Metabolic response to injury, fluid and electrolyte balance and shock

Published on 14/06/2015 by admin

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1 Metabolic response to injury, fluid and electrolyte balance and shock

The metabolic response to injury

In order to increase the chances of surviving injury, animals have evolved a complex set of neuroendocrine mechanisms that act locoregionally and systemically to try to restore the body to its pre-injury condition. While vital for survival in the wild, in the context of surgical illness and treatment, these mechanisms can cause great harm. By minimizing and manipulating the metabolic response to injury, surgical mortality, morbidity and recovery times can be greatly improved.

Factors mediating the metabolic response to injury

The metabolic response is a complex interaction between many body systems.

The acute inflammatory response

Inflammatory cells and cytokines are the principal mediators of the acute inflammatory response. Physical damage to tissues results in local activation of cells such as tissue macrophages which release a variety of cytokines (Table 1.1). Some of these, such as interleukin-8 (IL-8), attract large numbers of circulating macrophages and neutrophils to the site of injury. Others, such as tumour necrosis factor alpha (TNF-α), IL-1 and IL-6, activate these inflammatory cells, enabling them to clear dead tissue and kill bacteria. Although these cytokines are produced and act locally (paracrine action), their release into the circulation initiates some of the systemic features of the metabolic response, such as fever (IL-1) and the acute-phase protein response (IL-6, see below) (endocrine action). Other pro-inflammatory (prostaglandins, kinins, complement, proteases and free radicals) and anti-inflammatory substances such as antioxidants (e.g. glutathione, vitamins A and C), protease inhibitors (e.g. α2-macroglobulin) and IL-10 are also released (Fig. 1.1). The clinical condition of the patient depends on the extent to which the inflammation remains localized and the balance between these pro- and anti-inflammatory processes.

Table 1.1 Cytokines involved in the acute inflammatory response

Cytokine Relevant actions
TNF-α Proinflammatory; release of leucocytes by bone marrow; activation of leucocytes and endothelial cells
IL-1 Fever; T-cell and macrophage activation
IL-6 Growth and differentiation of lymphocytes; activation of the acute-phase protein response
IL-8 Chemotactic for neutrophils and T cells
IL-10 Inhibits immune function

(TNF = tumour necrosis factor; IL = interleukin)

The endothelium and blood vessels

The expression of adhesion molecules upon the endo-thelium leads to leucocyte adhesion and transmigration (Fig. 1.1). Increased local blood flow due to vasodilatation, secondary to the release of kinins, prostaglandins and nitric oxide, as well as increased capillary permeability increases the delivery of inflammatory cells, oxygen and nutrient substrates important for healing. Colloid particles (principally albumin) leak into injured tissues, resulting in oedema.

The exposure of tissue factor promotes coagulation which, together with platelet activation, decreases haemorrhage but at the risk of causing tissue ischaemia. If the inflammatory process becomes generalized, widespread microcirculatory thrombosis can result in disseminated intravascular coagulation (DIC).

Consequences of the metabolic response to injury

Hypovolaemia

Reduced circulating volume often characterizes moderate to severe injury, and can occur for a number of reasons (Table 1.3):

Table 1.3 Causes of fluid loss following surgery and trauma

Nature of fluid Mechanism Contributing factors
Blood Haemorrhage Site and magnitude of tissue injury
Poor surgical haemostasis
Abnormal coagulation
Electrolyte-containing fluids Vomiting Anaesthesia/analgesia (e.g. opiates)
Ileus
  Nasogastric drainage Ileus
Gastric surgery
  Diarrhoea Antibiotic-related infection
Enteral feeding
  Sweating Pyrexia
Water Evaporation Prolonged exposure of viscera during surgery
Plasma-like fluid Capillary leak/sequestration in tissues Acute inflammatory response
Infection
Ischaemia–reperfusionsyndrome

Decreased circulating volume will reduce oxygen and nutrient delivery and so increase healing and recovery times. The neuroendocrine responses to hypovolaemia attempt to restore normovolaemia and maintain perfusion to vital organs.

Fluid-conserving measures

Oliguria, together with sodium and water retention – primarily due to the release of antidiuretic hormone (ADH) and aldosterone – is common after major surgery or injury and may persist even after normal circulating volume has been restored (Fig. 1.2).

Secretion of ADH from the posterior pituitary is increased in response to:

ADH promotes the retention of free water (without electrolytes) by cells of the distal renal tubules and collecting ducts.

Aldosterone secretion from the adrenal cortex is increased by:

Aldosterone increases the reabsorption of both sodium and water by distal renal tubular cells with the simultaneous excretion of hydrogen and potassium ions into the urine.

Increased ADH and aldosterone secretion following injury usually lasts 48–72 hours during which time urine volume is reduced and osmolality increased. Typically, urinary sodium excretion decreases to 10–20 mmol/ 24 hrs (normal 50–80 mmol/24 hrs) and potassium excretion increases to > 100 mmol/24 hrs (normal 50–80 mmol/ 24 hrs). Despite this, hypokalaemia is relatively rare because of a net efflux of potassium from cells. This typical pattern may be modified by fluid and electrolyte administration.

