Fluid Therapy

Fluid Balance – General Principles

Posted by Kai Knudsen, Senior Physician in Anesthesia & Intensive Care. Sahlgrenska University Hospital. and Robert Hahn Professor of Anesthesia & Intensive Care. Southern University Hospital, Stockholm, Sweden.
Updated 2019-06-12

Fluid therapy in anesthesia and intensive care can usually be divided into preoperative, peroperative and postoperative fluid supply. In intensive care, fluid therapy is also provided to non-operated patients. Fluid therapy aims to maintain or restore as normal fluid balance and tissue perfusion as possible. The choice of fluid for infusion must be based on a good knowledge of the individual patient’s needs and the different contents of the fluid. Basal fluid requirement is approximately 30 ml/kg/day. A healthy adult has a daily need of water of about 1500-2500 ml. The basal need for electrolytes is about 80 mmol sodium, 40 mmol potassium, 20 mmol magnesium and 20 mmol phosphate.

In the choice of fluid administration for volume substitution, one should evaluate;

  1. Volume status and tissue perfusion. Fluid replacement refers to normovolemia, which can be perceived as the intravascular volume needed to achieve adequate cardiac output and perfusion pressure in peripheral tissues for sufficient microcirculation.
  2. Oxygen transport – target for hemoglobin level is based on age, oxygen demand, rheology and cardiovascular status.
  3. Coagulation – any possible coagulation defects should be evaluated and corrected.
  4. Colloid osmotic effect – the desired level in plasma may be weighed against any side effects of hypertonic fluids.
  5. Normothermia is desired for optimal physiology and coagulation.

Consideration should be given to known risks and adverse reactions with fluid therapy, especially in the case of abundant supply (> 3 L), congestive heart failure and at SIRS. Note that synthetic colloids have not been shown to affect survival positively compared to crystalloid fluids in controlled clinical trials. Any allergy should always be investigated. All fluid therapy has potential side effects, including naturally occurring substances. In the case of hypothermia, added fluids should be warm.

Note the following:

  1. Salt containing solutions can cause edema and impaired microcirculation if given in high volume.
  2. All synthetic colloids may cause renal impairment with impaired renal function in hypovolemia or hypoperfusion conditions.
  3. Hydroxyethyl starch (HES) should not be used in severe sepsis, at significant risk of renal failure or given to critically ill patients (ICU patients).
  4. Albumin is preferred as a colloidal solution in sepsis, head injury and liver failure.
  5. Gelatin produces relatively short-term volume expansion in plasma.
  6. Dextran must be preceded by the hapten inhibitor (Promiten) which must be prescribed specifically.
  7. Plasma and platelets are indicated only in case of volume deficiency with expected coagulation defect.

Electrolyte content in losses of various body fluids in mmol/l

Body fluidNa (mmol/l)K (mmol/l)Cl (mmol/l)HCO3 (mmol/l)H (mmol/l)
Stomach (Gastric content)20–601414060–80
Diarrhea / colostomy losses30–14030–7020–80
Losses from the ileum at high flows100–1404–575–1250–30
Losses from the ileum at low flows50–1004–525–750–30
Drainage or fistula from the pancreas125–13885685
Losses from the jejunum14051358
Ref: Neilson J, O’Neill F, Dawoud D, Crean P, Guideline Development G. Intravenous fluids in children and young people: summary of NICE guidance. BMJ (Clinical research ed). 2015;351:h6388

Clinical signs of dehydration

Symptoms / signsMildModerateSevere
Weight loss< 5 %5–10 %> 10 %
Deficit (ml/kg)< 5050–100> 100
General conditionThirsty and worriedThirsty, anxious or lethargic, halonatedVery sluggish to comatose, cold, gray, cyanotic
Mucous membranesNormal, moistyDryVery dry
Skin turgorNormalReducedPronounced impaired
PulseNormalTachycardiaTachycardia, weak pulse
Capillary refill< 2 secSlow > 2 secVery slow
Blood pressure (systolic)NormalNormal / lowLow
BreathingNormalDeepDeep and fast
Diuresis (urinary output)< 2 ml/kg/h< 1 ml/kg/h< 0,5 ml/kg/h
NOTE! Higher dehydration can be used without all the characters being met. In hyperosmolar conditions with dehydration, the symptoms may appear different. Drops in blood pressure will often be late and are ominous.

