Comparison of pressures measured from the proximal external jugular vein and from a central vein.

Sir,—The study by Shah, Swai and Latto (1986) comparing the venous pressures measured from the proximal external jugular vein and from a central vein was interesting. The authors found a mean difference in pressure of 3 mm Hg from the two sites which was not influenced by alterations in the head position or the side of jugular venous catheterization. Whether the reference zero is the right atrium or manubriosternal joint, the manometer scale or transducer must bear a constant relation to the zero (Lawlcr, 1980). The scale or transducer should be fixed to the patient or the operating table, rather than to a freestanding pole. The authors have failed to mention the position of the transducer in relation to the zero point. As the measurements were taken in the horizontal position, one is compelled to believe that the transducer was at the body level. Although a source of error did exist, it was probably equal for all the measurements. As long as the body position is horizontal, the measurement from a slightly distant vein like proximal external jugular may reflect near true venous pressures, as the authors have found. This is not necessarily true if the patient is tilted or slightly head-up on the operating table or intensive care bed. Then the position of the transducer becomes more important. Moreover, changes in the central venous pressure arc more important than a single value, particularly after fluid challenge. It remains to be seen what the effects of posture and fluid challenge would be on the pressures measured from the proximal external jugular vein and a central vein.