Assistant Professor, University of California, San Diego School of Medicine
Equation 3 has the advantage of expressing the dialysis effect as a volume equivalent of solute diffusing across the membrane per unit of time treatment 4th metatarsal stress fracture discount diltiazem 180mg without a prescription. The volume transferred per unit of time is constant; that is treatment qt prolongation cheap diltiazem 60mg fast delivery, a milliliter equivalent of solute is transferred per unit of time regardless of how much solute is contained in that milliliter treatment zinc deficiency purchase diltiazem 60 mg with visa. The rate of diffusion is directly proportional to the membrane surface area treatment sinus infection order diltiazem 180 mg otc, which is constant for any given model of dialyzer. Effects of Temperature, Pressure, and Molecular Weight Diffusion is a consequence of molecular motion, which is affected by pressure and heat energy and by molecular mass. The rate of diffusion is proportional to the absolute temperature, which is approximately 273 K at room temperature. Within the range of temperatures experienced in the dialysis center, the proportionate change in absolute temperature (260 to 280 K) is so small that its influence on diffusion across the dialysis membrane is negligible. More important are the physiological effects of temperature on blood flow and body water compartmentalization, which have significant effects on solute kinetics within the patient (see Quantifying Hemodialysis). Similarly, pressure effects have little influence on diffusion within the range of pressures recorded in modern dialyzers. Molecular mass plays a more significant role in determining the rate of diffusion, because at a given temperature and pressure, the heavier molecules move more slowly and collide with the semipermeable membrane less frequently. Smallmolecular-weight substances, such as urea and creatinine, diffuse readily across a semipermeable membrane, whereas larger substances, such as b2-microglobulin or albumin, diffuse slowly or not at all. The larger size of the heavier molecules further impedes diffusion through small pores. By convention, blood entering the hemodialyzer is designated arterial, whereas blood leaving the hemodialyzer is venous. The principal differences among the many available hemodialyzers are the membrane composition, membrane configuration, and membrane surface area. For hemodialysis, the solution must be prepared from properly treated water (see Chapter 24) and contain the solutes listed in Table 20-1 in concentrations comparable to those of plasma. Dialysate must have a low concentration of endotoxin to prevent pyrogen reactions in the patient, but, in contrast to peritoneal dialysate (see Chapter 27), sterility is not a requirement because the semipermeable membrane excludes large particles, such as bacteria and viruses. Vital electrolytes and glucose are added to the dialysate to reduce or abolish their concentration gradients, whereas bicarbonate or a bicarbonate precursor is added in higher concentrations to promote accumulation in the " Membrane surface area Properties of the solute # Molecular weight and size Shape # Charge Blood side # Unstirred blood layer " Blood flow Dialysate side # Dialysate channeling and unstirred layer " Dialysate flow " Countercurrent direction of flow " Increases clearance; # decreases clearance Chapter 20 Principles of Hemodialysis 287 Membrane Composition, Configuration, and Surface Area Composition of the Membrane Two major classes of membrane material are available commercially: 1) cotton fiber, or cellulose-based membranes, and 2) synthetic membranes. Cellulose-based membranes range from unmodified cellulose to substituted cellulose membranes. Unmodified cellulose membranes have many free hydroxyl groups, which are thought to be responsible for their bioincompatibility and propensity to activate white blood cells, platelets, and serum complement. In an effort to improve membrane biocompatibility while keeping costs down, the cellulose polymer is treated with acetate and tertiary amino compounds to form a covalent bond with the hydroxyl groups. The major polymers in commercial synthetic membranes are polyacrylonitrile, polysulfone, polycarbonate, polyamide, and polymethylmethacrylate. Despite their increased thickness, these membranes can be rendered more permeable than the cellulose membranes, allowing for greater fluid and solute removal. Because the pore sizes in the synthetic membranes can be made wider, largermolecular-weight substances, such as b2-microglobulin, can be removed more efficiently. As