Anatomy and physiology of the vascular system

Vessels in the thorax, abdomen, extremities, and extracranial cerebrovascular area constitute the circulatory system (Fig. 44-1). The ascending aorta, which originates from the left ventricle, carries oxygenated blood from the heart to the arteries. The major arteries leading to the head and upper extremities—the brachiocephalic trunk, left common carotid, and subclavian arteries—branch off from the aortic arch in the middle mediastinum above the heart. The thoracic aorta then descends through the posterior mediastinum at the left side of the vertebral column.

Passing through the diaphragm, the abdominal aorta descends to the level of the fourth lumbar vertebra, where it bifurcates (i.e., divides) to form the common iliac arteries that lead to the lower extremities. Arteries from the abdominal aorta carry blood to the kidneys and the abdominal and pelvic organs. The femoral artery, which originates from the iliac artery, is the main artery in each leg.

Oxygenated blood flows from the arterial system through the capillary network and returns deoxygenated to the heart via the venous system (Fig. 44-2). The venae cavae enter the right atrium. The superior vena cava, formed by the union of the two brachiocephalic veins, returns deoxygenated venous blood from the head, neck, upper extremities, and chest. The inferior vena cava, which begins at the level of the fifth lumbar vertebra, returns blood from the lower extremities, pelvis, and abdominal organs. There are two exceptions to this oxygenation/deoxygenation pattern:

Innervation of the arteries and veins is controlled by the efferent vasomotor fibers of the autonomic nervous system. The nerve fibers enter the vessel adventitia (i.e., covering) along the same route as the blood supply. Stimulation of these nerves can cause vasoconstriction that shunts blood to larger organ groups such as the skin or gastrointestinal tract. Some medications cause vasodilation, which relaxes the vessels and slows heart rate to decrease intraluminal pressure.

The vessels are lined with endothelial cells, which when intact do not support platelet aggregation. This property is interrupted when a vessel is injured, and a clot is allowed to form. The endothelial lining also secretes factors that resist clotting, stimulate tissue repair, and synthesize clotting factors.

Arterial anatomy

Arteries are elastic and constrict in response to hemorrhage. The arterial walls are made up of three layers that are separated by internal and external elastic membranes (Fig. 44-3). The innermost layer is the intima (tunica intima), a single layer of endothelial cells on a thin matrix of hyaluronic acid, collagen, and elastic fibers. The middle layer is the media (tunica media, or yellow fibrous). It is the thickest of the layers and is composed of a combination of smooth muscle fibers, yellow collagen, and some elastic fibers. The medial layer gives the arterial wall the flexibility and strength to withstand higher internal pressure.

The outermost layer is the adventitia (tunica adventitia, or white fibrous connective tissue). It is the thinnest layer, but provides most of the external support for the arterial wall and resistance to overexpansion.

The tissues of the adventitia and the outer third of the media are nourished by a series of capillaries called the vasa vasorum. The rest of the medial layer and the intimal layer are nourished by diffusion from the luminal flow. Terminal arteries, referred to as arterioles, end at the level of the capillary beds in the tissues.

Venous anatomy

Venous walls are structured in three layers. The innermost layer is the intima and is composed of endothelial cells that produce coagulation factors. The middle layer, or media, is much thinner in the deep venous system and may even be hard to identify microscopically. The superficial veins have a thicker medial layer composed mainly of smooth muscle fibers. This provides some resistance and response to intraluminal pressure changes. The external layer, or the venous adventitia, is composed of loose connective tissue (see Fig. 44-2). Because of lower flow pressure, larger veins have internal semilunar valves to maintain the direction of the blood flow.

The muscular component of the lower extremities provides force during contraction of large muscle groups, which facilitates venous return from the larger peripheral sinusoids. The cerebral veins are an exception; they have no valves. Veins are less elastic than arteries and tend to ooze instead of contract in response to hemorrhage.

Venous anatomy has three structural components: superficial veins, deep veins, and perforators (also known as communicating veins) (Fig. 44-4). The venous structure and flow of the lower extremities are examples of this system. At the tissue level, the origin of the venous system at the capillary bed is referred to as venules. The venous drainage of deoxygenated blood from the superficial veins returns through the perforators to the deep veins during muscular contraction. The deoxygenated blood then passes from the deep veins to the inferior vena cava.

