Nutrition and the surgical patient

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Contents

Introduction

There have been many improvements in the treatment of surgical patients during the 20th and early 21st century, leading to better safety, survival and scope of operative procedures. These improvements include anaesthesia, asepsis, blood transfusion, antibiotics, ventilatory support and parenteral nutrition. Nutritional advances have developed in parallel with a growing understanding of the metabolic response in injury and sepsis. This research field has recently been energised by new discoveries and techniques in molecular biology. The discovery that cytokines have effects as biological response modifiers, in addition to their immunologic mediatory role, the putative role of oxygen free radicals, and the identification of other mediators of the inflammatory response have been central to these developments. Knowledge of metabolism and expertise in nutrition are increasingly part of the stock-in-trade of the modern surgeon.

Nutrition features low in the body's homeostatic economy. Its priorities are oxygen delivery, regulation of acid-base balance and maintenance of fluid compartments. Threats to oxygen delivery are dealt with almost instantaneously by changes in minute ventilatory volume, alterations of cardiac output, and better efficiency in oxygen uptake and extraction by tissues. Acid-base abnormalities take longer to adjust, with both acute (buffering) and chronic (excretion) mechanisms. Changes in extracellular (including intravascular) and intracellular compartment volumes occur even more slowly, with the temporising expedient of intercompartmental fluxes.

The body's adjustments to starvation are slow, perhaps understandably, because they are not immediately life threatening. Nevertheless these changes are profound and critical to survival. Defective responses to nutritional deprivation are a major cause of morbidity and mortality in the surgical setting, complicated as it often is by sepsis and the response to injury. These subtle and drawn-out events can be overlooked by the clinician, resulting in a more protracted, and even jeopardised, surgical convalescence.

Body composition

The energy stores and body composition of an average 40-year-old man weighing 73 kg are shown in Table 10, “Energy stores and body composition in a 40-year-old 73-kg male”

Table 10. Energy stores and body composition in a 40-year-old 73-kg male
Mass (kg) Available energy (kcal)
Water 42 0
Fat 15 110 000
Protein 12 25 000
Glycogen 0.6 2 500
Minerals 3.4 0

Fat represents a high-energy source and can be hydrolysed to free fatty acids and glycerol. The metabolism of fat produces 9.4 kcal of energy per gram. Forty-five per cent of body protein is structural and not available for metabolic interchange, while the remaining 55% is contained in cells and circulating proteins. If this protein is lost it leads to loss of function, including muscle weakness and immune deficiency. The body's stores of carbohydrate are low and act as a rapidresponse provider of glucose, particularly in stress situations. The ratio of the fat-free mass (total bodyweight minus fat) to total body water (fat-free body hydration) is remarkably constant in a healthy person, but varies markedly in illness states.

Nutritional syndromes

Malnutrition and its complications are common but often unrecognised in surgical patients. Severe malnutrition can be defined as measured weight being greater than 20% less than recalled ‘well’ weight. Approximately 5% of patients coming to surgery are severely malnourished, and malnutrition is present to varying degrees in up to 50% of patients who have undergone surgery.

Studies of malnourished children, particularly in the context of developing countries, have recognised two broad syndromes of malnutrition that can be usefully transposed to the adult surgical setting. The first is marasmus, due to inadequate intake of an otherwise balanced diet (Table 11, “Comparisons between marasmus and kwashiorkor”). In the adult this is manifested as cachexia and is commonly termed protein-energy malnutrition (PEM).

Table 11. Comparisons between marasmus and kwashiorkor
Marasmus Kwashiorkor
Nutritional defect Impaired delivery Impaired utilisation
Protein catabolism Compensated Uncompensated
Aetiology No food Sepsis
No appetite Major trauma
Gut blocked, short, inflamed or fistulated Burns
Metabolic rate Normal or reduced Increased
Prognosis if untreated Months Weeks
Principles of treatment Replenish with standard nutrition Resuscitate and support
Use simplest available route Control sepsis
Treat underlying illnesses, if any Provide non-standard nutritional regimens
Clinical course Straightforward Complicated

The second entity is kwashiorkor, which results from an inadequate as well as unbalanced diet containing relatively more calories than protein (Table 11, “Comparisons between marasmus and kwashiorkor”). There is characteristic fluid retention, which may mask the commonly seen and often rapid erosion of muscle and fat stores. In the adult this is seen accompanying sepsis and after trauma.

