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This has diagnostic significance in the differential diagnosis between adenomas and meningiomas allergy eye swelling buy prednisone cheap. Direct cerebral angiography is adopted if a giant pituitary adenoma and mass lesions with intratumoural haemorrhages are present allergy pills for dogs purchase generic prednisone. The arch-shaped displacement of the anterior cerebral artery, rectification, and lateral displacement of the internal carotid artery siphon are the typical features of these tumours. The internal carotid artery siphons are included in the tumour structure, without their narrowing. Coronal 1-weighted imaging well defines intra-infralaterosuprasellar growth of the tumour. Direct angiography findings confirm the absence of compression of the internal carotid artery at the level of the cavernous sinus (,f) Sellar and Parasellar Tumours 555. Direct angiogram (c) visualises a typical bow- shaped upward displacement of the A1 segment of the anterior cerebral artery, the vascular net of the tumour is absent, and there is no compression of the internal carotid artery siphon. On direct angiograms in coronal (c,d) and lateral projections (,f), a typical bow-shaped upward displacement of the A1 segment of the anterior cerebral artery and rectification of the internal carotid artery siphon are seen. Tumour invasion into the depth of clivus is also well visualised on T1-weighted images if the disappearance of hyperintensive signal of bone marrow is found at this site. Large pituitary adenomas are often complicated by cystic degeneration and haemorrhages. Cystic degeneration of adenomas is demonstrated as regions of hypointensive signal on T1-weighted images and markedly hyperintensive signal on T2-weighted images. This is due to the high protein content in the cystic fluid, which shortens T1 relaxation time. Sometimes the sedimentation phenomenon may be observed in the cavity of a cyst, which is a specific sign of cystic degeneration. One of the serious problems in the diagnosis of pituitary adenomas is the identification of whether the diaphragm of the sella turcica was ruptured by a tumour. Presurgical diagnosis of these exophytic tumours is of utmost importance to plan tactics of surgical approach-transnasally or transcranially. However, it has not been yet explained how the remnants of diaphragm, dura mater, different tumour regions, and the adjacent dislocated structures may be correctly visualised or enhanced. Thus, subacute haemorrhage shows hyperintensive signal on 1-weighted images and on T2-weighted images. In some of these patients, clinical manifestations of the pituitary apoplexy may be the first sign of the disease (Ostrov et al. However, in contrast to adults the percentage of malignant tumours in children is high. Malignant tumours are characterised by rapid growth, frequent haemorrhages, and extended invasion of the surrounding structures. Gadolinium-chelate injection is feasible in large pituitary tumours when it is necessary to differentiate the tumour and the adjacent brain structures. In conclusion, it should be mentioned that the diagnosis of pituitary tumours has to resolve the following issues: to determine the location of a tumour in relation to the sella turcica, to visualise different directions of growth, to estimate the size of a tumour. Treatment tactics and optimal choice of surgical accesses or their combinations depend on the resolution of questions mentioned above. Coronal () and sagittal (b) 1-weighted image shows the dumbbells-like shape of the tumour. There are a few exophytic tumours of adenoma through the rupture of diaphragm of the sella turcica upwards. Sagittal (a) and coronal T1-weighted imaging (b): a tumour fills the cavity of enlarged sella turcica and the left half of the sphenoidal sinus, and has a suprasellar growth with invasion of the left cavernous sinus. Along with the isointensive signal of the tumour a hyperintensive focus of haemorrhage is seen in the left half of it. The signal of the most of tumour is isointensive to brain tissue; cystic components are hypointensive.

