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The rapid diastolic runoff can compromise coronary perfusion pressure purchase 50mg azathioprine with amex muscle relaxant before massage, thereby predisposing to subendocardial ischemia cheap azathioprine online muscle relaxant drug class. Many neurohormonal buy azathioprine 50mg online spasms in legs, renal, and vascular mechanisms interact to varying degrees to the pathogenesis and progression of the different hemodynamic forms of hypertension. Note that aortic baroreceptors, which also influence blood pressure, are not paced. Dotted arrows represent inhibitory neural influences, and solid arrows represent excitatory neural influences on sympathetic outflow to the heart, peripheral vasculature, and kidneys. Premise, promise, and potential limitations of invasive devices to treat hypertension. The efferent renal (sympathetic) nerves contribute to hypertension by causing renal vasoconstriction and vascular hypertrophy via alpha -adrenergic receptors, stimulate renin release via beta -adrenergic receptors, and1 1 enhance renal sodium and water reabsorption via alpha receptors (1 Fig. Renal afferent nerves contribute to hypertension by evoking reflex activation of central sympathetic outflow triggered to multiple tissues and vascular beds. Increased 26 sympathetic activity may also drive some drug-resistant hypertension. In these conditions, central sympathetic activation can result from deactivation of inhibitory neural inputs (e. Complete baroreflex failure causes labile hypertension, most often seen in throat cancer survivors as a late complication of radiation therapy, which causes a gradual destruction of the baroreceptor nerves (see Chapter 99). Partial baroreceptor dysfunction is common in elderly hypertensive patients and typically manifests with a triad of orthostatic hypotension, supine hypertension, and symptomatic postprandial hypotension—the last initiated by splanchnic pooling after carbohydrate-rich meals. Obesity-Related Hypertension With weight gain, reflex sympathetic activation is an important compensation to burn fat, but at the expense of sympathetic overactivity in target tissues such as vascular smooth muscle and kidney that produces hypertension. Hypertensive patients with the metabolic syndrome have near-maximal rates of sympathetic firing. Humans evolved in a low- sodium/high-potassium environment, and the human kidney handles poorly exposure to high sodium and low potassium. Salt retention also augments the smooth muscle contraction produced by endogenous vasoconstrictors. Department of Agriculture and Department of Health and Human Services recommend a daily intake of less than 5. All segments of the population would benefit, with blacks benefiting proportionately more, women benefiting particularly from stroke reduction, older adults from reductions in coronary heart disease events, and 29 younger adults from lower mortality rates. In hypertension, this pressure-natriuresis curve is shifted to the right, and in salt-sensitive hypertension, the slope is reduced. Pressure-natriuresis causes profound nocturia in rare patients with pure autonomic failure who have supine nocturnal hypertension. Nocturia also may be an unrecognized symptom of uncontrolled 30 primary hypertension. Low Birth Weight Because of fetal undernutrition, low birth weight with reduced nephrogenesis increases the risk for development of adult salt-dependent hypertension. Hypertensive adults have fewer glomeruli per kidney but very few obsolescent glomeruli, suggesting that nephron dropout with decreased total filtration surface area is the cause and not the consequence of the hypertension. When low-birth-weight children consume a fast-food diet, they are susceptible to rapid postnatal weight gain, leading to adolescent obesity and hypertension. Genetic Contributions Animal and human studies have implicated an important genetic contribution to salt-sensitive hypertension. A similar gene-environment interaction may explain why persons of sub-Saharan African ancestry remain normotensive on a sodium-restricted diet, but are predisposed to hypertension when they encounter a high-sodium diet. Vascular Mechanisms Alterations in the structure and function of small and large arteries are pivotal in the pathogenesis and progression of hypertension. Endothelial Cell Dysfunction The endothelial lining of blood vessels is critical to vascular health and constitutes a major defense against hypertension (see Chapter 57). Dysfunctional endothelium displays impaired release of endothelium-derived relaxing factors (e. Oxidative stress also contributes to endothelial cell vasodilator dysfunction in hypertension. An increase in the medial thickness relative to the lumen diameter (increased media-to-lumen ratio) is the hallmark of hypertensive remodeling in small and large arteries. The media/lumen ratio increases, but the medial cross-sectional area remains unchanged. Diagrams represent arteries in cross section showing the tunica adventitia, tunica media, and tunica intima. Antihypertensive therapy may not provide optimal cardiovascular protection unless it prevents or reverses