This Commentary Article describe about the Venous Tone: The Overlooked Physiology. Venous tone, the status of the smooth muscle lining veins under autonomic nervous system control has been largely overlooked. Because it is never monitored it is no wonder that its importance has been missed. The first, and to my knowledge, the only time that venous tone was observed was over sixty years ago when an investigator at the NIH threaded a double balloon catheter into the arm vein of a volunteer. Measuring the pressure in this isolated segment of vein revealed veins to be under delicate control by the autonomic nervous system. Venous pressure is commonly measured, but since veins are tubes open at both ends, we learn mean atrial pressure only. It says nothing at all about the status of venous tone. When I was a medical student at the University of Washington in the 1950's, I had the opportunity to see what happens when this mechanism suddenly quits working. I had just measured the venous pressure in a patient at 12 centimeters of saline when it suddenly fell to zero. Venous tone had suddenly collapsed. The patient insisted he felt fine, but over the next ten seconds he grew progressively pale and fainted. The blood flow back into his chest had all but stopped. Each cardiac stroke volume was less than the one before until the cerebral blood flow required for consciousness was no longer available. A short time later I had just measured the spinal fluid pressure on a patient when it too suddenly fell to zero. Realizing what was happening, I rolled the patient onto his back and lifted his legs, preventing the syncope that was underway. A few years later when I was a Cardiology Fellow with the late Dr. Robert Bruce, of treadmill fame, I had the opportunity to review all the indicator dilution studies he had done. These were done by injecting Evan's Blue dye (T-1824) into an arm vein and recording from an ox meter attached to an earlobe. I first had to review all the Korner and Shilling ford dye curve literature to understand how these curves revealed the cardiac output, the central blood volume (the blood in the chest), and the presence of intra-cardiac shunts. I soon saw that there was a close correlation between the central blood volume and the cardiac stroke volume. This suggested that the heart merely pumps whatever is made available to it, the blood in the chest, and that the status of venous tone is what regulates this. Eighty percent of our blood is in our veins, so there is plenty to work with. The evidence suggests that our body monitors oxygen delivery to the tissues somehow and when this falls short for any reason, the cardiac output is adjusted accordingly. Venous tone increases to put more blood in the chest and the cardiac output rises. How the body regulates its cardiac output is finally revealed. When I was an intern I had the opportunity to see what happens when a patient lacks this mechanism altogether. This Japanese patient with idiopathic autonomic insufficiency was brought to our hospital by his family because he fainted whenever he stood up.

I believe it is likely that venous tone physiology could be the key to understanding the long-standing mystery of high altitude pulmonary edema. Mountain climbers above 20,000 feet encounter a partial pressure of oxygen that is half what it is at sea level. The venous tone reflex would activate to raise the central blood volume. Most climbers can acclimate if given a little time, but if a climber's physical fitness involves an element of left ventricular hypertrophy, volume loads are poorly tolerated and pulmonary edema could occur. The word is spreading among climbing circles that a beta-adrenergic blocker may be helpful in this condition, even though no one knows why. It may be the same reason it is helpful in patients with ordinary heart failure. It interferes with the venous tone reflex as it attempts to put more blood in the chest. It is high time we recognized venous tone physiology and the important role it plays in our lives.