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Interventions for alcohol abuse allergy treatment in gurgaon prednisone 5 mg line, as is true for injectable drug abuse allergy symptoms 5 months buy prednisone in united states online, must be carefully orchestrated. Guidance from the local medical society or licensing authority is highly recommended. Ionizing Radiation Exposure the use of imaging equipment (eg, fluoroscopy) during surgery and interventional radiologic procedures exposes the anesthesiologist to the potential 12 risks of ionizing radiation. The three most important methods of minimizing radiation doses are limiting total exposure time during procedures, using proper barriers, and maximizing the distance from the source of radiation. Anesthesiologists who routinely perform fluoroscopic image guided invasive procedures should consider wearing protective eyeware incorporating radiation shielding. Lead glass partitions or lead aprons with thyroid shields are mandatory protection for all personnel who are exposed to ionizing radiation. The inverse square law states that the dosage of radiation varies inversely with the square of the distance. The health impact on operating room personnel of exposure to electromagnetic radiation remains unclear. The patient had a myocardial infarction 1 month earlier that was complicated by congestive heart failure. His blood pressure is 160/90 mm Hg, pulse 110 beats/min, respiratory rate 22 breaths/min, and temperature 38. Strangulation of the bowel begins with venous obstruction, but can quickly progress to arterial occlusion, ischemia, infarction, and perforation. Acute peritonitis could lead to severe dehydration, sepsis, shock, and multiorgan failure. Because of the history of recent myocardial infarction and congestive heart failure, an arterial line would be useful. Transesophageal echocardiography and pulse contour analysis monitors of cardiac output could be used. Pulmonary arterial flotation catheters have often been used in the past, but they are associated with significant complications and current evidence does not indicate that their use improves patient outcomes. Furthermore, information regarding myocardial supply (diastolic blood pressure) and demand (systolic blood pressure, left ventricular wall stress, and heart rate) should be continuously available. Central venous pressure may not track left atrial pressure in a patient with significant left ventricular dysfunction. What cardiovascular medications might be useful during induction and maintenance of general anesthesia Drugs causing severe tachycardia or extremes in arterial blood pressure should be avoided. During the laparotomy, gradual increases in heart rate and blood pressure are noted. The concentration of volatile anesthetic is increased, and metoprolol is administered intravenously in 1-mg increments. This results in a decline in heart rate to 115 beats/min, with no change in blood pressure. Suddenly, the rhythm converts to ventricular tachycardia, with a profound drop in blood pressure. As amiodarone is being administered and the defibrillation unit prepared, the rhythm degenerates into ventricular fibrillation. A differential diagnosis of pronounced tachycardia and hypertension might include pheochromocytoma, malignant hyperthermia, or thyroid storm. In this case, further inspection of the nitroglycerin infusion reveals a labeling error: although the tubing was labeled "nitroglycerin," the infusion bag was labeled "epinephrine. The net result is a decrease in heart rate, but a sustained increase in blood pressure. In addition, if the central venous catheter was malpositioned, with its tip in the right ventricle, the catheter tip could have stimulated ventricular arrhythmias. Incorrect drug labels are but one example of errors that can result in patient injury.

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The diagnosis of carbon monoxide poisoning requires cooximetry: pulse oximeters cannot reliably differentiate between carboxyhemoglobin and oxyhemoglobin allergy symptoms nose bleeds prednisone 20mg sale. The half-life of carboxyhemoglobin is reduced to 1 h with 100% oxygen; some clinicians advocate hyperbaric oxygen therapy if the patient does not respond to 100% oxygen allergy medicine elderly generic prednisone 10 mg visa. The diagnosis of cyanide poisoning is difficult because reliable measurements of cyanide are not readily available (normal levels are <0. The enzyme rhodanase normally converts cyanide to thiocyanate, which is subsequently eliminated by the kidneys. Sodium nitrite converts hemoglobin to methemoglobin, which has a higher affinity for cyanide than cytochrome oxidase; the cyanide, which is slowly released from cyanomethemoglobin, is converted by rhodanase to the less toxic thiocyanate. As with other forms of acute lung injury, nosocomial infectious pneumonias are common. More than one half of these deaths occur shortly after onset, usually due to arrhythmias (ventricular fibrillation). Most myocardial infarctions occur in patients with more than one severely narrowed (>75% narrowing of the cross-sectional area) coronary artery. The occlusion is nearly always due to thrombosis at a stenotic atheromatous plaque. The size and location of the infarct depend on the distribution of the obstructed vessel and whether collateral vessels have formed. Following brief episodes of severe ischemia, persisting myocardial dysfunction with only a slow and incomplete return of contractility can be observed. Relief of the ischemia in these areas can restore contractile function, albeit not immediately. Stunning may be observed following aortic cross-clamping during cardiopulmonary bypass and present as a reduced cardiac output upon attempted separation from bypass (see Chapter 22). When severe hypokinesis or akinesis is observed in the setting of severe chronic ischemia, the myocardium in these noninfarcted but poorly contractile areas may be said to be "hibernating. Based on local resources, timing, and anatomic findings during angiography, angioplasty, stenting, or coronary artery bypass surgery may be preferred. All patients without contraindications (such as acute heart failure) should receive blockers. If angina persists or if there is a contraindication to blockers, calcium channel blockers should be administered. Intraaortic balloon counterpulsation is usually reserved for hemodynamically compromised patients with refractory ischemia. Emergency treatment of arrhythmias constantly evolves and we recommend that the guidelines for Advanced Cardiac Life Support be followed. In general, ventricular tachycardia, if treated medically is best managed with amiodarone (150 mg intravenous bolus over 10 min). Synchronized cardioversion may be used in patients with ventricular tachycardia and with a pulse. Patients with a stable narrow-complex supraventricular tachycardia should be treated with amiodarone. These hormones constrict cutaneous muscle and splanchnic vasculature and promote salt and water retention.

