The measurement of blood pressure has become a standard diagnostic test in medicine but the pressure which is normally measured is arterial pressure.  The measurement can be made by inserting a catheter into an artery and connecting an inserted tube to a manometer, but is clinically accomplished by exerting pressure from outside of the artery until blood flow stops completely, the exerted pressure being a measure of the systolic pressure; and then detecting when flow is momentarily stopped at the lowest point of the cardiac wave, giving a measure of diastolic pressure.  An inflated cuff is normally used to exert the pressure from outside, and the existence or stoppage of flow is sensed by listening with a stethoscope. 
While arterial pressure measurements are and will remain important, measurements of venous pressure can indicate the possible existence, progress or incipience of abnormal conditions which are not revealed by arterial measurements alone and is important in the evaluation of patients with congestive heart failure, renal failure, septic shock, traumatic shock, cancer and other illnesses, especially as an aid in fluid management.  Unfortunately, measuring venous pressure is more difficult than arterial because the pressure is lower and the variations less distinct and because the flow rate in any readily accessible vein is lower than in an artery of equivalent accessibility.  In fact, the variations which do occur are respiratory rather than cardiac.  While arterial pressures are in the order of 80 mm Hg (diastolic) to 120 mm Hg or more (systsolic), venous pressure generally lies in the range of about 5 mm Hg tro about 35 mm Hg. 
Thus, venous pressure measuring has usually been limited to monitoring the pressure in patients who have a clear need, i.e., whose conditions require the information.  In such cases the measurements have been made using invasive techniques, i.e., by inserting a pressure transducer into a vein or by inserting a tube coupled to a pressure measuring device such as a manometer.  This is obviously a more sever technique than would be acceptable for normal clinical use and involves risks of infection and blockage. 
The following documents include examples of prior devices intended to measure pressures, discussions of the physiological aspects of venous pressure and background information on devices usable in connection with the present invention. 
______________________________________ U.S. Pat.  No.  Inventor ______________________________________ 3,605,723 King et al 3,123,068 Bigliano 3,102,534 Bigliano et al 3,099,262 Bigliano 4,030,484 Kuska et al 3,039,044 Dubsky et al 1,926,748 MacKenzie et al 3,585,987 Svensson 1,282,632 Roesch 3,087,488 Streimer 4,027,662 Lee ______________________________________