Fluid therapy during anaesthesia Dr M.G. Senekal Department of Anaesthesiology 3 Military Hospital Tempe Bloemfontein

Fluid balance To maintain an effective circulating volume To preserve oxygen delivery to the tissues To maintain electrolyte homeostasis On an individual patient basis

Fluid compartment physiology Total body water (60% of total body weight) Extracellular fluid (20% of total body weight) Intravascular fluid (5% of total body weight) Red cells, white cells and platelets (2% of total body weight) Plasma (3% of total body weight) Interstitial fluid (15% of total body weight) Intracellular fluid (40% of total body weight)

Fluid compartment physiology Plasma: solution in water inorganic ions (e.g. sodium chloride) simple molecules (e.g. urea) larger molecules (e.g. albumin) Capillary endothelium and blood vessel walls divide the extracellular compartment into the intravascular and interstitial compartments Cell walls separate the intracellular compartment from the extracellular compartment

Fluid compartment physiology Water moves freely through cell and vessel walls and is distributed throughout all the compartments The Na⁺/K⁺/ATPase in the cell wall extrude Na⁺ and maintains a sodium gradient across the cell membrane Capillary endothelium is freely permeable to small ions such as Na⁺, but relatively impermeable to larger molecules such as albumin This helps to retain fluid in the plasma due to the osmotic effect

Fluid compartment physiology Jv α [(Pc-Pi)-σ(πc-πi)] Jv – transcapillary fluid flux Pc – capillary hydrostatic pressure Pi – interstitial hydrostatic pressure πc – intravascular oncotic pressure πi – interstitial oncotic pressure σ – the reflexion coefficient Usually the net intracapillary pressures are more than the interstitial pressures This produces a slow flow to the interstitium Many of the effects of different fluid solutions are governed by their distribution within the physiological compartments of the body

Normal fluid and electrolyte requirements

Normal fluid and electrolyte requirements Adults lose 2.5-3 litres of water per day: 1300-1800ml urine 1000ml insensible loss from lungs and skin 100ml in faeces Normally fluid intake is orally and 200ml/day from metabolism Adults lose 1.5mmol/kg/day sodium ions and 1mmol/kg/day potassium ions

Assessment of hydration status and intravascular volume Present for surgery with conditions that result in altered fluid balance Hydration status and intravascular volume is determined by: history, examination, test results and response to iv fluids Water and solute depletion Decreased intake (fasting, anorexia, altered conscious level) Increased losses (diarrhoea, vomiting, bowel preparation, burns, pyrexia)

Assessment of hydration status and intravascular volume Dehydration: loss of water from extracellular and intracellular fluid Abnormal losses are often from the gastrointestinal tract: contains electrolytes depletes the extracellular fluid diarrhoea contain high [K⁺] Pyrexia ↑ insensible losses by 20%/°C Third space losses: from the intravascular compartment secondary to increased capillary permeability from the intestinal compartment into the peritoneal compartment into the pleural cavities

Assessment of hydration status and intravascular volume: Physical examination Most reliable preoperatively Clinical signs of hypovolaemia more difficult to determine during anaesthesia due to drugs and surgical stress Anaesthesia and surgery affect fluid balance General anaesthesia → vasodilatation and myocardial suppression Positive pressure ventilation → ↓ venous return and cardiac output Spinal and epidural anaesthesia → sympathetic blockade → vasodilatation → ↓ preload and blood pressure

Assessment of hydration status and intravascular volume: Physical examination Shock is defined as decreased oxygen delivery or utilisation by tissues that may lead to irreversible cellular damage if prolonged Patients who present in a state of shock require immediate fluid therapy!

Assessment of hydration status and intravascular volume: Physical examination Table 1: Grades of hypovolaemic shock secondary to intravascular volume loss Class l Class ll Class lll Class lV Blood loss (ml) Up to 750 750-1500 1500-2000 >2000 % blood loss Up to 15% 15-30% 30-40% >40% Pulse rate (/min) <100 >100 >120 >140 Blood pressure Normal Decreased Pulse pressure Respiratory rate Increased Rapid Urine output (ml/hour) >30 20-30 5-15 Negligible CNS/mental state Slight anxiety Mild anxiety Anxious, confused Confused, lethargic

Assessment of hydration status and intravascular volume: Physical examination Table 2: Grades of dehydration, relating to the % body weight lost and the resulting physical signs Mild <5% Moderate 5-10% Severe >10% Pulse rate Normal Increased Blood pressure Decreased Respiratory rate Rapid Capillary return <2 seconds 3-4 seconds >5 seconds Urine output Negligible/absent Mucous membranes Moist Dry Parched CNS/mental state Normal/restless Drowsy Lethargic/comatose