vascular remodeling by normalizing hemodynamic load, 35 restoring normal endothelial cell function, and eliminating the underlying neurohormonal activation. Interaction of aldosterone with cytosolic mineralocorticoid receptors in the renal collecting duct cells recruits sodium channels from the cytosol to the surface of the + renal epithelium. In normotensive individuals the risk for development of hypertension increases with increasing levels of serum aldosterone that are well within the normal range. By stimulating mineralocorticoid receptors in the heart and kidney, circulating 37 aldosterone may contribute to the development of cardiac and renal fibrosis in hypertension. Also, aldosterone contributes to sympathetic overactivity by stimulating mineralocorticoid receptors in the brainstem. The sympathetic activation promotes inflammation and damage in the kidney and other organs (especially the systemic vasculature) leading to severe hypertension. Hypertension control can improve exertional dyspnea caused by diastolic dysfunction, nocturia caused by resetting of pressure-natriuresis, and possibly even erectile dysfunction caused by endothelial dysfunction. An oscillometric monitor was set to take three readings at 1-minute intervals after the patient was unattended 42 by medical staff and unaccompanied by family members in the examination room for 5 minutes. The oscillometric method may not work well in patients with atrial fibrillation or frequent extrasystoles. Patients with white coat hypertension typically do not show exaggerated pressor reactions to stressful stimuli in their daily lives. Both the prevalence and the severity of white coat hypertension increase sharply with age (Fig. Many patients do not have pure white coat hypertension but rather “white coat aggravation,” a white coat reaction superimposed on a milder level of out-of-office hypertension that nevertheless requires treatment (eFig. Right, Pronounced “white coat” effect in an 80- year-old woman referred for evaluation of medically refractory hypertension. Victor, Heart Institute/Hypertension Center, Cedars-Sinai Medical Center, Los Angeles; Right, Provided by Dr. Wanpen Vongpatanasin, Hypertension Division, Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas. The vast majority of hypertensive patients meet current criteria for initiation of lipid-lowering therapy (see Chapter 48). This fluctuation is most common in elderly patients 55 and may represent a stiff aorta with impaired arterial baroreflexes and/or generalized anxiety disorder. Noninvasive Measurement of Aortic Stiffness and Central Aortic Pressure by Pulse Tonometry. The central aortic pressure waveform is the sum of the pressure wave generated by the left ventricle and reflected waves from the peripheral circulation.
Arthroscopic repair requires percutaneous anchor placement and arthroscopic suture-passing and knot-tying purchase azathioprine 50 mg overnight delivery muscle relaxant pharmacology. Bleeding is minimal with the arthroscopic technique purchase genuine azathioprine on line kidney spasms after stent removal, but may approach 400 mL with an open procedure order azathioprine 50mg mastercard 2410 muscle relaxant. Both require that the patient remain relaxed until all dressings are applied and he/she is fitted with an abduction sling. It involves simple resection of the distal 5 mm of the clavicle through an incision directly over the joint or through an accessory anterior portal. The coracoclavicular ligament often is repaired or reconstructed with tendon graft, or the coracoacromial ligament is transferred from the edge of the acromion to the clavicle. Following reduction and fixation, the deltoid is reattached to the clavicle if it has been avulsed, and the patient is placed in an immobilizer after skin closure. The operation is technically challenging, and the brachial plexus and subclavian vessels are at risk with screw placement and with inferior dislocations. Hata Y, Saitoh S, Murakami N, et al: A less invasive surgery for rotator cuff tear: mini-open repair. This involves “plication” of the capsule and/or labrum to decrease the capsular volume of the shoulder. Traumatic instability is usually anterior and is quite common in the young, active population. Recurrent dislocation in young, active patients is common (80–90%) and is associated with avulsion of the capsule/labrum from the anterior-inferior glenoid rim (Bankart lesion). The population undergoing a Bankart repair is almost invariably young and healthy. Posterior traumatic dislocation is much less common and is associated with high-energy trauma, seizures, or electrocution. Instability surgery is often preceded by exam under anesthesia and arthroscopic examination, either in the beachchair or lateral decubitus position. The essential feature of instability surgery, whether arthroscopic or open, is the reattachment of the anterior inferior capsulolabral complex back to the rim of the glenoid, thus reestablishing the normal “bumper” effect of the anterior-inferior labrum and decreasing the capsular volume of the shoulder. Nonanatomic procedures (reconstructive) are much less common, but are still performed