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Recurrent haemorrhage results in increased density of chronic subdural haematomas allergy shots under the tongue cheap 10 mg prednisone visa, and the sedimentation phenomenon is frequently marked allergy forecast in tampa florida order prednisone now. In chronic subdural haematoma connective tissue, trabeculae may be seen; they separate cavities of haematoma. Bilateral subdural haematoma is seen in 25% of cases; when it is large, it may lead to marked compression of lateral ventricles without midline shift. In several cases, subdural hygromas are hardly distinguished from chronic subdural haematomas, and more often, they are marked in the elderly and children; approximately in 20% of cases they are accompanied by other brain damage (Babchin et al. Damage of the internal skull bone lamina may lead to rupture of medial meningeal arteries or their branches; less frequently, it leads to bleeding from meningeal veins or venous sinuses (transverse or sigmoid). Blood penetrates the space between of internal skull bone lamina and dura mater, closely connected with sutures, and that is why epidural haematomas never cross sutures and have typical lens-like appearance compared with subdural haematomas. Epidural haematomas are more frequently seen in young people, they are usually unilateral (95%), and in most cases, they are located in the supratentorial space in the temporoparietal region. Hypodense areas inside haematoma are admixture of free blood and serum separated from a blood clot. Subacute and early chronic epidural haematomas are hyperintense on T1- and T2-weighted imaging. Large epidural haemorrhages with prominent mass effect may displace and compress the brain, leading to transtentorial herniation. Small epidural asymptomatic haematomas are situated along the convex surface, and they are smaller than 1. Similar to contusions, they are mainly localised in frontal and parietal regions; haemorrhage is accompanied by blood clot formation that penetrates into the deep white matter or traumatic rupture of perforans vessels occur. Haemorrhage spreads along axons, haematoma is formed, and blood clot is formed and retracted. Sagittal and coronal projections (e,f) add information about haematoma size and spread Head Trauma 847. Acute posttraumatic primary haematoma should be distinguished from haemorrhage into contusion area. Incidence of traumatic intracerebral haematomas is lower than that of haemorrhagic contusions, and they most frequently occur in adults. It look like a hyperdensive, well-delineated area, and perifocal oedema may be identified within the first 12 h after injury. More than two-week subdural haematomas mainly consist of intracellular ferritin and lysosomal haemosiderin. By the third week, haematomas become isodensive compared with brain parenchyma and are hardly identified. Continuing proteolysis, phagocytosis, and atrophy lead encephalomalacy in the haematoma place. Superacute haemorrhage (within the first hours) contains oxyhaemoglobin, acute haemorrhage (first three days) contains deoxyhaemoglobin, subacute (less than 3 weeks) contains methaemoglobin, and chronic haemorrhage (over 2 months) has a haemosiderin ring. Hy- podensive area on the haematoma periphery represents perifocal oedema formation divided into early and the late stages. Early subacute haemorrhage (intracellular methaemoglobin) develops on the 3d day and lasts for a week. Late (extracellular methaemoglobin) haemorrhage occurs at first week after injury. In the rest of the patients, causes of fatal outcome were different extracranial complications like septic pneumonia, myocardial infarction, pulmonary embolism etc. Interpeduncular cistern may be poorly visualised due to small quantities of isodensive blood. Acute obstructive hydrocephalus may develop within the first week due to ependymitis or intraventricular blood occluding the Sylvian aqueduct or the fourth ventricle. Communicating hydrocephalus may develop within the first hours at first week if arachnoid granulations are filled with blood, thus leading to fibroplastic proliferation in subarachnoid spaces and their blockage. Intracerebral haematomas may cause white matter tracts damage, with blood rupture into the ventricular system. Hypointense signal at haematoma periphery means accumulation of haemosiderin (the beginning of chronic phase). If these two injuries are isolated and hydrocephalus is absent, then they are clinically insignificant.