Vascular pathology

Vascular diseases that cause occlusion or stenosis are usually acquired diseases. Inadequate arterial blood supply causes ischemia in tissues and complete organ systems. If left untreated, this can lead to thrombus, embolus, ulceration, necrosis, or gangrene. Vessels are repaired, reconstructed, or replaced to improve peripheral (systemic) circulation.

Atherosclerosis is the most common arterial disease. It is a diffuse disease and begins as a disruption of the intima of a large artery. Cholesterol enters the media to stimulate muscle growth, and platelets accumulate around the disruption of the endothelial intima to form plaque or a thrombus. Often this process becomes localized around vessel orifices and branches (i.e., at bifurcations). The disease may produce stenosis (narrowing) and subsequent occlusion or ectasia (dilation) and aneurysm. The most common procedures are performed to revascularize a lower extremity for limb salvage, repair an aortoiliac aneurysm, and improve cerebral blood flow through the carotid arteries.

Atherosclerosis is the principal factor in transient ischemic attacks (TIAs, or “mini strokes”), cerebrovascular accidents (strokes, or “brain attacks”), myocardial infarctions (heart attacks), and aortic stenosis. Risk factors include familial history, a high level of serum cholesterol, smoking, and hypertension (Box 44-1).

Venous stasis disease or obstruction of venous return can cause hemodynamic imbalances that affect distal structures. Arterial blood enters the area, but cannot return through the venous system. Superior vena cava (SVC) syndrome is one example. Patients with compression on the SVC caused by benign or malignant tumors or mediastinal fibrosis have serious venous drainage obstruction of the head and upper extremities. SVC can be caused by venous thrombosis associated with pacer wires or central venous catheters. Patients experience visual disturbances and dyspnea.

Relief of the obstruction by conservative medical management is not always successful. SVC syndrome can be a surgical emergency. Endovascular treatment is usually the best choice.^7,14 In select patients a surgical bypass may be necessary.⁷

Diagnostic procedures

Preoperative assessment of cardiac risk is critically important in planning the care of a patient who requires major vascular surgery. Peripheral arterial and venous diseases are assessed by auscultation, palpation, and observation.

Table 44-1 compares the assessment factors of peripheral arterial and venous obstructive diseases in an extremity. Peripheral vascular laboratories, which are similar to cardiac cath labs, have been established in many health care facilities to perform noninvasive and invasive studies before and after surgical intervention. Interventional radiologists perform many of the diagnostic tests. X-ray studies, scans, imaging, ultrasound, and Doppler assessment reveal most pathologic conditions.¹

Noninvasive procedures

Computed tomography (CT scan) and magnetic resonance imaging (MRI) are noninvasive techniques of choice to confirm a diagnosis of aortic aneurysm, thrombus, or atherosclerotic plaque in arterial walls, especially in the abdominal and carotid circulation. CT scans are good tests to assess the sizes and stages of vessel occlusions. They can be performed with or without a contrast medium and very quickly in emergency situations. CT scans are somewhat expensive and expose the patient to x-rays.

MRI is useful for evaluating a three-dimensional image of the vessel being studied. This method is contraindicated for patients with stainless steel pacemakers, vena cava filters, or vessel clips, but many patients can benefit from its use. Nonmagnetic materials are not contraindicated. MRI is more expensive than CT scans and takes longer to perform.

Carotid phonoangiography and oculoplethysmography (OPG) are techniques to obtain cerebral blood flow measurements to localize obstructions in the vessels of the head and neck. OPG is contraindicated in patients with intraocular lens implants.

Pulse volume recording (PVR) or photoplethysmography is used to measure systolic pressure in the extremities and digital arterial systems. This test is affected by artifacts and patient positioning. A diagnosis of deep vein thrombosis (DVT) may be made by phleborheography (PRG), a plethysmographic technique that records the rhythmic changes in venous volume in the legs; these changes are associated with respiration. PRG is not useful for small thrombi or for deep iliac or femoral veins, and the process is time consuming and expensive.

Ultrasonography is a major diagnostic tool for measuring segmental arterial pressures and venous patency in the extremities; it also may be used for abdominal circulation. Doppler color-coded flow imaging and transcranial Doppler imaging are replacing carotid phonoangiography and OPG in the evaluation of carotid circulation. High-resolution, B-mode ultrasound provides real-time images of venous systems in the upper and lower extremities.

Saphenous and cephalic vein mapping accurately measures vein diameter, location, and quality to determine preoperatively if the vein is suitable for use as an arterial conduit in arterial reconstruction. Ultrasound also detects venous thrombosis. With a pulse Doppler blood flow detector, longitudinal and/or transverse cross-sectional scans are obtained and the images are recorded by oscilloscope. A computer-generated print of the image on the screen provides a permanent record of the arteriograph or venograph.