the area increases, solute transport, often called efficiency, of the dialyzer increases. To maximize membrane surface area, one can increase the length of the hollow fiber, increase the number of hollow fibers, or decrease the diameter of the hollow fiber while holding other parameters constant. Increasing the fiber length increases shear rate and magnifies the pressure drop between blood entering and exiting the dialyzer. Any decrease in ultrafiltration decreases its contribution to solute clearance and offsets the potential advantage of the increased surface area. Increasing the number of hollow fibers increases the volume of extracorporeal blood and may eventually compromise hemodynamic stability. Finally, as the diameter of the hollow fiber decreases, the increase in resistance to blood flow enhances filtration and backfiltration,115 but clotting is also enhanced. As fibers thrombose, effective surface area for diffusion decreases and solute clearances fall. Because of these adverse consequences, the minimal acceptable internal fiber diameter is 180 mm. The composition and the thickness of the membrane varies considerably and is often more important than the surface area in determining dialyzer efficiency. In general, the thinner the membrane, the more efficient is the transport of solutes and fluid across the membrane. Hollow Fiber Dialyzers Current hemodialyzers are constructed with a plastic casing, usually polycarbonate that encloses several thousand hollow fiber semipermeable membranes stretched from one end to the other, imbedded at each end into a plastic potting compound, usually polyurethane, that serves as the headers. The blood compartment fiber bundle volume of the hollow fiber dialyzer is 60 to 120 ml, which in contrast to older dialyzer designs, does not expand during dialysis. The potting material separates the blood compartment from the dialysate compartment where dialysate flows between and around each fiber in the direction usually opposite to blood flow. Blood flows to or from the open end of each fiber through a removable "header" attached to the blood tubing. In addition to a lower blood priming volume, the hollow-fiber design increases the area of contact between blood and dialysate, allowing for the most efficient exchange of solutes. Recent efforts to prevent loss of surface area by fiber-to-fiber contact include insertion of spacer yarns between fibers and a wavy Moire configuration of the fibers. Major disadvantages of the hollow-fiber design are thrombosis and the potting compound, which can absorb chemicals used to disinfect newly manufactured dialyzers. Effects of Flow on Clearance Blood Flow Dialyzer blood flow (Q b) is driven by a roller pump and generally ranges from 200 to 500 ml/min, depending on the type of vascular access. As Q b increases, more solute is presented per unit of time to the membrane, and solute removal increases.
A study in 2006 found that bone volume was low in 46% of patients who underwent bone biopsy symptoms pink eye diltiazem 60mg mastercard. Importantly medications bladder infections discount 60mg diltiazem mastercard, by adding the component of volume medications zanx effective 180mg diltiazem, one can see that long standing severe hyperparathyroid bone disease or disease on preexisting conditions of bone volume loss (postmenopausal osteoporosis or corticosteroid use) would be different (and likely more fragile with increased fractures) than newly diagnosed bone disease due to hyperparathyroidism symptoms bipolar disorder purchase diltiazem 180 mg without prescription. Ibels and colleagues in 1979105 demonstrated that both the renal and internal iliac arteries of patients undergoing a kidney transplant had increased atheromatous/intimal disease and increased calcification compared to transplant donors. In addition, the medial layer was thicker and more calcified in the recipients compared to the donors. In addition, morphometry of the arteries demonstrated increased medial thickening. Elevated phosphorus, uremic serum, hyperglycemia, oxidative stress, inflammatory cytokines, and other so-called nontraditional cardiovascular factors appear to initiate this transformation. These techniques are thought to be more reproducible than the older method of observing progression of vascular calcification on plain radiographs. In 1996 Braun and colleagues found that hemodialysis patients had twofold to fivefold greater coronary artery calcification than age-matched individuals with normal kidney function that had angiographically proven coronary artery disease. Individual patient parameters could be plotted on