Pathogenesis of metabolic events

In starvation, glycogen is initially broken down to produce glucose, to maintain brain function. However, glycogen is rapidly exhausted and in marasmus the body undergoes an important change, over several days, to use ketone bodies (keto-adaptation) from fat as brain fuel. This important adaptation preserves muscle protein. In sepsis and trauma, however, this does not occur and protein is catabolised to provide gluconeogenic precursors (glutamine and alanine), which in turn produce the glucose needed for the glucoseobligate tissues (including a healing wound). Accompanying this is a decrease in protein anabolism, and accelerated fat breakdown. In severe sepsis and burns this protein catabolism is even more marked and energy expenditure massively increases, fuelled by intense free fatty acid oxidation.

The metabolic response to injury is complex and was first described more than 60 years ago. However, in the past 15 years an explosion in knowledge of this field has led to important new therapeutic possibilities.

Initially it was thought that the metabolic response to injury was a neuroendocrine response, as within a few minutes of beginning an operation the level of counter-regulatory hormones (cortisol, glucagon and catecholamines) rises. In uncomplicated surgery these act only to initiate protein catabolism, as the endocrine response is relatively short-lived (lasting 24–48 hours). However protein catabolism continues for up to 1 month after major surgery. This has perplexed investigators, but recently, with advances in molecular biology, it has become clear that proinflammatory cytokines (e.g. tumour necrosis factor-alpha (TNF-α), interleukin (IL)-1, IL-6, IL-8) are important mediators of ongoing protein catabolism in injury and sepsis. These probably act locally, at the site of injury, and indirectly (via the bloodstream and in the central nervous system). Imbalances between pro- and anti-inflammatory cytokines also probably play a role in anorexia, pyrexia, fatigue, and fat catabolism. Arachidonic acid metabolites, oxygen free radicals and nitric oxide are also important mediators.

Contributions of the disease

The disease itself may play an important role in malnutrition associated with surgical illness. Gastrointestinal disease can produce obstruction, malabsorption and fistulas. Inflammatory mediators associated with the inflammatory phlegmon may secondarily lead to PEM and worsen fluid and electrolyte disturbances. AIDS leads to severe cachexia, similar to that seen in cancer. This is probably mediated by cytokines such as TNF-α and is complicated by chronic infection and malignancies. In cancer there is a rise in resting energy expenditure and the tumour avidly retains nitrogen as well as operating at a glucose-wasteful, high rate of anaerobic metabolism. Unlike the situation in experimental animal models, these tumour effects are unlikely to explain the degree of cachexia often seen in humans. Cancer-induced anorexia and host cytokine production are probably involved.

Complications of the metabolic response to injury and malnutrition

Malnutrition is complicated by immune incompetence and decreased wound healing ability. Protein-energy metabolism may be accompanied by physiological changes such as poor muscle function, manifest as physical weakness and poor respiratory muscle function. These changes increase the likelihood of postoperative pneumonia and difficulty in weaning from ventilators.

Fatigue is a common concomitant of surgical illness and is characterised by prolonged mental and physical tiredness. After surgery it is most pronounced at 1 week, and slowly improves for up to 3 months. It is worse in the elderly, in patients who were tired prior to surgery, and in patients with cancer. It appears to have an important psychological component because it only occurs in humans.

Nutritional assessment

There is no single clinical or laboratory test that defines nutritional status exactly (Some nutritional markers for nutritional assessment). The aim of nutritional assessment is to define how much the patient has lost from his or her body stores of protein and fat and, as a corollary, how much remains. Some sort of assessment of physiological impairment is important, as it has been found that the PEM is clinically significant only when associated with impairment of physiological function.

Nutritional assessment begins with a careful clinical evaluation. Important features of the history are weight loss greater than 10% during the past 3 months and a change in exercise tolerance. Physical examination may reveal non-healing wounds, oedema and fistulas.