occasionally. These include transfer of the coracoid process to the anterior glenoid rim (Bristow or Latarjet procedure). T h e open Bankart repair is performed in the beachchair position using the deltopectoral approach, with the interval between the deltoid and pectoralis major. The glenoid rim is decorticated, providing bleeding bone to promote healing, and the anterior capsule is reattached through drill holes in the glenoid or with suture anchors. Cross section of the joint: The joint capsule is redundant inferiorly to allow abduction. The tendon is surrounded by synovium and, therefore, is anatomically intracapsular but extrasynovial. The musculocutaneous nerve may be stretched by excessive medial retraction of the coracobrachialis (especially if a coracoid osteotomy is used) and the axillary nerve may be injured if the surgeon strays too far inferiorly. If a subscapularis-releasing technique is used, the muscle is reattached and must be protected postop. External rotation of the shoulder is prevented for several weeks while the repair heals, and the surgeon prefers that the patient remain anesthetized until a shoulder immobilizer is applied. The arthroscopic Bankart repair is similar to the open procedure but is performed through two anterior portals with the scope coming in posteriorly. This procedure is less painful postop and allows for more rapid rehabilitation, because the subscapularis is not detached. Open surgery for posterior dislocation is similar to the open Bankart repair, but it is done in the lateral position and utilizes the interval between the infraspinatus and teres minor. Individuals presenting for repair of shoulder dislocations also may include those with a joint hypermobility syndrome (e. A suprascapular block (when interscalene block is contraindicated) can be used for intraop → postop pain control in arthroscopic shoulder procedures. Unless contraindicated, a long-acting local anesthetic should be used in regional anesthesia for shoulder surgery to ameliorate postop pain. Borgeat A: Acute and nonacute complications associated with interscalene block and shoulder surgery. Sperber A, Hamberg P, Karlsson J, et al: Comparison of an arthroscopic and open procedure for posttraumatic instability of the shoulder: a prospective, randomized multicenter study. Primary osteoarthritis (wear-and-tear arthritis) is much less common in the shoulder than in the weight-bearing joints, such as the hip and knee. Both components may be cemented or uncemented, depending on the surgeon’s preference. Some revision cases require glenoid bone grafting, which increases the complexity and potential blood loss. Shoulder arthroplasty utilizes the beachchair position and the deltopectoral incision (Fig. The humeral head is dislocated anteriorly, and the head is removed with an oscillating saw. If the glenoid is to be resurfaced, it is done before implantation of the final humeral component. The labrum is excised and a motorized reamer is used to remove the cartilage of the glenoid. The glenoid prosthesis is cemented into place, with the component held in position manually until the cement hardens (~15 min). Trial humeral components are placed, and the appropriate sizing of the head and stem are assessed. Unless contraindicated, a long- acting local anesthetic should be used in regional anesthesia for shoulder surgery to ameliorate postop pain. Borgeat A: Acute and nonacute complications associated with interscalene block and shoulder surgery. Some of these injuries include common athletic injuries, such as acromioclavicular joint separations, which rarely require surgery unless there are associated acromial or clavicular fractures. Posterior sternoclavicular dislocations may warrant surgical stabilization if the trachea is compressed. Clavicle fractures, frequently associated with scapular fractures, occasionally require open reduction. Extreme fractures involving the shoulder girdle (scapulothoracic dissociations) include scapular fracture, clavicle fracture, subclavian or axillary artery disruption, and brachial plexus injury. These may coexist with proximal humerus fractures, rib fractures, and pneumothorax. In the older, debilitated patient, the most common injury is proximal humeral fracture, which may be amenable to surgical stabilization or may be so comminuted as to warrant hemiarthroplasty. A displaced proximal humerus fracture may require open reduction internal fixation with a plate and screws or hemiarthroplasty through a deltopectoral approach utilizing a beachchair position (see Surgery for Shoulder Instability, p. A displaced clavicle fracture may require open reduction internal fixation with a plate and screws utilizing a beachchair or supine position.