Assessment of hydration status and intravascular volume: Physical examination

Assessment of hydration status and intravascular volume: Laboratory evaluation To evaluate intravascular volume and adequacy of tissue perfusion: Serial hematocrits Arterial blood pH Urinary specific gravity or osmolality Urinary sodium or chloride concentration Serum sodium The serum creatinine to blood urea nitrogen (BUN) ratio

Assessment of hydration status and intravascular volume: Hemodynamic measurements Central venous pressure monitoring Pulmonary artery pressure monitoring Transesophageal monitoring

Crystalloids Colloids Blood products Intravenous fluids Three types of intravenous fluids: Crystalloids Colloids Blood products Different fluids influences: The magnitude and duration of intravascular volume expansion Hemorrheology Hemostasis Vascular integrity Inflammatory cell function

Intravenous fluids: Crystalloid solutions Inorganic ions and small organic molecules in water The main solute is glucose or sodium chloride Isotonic, hypotonic or hypertonic compared to plasma Potassium, calcium or lactate may be added to more closely replicate plasma Crystalloids with an ionic composition close to that of plasma is referred to as “balanced” or “physiological”

Intravenous fluids: Crystalloid solutions A crystalloid with [Na⁺] similar to plasma rapidly distribute through the extracellular space – only 25-30% of 0.9% saline or Ringer’s remains in the intravascular compartment A crystalloid with a lower [Na⁺] than plasma distribute throughout the total body water – less than 10% of 5% glucose or 0.18% saline with 4% glucose remains in the intravascular compartment after the glucose has been metabolised Crystalloids exerts short lived haemodynamic effects

Intravenous fluids: Colloid solutions Suspensions of high molecular weight particles, derived from gelatine, protein or starch in solutions of saline or glucose Remain longer intravascularly and expand plasma volume Divided into semisynthetic colloids (gelatins, dextrans and hydroxyethyl starches) and human plasma derivatives (human albumin solutions and fresh frozen plasma) Vary in magnitude and duration of plasma volume expansion, effects on hemorrheology and hemostasis, interaction with endothelial and inflammatory cells, adverse drug reactions and cost

Intravenous fluids: Colloid solutions Duration of plasma volume expansion is dependent on the rate of molecule loss from the circulation and their metabolism – determined by molecular size and surface charge characteristics Reduce blood viscosity by hemodilution Semisynthetic colloids influence plasma viscosity and red cell aggregation Affect hemostasis because of hemodilution of clotting factors and because of effects on the hemostatic mechanism The dextrans are effective antithrombotic agents Anaphylaxis has been described The human derived colloids is expensive and there is a risk infection

Intravenous fluids: Blood Transfusions Packed red blood cells Complications: Haemolytic reactions Anaphylaxis Fever Urticaria Pulmonary oedema (TRALI) Graft-versus-host disease Purpura Immune suppression Infections Coagulopathy Citrate toxicity Hypothermia Acid-base balance disturbances Hyperkalaemia

Intravenous fluids: Crystalloids versus Colloids Water is “pulled” along osmotic gradients Solute distribution determines the water content of each compartment Solute distribution is determined by the properties of the membranes separating the compartments Solutes that pass freely across a semipermeable membrane do not generate any osmotic pressure The volume of distribution of infused fluids is therefore dictated by their solute content Plasma volume expansion = Volume infused/Volume of distribution

Intravenous fluids: Crystalloids versus Colloids Infusion of water expand all compartments in proportion to their total volume Only 7% of infused water remain in the intravascular space Infusion of isotonic glucose solution (5% glucose) is rapidly equivalent to infusion of water Infusion of isotonic crystalloid solution (0.9% saline or Ringer’s) expand the extracellular volume Only 20% of the infused volume remain in the intravascular space Infusion of an “ideal colloid”, containing large molecules that do not escape from the circulation, will expand the intravascular volume by 100% of the volume infused

Intravenous fluids: Crystalloids versus Colloids Significant plasma volume expansion requires large volume crystalloid infusion → significant expansion of the extracellular volume → tissue oedema → increasing diffusion distances and compression of small vessels and capillaries → compromised end organ perfusion and oxygenation Colloid may result in less oedema and better recovery in the postoperative period due to less tissue oedema Hypertonic crystalloid and colloid solutions draw tissue fluid into the intravascular space and provide a significant plasma volume expansion with minimal tissue oedema

Intravenous fluids: Crystalloids versus Colloids When prescribing fluid replacement, it is important to identify which compartment is depleted Specific losses should be replaced with the appropriate fluid

Peri-operative fluid management Provision of maintenance fluids Replacement of pre-existing deficits Replacement of intra-operative losses To maintain the circulating volume and tissue oxygen delivery Fluid regimes must be individualised for each patient Fluids should be warmed to help maintain a normal body temperature

