Anaesthesia

Body Fluid Compartments

Understanding fluid therapy starts with the body fluid compartments. Total body water is about 60% of body weight, divided into the intracellular fluid (ICF, ~2/3) and the extracellular fluid (ECF, ~1/3); the ECF is further split into the interstitial fluid and the plasma (intravascular) volume. Where an infused fluid distributes — and therefore how much stays in the circulation — depends on its composition.

Crystalloids vs Colloids

Crystalloids (e.g. normal saline, Ringer’s lactate/Hartmann’s) are salt solutions that distribute throughout the ECF, so only about a quarter to a third stays intravascular — a larger volume is needed to expand the circulation. Colloids (e.g. gelatins, albumin) contain large molecules that stay in the plasma, expanding the intravascular volume more efficiently (though concerns about cost, allergy and, for starches, kidney injury have reduced synthetic colloid use).

Maintenance & Replacement

Perioperative fluids meet several needs: maintenance (normal daily water and electrolytes), replacement of deficits (from fasting, vomiting, bowel prep), and ongoing losses (blood loss, evaporation, third-space sequestration). Therapy is guided by assessment — heart rate, blood pressure, urine output, capnograph, and, in major cases, dynamic/cardiac-output measures — aiming to keep the patient normovolaemic (goal-directed therapy).

Third-Space Loss & Ongoing Losses

A concept peculiar to the surgical patient is the ‘third space’ — the functional loss of extracellular fluid that becomes sequestered in traumatised or oedematous tissue and in the gut lumen during major surgery, and which is not immediately available to the circulation even though it remains within the body. This sequestration, together with evaporative loss from exposed surfaces during long open operations and the obligatory losses of urine and insensible perspiration, adds to the fluid requirement above simple maintenance, and it later mobilises back into the circulation during recovery, which is one reason careful fluid balance matters both during and after surgery. Modern enhanced-recovery practice, however, cautions against the historically generous replacement of these losses, favouring a more restrictive, individualised approach that avoids the harms of overload.

• Total body water ~60%: ICF (~2/3) + ECF (~1/3 = interstitial + plasma).
• Crystalloids distribute through ECF (~1/4–1/3 intravascular → larger volume needed).
• Colloids stay in plasma (efficient volume expansion; cost/allergy/renal concerns limit synthetic use).
• Fluids provide maintenance, deficit replacement, and ongoing-loss replacement.
• Aim for normovolaemia; guide by HR, BP, urine output, dynamic measures (goal-directed); avoid over/under-load.

Crystalloids

Crystalloids are the first-line perioperative fluids. 0.9% ‘normal’ saline (Na⁺ 154, Cl⁻ 154 mmol/L) is isotonic but its high chloride can cause a hyperchloraemic metabolic acidosis in large volumes. Balanced solutions — Ringer’s lactate/Hartmann’s — have an electrolyte content closer to plasma (with lactate as a buffer) and are preferred for larger-volume resuscitation. Dextrose solutions (5% dextrose) provide free water (the glucose is metabolised) and are used for maintenance water, not for volume resuscitation.

Colloids

Colloids contain large molecules retained in the plasma. Human albumin is a natural colloid used in specific situations. Synthetic colloids include gelatins and — historically — starches (HES) and dextrans. Their theoretical advantage is efficient plasma expansion with less volume, but they are more expensive, can cause anaphylactoid reactions, and starches are now largely avoided because of renal injury and coagulopathy. Evidence has not shown colloids to improve survival over crystalloids for most resuscitation.

Choosing a Fluid

The choice depends on the purpose: a balanced crystalloid for most resuscitation and replacement, dextrose-containing fluids for maintenance water, and blood products for significant blood loss. Colloids are used selectively. The volume and rate are titrated to the clinical response.

Tonicity & Its Consequences

The behaviour of an intravenous fluid is governed by its tonicity relative to plasma. An isotonic crystalloid such as saline or Hartmann’s stays within the extracellular space and does not shift water into or out of cells, which is why it is used for volume replacement; a solution of 5% dextrose, once the glucose is metabolised, leaves behind free water that distributes across all body compartments and so is useless for expanding the circulation and can, if given in excess, cause dangerous hyponatraemia. Hypertonic solutions draw water out of cells and are reserved for specific indications such as raised intracranial pressure. Understanding tonicity therefore explains both which fluid to choose for a given purpose and the electrolyte disturbances that inappropriate choices can cause.

The Colloid–Crystalloid Debate

Whether colloids offer any real advantage over crystalloids for resuscitation has been one of the long-running debates in anaesthesia and intensive care. The theoretical appeal of colloids is that, by remaining in the plasma, they expand the circulation with a smaller infused volume and less tissue oedema; in practice, large trials have shown no consistent survival benefit over crystalloids, and specific colloids have fallen out of favour because of harm — the hydroxyethyl starches because of an association with acute kidney injury and coagulopathy, and the dextrans because of bleeding and anaphylaxis. The current consensus is that balanced crystalloids are the sensible default for most resuscitation, with albumin reserved for particular indications and synthetic colloids used sparingly if at all.