Transection of the jejunum usually occurs just distal to the ligament of Treitz order genuine azathioprine line spasms medication, where the jejunum is ﬁrmly attached to the posterior abdominal wall purchase azathioprine 50mg otc spasms in 7 month old. In transection of the small bowel discount azathioprine 50mg overnight delivery muscle relaxant drugs over the counter, there is usually associated injury to the mesentery. Spontaneous rupture of the small bowel may occur due to infarctions secondary to incarceration, strangulation, various ulcer- ative diseases of the mucosa, and thrombosis of the mesenteric vasculature. With severe blunt trauma to the abdomen and injury to internal organs, the mesentery of the small intestine is often contused or torn. The mesentery appears to be torn most often by a tangential blow to the abdomen that exerts traction on the mem- brane. Death could occur solely from injury to the mesentery if there is laceration of one of the large blood vessels coursing through the mesentery. The large intestine differs from the small intestine in its larger caliber, more ﬁxed position and less vulnerability to trauma. The midportion or transverse colon is the most open to trauma because of its relation to the vertebral column and its exposed position in the mid abdominal cavity. A severe impact to the anterior abdominal wall may crush the midportion of the transverse colon between the anterior abdominal wall and the lumbar vertebrae. The resulting traumatic lesion depends on the severity of the blunt force and might range from a contusion to a laceration to transection. Rup- ture of the colon may also occur following insertion of foreign objects, hands, or animals for sexual stimulation. Blunt Trauma Injuries of the Trunk and Extremities 139 Kidneys The kidneys are situated in the posterior part of the abdomen on either side of the vertebral column behind the peritoneum. The posterior surface and upper portion of the right kidney rest on the 12th rib; the left kidney usually rests on the 11th and 12th ribs. The anterior surface of the right kidney is in contact with the right adrenal gland, liver, and the right colic ﬂexure. The anterior surface of the left kidney is in contact with the left adrenal gland, stomach, spleen, jejunum, colon, and, medially, the pancreas. They are usually seen following motor vehicle accidents or falls from great heights when there is massive blunt force trauma to the abdominal cavity. Blunt force applied to the ﬂank may crush the kidney between the abdominal wall and the lumbar vertebrae. Aside from contusions, the major- ity of injuries to the kidney are small transverse lacerations beneath an intact capsule with minimal hemorrhage. Injuries producing massive lacerations of the kidneys up to fragmentation are uncommon and are associated with massive injury to the other abdominal organs. Urinary Bladder In adults, the empty urinary bladder is placed entirely within the pelvis, behind the pubic symphysis. In children, the anterior surface of the bladder is in contact with the lower two-thirds of the abdominal wall between the sym- physis pubis and the umbilicus. Beginning at puberty, it slowly begins to descend to its ﬁnal position in the pelvis. Iatrogenic rupture of the urinary bladder may occur during instrumentation for diagnostic or therapeutic purposes. More commonly, severe blunt trauma to the pelvis and lower abdomen causes rupture. The degree and type of injury that occurs usually depends on the volume of urine in the bladder. Extraperitoneal occurs when the bladder is empty or contains only a small amount of urine. In extraperitoneal rupture, the bladder lies within the pelvis and is protected by the strong bony pelvis. Here lacerations of the urinary bladder are associated with fractures of the pelvis. This is when blunt force is applied to the lower abdominal wall in a downward direction. Intraperitoneal rupture of the urinary bladder occurs when the bladder is markedly distended by urine. At this time, a kick, a blow, or any blunt force to the lower abdominal wall can compress the posterior wall of the 140 Forensic Pathology bladder against the sacrum, raising the pressure within the bladder lumen and rupturing it, with urine entering the abdominal cavity. When they do occur, they are usually associated with extensive fractures of the pelvis. Blunt trauma injuries to the pregnant uterus and/or fetus are usually caused by automobile accidents, with falls and assaults accounting for a signiﬁcantly smaller num- ber of cases. Sep- aration occurs at the moment of trauma but may not become evident for a few hours. This is probably due to a small separation at the edge of the placenta, with development of a retroplacental hematoma that takes a while to grow and kill the fetus. In the absence of any direct trauma, the cause for the separation is severe distortion of the uterus that can occur with violent motion. Following the death of the fetus, labor usually begins within 48 h, though it may be delayed up to a few weeks. During this time, the mother may develop a disseminated intravascular coagulopathy. With fractures of the pelvis, there may be not only placental separation but direct fetal injury, for example, fracture of the fetal skull and/or internal injuries to the fetus. Blunt Force Injuries of the Extremities These injuries may be limited to the skin and subcutaneous tissues or extend to muscles, blood vessels, nerves, bones, and joints. Avulsive wounds of the lower extremities are most frequently seen in automobile–pedestrian accidents. If an automobile wheel passes over the lower extremities, it