the graph, or means and ranges of grouped data could be shown. For example, many patients with renal osteodystrophy cluster in areas shown by the bars. The bone volume may be low to medium, depending on the severity and duration of the process and other factors that affect bone. In the present graph, it is depicted as highturnover, normal bone volume, with abnormal mineralization. Plain radiographs of the thigh demonstrate calcification of the femoral artery in a plaquelike arrangement (termed intimal) or a medial (circumferential) arrangement. Using these radiographs, patients were classified into medial or intimal (including mixed medial and intimal lesions) or no calcification, and followed prospectively. There was lowest survival for patients with intimal calcification, followed by medial calcification, followed by no calcification. Mineral metabolism abnormalities including hyperphosphatemia, elevated calciumphosphorus product, or excessive calcium load from phosphate binders have been identified as additional risk factors in several, but not all, studies. Two small studies have demonstrated an increase in mortality with increased coronary artery calcification. Alterations in mineral metabolism appear to be associated with increased calcification in peripheral arteries in the majority of studies. Thereafter, bone mineral content decreases gradually, with a 5-year acceleration at the time of menopause in women. Interestingly, coronary artery calcification progresses from the age of 25 to 35 until death. The bone changes that ensue are associated with these biochemical alterations and other mechanisms. Both the biochemical changes and bone abnormalities contribute to vascular calcification. The remainder of this chapter discusses some of the studies that led to these recommendations. Unfortunately, high-quality, randomized, controlled clinical trials with patient level outcomes are lacking in this field. Unfortunately, it remains challenging and requires a combination of dietary restriction, phosphate binders, and enhanced dialytic removal. Diet Phosphorus is an inherent element in plant and animal cells; however, the content of phosphorus in protein foods and the proportion that can be absorbed vary greatly. For instance, plant sources of food are high in phosphorus, but the enzyme phytate is required for the breakdown of ingested phosphorus, and because this enzyme is absent in humans, phosphorus absorption of proteins derived from plant foods is less complete. Phosphorus is also added to processed foods including meats, spreads, puddings, caramelized colas, and many of the "fast foods" and less expensive foods. A randomized trial found that counseling dialysis patients to avoid processed foods can reduce the serum phosphorus. Although the theoretical and experimental data demonstrating that this prevents the development of secondary hyperparathyroidism are compelling, definitive evidence of sustained efficacy of dietary phosphorus restriction in preventing or treating secondary hyperparathyroidism in humans is lacking. They are used by clinicians, and also by insurance providers, governments, the United States Food and Drug Administration, and other regulatory agencies to establish standards of care for clinical practice. They are usually developed through a series of steps and are led by an evidence review team and a panel of experts. These individuals define the populations, predictors, interventions, and outcomes of interest and develop literature search strategies. The evidence review team then grades the quality of evidence for the outcomes of each study and provides an overall quality of evidence. The work group then writes recommendations and grades the strength of that recommendation. The process of developing guidelines and grading the level of evidence is under evaluation by multiple international organizations. Stage 5D: Use calcitriol, vitamin D analog, and/or calcimimetics (2B) with choice dependent on calcium and phosphorus levels. Phosphate Binders the use of aluminum as a phosphorus binder was popular throughout the 1970s and 1980s. In the decade that followed, it was noted that calcium acetate would reduce the elemental load of calcium while controlling the serum phosphorus level when compared to calcium carbonate, although there was no difference in the incidence of hypercalcemia. Among the 84 patients who completed the study, final coronary artery calcification scores were greater than initial scores in those receiving diet alone (P < 0. Subjects in both groups received atorvastatin to achieve an low-density lipoprotein goal of 70 mg/dl (1.
