Page 7 - Medicine and Surgery
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                                                                       Chapter 1: Fluid and electrolyte balance 3


                  with renal or cardiac failure. Oliguria (urine output  cardiac failure, and these patients may require in-
                  below 0.5 mL/kg/h) requires urgent assessment and in-  otropic support.
                  tervention. A lowurine output may be due to prere-  Further investigations and management depend on the
                  nal (decreased renal perfusion due to volume depletion  underlying cause. Baseline and serial U&Es to look for
                  or poor cardiac function), renal (acute tubular necrosis  renal impairment (see page 230) should be performed.
                  or other causes of renal failure) or postrenal (urinary or  Where there is suspected bleeding, the initial FBC may
                  catheter obstruction) failure.                be normal, but this will fall after fluid replacement is
                    In fluid depletion, the management is fluid resusci-  givendue to the dilutional effect of fluids. Chest X-
                  tation. In previously fit patients, particularly if there is  raymay show cardiomegaly and pulmonary oedema.
                  hypotension, more than 1 L/h of colloids or crystalloids  Arterial blood gases can be helpful in identifying any
                  (usually saline) may be needed and several litres may be  acid–base disturbance due to renal failure or degree of
                  required to correct losses. However, the management is  hypoxia due to underlying lung disease or pulmonary
                  very different in fluid overload or in oliguria due to other  oedema.
                  causes.Inmostcases,clinicalassessmentisabletodistin-
                  guish between these causes. In cases of doubt (and where  Hypernatraemia
                  appropriate following exclusion of urinary obstruction)
                  afluidchallengeof∼500mLofnormalsalineoracolloid  Definition
                  (see page 9) over 10–20 minutes may be given. However,  Aserum sodium concentration >145 mmol/L.
                  care is required in patients at risk of cardiac failure (e.g.
                                                                Incidence
                  previous history of cardiac disease, elderly or with renal
                                                                This occurs much less commonly than hyponatraemia.
                  failure), when smaller initial volumes and more invasive
                  monitoring (such as a central line to allow central ve-
                                                                Age
                  nous pressure monitoring) and frequent assessment is
                                                                Any. Infants and elderly at greatest risk.
                  needed. Patients should be reassessed regularly (initially
                  usually within 1–2 hours) as to the effect of treatment on  Sex
                  fluid status, urine output and particularly for evidence  M = F
                  of cardiac failure:
                    If urine output has improved and there is no evidence
                                                                Aetiology
                    of cardiac failure, further fluid replacement should be  This is usually due to water loss in excess of sodium loss,
                    prescribed as necessary.                    often in combination with reduced fluid intake. Those
                    If the urine output does not improve and the patient
                                                                at most risk of reduced intake include the elderly, infants
                    continues to appear fluid depleted, more fluid should  and confused or unconscious patients.
                    be given. However, in patients who are difficult to as-     Causes of water loss include burns, sweat, hyperven-
                    sess, clinically more invasive monitoring such as cen-  tilation, vomiting and diarrhoea, diabetes insipidus
                    tral venous pressure (CVP) monitoring may be re-  (see page 445) and hyperosmolar non-ketotic coma
                    quired. This is performed via a central line, usually  (see page 461).
                    placed in the internal jugular vein. A normal CVP is     Hypernatraemia may be iatrogenic due to osmotic di-
                    5–10 cm of water above the right atrium. The CVP  uretics which cause more water than sodium loss or
                    is either monitored continuously or hourly and fluids  excessive administration of sodium, usually in intra-
                    are titrated according to the results. However, CVP  venous fluids.
                    measurements should only form part of the clinical     Ararer cause of hypernatraemia is Conn’s syndrome
                    assessment and in practice they can be unreliable.  (see page 442) or ectopic ACTH syndrome.
                    If there is any evidence of cardiac failure, fluid ad-

                    ministration should be restricted and diuretics may  Pathophysiology
                    be required.                                The normal physiological response to a rise in extracel-
                    If hypotension persists despite adequate fluid replace-  lular fluid osmolality is for water to move out of cells. Pa-

                    ment, this indicates poor perfusion due to sepsis or  tients become thirsty and there is increased vasopressin
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