• Crystalloids first-line: normal saline (hyperchloraemic acidosis in volume), Ringer’s lactate (balanced, for resuscitation).
• 5% dextrose = free water (maintenance), NOT for volume expansion.
• Colloids (albumin, gelatins; starches/dextrans historical) expand plasma efficiently but costlier/allergy.
• Starches avoided (renal injury, coagulopathy); colloids don’t clearly improve survival vs crystalloid.
• Choose by purpose: balanced crystalloid to resuscitate, dextrose to maintain, blood for blood loss.

Indications & Products

Blood transfusion replaces blood components lost or deficient. Whole blood is rarely used; instead component therapy gives the specific part needed. Packed red cells restore oxygen-carrying capacity (for significant anaemia/haemorrhage); fresh frozen plasma (FFP) replaces clotting factors; platelets treat thrombocytopenia/platelet dysfunction; and cryoprecipitate supplies fibrinogen and specific factors.

Compatibility & Cross-matching

Red cells must be ABO- and Rhesus-compatible to avoid a haemolytic reaction. Group O is the ‘universal donor’ for red cells (no A/B antigens) and group AB the universal recipient; Rh-negative blood is given to Rh-negative patients (especially women of childbearing age). Before transfusion, samples are grouped and cross-matched, and a strict bedside identity check of patient and unit is performed to prevent the commonest fatal error — giving the wrong blood.

Transfusion Trigger

Transfusion of red cells is guided by a restrictive threshold in most patients (e.g. a haemoglobin around 7–8 g/dL), balancing oxygen delivery against transfusion risks; the trigger is higher in active bleeding or significant cardiac disease. Each unit is given for a defined indication, not routinely.

The Transfusion Process & Safety Steps

Safe transfusion is as much about process as about the blood itself, because the great majority of serious incidents arise from human error rather than from the blood. The chain of safety runs from correct patient identification and labelling of the sample at the bedside, through accurate laboratory grouping and cross-matching, to a final, rigorous bedside check immediately before the unit is connected, in which the patient’s identity and the details on the unit and its compatibility label are confirmed against one another. The transfusion is then observed, particularly in its first minutes when acute reactions declare themselves, and each unit is given for a specific, documented indication rather than as a routine, so that patients are neither under- nor over-transfused.

Special Situations & the Rh System

Two aspects of compatibility deserve emphasis. The Rhesus (D) system matters especially in women of childbearing potential, because a Rh-negative woman exposed to Rh-positive cells can form antibodies that cause haemolytic disease in a future Rh-positive fetus, so Rh-negative blood is given to Rh-negative recipients wherever possible. In an emergency when there is no time to cross-match, group O Rh-negative red cells can be given as the universal donor, moving to group-specific and then fully cross-matched blood as soon as it becomes available. These principles ensure that the life-saving benefit of transfusion is delivered without provoking the very reactions that compatibility testing exists to prevent.

• Component therapy: packed red cells (O₂ carriage), FFP (clotting factors), platelets, cryoprecipitate (fibrinogen).
• Red cells must be ABO/Rh compatible; O = universal red-cell donor, AB = universal recipient.
• Group & cross-match; strict bedside identity check (wrong-blood is the commonest fatal error).
• Restrictive transfusion trigger (Hb ~7–8 g/dL) in most; higher with active bleeding/cardiac disease.
• Give each component for a defined indication.

Overview

Blood transfusion carries important complications, grouped into immunological reactions, transfusion-transmitted infection, and the effects of massive transfusion. Many are preventable with correct practice.

Immunological Reactions

Acute haemolytic reaction — from ABO incompatibility — is the most dangerous: fever, loin/chest pain, hypotension, haemoglobinuria and disseminated intravascular coagulation; stop the transfusion immediately and support. Febrile non-haemolytic and allergic/anaphylactic reactions are commoner and usually milder (though anaphylaxis can be severe). Delayed haemolytic reactions occur days later. Transfusion-related acute lung injury (TRALI) causes acute respiratory distress.

Infection, TACO & Others

Transfusion-transmitted infection (hepatitis B/C, HIV, and others) is now rare with screening but not zero. Transfusion-associated circulatory overload (TACO) — fluid overload/pulmonary oedema, especially in the elderly/cardiac patient. Others: hyperkalaemia, hypocalcaemia (citrate), hypothermia, and coagulopathy (more with massive transfusion), and rarely graft-versus-host disease.