Preoperative Fluid Administration

Preoperatively, fluid is usually given to fasting patients waiting for surgery for more than 6 hours. This fluid delivery is advantageously administered orally until 2 hours prior to surgery. Alternatively, a balanced electrolyte solution with glucose is given intravenously. A common regimen for patients who have to wait more than 6 hours is Glucose 2.5-5% (25-50 mg/ml) with electrolyte additives, with sodium at 40-80 mmol/l and potassium 20-40 mmol/l. The glucose solution can be given at a rate of 1 liter per 12 hours, which corresponds to about 80 ml/h.

Basic Fluid Needs

 Daily requirementDaily requirement at body weight 70 kg
Water30 ml/kg/day2100 ml
Sodium1,1-1,4 mmol/kg80-100 mmol
Potassium0,6-0,7 mmol/kg40-50 mmol
Glucose2 g/kg140 g
Calories (kcal)25 kcal/kg1750 kcal

Patients who have scheduled surgery during the day can also be given Ringer’s acetate preoperatively at an infusion rate of approximately 100 ml/h or 1.5 ml/kg/hour. The preoperative fluid supply must take into account the patient’s current fluid status and acid base balance.

Peroperative Fluid Administration

Peroperatively, balanced electrolyte solutions (isotonic crystalloid solutions) are usually given at a rate of 3-5 ml/kg/h without glucose. This fluid supply is normally given without any volume pump and is manually dispensed by means of a hose clamp. The fluid compensates for a minimal operating bleeding of 50-100 ml. In case of greater bleeding, the rate of infusion should be increased.

Common fluids given peroperatively are Ringer’s acetate or similar crystalloid solutions. Isotonic sodium chloride is reserved for patients with hyponatremia or metabolic alkalosis due to vomiting. Blood sugar level should always be checked during major surgery or prolonged surgery over 3 hours. Normally, glucose containing solutions are not given peroperatively. In hypoglycemia though, glucose is given in the form of 5 or 10% solution. In severe hypoglycemia with blood levels lower than 3.5 mmol/L, glucose can be given as bolus in a 20 or 30% solution and then in 10% solution.

Fluid therapy in moderate dehydration

Percentual deficit of total body weightSymptomExampel at body weight 70 kg4 hours (2/3 of need)
Two percent deficitDry mucous membranes, thirst1,4 liter0,9 liter
Five percent deficitTachycardia, pronounced thirst, oliguria3,5 liter2,3 liter
Ten percent deficitHypotension, vasoconstriction, cognitive impairment, pronounced orthostatism7 liter5 liter

Ringer’s Acetate or sodium chloride is given in a volume that is 2-3 times lost blood volume. The supply of crystalloid solutions replaces lost blood volume by approximately 20% of blood loss. These solutions are distributed from the blood volume to the interstitial fluid (ISV) within half an hour, which means that the initial expansion of the blood volume is quite good.

If the patient is diagnosed as hypovolemic during surgery with administration of more than 3 liters of Ringer’s acetate, a colloidal solution should be given in addition in the form of dextran, albumin, hydroxyethyl starch, gelatin or plasma. Colloids with hydroxyethyl starch can be given up to 20 ml/kg body weight per hour (equivalent to 0.33 ml/kg/min or 1.2 g hydroxyethyl starch/kg body weight per hour), but not to critically ill patients with sepsis or renal failure. The addition of hydroxyethyl starch should be given restrictively and terminate as soon as possible. The colloid osmotic effect of human albumin 20% (such as Albumin 200 g/L) is about four times as high as for blood plasma. In healthy subjects, less than 10% of the administrated albumin leaves the intravascular space for the first 2 hours after the infusion.

The assessment of hypovolemia during surgery is difficult as anesthetic agents inhibitory effect on the autonomic nervous system causes hypotension. Indication of hypovolemia may be sudden blood pressure drop, weak pulse, low-filling arterial pressure curves, high stroke volume variation, high pulse pressure variation, posture dependent blood pressure drop (orthostatism), low central filling pressures, low cardiac output (if measured), high EVF or hemoglobin  and circulatory instability when administering anesthetic agents.

If possible, check the blood pressure reaction on leg lift. In case of suspected hypovolemia give 500 ml of Ringer’s acetate as a fluid bolus and do not repeat if there is no positive effect.

Give special attention to twists and turns of body position in hypovolemic patients as this can easily cause severe blood pressure drop. Older hypovolemic patients are extra sensitive. A simple fluid balance calculation peroperatively is to count the amount of fluid in the form of colloids and crystalloids including blood replacement minus urine and peroperative blood loss. Determination of urinary output is therefore required for major surgery or long surgery over 3 hours, aiming for a diuresis over 0.5 ml/kg/h, although this low flow does not seem to cause kidney damage. The reduction in arterial pressure during anesthesia strongly inhibits urine production, which should be included in the assessment. In case of minor surgery or surgery less than 3 hours, measurement of urinary output is usually not required. In the case of overhydration peroperatively without good urinary output, diuretics can be used intravenously, for example furosemide 5-10 mg i.v.