Catabolism and starvation

Catabolism is the breakdown of complex substances to their constituent parts (glucose, amino acids and fatty acids) which form substrates for metabolic pathways. Starvation occurs when intake is less than metabolic demand. Both usually occur simultaneously following severe injury or major surgery, with the clinical picture being determined by whichever predominates.

Catabolism

Carbohydrate, protein and fat catabolism is mediated by the increase in circulating catecholamines and proinflammatory cytokines, as well as the hormonal changes observed following surgery.

The mechanisms mediating muscle catabolism are incompletely understood, but inflammatory mediators and hormones (e.g. cortisol) released as part of the metabolic response to injury appear to play a central role. Minor surgery, with minimal metabolic response, is usually accompanied by little muscle catabolism. Major tissue injury is often associated with marked catabolism and loss of skeletal muscle, especially when factors enhancing the metabolic response (e.g. sepsis) are also present.

In health, the normal dietary intake of protein is 80–120 g per day (equivalent to 12–20 g nitrogen). Approximately 2 g of nitrogen are lost in faeces and 10–18 g in urine each day, mainly in the form of urea. During catabolism, nitrogen intake is often reduced but urinary losses increase markedly, reaching 20–30 g/day in patients with severe trauma, sepsis or burns. Following uncomplicated surgery, this negative nitrogen balance usually lasts 5–8 days, but in patients with sepsis, burns or conditions associated with prolonged inflammation (e.g. acute pancreatitis) it may persist for many weeks. Feeding cannot reverse severe catabolism and negative nitrogen balance, but the provision of protein and calories can attenuate the process. Even patients undergoing uncomplicated abdominal surgery can lose ~600 g muscle protein (1 g of protein is equivalent to ~5 g muscle), amounting to 6% of total body protein. This is usually regained within 3 months.

Changes in red blood cell synthesis and coagulation

Anaemia is common after major surgery or trauma because of bleeding, haemodilution following treatment with crystalloid or colloid and impaired red cell production in bone marrow (because of low erythropoietin production by the kidney and reduced iron availability due to increased ferritin and reduced transferrin binding). Whether moderate anaemia confers a survival benefit following injury remains unclear, but actively correcting anaemia in non-bleeding patients after surgery or during critical illness does not improve outcomes.

Following tissue injury, the blood typically becomes hypercoagulable and this can significantly increase the risk of thromboembolism; reasons include:

Factors modifying the metabolic response to injury

The magnitude of the metabolic response to injury depends on a number of different factors (Table 1.6) and can be reduced through the use of minimally invasive techniques, prevention of bleeding and hypothermia, prevention and treatment of infection and the use of locoregional anaesthesia. Factors that may influence the magnitude of the metabolic response to surgery and injury are summarised in table 1.6.

Table 1.6 Factors associated with the magnitude of the metabolic response to injury

Factor Comment
Patient-related factors
Genetic predisposition Gene subtype for inflammatory mediators determines individual response to injury and/or infection
Coexisting disease Cancer and/or pre-existing inflammatory disease may influence the metabolic response
Drug treatments Anti-inflammatory or immunosuppressive therapy (e.g. steroids) may alter response
Nutritional status Malnourished patients have impaired immune function and/or important substrate deficiencies. Malnutrition prior to surgery is associated with poor outcomes
Acute surgical/trauma-related factors
Severity of injury Greater tissue damage is associated with a greater metabolic response
Nature of injury Some types of tissue injury cause a disproportionate metabolic response (e.g. major burns),
Ischaemia–reperfusion injury Reperfusion of ischaemic tissues can trigger an injurious inflammatory cascade that further injures organs.
Temperature Extreme hypo- and hyperthermia modulate the metabolic response
Infection Infection is associated with an exaggerated response to injury. It can result in systemic inflammatory response syndrome (SIRS), sepsis or septic shock.
Anaesthetic techniques The use of certain drugs, such as opioids, can reduce the release of stress hormones. Regional anaesthetic techniques (epidural or spinal anaesthesia) can reduce the release of cortisol, adrenaline and other hormones, but has little effect on cytokine responses

Fluid and Electrolyte Balance

In addition to reduced oral fluid intake in the perioperative period, fluid and electrolyte balance may be altered in the surgical patient for several reasons:

Careful monitoring of fluid balance and thoughtful replacement of net fluid and electrolyte losses is therefore imperative in the perioperative period.

Normal water and electrolyte balance

Water forms about 60% of total body weight in men and 55% in women. Approximately two-thirds is intracellular, one-third extracellular. Extracellular water is distributed between the plasma and the interstitial space (Fig. 1.5A).

The differential distribution of ions across cell membranes is essential for normal cellular function. The principal extracellular ions are sodium, chloride and bicarbonate, with the osmolality of extracellular fluid (normally 275–295 mOsmol/kg) determined primarily by sodium and chloride ion concentrations. The major intracellular ions are potassium, magnesium, phosphate and sulphate (Fig. 1.5B).