can exert tangential pressure on the skin and subcu- taneous tissues, separating them from the underlying muscles. In other instances, the skin and subcutaneous tissue are also torn, forming a large ﬂap of skin (Figure 5. A blood-ﬁlled pocket may also be produced in the back and/or lateral (outer) aspect of the thigh in pedestrians impacted by the front of the hood. The tangential force of the hood impacting the thigh strips the skin and subcutaneous tissue from the muscle, creating a blood-ﬁlled pocket (Figure 5. Complications of Blunt Force Injuries to the Lower Extremities Shock — caused by severe crushing, soft tissue injuries, and/or com- pound fracture. Hemorrhage — occurs from traumatic amputation, compound fracture with severing of a large vessel, multiple lacerations, or severe avulsive wounds Blunt Trauma Injuries of the Trunk and Extremities 141 A B C Figure 5. Venous thrombosis with fatal pulmonary embolism — Veins may be injured directly by fracture of the lower extremity, with resultant thrombosis. Thrombosis may also be secondary to venous stasis following prolonged immobilization of the lower extremity when the patient is conﬁned to bed with a fractured extremity. There may be crushing injuries rather than frac- tures of the lower extremity with either direct injury to the veins or stasis bv compressing hemorrhage and edema resulting from the leg injury. Fat embolism — Fat embolism follows mechanical trauma that mobi- lizes the fat from an injured fat deposit in the body.
Then the external iliac artery is clamped and an artery-to renal-artery anastomosis is performed buy azathioprine overnight muscle relaxant antidote. The patient should be euvolemic at this point; mannitol and/or furosemide can be given discount 50 mg azathioprine free shipping muscle relaxant urinary retention. The bladder is filled with an antibiotic irrigation solution to facilitate the implantation of the ureter buy azathioprine with american express muscle relaxant pills over the counter. The detrusor muscle is then reapproximated over 3–4 cm of ureter to create an antireflux valve. Sketch of a kidney transplant labeled 1, 2, 3 in the order of the surgical anastomoses. Less commonly, pancreas transplantation is done for patients with brittle diabetes or with impending complications while they still enjoy normal or near-normal kidney function. The pancreas transplant is placed in the right iliac fossa, and the kidney transplant is placed in the left iliac fossa. This can be done through a transperitoneal lower midline incision or through two separate extraperitoneal lower-quadrant incisions in the same manner as kidney transplantation. For arterial in-flow, a Y-graft is fashioned using the donor iliac artery bifurcation. The portal vein coming off the pancreatic graft is anastomosed to the external iliac vein. The Y extension vascular graft is then anastomosed to the recipient external or common iliac artery. The donor duodenum is anastomosed to a loop of small bowel or to the urinary bladder to drain the exocrine secretions (Fig. With pancreas transplantation, there may be significant blood loss if the graft mesenteric vessels are not occluded properly. After the pancreas is implanted, the kidney transplant is placed into the opposite iliac fossa (as described in Kidney Transplantation, p. In normal individuals, 50% of secreted insulin is extracted from the circulation in the first pass through the liver. This more physiologic approach, however, is associated with a higher technical failure rate and requires a long upper midline incision (Fig. Pancreatic islet cells may be infused via a radiological portal vein approach, a procedure that is usually performed in the radiology/angio suite. Rarely, patients will present for transplant surgery + without adequate preparation (e. Spinal, epidural, or combined spinal- epidural anesthesia may be considered for renal transplantation, if coagulation and platelet function acceptable. Ilioinguinal-iliohypogastric and intercostal nerve blockade can be utilized as an alternative method for postop pain control. Hadimioglu N, Ertug Z, Bigat Z, et al: A randomized study comparing combined spinal epidural or general anesthesia for renal transplant surgery. Kidney transplantation from living donors is associated with a better patient and graft survival rate. Initial concerns regarding ureteral complications and longer warm ischemic time have mostly subsided with the improvement of the surgical technique and greater experience. The patient is positioned in lateral decubitus over a cushioned beanbag, the kidney rest is slightly elevated, and pillows and an axillary roll are used to prevent compression injuries. The hand-assisted approach, however, has gained popularity over the years and is currently the preferred technique of the majority of Transplant Centers in the United States. In this approach two or three ports are used with a 6–8 cm incision made at the level of the umbilicus or infra-umbilical. The pneumoperitoneum is kept < 15 mm Hg to avoid decreased perfusion to the kidney. Aggressive hydration and intermittent use of iv mannitol help improve kidney perfusion. On the left