In peroperative fluid therapy, you always get some hemodilution. A hemoglobin level of 10-11 and EVF 30-35% is considered good for rheology and microcirculation, but which hemoglobin is optimal is controversial. Younger and stronger patients tolerate hemodilution better than older and weak patients. Dilution to a hemoglobin value of less than 8 g/l may adversely affect the blood’s coagulation ability and the oxygen supply dependent on hemoglobin and cardiac output may be insufficient. At hemoglobin level below 5 there is an immediate transfusion need and it is common with myocardial ischemia that can be seen in ECG changes. Note that while administering colloidal dextran, it decreases platelet adhesion and bleeding tendency may increase, especially when administering more than one liter of dextran to normal weighty adults.

Post Operative Fluid Administration

Postoperatively, a combination of crystalloid solutions and glucose solutions is usually given. In order to give the patient a basic need for energy during the day of operation, glucose solution is usually administered postoperatively in the form of 2.5 or 5% glucose with electrolyte additives. Continued delivery can be given at a rate of about 100 ml/h (80-120 ml/h) or 1.5 ml/kg/hour. Glucose solutions should not be given faster than 10 ml/kg and hour, i.e. a 70 kg patient should never get 2.5% glucose faster than 700 ml/hour.

In glucose solutions, electrolytes in the form of sodium are usually added at 80 mmol/l (40-120) and potassium at 40 mmol/l (20-40). Additives of sodium and potassium are adjusted according to current blood levels. Normal values ​​are usually best but not always. Pronounced deviations of electrolytes should not be fully corrected too fast. Potassium should never be given in a peripheral venous catheter in a concentration higher than 40 mmol/l as it is vascular irritating and may be painful. If needed, a large amount of potassium (> 80 mmol/d) is best supplied in a concentrated solution (1 mmol/ml) in a central venous catheter (5-20 mmol/h). Potassium should not be given faster than 10 mmol/h, which can be extended to 20 mmol/h if ECG is monitored continuously. Blood sugar level should be kept between 6 and 10 mmol/l. In case of large amounts of urinary output (> 3 liters), serum and urine osmolality and serum and urine electrolytes should be checked. In case of severe head injury, it is not uncommon with a diabetes insipidus, with extra fluid being needed or occasionally desmopressin (vasopressin).

The fluid supply is postoperatively controlled by the patient’s blood volume, metabolism and current losses. Normal urinary output is around 80-200 ml/h. Urine production should amount to at least 1000-1500 ml/day. Perspiration (fluid loss through sweating and breathing) usually amounts to approximately 700 ml/day. A patient on a ventilator usually has a lower perspiration rate, approximately 400-600 ml/day. At temperature above or under 37º, perspiration increases or is reduced by 25% per degree of temperature change. The fluid supply postoperatively shall replace current losses. In case of fever, extra fluid should be given, usually 2-300 ml extra per degree above 38 degrees. In case of vomiting, diarrhea and drainage, these must be compensated. Vomiting and diarrhea can usually be compensated with one liter of extra crystalloid fluid, such as Ringer’s Acetate. In the event of drainage losses, these can be compensated with colloids, e.g. hydroxyethyl starch containing solutions in the same volume as the loss.

If the patient is assessed as normovolemic, a balance sheet of minus 500 ml/day can provide a good daily fluid balance. This fluid balance is checked against body weight. It is common for body weight to increase significantly on the first day of surgery due to peroperatively supplied fluid. This increase in body weight can amount to several kilograms. A weight gain of 0-3 kg can be considered normal after major surgery, a weight gain of 3-5 kg ​​is considered to be large, while a weight gain of 5-10 kg can be considered very large and more than 10 kg as extraordinary and dangerous. Weight gain over 10 kg often causes fluid outflow in the lungs and other vital organs that threatens both respiration, microcirculation and organ function. A weight gain of less than 3 kg usually does not need to be actively addressed but small amounts of diuretics can be given. A weight gain of 3-5 kg ​​can be normalized within a couple of days; a negative fluid balance should then be sought through treatment with diuretics, such as furosemide 10 mg x 3. The whole weight gain should not normally be restored in a day, one should not be in a hurry.

