Managing Physiological Change in the Surgical Patient

Factors Responsible for Systemic Responses ( Box 2.1 )

Surgical patients are subject to a variety of major stressors that make massive demands on the body’s ability to maintain physiological equilibrium and sustain life. Examples of such stressors include:

• Major operations —tissue trauma, blood and fluid loss, anaesthesia (particularly Trendelenburg head-down position + pneumoperitoneum for laparoscopic surgery), healing and repair

• Major trauma including fractures and burns; head, abdominal and chest injuries

• Major cardiovascular events, for example, myocardial infarction, pulmonary embolism, stroke

• Haemorrhage and fluid infusion including blood; fluid and electrolyte abnormalities

• Infection, inflammation and sepsis

• Hypoxia and hypotension

The way the body responds to major systemic insults depends on several factors: the physiological reserve of the patient’s vital organ systems (i.e., basic fitness), the nature of the injurious process, the severity of physiological disruption, the duration of delay before resuscitation and the virulence of any microorganisms involved. Most patients are remarkably resilient given good basic care but in a deteriorating patient, several physiological systems are likely to be impacted upon simultaneously, evoking a range of complex homeostatic mechanisms.

Managing the Deteriorating Patient

The aim is always to recognise problems early by regular clinical observation, and to correct abnormal physiology rapidly and accurately to prevent intrinsic compensatory mechanisms becoming overwhelmed. If this happens in one organ system without correction, escalating decompensation of other organ systems follows.

Management requires careful monitoring, often in a high-dependency or intensive care unit, with repeated investigations of organ function and dysfunction. In most elective operations, many of the responses discussed subsequently can be mitigated by good preoperative assessment, preoperative optimisation, appropriate perioperative fluid management, ensuring oxygenation, adequate analgesia, reducing psychological stress, preventing infection and using best operative technique to minimise tissue trauma, blood loss and complications. Enhanced recovery programmes (Enhanced Recovery After Surgery Programmes [ERAS] have been introduced which give special attention to these factors before, during and after operation and benefits accrue with attention to each of many small details ( Table 2.1 ). A list of ERAS society guidelines is given at: http://erassociety.org.loopiadns.com/guidelines/list-of-guidelines/ . ‘Prehabilitation’ exercise training has been used but has so far failed to translate into improved outcomes.

The individual variables responsible for potentially excessive systemic responses to severe injury or major surgery are summarised in Box 2.1 .

Stressors in the Surgical Patient

Metabolic Responses to Pathophysiological Stress

In severe trauma or extensive operative surgery, particularly if complicated by sepsis, the key factors in the systemic response are increased sympathetic activity plus increased circulating catecholamines and insulin . Cytokine responses signal other cells to prepare for action (e.g., polymorphs, T and B cells), to compensate for starvation, provide additional energy and building blocks for tissue repair, and conserve sodium ions and water.

Glucose production is massively increased by gluconeogenesis under the influence of catecholamines. There is also enhanced secretion of ACTH, glucocorticoids (cortisol), glucagon and growth hormone, all contributing to the general catabolic response . Insulin acts as an antagonist of most of these and is secreted in increased amounts from the second or third day after injury.

The sum of these factors is to cause inevitable catabolism and potentially extreme changes in fluid balance and electrolytes. These metabolic changes are shown in Fig. 2.1 .

Effects on Carbohydrate Metabolism

The overall effect is rising blood glucose (levels may reach 20 mmol/L); often resulting in hyperglycaemia and a pseudo-diabetic state, and glucose may appear in the urine. This is in marked contrast to simple fasting, in which glucose levels are normal or low and glycosuria does not occur.

Effects on Body Proteins and Nitrogen Metabolism

In a normal healthy adult, nitrogen balance is constantly maintained. Protein turnover results in daily excretion of 12 to 20 g of urinary nitrogen which is made good by dietary intake. In a hypercatabolic state, nitrogen losses can increase three- or fourfold. Most importantly, this metabolic environment prevents proper use of food or intravenous nutrition. There is therefore huge destruction of skeletal muscle. This state of negative nitrogen balance contrasts markedly with simple starvation in which body protein is preserved.

Effects on Lipid Stores and Metabolism

The effects of major body insults on lipid metabolism are little different from simple starvation; most of the energy requirements are met from fat stores.

Surgical catabolism reverses only as the patient recovers from the illness and therefore early parenteral nutrition has little effect, although carbohydrate administration may spare some protein loss.

Note that when patients have been severely ill, carbohydrate metabolism is minimal and energy comes from catabolism of protein and fat. Once feeding recommences, there is a danger of refeeding syndrome which should be anticipated (see later).

Introduction

Fluid, electrolyte and acid–base derangements can be minimised if high-risk patients are assessed before operation and closely monitored before, during and after operation. If abnormalities do develop, the diagnosis and management can be worked out with reasoning and common sense. Plasma urea and electrolytes should be checked at least daily in patients undergoing major surgery or those receiving intravenous fluids for more than a day or two.

In general, patients who are unable to meet their fluid or electrolyte needs require maintenance replacement therapy equivalent to 25 to 30 mL/kg per day of water, 1 mmol/kg per day of sodium, potassium and chloride, and 50 to 100 g/day of glucose (noting that 5% glucose contains 5 g glucose per 100 mL).

Severely ill patients with abdominal infection, sepsis and fistulae, and patients with severe burns are likely to suffer major problems of fluid balance (and nutrition, see later). These are best managed with the help of experienced anaesthetists and intensivists in high dependency or intensive care units, where monitoring and therapy can be rigorously managed.

Normal Fluid and Electrolyte Homeostasis

The body of an average 70 kg adult contains 42 L of fluid, distributed between the intracellular compartment, the extracellular space and the bloodstream ( Fig. 2.2 ). Fluid input is mainly by oral intake of fluids and food but about 200 mL/day of water is produced during metabolism. Normal adult losses are between 2.5 and 3 L/day. About 1 L is lost insensibly from skin and lungs, 1300 to 1800 mL are passed as urine (about 60 mL/h or 1 mL/kg per h) and 100 mL are lost in faeces. About 100 to 150 mmol of sodium ions and 50 to 100 mmol of potassium ions are lost each day in urine and this is balanced by the normal dietary intake ( Table 2.2 ).