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Some cases of "catheter migration" may represent delayed recognition of an improperly positioned catheter allergy forecast waco tx prednisone 20mg otc. If a clinician uses an initial test dose allergy symptoms to peanuts cheap prednisone 20 mg with visa, is diligent about aspirating prior to each injection, and always uses incremental dosing, major systemic toxic side effects and/or total spinal anesthesia from accidental intrathecal injections will be rare. Additives to the local anesthetic, particularly opioids, tend to have a greater effect on the quality of epidural anesthesia than on the duration of the block. Epinephrine in concentrations of 5 mcg/mL prolongs the effect of epidural lidocaine, mepivacaine, and chloroprocaine more than that of bupivacaine, levobupivacaine, etidocaine, or ropivacaine. In addition to prolonging the duration and improving the quality of block, epinephrine delays vascular absorption and reduces peak systemic blood levels of all epidurally administered local anesthetics. Epidural Anesthetic Agents the epidural agent is chosen based on the desired clinical effect, whether it is to be used as a primary anesthetic, supplementation of general anesthesia, or analgesia. Commonly used short- to intermediate-acting agents for surgical anesthesia include chloroprocaine, lidocaine, and mepivacaine. Only preservative-free local anesthetic solutions or those specifically labeled for epidural or caudal use are employed. Once some regression in sensory level has occurred, one-third to one-half of the initial activation dose can generally safely be reinjected in incremental doses. It should be noted that chloroprocaine, an ester with rapid onset, short duration, and extremely low toxicity, may interfere with the analgesic effects of epidural opioids. Current preparations Factors Affecting Level of Block Factors affecting the level of epidural anesthesia may not be as predictable as with spinal anesthesia. This is probably a result of age-related decreases in the size or compliance of the epidural space. Although there is little correlation between body weight and epidural dosage requirements, patient height affects the extent of cephalad spread. Thus, shorter patients may require only 1 mL of local anesthetic per segment to be blocked, whereas taller patients generally require 2 mL per segment. Although less dramatic than with spinal anesthesia, spread of epidural local anesthetics tends to be partially affected by gravity. The lateral decubitus, Trendelenburg, and reverse Trendelenburg positions can be used to help achieve blockade in the desired dermatomes. Some experts believe that local anesthetics, when injected in very large doses intrathecally may have been at least partly responsible for neurotoxicity. Compared with bupivacaine, ropivacaine may produce less motor block at similar concentrations while maintaining a good sensory block. Addition of sodium bicarbonate (1 mEq/10 mL of local anesthetic) immediately before injection may therefore accelerate the onset of the neural blockade. Sodium bicarbonate is typically not added to bupivacaine, which precipitates above a pH of 6. Also, the more variable anatomy of the epidural space and less predictable spread of local anesthetic make epidural anesthesia inherently less predictable than spinal anesthesia. In some patients, the spinal ligaments are soft, and either good resistance is never appreciated or a false loss of resistance is encountered. Similarly, entry into the paraspinous muscles during an off-center midline approach may cause a false loss of resistance. Other causes of failed epidural anesthesia (such as intrathecal, subdural, and intravenous injection) are discussed in the section of this chapter on complications. Local Anesthetic pH Adjustment Local anesthetic solutions have an acidic pH for chemical stability and bacteriostasis. Local anesthetic solutions that are formulated with epinephrine by the manufacturer are more acidic than the "plain" solutions that do not contain epinephrine. Because they are weak bases, they exist primarily in the ionic form in commercial preparations.

