Lower limb blood supply

Arteries and pulses

The external iliac artery becomes the femoral artery behind the inguinal ligament, the femoral becomes the popliteal at the adductor hiatus, and the popliteal divides at the lower border of popliteus into the anterior and posterior tibial arteries.

• The dorsalis pedis is absent or tiny in a minority of normal people; the perforating branch of the peroneal artery then supplies the dorsum. So an absent dorsalis pedis with a good posterior tibial pulse may be a normal variant. When the dorsalis pedis is blocked, the peroneal (an artery of the back of the leg) and the plantar arch keep the dorsum supplied.
• The deep plantar arch is formed by the lateral plantar artery (from the posterior tibial) and completed by the deep plantar branch of the dorsalis pedis.
• Pulse amplification: as the pressure wave travels to smaller distal arteries, reflected waves add to it, so systolic pressure and pulse pressure are higher in the dorsalis pedis than in the aorta or radial artery, while diastolic is lower and mean pressure falls slightly. Standing adds the hydrostatic column, so foot pressures rise further.
• Ankle–brachial pressure index (ABPI): ankle systolic divided by brachial systolic; normal about 1.0–1.2; below 0.9 indicates peripheral arterial disease; below 0.5 suggests critical ischaemia. It is falsely high in diabetes and renal failure because calcified vessels resist compression.
• Claudication: calf pain on walking relieved by rest points to femoropopliteal disease; buttock and thigh pain with erectile dysfunction points to aortoiliac disease (Leriche syndrome).
• Atherosclerosis is most severe, in order, in the lower abdominal aorta, the coronary arteries, the popliteal arteries, the descending thoracic aorta, the internal carotids and the circle of Willis.

Superficial veins

• The great saphenous vein is the longest vein in the body, has many one-way valves (commonly quoted as about 10–20, more below the knee) and is connected to the deep veins by perforators. It is used for coronary bypass grafts and venous cut-down.
• Stripping the great saphenous vein below the knee risks the saphenous nerve (numb medial leg); small saphenous surgery risks the sural nerve (numb lateral foot).

Deep veins and the calf muscle pump

• Deep veins accompany the arteries; below the knee they are paired venae comitantes. Large valveless soleal sinuses lie within soleus.
• Perforating veins pass through the deep fascia from superficial to deep, with valves that direct flow inwards.
• When the calf contracts in walking, the calf muscle pump squeezes deep veins and drives blood upwards; valves stop backflow. When the calf relaxes, deep pressure falls and blood is drawn in from the superficial veins through the perforators.

Varicose veins

• Dilated, tortuous superficial veins caused by incompetent valves, so blood refluxes down the superficial system under the full hydrostatic pressure of the column. In an otherwise healthy middle-aged adult this is primary valve failure, most often at the saphenofemoral junction.
• Secondary varicose veins follow DVT (damaged deep valves), pelvic masses or arteriovenous fistula.
• Risk factors: female sex, pregnancy, prolonged standing, family history, obesity.
• Assessment: duplex ultrasound is the standard; Trendelenburg and tourniquet tests are older bedside methods.
• Complications of venous hypertension: oedema, haemosiderin pigmentation, venous eczema, lipodermatosclerosis, venous ulcers above the medial malleolus (gaiter area), bleeding, superficial thrombophlebitis.
• Treatment: compression, endovenous laser or radiofrequency ablation, foam sclerotherapy, surgery (high tie and stripping).

Deep vein thrombosis and embolism

• Virchow's triad: stasis, endothelial injury, hypercoagulability (surgery, immobility, pregnancy and the puerperium, cancer, oral oestrogens, thrombophilia).
• Thrombi usually begin in calf veins, especially the soleal sinuses; extension to the popliteal and femoral veins carries the main risk of pulmonary embolism. A detached leg embolus passes through the right heart and lodges in the pulmonary arteries.
• Diagnosis: Wells score, D-dimer, compression ultrasound. Prevention after surgery: early mobilisation, stockings, intermittent pneumatic compression, low molecular weight heparin.
• Phlegmasia cerulea dolens: massive iliofemoral thrombosis causing a blue, painful, swollen leg with risk of venous gangrene.

Remember Valves fail and the superficial veins dilate; deep clots break off and lodge in the lungs.

Lymphatic drainage

• Superficial inguinal nodes lie in the superficial fascia below the inguinal ligament. The horizontal group drains the lower anterior abdominal wall below the umbilicus, the buttock, the perineum, the anal canal below the pectinate line, the skin of the scrotum and penis, the vulva and lower vagina, and (via the round ligament) the region of the uterine cornu. The vertical group follows the great saphenous vein and drains most of the limb.
• Superficial inguinal nodes drain into the deep inguinal nodes and then the external iliac nodes.
• Lymph from the lateral foot, heel and back of the calf follows the small saphenous vein to the popliteal nodes, then the deep inguinal nodes.
• The testis drains to para-aortic nodes and the cervix to the internal and external iliac and obturator nodes, not to the inguinal nodes.
• Lymphoedema: primary (Milroy disease) or secondary (filariasis, node dissection, radiotherapy, malignancy).

Exam trap The great saphenous vein passes in front of the medial malleolus; the small saphenous passes behind the lateral malleolus.

Exam trap Scrotal skin drains to inguinal nodes, but the testis drains to para-aortic nodes.

How FCPS asks about this

• Where to feel the dorsalis pedis: just lateral to the EHL tendon; an absent pulse with a good posterior tibial can be normal.
• Artery supplying the dorsum when dorsalis pedis is absent: perforating branch of the peroneal artery.
• Pressure in the dorsalis pedis versus radial: higher systolic and pulse pressure distally (wave reflection); higher still when standing.
• Varicose veins in a healthy middle-aged man: incompetent valves, usually at the saphenofemoral junction.
• Great saphenous vein course: in front of the medial malleolus with the saphenous nerve, ending in the femoral vein at the saphenous opening.
• Superficial inguinal node drainage and onward route: vertical group follows the great saphenous vein; all drain to deep inguinal then external iliac nodes.
• Detached embolus from deep leg veins: lodges in the pulmonary arteries.