Body composition is assessed by simple clinical tests and a standard blood test. Loss of body fat is often apparent from observations of the patient but is also assessed by palpating the triceps' and biceps' skinfolds. If the dermis can be felt between finger and thumb then it is likely that the body mass is composed of less than 10% fat.

Protein stores are assessed by observation and palpation of the temporalis, deltoids, suprascapular and infrascapular muscles, the bellies of biceps and triceps and the interossei of the hands. If the tendons are palpable or the bony shoulder girdle is sharply outlined (tendon-bone test) then the patient is likely to have lost more than 30% of total body protein stores.

Plasma albumin levels are of assistance in determining the type of PEM. In kwashiorkor, the albumin may be low, reflecting the expansion of the extracellular fluid space, and this may manifest clinically as pitting oedema.

Assessment of physiological function is of vital importance because weight loss without evidence of physiological abnormality is probably of no consequence. Function is observed while performing a physical examination and then by watching the patient's activity on the ward. Grip strength is assessed and respiratory muscle strength is assessed by asking the patient to blow hard holding a strip of paper 10 cm from the lips. Severe impairment is present when the paper fails to move.

Metabolic stress will be revealed by history and examination. It is present if the patient has had major surgery or trauma in the preceding week and where there is evidence of sepsis or ongoing inflammation, such as inflammatory bowel disease.

These findings are recorded on a standard form (an example is shown in Table 12, “A standard nutritional impairment record”) and an assessment of the type of nutritional impairment is made. Marasmic patients will have weight loss greater than 10% and normal albumin, and will resemble a ‘walking skeleton’. Patients with kwashiorkor have suffered from major trauma or serious sepsis and are not eating. There will be clear signs of major metabolic stress, albumin will be low and oedema is likely to be present. They may have near-normal stores of muscle and fat but this will not last for long if the situation persists. A mixture of the two conditions is termed marasmic kwashiorkor.

Table 12. A standard nutritional impairment record
Energy and protein balance Normal/mild Moderate Severe
Weight loss (degree and pattern) (for 3 months) <10% >10–20% >20%
Meal size, frequency and type (for 3 months) 2/3 size 1/3 size
Output (vomiting, diarrhoea, stoma)
Summary
Body composition Normal/mild Moderate Severe
Fat store depletion (finger-thumb test)
Protein store depletion (tendon-bone test)
Oedema
Albumin 35.47 g/L <35 g/L <30 g/L
Summary
Physiologic function Normal/Mild Moderate Severe
Exercise tolerance/watching the patient's activity
Grip strength (squeeze examiner's fingers)
Respiratory function (blow a strip of paper)
Shortness of breath, respiratory excursion
Wound healing: unhealed wounds, scratches or sores, and infection
Summary
Metabolic stress Normal/Mild Moderate Severe
Temperature >38°C in past 24 hours ( ) Pulse rate 100/min in past 24 hours ( )
WCC 12 000 or 3000 in past 24 hours ( ) Respiratory rate 30/min in past 24 hours ( )
Positive blood culture ( ) Surgery, trauma and sepsis ( )
Defined focus of infection ( ) Active inflammatory bowel disease ( )
Summary
Type of protein energy malnutrition
Normal ( ) Marasmus ( ) Marasmus/Kwashiorkor ( ) Kwashiorkor ( )
Severity of protein energy malnutrition
Normal/Mild ( ) Moderate ( ) Severe ( )
Nutritional metabolic goal

Determining the intensity and type of malnutrition is of great importance in setting nutritional goals. When PEM is severe and affects physiological function, post-operative complications are more common and post-operative stay is prolonged. The identification of metabolic stress is also important; because the extracellular water is expanded the response to standard nutritional intervention is impaired and the type of malnutrition is predictable.

Nutritional intervention

The principles of nutritional intervention are summarised in Principles of nutritional intervention.

Indications for nutritional intervention

Nutritional intervention is indicated prior to surgery only in severely malnourished patients with physiological impairment. Nutritional support is required in patients who cannot eat, in whom intake is insufficient for their needs, in whom the gastrointestinal tract cannot be used, and in those with accelerated losses (Table 13, “Indications for nutritional intervention in surgical patients”).