The distribution of fluid between the intra- and extravascular compartments is dependent upon the oncotic pressure of plasma and the permeability of the endothelium, both of which may alter following surgery as described above. Plasma oncotic pressure is primarily determined by albumin.

The control of body water and electrolytes has been described above. Aldosterone and ADH facilitate sodium and water retention while atrial natriuretic peptide (ANP), released in response to hypervolaemia and atrial distension, stimulates sodium and water excretion.

In health (Table 1.7):

In the absence of sweating, almost all sodium loss is via the urine and, under the influence of aldosterone, this can fall to 10–20 mmol/24 hrs. Potassium is also excreted mainly via the kidney with a small amount (10 mmol/day) lost via the gastrointestinal tract. In severe potassium deficiency, losses can be reduced to about 20 mmol/day, but increased aldosterone secretion, high urine flow rates and metabolic alkalosis all limit the ability of the kidneys to conserve potassium and predispose to hypokalaemia.

In adults, the normal daily fluid requirement is ~30–35 ml/kg (~2500 ml/day). Newborn babies and children contain proportionately more water than adults. The daily maintenance fluid requirement at birth is about 75 ml/kg, increasing to 150 ml/kg during the first weeks of life. After the first month of life, fluid requirements decrease and the ‘4/2/1’ formula can be used to estimate maintenance fluid requirements: the first 10 kg of body weight requires 4 ml/kg/h; the next 10 kg 2ml/kg/h; thereafter each kg of body requires 1ml/kg/h. The estimated maintenance fluid requirements of a 35 kg child would therefore be:

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The daily requirement for both sodium and potassium in children is about 2–3 mmol/kg.

Assessing losses in the surgical patient

Only by accurately estimating (Table 1.8) and, where possible, directly measuring fluid and electrolyte losses can appropriate therapy be administered.

Table 1.8 Sources of fluid loss in surgical patients

  Typical losses per 24 hrs Factors modifying volume
Insensible losses 700–2000 ml ↑ Losses associated with pyrexia, sweating and use of non-humidified oxygen
Urine 1000–2500 ml ↓ With aldosterone and ADH secretion;
↑ With diuretic therapy
Gut 300–1000 ml ↑ Losses with obstruction, ileus, fistulae and diarrhoea (may increase substantially)
Third-space losses 0–4000 ml ↑ Losses with greater extent of surgery and tissue trauma

The effect of surgery

Intravenous fluid administration

When choosing and administering intravenous fluids (Table 1.10) it is important to consider:

Types of intravenous fluid

Crystalloids

Dextrose 5% contains 5 g of dextrose (d-glucose) per 100 ml of water. This glucose is rapidly metabolized and the remaining free water distributes rapidly and evenly throughout the body’s fluid compartments. So, shortly after the intravenous administration of 1000 ml 5% dextrose solution, about 670 ml of water will be added to the intracellular fluid compartment (IFC) and about 330 ml of water to the extracellular fluid compartment (EFC), of which about 70 ml will be intravascular (Fig. 1.6). Dextrose solutions are therefore of little value as resuscitation fluids to expand intravascular volume. More concentrated dextrose solutions (10%, 20% and 50%) are available, but these solutions are irritant to veins and their use is largely limited to the management of diabetic patients or patients with hypoglycaemia.

Sodium chloride 0.9% and Hartmann’s solution are isotonic solutions of electrolytes in water. Sodium chloride 0.9% (also known as normal saline) contains 9 g of sodium chloride dissolved in 1000 ml of water; Hartmann’s solution (also known as Ringer’s lactate) has a more physiological composition, containing lactate, potassium and calcium in addition to sodium and chloride ions. Both normal saline and Hartmann’s solution have an osmolality similar to that of extracellular fluid (about 300 mOsm/l) and after intravenous administration they distribute rapidly throughout the ECF compartment (Fig. 1.6). Isotonic crystalloids are appropriate for correcting EFC losses (e.g. gastrointestinal tract or sweating) and for the initial resuscitation of intravascular volume, although only about 25% remains in the intravascular space after redistribution (often less than 30–60 minutes).

Balanced solutions such as Ringers lactate, closely match the composition of extracellular fluid by providing physiological concentrations of sodium and lactate in place of bicarbonate, which is unstable in solution. After administration the lactate is metabolised, resulting in bicarbonate generation. These solutions decrease the risk of hyperchloraemia, which can occur following large volumes of fluids with higher sodium and chloride concentrations. Hyperchloraemic acidosis can develop in these situations, which is associated with adverse patient outcomes and may cause renal impairment. Some colloid solutions are also produced with balanced electrolyte content.

Hypertonic saline solutions induce a shift of fluid from the IFC to the EFC so reducing brain water and increasing intravascular volume and serum sodium concentration. Potential indications include the treatment of cerebral oedema and raised intracranial pressure, hyponatraemic seizures and ‘small volume’ resuscitation of hypovolaemic shock.