side, the descending colon and spleen are mobilized medially; the renal vessels are exposed; the adrenal, lumbar, and gonadal veins are clipped and divided; the ureter is mobilized en bloc, along with the gonadal vein, down to the pelvic inlet. The artery is freed from surrounding lymphatic and neural tissue as it comes off the aorta (Fig. If the procedure is done purely laparoscopically, a 6-cm suprapubic incision is then made, the peritoneum is exposed in the midline, and an 18-mm port is used to insert a 15-mm Endocatch retrieval bag. The kidney is placed in the bag as it continues to be perfused, avoiding warm ischemia. With the hand-assisted approach the incision is already made for retrieval with the surgeon’s hand. The heparin is reversed with protamine, the suprapubic incision is closed, and homeostasis is verified before extracting the ports. For a right nephrectomy, the right colon and duodenum are mobilized medially and the liver is retracted upward. An incision is made from the rectus muscle, angling slightly cephalic to cross into the flank just below the tip of the 12th rib. Just before clamping the renal artery, furosemide and/or mannitol may be given to stimulate diuresis. It is important to keep the vascular volume expanded in these patients before kidney removal. The kidney is removed and taken to the back table, where it is flushed with a cold preservation solution. Some surgeons use a full dose of heparin (75 U/kg) before clamping and use protamine afterward. Smaller incisions and muscle- sparing incisions are now used to improve postop recovery. This operation is divided into categories: early nephrectomy, performed during the first month posttransplant, and late nephrectomy, thereafter. Early transplant nephrectomy may be required for primary nonfunction, vascular thrombosis, and, rarely, refractory rejection. In these cases, an extracapsular approach, through the original transplant incision, is used. Kidney Transplant NephrectomyThe kidney is freed from the surrounding adhesion to obtain vascular control of the renal artery and renal vein. The ureter is ligated as close as possible to the bladder and excised completely, with primary repair of the bladder. Late transplant nephrectomy is performed most commonly for acute, irreversible rejection with failure of the renal allograft. Most of these patients have returned to dialysis, and the immunosuppressive medications are stopped.
In contrast purchase azathioprine cheap spasms kidney stones, if activation proceeds in a direction perpendicular to that direction (cosine equals 0) purchase azathioprine 50mg line spasms in spanish, the sensed potential will be zero buy cheapest azathioprine and azathioprine spasms thoracic spine. The cardiac electrical field during recovery phases differs in several important ways from that during activation. First, the gradient of intercellular potentials and thus the direction of current flow during recovery are the opposite of those described for activation. As a cell undergoes recovery, its intracellular potential becomes progressively more negative. For a cardiac fiber, the intracellular potential of the region whose recovery has progressed further is more negative than that of the adjacent, less recovered region. Intracellular currents then flow from the less recovered toward the more recovered portion of the fiber. That is, recovery wavefronts will have an orientation opposite that of activation wavefronts. The strength of the recovery front also differs from that of the activation front. As noted, the strength of a wavefront is proportional to the rate of change in transmembrane potential. Rates of change in transmembrane potential during the recovery phases of the action potential are considerably slower than during activation, and thus the strength of the recovery wavefronts during recovery is less than during activation. The rate of movement of the activation and recovery wavefronts is a third difference between activation and recovery. Activation is rapid (as short as 1 millisecond in duration) and occurs over only a small distance along the fiber. Recovery, by contrast, lasts 100 milliseconds or longer and occurs simultaneously over extensive portions of the heart. The activation and recovery fields are perturbed by the complex three-dimensional physical environment in which they are generated. These transmission factors include the biophysical characteristics of the heart itself as well as those of the surrounding organs and tissues. The most important cardiac factor is the presence of connective tissue between cardiac fibers that disrupts efficient electrical coupling of adjacent fibers. Waveforms recorded from fibers with little or no intervening connective tissue are narrow in width and smooth in contour, whereas those recorded from tissues with abnormal fibrosis are prolonged and sometimes exhibit prominent notching. Extracardiac factors include the effects of all the tissues and structures that lie between the activation region and the body surface, including intracardiac blood, lungs, skeletal muscle, subcutaneous fat, and skin. These tissues alter the intensity and the orientation of the