What are the causes of venous disease?
Pressure gradients vary dramatically
In the most basic analysis, venous disease results from the failure of valves in the deep venous system of the legs. When a person with venous disease stands up, for instance, blood pools in the lower extremities, and pressure levels in the ankle and lower leg increases precipitously, which is venous hypertension. "When the valves of the venous system break down, it causes everything else down to distend, leading to vein dilatation, varicosity, venous hypertension, edema, tissue breakdown, and finally ulceration," explains John Macdonald, MD, FACS, medical director of the Wound Healing and Lymphedema Center in Ft. Lauderdale, FL.
Leg veins are categorized depending on their relationship to the fascia. Deep veins join to form the popliteal and femoral veins, which are primarily responsible for venous return from the leg muscles. In people with normal venous systems, when the leg is at rest, blood pools inside valveless sinusoids located in the soleus and gastrocnemius muscles. The combined volume of the deep veins and the sinusoids ranges from 100 ml to 140 ml, which is roughly equivalent to the volume of one of the heart ventricles.
Lying closer to the fascia are the veins of the superficial venous system. The key players in the superficial system are the long and short saphenous veins, which run, respectively, from the foot to the groin and from the foot to the knee. Through these veins, blood returns from the skin and subcutaneous fat to the deep veins. A series of communicating veins connects the deep and superficial systems, maintaining a one-way flow toward the deep veins.
The pressure gradient within leg veins can change dramatically depending on a person's position and activity of the moment. Venous pressure hovers around 10 mm Hg when a person is lying down, but jumps to around 80 mm Hg as soon as she stands. While walking, calf and foot muscle flexion compresses the deep system veins and soleal sinuses, temporarily boosting the pressure to as high as 250 mm Hg. In about half a minute, the system achieves an equilibrium of 30 mm Hg to 40 mmHg.
Damaged valves allow blood to backtrack
In a normally functioning venous system, the proximal valves close, ensuring that the blood flows in its intended upward direction toward the heart. But in a damaged system, the valves do not keep the blood moving in one direction, so it collects in the veins.
If the superficial veins are malfunctioning, the results can be distention and varicose veins. Damaged communicating veins will allow blood to flow improperly from the deep system to the superficial veins, possibly damaging the superficial vessels. Deep vein obstructions lead to high pressures caused by calf muscle contractions, possibly leading to failure of the valves in the communicating veins and a subsequent jump in pressure in the superficial veins.
Severe hypertensive responses in the superficial venous system can spur a breakdown of red blood cells, which manifests as skin reddening - an indication that a stasis ulcer is about to form.