Intraoperative assessment of shunt performance or vessel patency or stenosis, as well as identification of an arteriovenous fistula (AVF), is easily performed with a sterile Doppler probe (Fig. 44-5). An audible signal is transduced and is similar to the sounds transmitted by a Geiger counter.

Invasive procedures

Selective angiography permits the x-ray study of a particular segment of the vascular system.^1,4 Aortography visualizes the aorta (Fig. 44-6). Arteriography shows the patency of an artery or a branch of the aorta and its collateral circulation.

Phlebography detects DVT, and a venogram visualizes the veins. An angiogram requires the injection of a nontoxic contrast media. The pain associated with injection of intravascular contrast material can be intense. The procedure may be performed under continuous epidural anesthesia or general anesthesia.

Angioscopy is an endoscopic technique used to visualize the interior of vessels. A small (1.5- to 3-mm) flexible fiberoptic angioscope is coupled to a camera, which allows the view from the angioscope to be seen on a monitor. The lining and structures within the blood vessels are visualized as the scope is advanced within each vessel. For many patients, angioscopy is an alternative preoperative diagnostic technique to angiography and may be used to evaluate the effectiveness of therapy intraoperatively. It can reveal retained atherosclerotic plaque or thrombi and suture lines.

Intravascular ultrasonic scanning of the coronary or peripheral vasculature uses a miniaturized ultrasonic probe at the end of a 3.5 French catheter. The probe is introduced over a guidewire. The probe is irrigated with heparinized saline as it is introduced into the vessel percutaneously. Images of the entire circumference are obtained and viewed on a monitor to determine the thickness of the vessel wall and the distribution of plaque within the wall. This technique may be performed percutaneously or during a surgical procedure. This device is single use and not reprocessed. This device is not recommended for use on cerebral vessels.

Special features of vascular surgery

Circulation within the peripheral vascular system affects the brain, internal organs, and extremities. An expanding body of knowledge relating to vascular physiology and the development of the art of vascular surgery has improved the quality of life for many patients with peripheral vascular diseases. Circulatory problems may affect any part of the body, but this discussion focuses on the most common pathologic conditions amenable to vascular procedures performed by vascular surgeons and interventional radiologists. Current surgical trends include open procedures and endovascular techniques.

Vascular injury can occur during invasive diagnostic tests, intravascular monitoring, or therapeutic procedures. Iatrogenic arterial injuries, those resulting from an unexpected outcome of a procedure, can cause loss of function or even death from ischemia, hemorrhage, or embolus. The patient must be carefully observed and monitored for signs of complications during and after vascular procedures. Infection is a devastating postoperative complication that must be avoided through strict adherence to aseptic and sterile techniques. Other considerations include the following:

• A thorough understanding of the principles of general surgery should be combined with special training in vascular surgical techniques. Speed and accuracy are imperative.

• Local or monitored anesthesia care (MAC) is usually preferred for most conservative interventional procedures. General anesthesia or regional block is used for longer or more extensive procedures.

• Skin preparation is performed very gently. Vascular pathology (i.e., carotid stenosis, aneurysms, or venous thrombosis) should never be rubbed or pressed during the prep because plaque or clots could embolize, causing serious limb or brain damage. Aneurysms could rupture.

• Temperature regulation may be a problem during long procedures or when multiple blood transfusions are given. Warmed fluids can be administered via rapid infusing pumps. Forced-air warming blankets provide a normothermic temperature.

Intentional hypothermia can be used to preserve the spinal cord during aortic surgery.¹³ The low temperatures decrease the metabolic needs of the cord by minimizing the effects of temporary ischemia during cross-clamping of the aorta.

• Meticulous care is exercised during anastomosis of vessels to avoid the danger of postoperative thrombosis and stenosis. To prevent undue trauma to vessels, an assortment of curved and angled scissors, noncrushing vascular clamps, and forceps specifically designed for vascular surgery is included in the instrument setup (Fig. 44-7). Umbilical tape (also called hernia tape) or synthetic vessel loops are used for retraction and vessel control (Fig. 44-8). An operating microscope may be used for anastomosis of vessels. Appropriate instrumentation for microsurgery is made available.

