (some performed as early as 1890) on both men and animals, that the pressure in the venous side, may be and probably is, dominated by a special nervous mechanism. For we now know that the veins are supplied with motor nerves and it has been shown that the veins respond to epinephrin by constricting while Crawford and Twombly have shown, in addition to this action, that marked constriction of the vein occurs when subjected to a solution of epinephrin of the strength of 1 to 60,000, even when entirely freed from vasomotor control.
1 Dunn and Chavasse, Proc. Royal Society, London, lxxxvi, 1912. * A. C. Crawford and M. M. Twombly, N. Y. Med. Jour., Aug. 16, 1913.