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A meaningful analysis of fluid, electrolyte, and acid–base abnormalities must be based on the clinical characteristics of each patient. Although a rigorous appraisal of laboratory parameters often yields the correct differential diagnosis, it is the history and physical examination that provides an understanding of the extracellular fluid volume (ECFV) and pathophysiology, and as well as development of the appropriate treatment strategy. Thus, the evaluation always begins with an overall assessment of the patient's status.

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History

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The history should be directed toward clinical questions associated with fluid and electrolyte abnormalities. Xenobiotic exposure commonly results in fluid losses through the respiratory system (hyperpnea and tachypnea), gastrointestinal tract (vomiting and diarrhea), skin (diaphoresis), and kidneys (polyuria). Patients with ECFV depletion may complain of dizziness, thirst, and weakness. Usually the patients can identify the source of fluid loss.

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A history of exposures to nonprescription and prescription medications, alternative or complementary therapies, and other xenobiotics may suggest the most likely electrolyte or acid–base abnormality. In addition, premorbid conditions and the ambient temperature and humidity should always be considered.

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Physical Examination

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The vital signs are invariably affected by significant alterations in ECFV. Whereas hypotension and tachycardia may characterize life-threatening ECFV depletion, an initial finding may be an increase of the heart rate and a narrowing of the pulse pressure. Abnormalities may be recognized through an ongoing dynamic evaluation, realizing that the measurement of a single set of supine vital signs offers useful information only when markedly abnormal. Orthostatic pulse and blood pressure measurements provide a more meaningful determination of functional ECFV status (Chaps. 3 and 23).

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The respiratory rate and pattern can give clues to the patient's metabolic status. Hyperventilation (manifested by tachypnea, hyperpnea, or both) may be caused by a primary respiratory stimulus (respiratory alkalosis) or may be a response to the presence of metabolic acidosis. Although hypoventilation (bradypnea or hypopnea or both) is present in patients with metabolic alkalosis, it is rarely clinically significant except in the presence of chronic lung disease. More commonly, hypoventilation is associated with a primary depression of consciousness and respiration, and respiratory acidosis. Unless the clinical scenario (ie, nature of the overdose or poisoning, presence of renal or pulmonary disease, findings on physical examination or laboratory testing) is diagnostic, arterial blood gas analysis is required to determine the acid–base disorder associated with a change in ventilation.

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The skin should be evaluated for turgor, moisture, and the presence or absence of edema. The moisture of the mucous membranes can also provide valuable information. These are nonspecific parameters and may not correlate directly with the status of hydration. This dissociation is especially true with xenobiotic exposure, as many xenobiotics alter skin and mucous membrane moisture without necessarily altering ECFV status. For example, antimuscarinics and anticholinergics commonly result in dry mucous membranes and skin without producing ECFV depletion. Conversely, patients exposed to sympathomimetics ...

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