Hypovolaemia and Fluid Resuscitation

Introduction

Hypovolaemia is a reduction in effective circulating blood volume sufficient to compromise tissue perfusion if uncorrected. Recognising it, and prescribing intravenous fluid safely in response, is one of the most common tasks in acute medicine - and one of the most frequently done poorly, since fluid is easy to prescribe but, given in the wrong volume or composition, causes real harm.

It is worth distinguishing hypovolaemia from dehydration, terms that are often used interchangeably but describe different things: dehydration is a deficit of total body water (and tends to raise plasma sodium), while hypovolaemia is a deficit of circulating volume, which may occur with normal, low or high total body water depending on the cause. A patient can be hypovolaemic and oedematous at the same time - heart failure or hypoalbuminaemia can leave the intravascular space depleted while fluid accumulates elsewhere, which is exactly why fluid status is assessed clinically rather than inferred from the presence or absence of visible swelling.

NICE CG174 structures every fluid prescription around five questions, known as the 5 Rs: does this patient need Resuscitation, Routine maintenance, Replacement of ongoing losses, correction of fluid Redistribution, and how will the response be Reassessed. Working through these in order prevents the common error of prescribing a generic maintenance regimen to a shocked patient, or an aggressive resuscitation regimen to someone who simply cannot swallow their tablets.¹

Causes of hypovolaemia

• Haemorrhage - traumatic, gastrointestinal, obstetric, or surgical
• Gastrointestinal losses - vomiting, diarrhoea, high-output stoma or fistula, nasogastric drainage
• Renal losses - diuretics, osmotic diuresis (hyperglycaemia in DKA/HHS), diabetes insipidus, salt-wasting nephropathy
• Third-spacing - fluid shifts out of the intravascular space into the interstitium or a body cavity, as in pancreatitis, bowel obstruction, sepsis (capillary leak) and burns; this fluid is lost from the circulation but not from the body, and is not corrected simply by conventional fluid replacement alone
• Skin losses - extensive burns, excessive sweating, fever
• Inadequate intake - reduced oral intake from illness, reduced consciousness, or difficulty swallowing, particularly in frail or elderly patients

Clinical assessment

Bedside assessment remains the foundation, since it is immediate and does not depend on waiting for laboratory results.

• Heart rate and blood pressure - tachycardia often precedes hypotension; a postural drop in systolic blood pressure of 20 mmHg or more (or a rise in heart rate of 20 beats/min) on standing is a sensitive early sign in a patient who can stand
• Capillary refill time - centrally, over the sternum, should be under 2 seconds
• Skin turgor and mucous membranes - reduced turgor and dry mucous membranes suggest volume depletion, though this is less reliable in older patients with reduced skin elasticity
• JVP - low or not visible in hypovolaemia, though this sign is affected by the patient's positioning and is operator-dependent
• Urine output - falling output (below 0.5 mL/kg/hour) is a sensitive marker of inadequate renal perfusion
• Weight change, where a recent baseline is known, can quantify fluid loss directly

Resuscitation

For a patient who is haemodynamically unstable - hypotensive, markedly tachycardic, or otherwise showing signs of inadequate perfusion - give a fluid bolus of 500 mL of balanced crystalloid over up to 15 minutes, then reassess before deciding whether to repeat it.¹ Balanced crystalloids (for example Hartmann's solution or Plasma-Lyte) are generally preferred over 0.9% saline, since large volumes of saline can cause a hyperchloraemic metabolic acidosis.

• Use smaller boluses (250 mL) with closer reassessment in patients with, or at risk of, heart failure, renal impairment, or who are frail or elderly, since they tolerate rapid volume shifts poorly and are more prone to pulmonary oedema
• Reassess after every bolus - heart rate, blood pressure, capillary refill, respiratory rate, chest auscultation for new crackles, and JVP
• Consider up to 2 litres of crystalloid in boluses while continuing to reassess; if the patient remains shocked beyond this, seek senior/critical care input and reconsider the diagnosis and whether blood products, vasopressors or a different underlying cause need addressing
• Suspected haemorrhage is managed with early blood products rather than escalating volumes of crystalloid - see the major haemorrhage pathway

Routine maintenance

For a patient who is not shocked but cannot meet their fluid and electrolyte needs orally or enterally, prescribe maintenance fluid to match normal daily requirements:

A common practical regimen is around 2-2.5 litres of fluid a day in an average adult, adjusted down for older, frail or smaller patients and up for those with ongoing abnormal losses - the figures above are a starting point, not a fixed prescription, and should be reassessed daily against clinical status and U&Es.

Replacement and redistribution

Replacement fluid is prescribed on top of maintenance to match ongoing abnormal losses - nasogastric aspirate, stoma output, diarrhoea, drain losses - matching volume and, where possible, the electrolyte composition of what is being lost (for example higher potassium replacement for large gastrointestinal losses).

Redistribution refers to fluid shifted within the body rather than lost from it - third-spacing in sepsis, pancreatitis or bowel obstruction, where intravascular volume is depleted despite the patient sometimes appearing oedematous overall. This is why a patient can be simultaneously fluid-overloaded in the interstitium and intravascularly volume-depleted, and why clinical assessment of perfusion (not just weight or visible oedema) determines whether further fluid is needed.

Reassessment

Every fluid prescription needs a plan for reassessment, not just an initial decision. Reassess clinical status, fluid balance charts, daily weight where practical, and U&Es at least daily (more often during active resuscitation), and adjust the prescription according to trend rather than continuing a fixed regimen on autopilot.

Choice of fluid

Special situations

• Diabetic ketoacidosis - fluid resuscitation follows its own specific protocol, since rapid correction alongside insulin therapy risks cerebral oedema (particularly in children and young adults) and hypokalaemia as potassium shifts intracellularly with insulin treatment
• Sepsis - an initial 500 mL crystalloid bolus (or per local sepsis pathway) is given promptly alongside antibiotics and source control, but ongoing large-volume resuscitation is titrated carefully, since capillary leak in sepsis means excess fluid readily moves into the interstitium and worsens outcomes if given indiscriminately
• Chronic kidney disease and heart failure - smaller boluses, closer monitoring, and earlier involvement of renal or cardiology input where the fluid balance is finely poised
• Children - maintenance and resuscitation volumes are calculated on a weight-band basis rather than the flat adult figures used here, and the margin for error is narrower given smaller absolute circulating volumes

Complications of fluid therapy

• Pulmonary oedema from over-resuscitation, particularly in heart failure, renal impairment or the elderly
• Hyperchloraemic metabolic acidosis from large-volume 0.9% saline
• Electrolyte disturbance - hyponatraemia from excessive hypotonic fluid, hyperkalaemia from excessive potassium replacement in renal impairment
• Peripheral and interstitial oedema, impairing wound healing and gas exchange
• Abdominal compartment syndrome in extreme over-resuscitation, particularly recognised in severe sepsis and major burns

Red flags

Prognosis

Outcome from hypovolaemia depends on how quickly the underlying cause is identified and treated, and on the fluid prescription being matched to the patient rather than applied as a default regardless of context. Both under- and over-resuscitation independently worsen outcomes, which is why fluid is best thought of as a prescribed drug - with an indication, a dose, and a plan for reassessing its effect - rather than a routine, risk-free addition to every admission.