cardiac field because of differences in the electrical resistivity of adjacent tissues within the torso. For example, intracardiac blood has much lower resistivity (approximately 160 Ω cm) than the lungs (~2150 Ω cm). Potential magnitudes change in proportion to the square of the distance between the heart and recording electrode. One consequence of this principle is that eccentricity of the heart within the chest affects the surface waveforms. The right ventricle and anteroseptal aspect of the left ventricle are closer to the anterior chest wall than are other parts of the left ventricle and atria. An additional physical factor affecting the recording of cardiac signals is cancellation. When two or more wavefronts are simultaneously active during activation (or repolarization) and have different orientations, the vectorial components of the wavefronts may augment (if oriented in the same directions) or cancel (if oriented in opposite directions) each other when viewed from remote electrode positions. As a result of these transmission factors, body surface potentials (1) have an amplitude of only 1% of the amplitude of transmembrane potentials, (2) are smoothed in detail so that they have only a general spatial relationship to the underlying cardiac events, (3) preferentially reflect electrical activity in some cardiac regions over others, and (4) represent only limited amounts of total cardiac electrical activity. These electrodes are connected to form leads that record the potential difference between two electrodes. The potential at the other (negative) electrode is subtracted from the potential at the positive electrode to yield the bipolar potential. The actual potential at either electrode is not known; only the difference between them is recorded. In some cases, as described later, multiple electrodes are electrically connected together to form the negative member of the bipolar pair. This electrode network is commonly referred to as a compound or reference electrode. The lead then records the potential difference between a single electrode serving as the positive input (the exploring electrode) and the potential in the reference electrode. Specifics of electrode placement and definitions of the positive and negative inputs for each lead are presented in Table 12. The standard limb leads record the potential differences between two limbs, as detailed in Table 12. The electrode on the right leg serves as an electronic reference that reduces noise and is not included in these lead configurations. Bottom, Electrode locations and electrical connections for recording a precordial lead. Right, Connections to form the Wilson central terminal for recording a precordial (V) lead. Five-thousand ohm resistors (5kΩ) are connected to each limb electrode when constructing the Wilson central terminal. The electrical connections for each of these leads can be represented as a vector oriented from its negative toward the positive pole. The precordial leads register the potential at each of the six specific torso sites (see Fig. For this purpose, an exploring electrode is placed at each of six specific precordial sites and connected to the positive input of the recording system (see Fig. The reference potential for these leads is formed by connecting the two limb electrodes that are not used as the exploring electrode. Thus, and This modified reference system produces a larger-amplitude signal than if the full Wilson central terminal were used as the reference electrode. When the Wilson central terminal was used, the output was small, in part because the same electrode potential was included in both the exploring and the reference potential inputs. The three standard limb leads and the three augmented limb leads are aligned in the frontal plane of the torso. The 12 leads are usually divided into subgroups corresponding to the cardiac regions to which they may be most sensitive. Expanded lead systems that are frequently used include recordings from additional electrodes placed on 2 the right precordium to assess right ventricular abnormalities such as right ventricular infarction, and on the left posterior torso (see Table 12. Electrodes placed higher on the anterior torso than normal may also help detect abnormalities such as the Brugada pattern and its variants (see Chapters 33 and 37). Other lead sets have sought to minimize movement artifacts during exercise and long-term monitoring (see Chapters 13 and 35) by placing limb electrodes on the torso rather than near the ankles and wrists as recommended. Electrode sets including 80 or more electrodes that sense cardiac potentials over large portions of the torso have been used to display the spatial distributions as well as the amplitudes of potentials throughout the cardiac cycle. Also, electrodes may be passed into the esophagus to enhance detection of atrial activity in, for example, the diagnosis of various arrhythmias (see Chapter 35). For an augmented limb and for a precordial lead, the origin of the lead vector passes through the midpoint of the axis connecting the electrodes that comprise the reference electrode.
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