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There is a loss of oligodendroglia and many macrophages with lipid inclusions appear allergy symptoms yeast discount prednisone 40mg without a prescription. Vessels within and outside of necrotic foci do proliferate; hyalinosis is seen in their walls allergy shots for yeast order genuine prednisone on line. Neuroimaging in the acute stage includes diffuse oedema of corpus callosum with hyperintensity on 2-weighted imaging. Confluent hyperintense foci are seen on T2-weighted imaging in subcortical and periventricular white matter. According to several authors, extracallosal lesions may disappear with time and thus they are likely to correspond to oedema and not demyelination. Subacute combined degeneration of the spinal cord develops with demyelination and vacuolisation of posterior and lateral columns. The most frequent causes of cobalamine deficiency are pernicious anaemia and autoimmune gastritis in which the internal Castle factor is absent. In addition, vitamin 12 deficiency develops after gastric surgery and in malabsorption syndrome. The symptoms of funicular myelosis are general fatigue and paraesthesia in the arms and feet. As the disease progresses, gait disturbances appear, and palsies in lower extremities with further development of spastic and ataxic paraplegia with contractures occur. The most important factor determining the treatment efficacy is a time of onset of the disease. Posterior and lateral columns have greyish pallor, and are thinned and semi-transparent. Early pathological signs are swelling of myelin sheaths, with relative preservation of axons. Foamy macrophages and perivascular lymphocytic infiltration are found in the foci of necrosis. In the severe cases of disease, the spinal cord is vacuolated, demyelinating lesions affect posterior and lateral columns throughout its length, and sometimes anterior columns are affected. Posterior column signal changes are distributed throughout the length of spinal cord, which differentiates it from demyelination of other origin, whose foci are disseminated (for instance, in multiple sclerosis). However, there are cases when patients undergo only radiation (tumours, vascular malformations) or only chemotherapy; sometimes postchemotherapy encephalopathy develops in patients underwent chemotherapy for an extraneural tumour. Consequences of isolated brain exposure to radiation, such as radiation necrosis, postradiation leukoencephalopathy, and myelopathy are discussed in the chapters on neuro-oncology. Here we discuss only postchemotherapy leukoencephalopathy and combined involvement of brain after radiation and chemotherapy. The drugs that cause leukoencephalopathy are methotrexate, cisplatin, arabinosylcytosine, carmustin, and thiotepa. On 2-weighted imaging (a) there is hyperintense signal in dorsal parts of the spinal cord throughout all the study. A typical triangle-shaped, symmetrical involvement of the posterior funiculi of the spinal cord is seen (b,c arrows) Toxic and Metabolic Disorders 1027. In contrast to focal white matter involvement seen after postradiation leukoencephalopathy, areas with 1 and 2 prolongation, occurred in a delayed period after chemotherapy, are usually symmetrical, diffuse, and occupy large territories. T2 prolongation occurs first in the central and periventricular white matter; subcortical U-shaped fibres are relatively spared (Stemmer et al. Signal changes in the white matter usually reverse after cancellation of treatment, but may persist for life with adjoining of secondary nonspecific changes (gliosis and moderate ventricular dilatation). It is important to understand that combined exposure to radiation and chemotherapy causes damage more severe to brain tissue than that of these treatments separately (Scott et al. It may be difficult to differentiate a focus of radiation necrosis from relapse of a tumour. Leukoencephalopathy after combined treatment develops usually later than a tumour relapse, is usually multifocal, and often lesions are located very distant from each other, even contralaterally.

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Inherited myelinopathies and secondary demyelinating disorders of toxic and metabolic origin are discussed in Chap allergy symptoms of gluten prednisone 5mg with visa. However allergy medicine brand names buy 10mg prednisone fast delivery, they are not related to demyelinating disorders at present as they have another pathogenesis. They allow judging the stage of demyelination, and can be used as diagnostic tools for the follow-up of treatment efficacy. The history of research in this disorder started in 1835, when the great French pathologist J. Charcot in 1866, who characterised the disease as a combination of spastic paraplegia, intention tremor, speech impairment, visual loss, and nystagmus. Exogenous factors are probably responsible for the development of the disease, and these factors act because of genetic predisposition. Aetiological hypotheses consider infection, autoimmune reactions to normal myelin, a combination of infectious and autoimmune processes, and toxic and metabolic causes. A lot of evidence suggests viral aetiology in genetically predisposed individuals (Gusev et al. Sensory loss is often seen at onset, and pain is not typical and may occur as a rare occasion. In the early stages; complete recovery may occur after exacerbation; however, the subsequent course of the disease may become progressive. In patients with chronic progressive course, more prominent involvement of the spinal cord than that of the brain is seen. In the late stages of the classic variant, severe neurological deficit is present with cognitive decline irrespective to the disease duration (tolyan et al. Demyelinating Diseases of the Central Nervous System 1035 In addition, in children a monosymptomatic disease may be seen as a single episode of a neurological deficit, such as optic neuritis, transverse myelitis, or brainstem syndrome. In the chronic stages, plaques may occupy larger territories of the white matter of cerebral hemispheres. The majority of plaques are located in typical sites, connected probably with subependymal veins (Newcombe et al. About 50% of lesions are periventricular and mainly surround the horns of lateral ventricles. The periaqueductal grey matter of brainstem and the bottom of the fourth ventricle are also often affected. Plaques typically undergo acute, subacute stages, and a transformation to gliosis. In the acute stage, plaques are moist and pink; in the subacute stage, they become less vivid and acquire chalky colour, and the number of macrophages within them increases. Cortical plaques are hardly seen largely, except cases when they occupy subcortical white matter. In the spinal cord, lesions are usually oval with longitudinal orientation along their lengths. In acute-stage oedema, fragmentation of myelin and formation of myelin "globules" are seen, well detected in stained preparations. Perivascular lymphocytic infiltrates mainly containing T cells is a typical but not a constant feature. With the progression of lesions, lymphocytes are replaced by macrophages with lipid inclusions. Sudanophilic material is seen inside macrophages as well as in the extracellular space. Despite the fact that axons are relatively intact, their number decreases, and they become situated less compactly than in adjacent intact areas (Adams and Victor 1993). In some plaques, incomplete destruction of myelin is seen and owing to their irregular margins, they are so-called shadowlike plaques (Poser et al. It was shown that remyelination also occurs in acute lesions; however, myelin recovers incompletely.