Instruments and endoscopes There are three main methods for sterilizing instruments and endoscopes: heat chemicals radiation medications 25 mg 50 mg diltiazem 60 mg free shipping. The area around the incision is then isolated with sterile drapes made of linen or disposable material medicine xarelto buy diltiazem 60mg low cost. Usually the process is carried out at a central site 300 medications for nclex order generic diltiazem canada, away from the theatre suite schedule 8 medications list cheap diltiazem 60mg on line. The most effective of the chemical methods is prolonged soaking in gluteraldehyde, but the solution is toxic and requires special facilities for its use. Radiation with gamma-rays is highly effective but is not available in most hospitals. Its main use is to sterilize pre-packed mass-produced disposable items such as meshes for hernia repair and artificial joints. There are many other methods of sterilization but a full discussion of their merits is beyond the scope of this book. The environment the modern operating theatre is air-conditioned and kept scrupulously clean. Laminar airflow is used for various forms of surgery, particularly involving the implantation of synthetic materials such as artificial joints and heart valves. The maximum benefit of laminar flow systems is seen with operations where the consequence of infection is potentially disastrous and the infecting organisms come from the theatre environment, not from the patient. In most cases the organisms come from the patient, either from the organ being operated upon or the skin. Bacteria usually enter a wound directly, but it is possible for a wound to become infected by haematogenous spread with organisms coming from infected invasive monitoring and intravenous lines. It is often possible to deduce the source of infection from the type of bacteria isolated. The precise definition of a wound infection is surprisingly difficult as wounds may become inflamed without infection. For most purposes, including the measurement of wound infection rates, a wound is declared infected if, after an operation, it discharges pus or bacteria are isolated from an exudate. The operating team the members of the surgical team should wash their hands and scrub their nails using antiseptic solutions according to local protocol, and wear sterilized clothing and rubber gloves. Facemasks are widely used but there is no evidence that they reduce the risk of infection. Antibiotic prophylaxis against wound infection 45 Risk factors that influence the incidence of wound infection after surgery Contamination the risks of wound infection after an operation depend upon the field through which the operation is being performed. The basic principles guiding the use of prophylactic antibiotics are set out in Table 3. The most effective way to give a prophylactic antibiotic is intravenously at the induction of the anaesthetic. The illogical and indiscriminate use of antibiotics is a major factor in the induction of resistant strains of bacteria. The choice of antibiotic depends on the surgery and the likely infecting organisms. For example, with bowel surgery it is vital to include an agent such as metronidazole, which is active against anaerobic organisms. There is good evidence for the use of antibiotic prophylaxis against wound infection. It is however important to appreciate that this is prophylaxis not treatment and it must be confined to the operative period. Nevertheless, the consequences may be significant if implanted synthetic materials become infected and have to be removed. A clean wound may become contaminated by an intra-operative procedure that opens a contaminated area. The gall bladder is part of the gut and in 40 per cent of the gall bladders that contain gall stones the bile contains bacteria. If the operative field contains pus or bowel content at the time of surgery the risk of postoperative infection is at least 50 per cent. Diabetes Immunosuppression caused by disease or chemotherapy Heavy blood loss during the operation Foreign bodies in the wound Postoperative haematoma Table 3. Whenever possible collect any pus or exudate and take a wound swab for bacterial culture and sensitivity. Only if the patient is systemically ill, or the consequences of infection serious, should antibiotics be started blindly. Otherwise await identification of the organism and then give an antibiotic to which it is sensitive. An exception to this is rapidly spreading erythema and tenderness around a wound, signs which indicate a streptococcal infection. In this event there will be no exudate or pus formation and usually a negative wound swab, so treatment with penicillin in adequate doses should be started at once. The skin is more resistant than subcutaneous fat but if it slowly undergoes necrosis, a dark area appears through which the abscess will eventually burst. Natural history of an abscess An abscess may: burst, either externally, or internally into a hollow viscus or a serous cavity become chronic resolve spontaneously. With major wound infection a drastic but effective measure is to return the patient to the operating theatre and lay the wound open down to the deep fascia. The wound may subsequently be allowed to granulate or may be re-sutured once it is clean.