Reducing Transfusion & Its Risks

Because every transfusion carries some risk, an important principle is to avoid unnecessary transfusion altogether wherever possible, through what is termed patient blood management: optimising the patient’s own haemoglobin before surgery by treating anaemia and iron deficiency, minimising blood loss with careful surgery and antifibrinolytic drugs such as tranexamic acid, using cell salvage, and applying a restrictive transfusion threshold. When transfusion is genuinely needed it is given as the specific component required, in the minimum effective amount, so that the benefit clearly outweighs the immunological, infective and circulatory hazards that this topic describes.

Recognising a Reaction Early

Because transfusion reactions can be life-threatening and their early features overlap, the transfused patient is observed closely, particularly in the first fifteen minutes of each unit, when the most dangerous acute haemolytic and anaphylactic reactions tend to declare themselves. A rise in temperature, rigors, flushing, breathlessness, pain, hypotension or dark urine prompts the transfusion to be stopped and the patient assessed, and the response is then tailored to the likely reaction — immediate resuscitation and renal protection for a haemolytic reaction, adrenaline for anaphylaxis, respiratory support for TRALI, diuresis for TACO. This vigilance, combined with the identity checks that prevent the worst reactions, is what makes transfusion as safe as it is.

• Immunological: acute haemolytic (ABO — fever, loin pain, hypotension, haemoglobinuria, DIC), febrile, allergic/anaphylactic, TRALI, delayed haemolytic.
• Acute haemolytic reaction → STOP transfusion immediately + support.
• Infection (hepatitis, HIV) now rare with screening; TACO = circulatory overload/pulmonary oedema.
• Massive transfusion: hyperkalaemia, hypocalcaemia (citrate), hypothermia, coagulopathy.
• Distinguish TACO (overload) from TRALI (lung injury) in the breathless transfused patient.

Estimating Blood Loss

Managing haemorrhage begins with estimating blood loss and recognising its severity. Total blood volume is about 70 mL/kg in adults (~5 L). Loss is estimated from swab weighing, suction volumes, the surgical field, and the physiological response — tachycardia and a narrowed pulse pressure appear early, with hypotension a late sign (young patients compensate until they suddenly decompensate).

Management of Haemorrhage

Management follows resuscitation principles: large-bore IV access, fluids/blood to restore volume, control the bleeding source, and monitoring (including cross-match and coagulation). Warmed fluids and blood, and attention to temperature and calcium, are important. Restore oxygen-carrying capacity (red cells) and haemostasis (FFP, platelets, cryoprecipitate) as needed.

Massive Transfusion

Massive transfusion (e.g. replacing the patient’s blood volume in 24 hours, or rapid large-volume transfusion) requires a protocol: giving red cells, FFP and platelets in balanced ratios to prevent dilutional coagulopathy, plus cryoprecipitate/fibrinogen and tranexamic acid. Its complications — hypothermia, hypocalcaemia (citrate), hyperkalaemia, acidosis and coagulopathy (the ‘lethal triad’ of hypothermia, acidosis and coagulopathy) — must be actively prevented and treated.

Grading the Severity of Haemorrhage

It is useful to grade acute blood loss by its physiological effects, because the signs guide the urgency of resuscitation. Small losses of up to around fifteen per cent of blood volume are usually well tolerated with little more than mild tachycardia; as loss increases towards a third of the blood volume the tachycardia becomes marked, the pulse pressure narrows as diastolic pressure rises with vasoconstriction, and the patient becomes anxious; only when loss exceeds this does frank hypotension appear, and beyond about forty per cent the patient is severely shocked with a depressed conscious level. Recognising that the compensated patient with a normal blood pressure but a rising heart rate and narrow pulse pressure is already significantly hypovolaemic is the key to intervening before sudden decompensation.

Restoring Oxygen Delivery & Haemostasis

The two goals of resuscitating a bleeding patient are to restore oxygen delivery and to secure haemostasis, and these require different interventions given in parallel with surgical control of the bleeding source. Oxygen delivery depends on an adequate circulating volume and enough red cells to carry oxygen, restored with balanced crystalloid initially and with red cells as loss becomes significant; haemostasis depends on adequate clotting factors, fibrinogen and platelets, replaced with plasma, cryoprecipitate and platelet concentrates and supported by tranexamic acid, while hypothermia and acidosis — which themselves impair clotting — are corrected. Balancing these while the surgeon controls the source is the essence of managing major haemorrhage.

• Blood volume ~70 mL/kg (~5 L adult); estimate loss from swabs/suction/field + physiology.
• Tachycardia & narrow pulse pressure early; hypotension LATE (compensation then sudden decompensation).
• Manage: large-bore access, restore volume (fluids/blood), control source, monitor, warm, watch calcium.
• Massive transfusion protocol: balanced red cell/FFP/platelet ratios + cryoprecipitate + tranexamic acid.
• Prevent the lethal triad: hypothermia, acidosis, coagulopathy (+ hypocalcaemia, hyperkalaemia).