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Larger chambers allow for the simultaneous treatment of multiple patients and for the presence of medical personnel in the chamber with patients allergy treatment using cold laser for drug withdrawal buy genuine prednisone online. Multiplace chambers use air to pressurize the chamber allergy forecast stockton ca buy prednisone 20 mg without prescription, whereas patients receive 100% oxygen by mask, hood, or tracheal tube. Common indications for hyperbaric oxygen include decompression sickness (the "bends"), certain forms of gas embolism, gas gangrene, carbon monoxide poisoning, and treatment of certain wounds. High concentrations of oxygen can cause pulmonary atelectasis in areas of low V/Q ratios. As nitrogen is "washed out" of the lungs, the lowered gas tension in pulmonary capillary blood results in increased uptake of alveolar gas and absorption atelectasis. Toxicity is dependent both on the partial pressure of oxygen in the inspired gas and the duration of exposure. Alveolar rather than arterial oxygen tension is most important in the development of oxygen toxicity. This gives the molecule the paramagnetic property that allows precise measurements of oxygen concentration. Notably, internal rearrangement of these electrons or their interaction with other atoms (iron) or molecules (xanthine) can produce potentially toxic chemical species. Oxygen toxicity is thought to be due to intracellular generation of highly reactive O2 metabolites (free radicals) such as superoxide and activated hydroxyl ions, singlet O2, and hydrogen peroxide. Two cellular enzymes, superoxide dismutase and catalase, protect against toxicity by sequentially converting superoxide first to hydrogen peroxide and then to water. Additional protection may be provided by antioxidants and free radical scavengers; however, clinical evidence supporting the use of these agents in preventing pulmonary toxicity is lacking. Hazards of Oxygen Therapy Oxygen therapy can result in both respiratory and nonrespiratory toxicity. These patients develop an altered respiratory drive that becomes at least partly dependent on the maintenance of relative hypoxemia. Elevation of arterial oxygen tension to "normal" can therefore cause severe hypoventilation in these patients. Oxygen therapy can be indirectly hazardous for patients being monitored with pulse oximetry while receiving opioids for pain. Pulmonary O2 toxicity in newborn infants is manifested as bronchopulmonary dysplasia. The potential for oxygen enriched gas mixtures to promote fires and explosions is discussed in Chapter 2. In other instances, mechanical ventilation may be used as an adjunct (usually to positive-pressure therapy; see below) in the treatment of hypoxemia. The risk and expected degree of toxicity are directly related to the pressures used as well as the duration of exposure. This may present initially with retrosternal burning, cough, and chest tightness and will result in progressive impairment of pulmonary function with continued exposure. Although negative-pressure ventilation does not require tracheal intubation, it cannot overcome substantial increases in airway resistance or decreases in pulmonary compliance, and it also limits access to the patient. During positive-pressure ventilation, lung inflation is achieved by periodically applying positive pressure to the upper airway through a tightfitting mask (noninvasive mechanical ventilation) or through a tracheal or tracheostomy tube. Increased airway resistance and decreased lung compliance can be overcome by manipulating inspiratory gas flow and pressure. The major disadvantages of positive-pressure ventilation are altered ventilation-to-perfusion relationships, potentially adverse circulatory effects, and risk of pulmonary barotrauma and volutrauma. Positive-pressure ventilation increases physiological dead space because gas flow is preferentially directed to the more compliant, nondependent areas of the lungs, whereas blood flow (influenced by gravity) favors dependent areas. Reductions in cardiac output are primarily due to impaired venous return to the heart from increased intrathoracic pressure.

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