Various instruments and telescopes have been available for many years 97140 treatment code diltiazem 60mg visa, but a breakthrough came in the 1980s with the invention of small colour-accurate television cameras treatment endometriosis buy cheapest diltiazem and diltiazem. Previously medications zithromax diltiazem 60mg cheap, because the surgeon had to hold a telescope to one eye symptoms 2 days after ovulation order on line diltiazem, he had only one hand available to manoeuvre the instruments. Once the operative field could be displayed on a television monitor, both hands, and those of the assistants, could be used. Indeed the view and access is usually better than with open surgery as it does not depend on the size of the entry wound. Thoracoscopy this is used in the staging of lung the mouth, puncturing the wall of the stomach the vagina, entering through a fornix the anus, with access though the colon. Sophisticated, largely flexible instruments are required, and special techniques needed for the safe closure of the puncture wound in the wall of the organ of entry. These techniques are experimental and it is impossible to predict how they will develop. The advantages and disadvantages of minimal access surgery are set out in Table 2. The purpose is to alert the anaesthetist and surgeon to any problems likely to arise from concomitant medical illness or treatment. In general, no precautions are necessary if the condition is diet controlled but those on oral hypoglycaemic drugs should discontinue them and be given insulin if required. The complication to be avoided at all costs is hypoglycaemia and its associated unconsciousness, which may be fatal. This is prevented by giving intravenous glucose, and erring on the side of a high rather than a low blood sugar. Coagulopathy Coagulopathy, particularly that associated with obstructive jaundice, should be corrected, usually with vitamin K. Chronic obstructive airways disease Such patients may do better if given elective postoperative ventilation. Myocardial infarction If an operation is performed shortly after a patient has had a myocardial infarct there is a significant chance of a further infarct, which is quite likely to be fatal. This risk is greatest shortly after the original infarct, but persists for at least 6 months. Elective surgery is thus contraindicated within 6 months of a proven myocardial infarction. Haemoglobinopathies Sickle cell anaemia and other haemoglobinopathies provide major problems for the anaesthetist. There are six levels, with the risk of death increasing as the grade increases (Table 2. Obesity Obesity increases the incidence of almost every postoperative complication. The current consensus view is: stasis in the veins damage to the venous endothelium, produced by immobility hypercoagulability of the blood, a normal response to any form of injury such as an operation or acute illness. It should be stopped before elective surgery but, if this is not possible, prophylaxis should be stepped up one level. Hormone replacement therapy does not increase normal oestrogen levels; it replaces something that is lacking. There should therefore be no increased risk and it need not be stopped before surgery. The rest develop in the following 5 days while the hypercoagulable state persists. For example, almost half of patients undergoing a major open abdominal procedure, such as a hemicolectomy, pulmonary embolism, which may be fatal the post-thrombotic syndrome, which takes many years to appear but is a cause of chronic morbidity. The risk is much higher in emergency patients and in those who have been seriously ill and/or immobile in bed before operation. Mechanical methods aim to overcome stasis and to a certain extent prevent changes in the venous endothelium. They should be donned before operation and ideally worn for a week or so afterwards. The method is effective but cumbersome and cannot be used when the operation is on the leg. It is impossible to overemphasize the importance of a surgical unit having a rigorous policy based on simple protocols to prevent thromboembolic disease (Table 2. Prophylactic measures are much more likely to reach every patient at risk if they are applied to all, i. When the edges of a clean incised wound are approximated, there are histological signs of healing within an hour. However, the scar does not achieve its final full strength for 6 months, longer in specialized tissues such as bone. If they lie close together without tension or compression, this layer helps stick the edges of the wound together. Pharmacological methods aim to inhibit thrombosis at the beginning of the clotting cascade (Factors X to Xa) not the coagulation at its end (prothrombin to thrombin), which would lead to increased bleeding during and after surgery. They must be given by daily injection and it is important to give the first dose an hour or so before surgery commences. Macrophages and fibroblasts pass into the layer of clotted blood between the wound edges and begin to lay down collagen. This can be seen in every patient who has an external stoma or when bowel is used to substitute for ureter or bladder. A hypertrophic scar is thickened, but the scar tissue remains between the edges of the original wound and tends to regress. Their management involves excision followed by local steroid injections to try to suppress the overgrowth and local invasion.
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