Peri-operative fluid management: Maintenance Normal maintenance requirements: 1.5ml/kg/hour Usually 0.9% saline or Ringer’s and 5% glucose Patients undergoing elective minor surgery often do not need supplementary intravenous fluids

Peri-operative fluid management: Replacement of pre-existing deficits Stabilise the patient before the anaesthetic is performed Fluid deficits, electrolyte and acid-base imbalances should be corrected The fluid deficit is the maintenance requirement (x hours since last oral intake) + preoperative external and third space losses Hard to measure accurately Replacement should be based on response 10-20ml/kg fluid boluses in dehydration followed by reassessment and further fluids guided by clinical signs

Peri-operative fluid management: Replacements of intra-operative and postoperative losses Maintenance of an effective intravascular volume to maintain tissue perfusion and cellular oxygenation Inaccuracy in the observation of blood and other fluid losses The physiological stress response to surgery or trauma results in water and sodium retention - more water than sodium is retained with a risk of hyponatraemia

Peri-operative fluid management: Replacements of intra-operative and postoperative losses Postoperatively maintenance requirements, abnormal insensible losses, visible external losses, third space loss and concealed blood loss should be measured or estimated. 0.9% saline or Ringer’s because of the risk of hyponatraemia

Peri-operative fluid management: Blood transfusion Oxygen delivery is a function of haemoglobin level, haemoglobin oxygen saturation and cardiac output Ensuring an adequate haemoglobin level and intravascular volume is vital Haematocrit or Hb may reduce after fluid replacement Blood loss is replaced initially with 3ml of 0.9% saline or Ringer’s per ml blood lost Transfusion is indicated when Hb falls to 7.5g/dl in fit patients Patients with ischaemic heart disease may need levels of more than 9g/dl

Peri-operative fluid management: Blood transfusion The volume of red cells to be transfused: Calculate the patient’s blood volume (patient’s weight in kg x 65ml/kg) Calculate the red blood cell volume at the ideal hematocrit (ideal hematocrit in % x patient’s blood volume) A Calculate the red blood cell volume at the current hematocrit (hematocrit in % x patient’s blood volume) B A – B = red cell volume to be transfused (x 1.5 if transfusing packed red blood cells or x 3 if transfusing whole blood) A unit of platelets may be expected to increase the count by 5000-10000 x 10⁹/l The initial therapeutic dose of Fresh Frozen Plasma is usually 10-15ml/kg

The end is near

How fluids should be administered/Assessment of fluid administration The “recipe book” approach is a continuous predetermined rate of fluid infusion with additional replacement of observed losses However, different magnitudes of surgical insult require very different amounts of fluid therapy

How fluids should be administered/Assessment of fluid administration Adverse outcomes is associated with inadequate or excessive fluid administration Fluids should be titrated to physiological end-points that can be monitored and responded to Fluids should therefore be administered with adequate monitoring Avoid underuse of fluid resulting in hypovolaemia and inadequate tissue perfusion Avoid the administration of excess fluid with the risk of pulmonary and peripheral oedema

How fluids should be administered/Assessment of fluid administration Maintain a mean arterial blood pressure above a level defined by an individual’s pre-operative mean arterial blood pressure Variations in systolic blood pressure and pulse pressure with positive pressure ventilation predict circulatory responses to a fluid challenge (A decrease in systolic pressure of >5mmHg during a positive pressure mechanical breath is predictive of a positive response to a colloid volume challenge)

How fluids should be administered/Assessment of fluid administration The response of CVP to a fluid challenge: A volume of colloid (200ml) is infused over 10-15min No change in CVP or a decrease suggests hypovolaemia and the fluid challenge should be repeated (The combination of an increase in intravascular volume without a related increase in pressure indicates that vascular compliance has increased, suggesting a reduction in vasoconstrictor tone) A sustained increase of >3mmHg suggests that the limits of intravascular compliance have been reached and fluid challenges should be discontinued

How fluids should be administered/Assessment of fluid administration Physiological goals: Normal pulse rate (<100/min) Normal blood pressure (within 20% of normal) Urine output 0.5-1ml/kg/hour CVP 6-12cmH₂O Normal pH, PaO₂, base excess, serum lactate Haemoglobin >7.5g/dl in fit patients, >9g/dl in patients with ischaemic cardiac disease Where advanced monitoring is used, fluids may be targeted to maintain cardiac output

How fluids should be administered/Assessment of fluid administration Knowledge of fluids should guide their administration Correct dosage of fluid improves patient outcome Accurate dosing of fluid in major surgery requires monitoring of arterial blood pressure and blood flow