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Chapter: Medicine and surgery: Principles and practice of medicine and surgery

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Assessing fluid balance - Fluid and electrolyte balance

This is an important part of the clinical evaluation of patients with a variety of illnesses, which may affect the circulating volume or sodium and water balance.

Assessing fluid balance

This is an important part of the clinical evaluation of patients with a variety of illnesses, which may affect the  circulating volume or sodium and water balance. Examples include patients with any history of cardiac, liver or renal failure, those with symptoms such as vomiting and diarrhoea, perioperative patients or any patient who has other losses, e.g. from bleeding or drains. Clinical evaluation of fluid balance requires the observation of several signs that together point to whether the patient is euvolaemic (normal fluid balance), fluid depleted (reduced extracellular fluid) or fluid overloaded (increased extra-cellular fluid). In most cases when the patient is fluid depleted, there is decreased circulating volume; however in fluid overload, there may either be increased circulating volume or decreased circulating volume depending on the mechanism.


·        Fluid depletion may be suggested by an appropriate history of losses or reduced intake, but this can be un-reliable. Symptoms of thirst and any postural dizziness should be enquired about. Signs of volume depletion include a mild tachycardia, reduced peripheral perfusion (cool dry hands and feet, increased capillary refill time >3 seconds), postural hypotension and/or hypotension, and reduced skin turgor (check over the anterior chest wall as the limbs are unreliable, particularly in the elderly). The jugular venous pressure is low and urine output reduced.


·        Fluid overload is more likely to occur in patients with cardiac, liver or renal failure, particularly if there has been over-enthusiastic fluid replacement. Breathlessness is an early symptom. Tachypnoea is common and there may be crackles heard bilaterally at the bases of the chest because of pulmonary oedema. The jugular venous pressure is raised and sacral and/or ankle oedema may be present (bedbound patients often have little ankle oedema, but have sacral oedema). The blood pressure is usually normal (occasionally high), but blood pressure and heart rate are often unreliable because of underlying cardiac disease: in heart failure the blood pressure often falls with worsening fluid overload. Pleural effusions and ascites suggest fluid overload, but in some cases there may be increased interstitial or third space fluid with reduced intravascular fluid so that the patient has decreased circulating volume with signs of intravascular hypovolaemia.


Urine output monitoring and 24-hour fluid balance charts are essential in unwell patients. Daily weights are useful in patients with fluid overload particularly those with renal or cardiac failure. Oliguria (urine output below 0.5 mL/kg/h) requires urgent assessment and intervention. A low urine output may be due to prerenal (decreased renal perfusion due to volume depletion or poor cardiac function), renal (acute tubular necrosis or other causes of renal failure) or postrenal (urinary or catheter obstruction) failure.


In fluid depletion, the management is fluid resuscitation. In previously fit patients, particularly if there is hypotension, more than 1 L/h of colloids or crystalloids (usually saline) may be needed and several litres may be required to correct losses. However, the management is very different in fluid overload or in oliguria due to other causes. In most cases, clinical assessment is able to distinguish between these causes. In cases of doubt (and where appropriate following exclusion of urinary obstruction) a fluid challenge of 500 mL of normal saline or a colloid  over 10–20 minutes may be given. However, care is required in patients at risk of cardiac failure (e.g. previous history of cardiac disease, elderly or with renal failure), when smaller initial volumes and more invasive monitoring (such as a central line to allow central venous pressure monitoring) and frequent assessment is needed. Patients should be reassessed regularly (initially usually within 1–2 hours) as to the effect of treatment on fluid status, urine output and particularly for evidence of cardiac failure:


·        If urine output has improved and there is no evidence of cardiac failure, further fluid replacement should be prescribed as necessary.


·        If the urine output does not improve and the patient continues to appear fluid depleted, more fluid should be given. However, in patients who are difficult to assess, clinically more invasive monitoring such as central venous pressure (CVP) monitoring may be required. This is performed via a central line, usually placed in the internal jugular vein. A normal CVP is 5–10 cm of water above the right atrium. The CVP is either monitored continuously or hourly and fluids are titrated according to the results. However, CVP measurements should only form part of the clinical assessment and in practice they can be unreliable.


·        If there is any evidence of cardiac failure, fluid administration should be restricted and diuretics may be required.


·        If hypotension persists despite adequate fluid replacement, this indicates poor perfusion due to sepsis or cardiac failure, and these patients may require in-otropic support.


Further investigations and management depend on the underlying cause. Baseline and serial U&Es to look for renal impairment  should be performed. Where there is suspected bleeding, the initial FBC may be normal, but this will fall after fluid replacement is given due to the dilutional effect of fluids. Chest X-ray may show cardiomegaly and pulmonary oedema. Arterial blood gases can be helpful in identifying any acid–base disturbance due to renal failure or degree of hypoxia due to underlying lung disease or pulmonary oedema.


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