Synthetic nonabsorbable monofilament suture materials are preferred because they are strong and pass through vessel walls and grafts easily with minimal trauma and tissue reaction. Swaged needles also minimize trauma. A larger swaged suture-to-needle ratio is advantageous to avoid leakage. Holes made in graft materials by the needle are occluded by the larger suture. Double-armed needle sutures are often used for vessel anastomosis.

Structurally the surgeon may use a triangulating technique in which the vessel edges are gently straightened to create a straight sutured angle instead of suturing a circular stitch.

• Heparinized solution is available for use as an anticoagulant irrigation. Preoperatively, heparin (5000 units) may be given subcutaneously for DVT prophylaxis 1 hour before the surgical procedure.⁶ Intraoperatively, the optimal dose is 70 to 100 units/kg of body weight if given intravenously for immediate systemic effect. Thromboelastography may be used intraoperatively to monitor the effects of heparin administration.

Sensitivity to bovine sources of heparin can result in heparin-induced thrombocytopenia (HIT), also known as white clot syndrome. Clots composed of fibrin and platelets form after 4 to 15 days of heparin therapy. A severe decrease in the platelet count predisposes the patient to thrombosis and acute arterial occlusion.

• Patients with preoperative anticoagulation with warfarin for known blood coagulation pathology are at risk for extreme blood loss. Temporary reversal of the therapeutic anticoagulation for the duration of an emergent surgical procedure is accomplished by the administration of fresh frozen platelets (FFP).⁶ Onset of FFP action is immediate and easily amenable to postoperative anticoagulation as necessary. The activity of FFP is short term.

Less urgent surgical procedures for the anticoagulated patient can be performed with minimal blood loss after the administration of vitamin K. Studies have shown that 1 mg of intravenous vitamin K reversed the effects of therapeutic anticoagulation within 27 hours of administration.⁶ Subcutaneous vitamin K injections are absorbed at an unpredictable rate and are rarely used. Low-dose vitamin K does not interfere with postoperative therapeutic anticoagulation regimes.

• Before closure at the end of the procedure, protamine sulfate, a heparin antagonist, is given to reverse the anticoagulant effect; 1 mg of protamine is given to counteract 100 units of heparin. Protamine is derived from fish semen and testicular tissue and may elicit a sensitivity reaction in patients who are allergic to fish. Patients with type 1 diabetes who take NPH insulin also may be predisposed to a sensitivity reaction to protamine. Some men who have had a vasectomy may exhibit allergic or hypersensitivity reactions.

• Hemostatic agents can be used independently or in combination (Box 44-2). Care is taken not to permit hemostatic materials to be suctioned into the blood-salvage or cell-saving device. These products can be hazardous if permitted to enter the patient’s vascular system. Containers and delivery devices for thrombin should be carefully labeled with concentration and dosage to prevent accidental injection.

BOX 44-2   Hemostatic Agents Used in Vascular Surgery

• Absorbable gelatin sponge (Gelfoam) can be used dry or dipped in saline. According to the manufacturer, the patient may have an antigenic reaction when the sponge is soaked in thrombin. The sponge can be applied to the site and removed after 20 seconds or left in the wound; it is absorbed after 4 to 6 weeks. Because it may provide a favorable microbiologic growth medium, it is not used in the presence of infection. It is derived from porcine gelatin and has more of a mechanical than a chemical hemostatic property.

• Absorbable collagen (Avitene, Hemopad, Helistat), either in powder, foam, woven, or nonwoven form, induces platelet adhesion and results in the formation of fibrin. It provides both a chemical and a mechanical hemostasis. Because it may provide a favorable microbiologic growth medium, it is not used in the presence of infection. The long-term effects of an in situ collagen hemostat are unknown, but animal studies indicate that it absorbs. It is of bovine origin and can be moistened, but it works better when dry.

• Oxidized cellulose (Surgicel, Oxycel) is applied dry over a bleeding site for hemostasis. Although a single layer can be left in the wound and absorbed, it is preferred that a large wad be removed because it may interfere with healing or cause pressure. It chemically destroys thrombin and is not used concurrently with it. Because it may provide a favorable microbiologic growth medium, it is not used in the presence of infection. It has some bactericidal properties against gram-positive and gram-negative microorganisms. The mode of action is mechanical hemostasis when applied to an area of bleeding.

• Fibrin glue is applied by simultaneously placing a combination of cryoprecipitate, bovine thrombin, and/or calcium chloride on the bleeding tissue surface to form a fibrin patch.

• Topical thrombin should be available. Preparations from bovine origin are contraindicated in patients who are sensitive or allergic to these products.

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