At a weight gain of 5-10 kg, this should be treated actively by the administration of diuretics, such as furosemide intermittently or in continuous infusion. The supply of fluid should be restrictive. Glucose should be given in more concentrated solutions to limit fluid supply.

At a weight gain of more than 10 kg, the treatment must be active and immediate. Normally, diuretics do not suffice, but dialysis need to be used, for example, a veno-venous continuous dialysis (CRRT) via a dialysis catheter (CDC).

In postoperative fluid treatment, glucose solutions can be used for up to three days. Glucose delivery should be distributed evenly throughout the day to avoid hyperglycemia. In addition to the addition of sodium and potassium, additives of magnesium (20 mmol/d) of phosphate (20 mmol/d) and sometimes zinc are needed. However, three liters of 5% glucose contain only 600 kcal, so after the third day, calorie and nutritional supply should be more effective by switching to parenteral nutrition, including amino acids and fat, intravenously.

Composition of Colloidal Solutions (Colloids)

SolutionActive substanceSodium content (mmol/l)Potassium content (mmol/l)Chloride content (mmol/l)Acetate content (mmol/l)Osmolality (mosm/l)
Macrodex 60 mg/mlDextrane 7015401540300
RheomacrodexDextrane 4015401540350
Ringer-DextranDextrane 60130411030270
PlasmodexDextrane 60130411030270
RescueFlowDextrane 7012800128001580
HesraHES 130/0,41305,311227277
HyperHAESHES 200/0,512320123202464
Tetraspan 60 mg/mlHES 130/0,4140411824310
VolulyteHES 130/0,4154411824310
Voluven 60 mg/mlHES 130/0,415401540310
Venofundin 60 mg/mlHES 130/0,4215401540310
Albumine 4% 40 g/lHuman albumine83< 2(?)0310
Albumine 20% 200 g/lHuman albumine125< 2100 (?)0 

Composition of Crystalloidal Solutions (Crystalloids)

SolutionSodium Content (mmol/l)Potassium Content (mmol/l)Chloride Content (mmol/l)Acetate Content (mmol/l)Energy Content (kcal/l)
Sodium Chloride 0,9 % (9 mg/ml)154015400
Ringer's Acetate1314110300
Glucose Buffered 2.5 % (25 mg/ml)7004525100
Ringer-glucose 2.5 % (25 mg/ml)7327830100
Glucose 5% (50 mg/ml)0000200
Glucose 10% (100 mg/ml)0000400
Glucose 20% (200 mg/ml)0000800
Glucose 5 % with sodium and potassium (50 mg/ml)4020601200
Glucose 10 % with sodium and potassium (100 mg/ml)4020601400
Glucose 2,5 % buffered (25 mg/ml)7004525100
Glucose 5 % buffered (50 mg/ml)7004525200
Rehydrex with Glucose 2,5 % (25 mg/ml)7004525100
Plasmalyte Glucose 14059827200

Volume Expansion with Fluid Administration

The expansion effect of blood volume supplement decreases over time at different rates for different crystalloid and colloidal solutions. The effect also varies depending on the patient’s condition, cardiac condition and renal function. Increased capillary permeability and less plasma retention are seen mainly in sepsis and inflammatory conditions (SIRS) but can also be seen in trauma and other serious medical conditions. The expansion effect can be significantly altered in severe disease states with increased capillary permeability as in sepsis and SIRS. Initially, blood volume increases approximately in the following way.

Volume-expanding effect of different solutions:

  • 1000 ml crystalloid solution increases the plasma volume by 180-330 ml. Initial volume expansion is 500 ml, but it drops within 30 minutes by distribution to the interstitial space.
  • 500 ml dextran increases the plasma volume by about 400-500 ml, volume effect for 6-24 hours.
  • 500 ml of hydroxyethyl starch (HES) increases blood volume by 375-500 ml, volume effect for 4-6 hours.
  • 500 ml albumin 4% increases blood volume by 375-400 ml, volume effect for 4-6 hours.
  • 100 ml albumin 20% increases blood volume with 300 ml, volume effect for 6-8 hours.
  • 500 ml of gelatin increases the plasma volume by about 400-500 ml. Gelatin has a volume effect for 2-3 hours.


To expand the blood volume by one liter to an adult 70 kg patient, approximately the following volumes of fluid replacement are required:

  • Crystalloid solution: approximately 3000 ml.
  • Dextran solution: 1000-1250 ml.
  • Hydroxyethyl starch: 1000-1330 ml.
  • 4% albumin: 1250-1420 ml.
  • 20% albumin: 333 ml.
  • Gelatin solution: 1000-1250 ml.