Table 13. Indications for nutritional intervention in surgical patients
Indications for Indications for total Indications for
pre-operative nutrition parenteral nutrition enteral nutrition
Severe malnutrition with Gut is obstructed Malnutrition with a
physiological impairment Gut is short functioning gut
Gut is fistulated Post-operative feeding
Gut is inflamed
Gut cannot cope

There are six components of adequate nutrition: protein, water, energy (as fat and carbohydrate), electrolytes, minerals and vitamins (Table 14, “Components of a nutrition regimen for 25- to 55-year-olds per day”).

Table 14. Components of a nutrition regimen for 25- to 55-year-olds per day
Component Requirement in health Requirement after major surgery
Protein 1.0–1.5 g/kg 1.5–2.0 g/kg
Water 40 mL/kg Variable according to losses
Energy 40 kcal/kg 40 kcal/kg
Electrolytes 75 mmol sodium Variable according to losses
50 mmol potassium
Minerals 15 mEq calcium Variable according to losses
40 mmol phosphate
10 mEq magnesium
Vitamins B group, C, fat soluble Some vitamins may be of benefit in surgical illness

The requirements for these different components vary according to the patient and the clinical condition. A detailed discussion regarding this complex topic is beyond this text and interested readers are referred to the recommended reading list.

Intravenous nutrition

Intravenous nutrition (also known as total parenteral nutrition (TPN) is useful if the gut is obstructed, too short, fistulated, inflamed or simply cannot cope, such as in post-operative ileus. Total parenteral nutrition is administered by a dedicated central venous catheter inserted under sterile conditions. Central venous catheter infection is potentially life threatening and therefore care must be meticulous. Approximately 50 kcal/kg bodyweight per day and 0.3 g of nitrogen as amino acids per kilogram per day is required to achieve gain in body protein. Use of nutritional intervention must be preceded by correction of anaemia, hypoalbuminaemia, fluid and electrolyte abnormalities, and deficits in trace metals. Vitamins must be dealt with by appropriate infusions so that administered nutrients will be used efficiently.

In certain situations, intravenous nutrition can be administered by peripheral infusion. This is not common because it is not possible to provide full energy intake using this technique and it requires frequent intravenous catheter changes.

Enteral nutrition

In circumstances where the gut is functional, enteral nutrition should be used. Enteral nutrition may be important in maintaining gut barrier function, demonstrated to be of critical importance in laboratory models. Enteral nutrition is administered by mouth if possible (as high-energy nutritional supplements), but may also be delivered by a fine-bore feeding tube introduced under fluoroscopic control or using an endoscope. Fine tubes can also be placed into the jejunum at surgery and feeding can begin in the recovery room after the operation is complete. If prolonged enteral feeding is anticipated, a gastrostomy should be created, usually via the percutaneous endoscopic route.

Metabolic response modification

Although provision of adequate nutrition to patients, both pre-operatively and post-operatively, has been of immense benefit, it has proven difficult for the injured patient to gain nitrogen. Several advances in the past few years have made this goal achievable and there have been important improvements in patient recovery.

With the recognition of the importance of glutamine, several studies have demonstrated that it is beneficial to include this amino acid in solutions administered to patients. Other amino acids have important roles in particular states, such as liver and renal failure. The role of growth hormone may be shown to be important in the future, and recently its usefulness in enabling patients to become anabolic earlier has been demonstrated. Anticytokine therapy awaits a better understanding of cytokine biology, and single-agent trials have so far been disappointing. This probably relates to the complexity of cytokine production and function, and the sites of action of cytokines in injury and sepsis.

Several recent studies from Europe have shown that specialised immune-enhancing formulas improve immune function, modulate cytokine production, decrease wound infections, and improve patient recovery in patients undergoing major surgery.

Epidural anaesthesia blocks much of the early stress response to surgery and this has been postulated to be of critical importance in slowing protein loss. What may be of more importance is the mobility that epidural anaesthesia permits the surgical patient in the immediate post-operative period and the ability of the epidural block to limit post-operative ileus.

Non-steroidal anti-inflammatory drugs (NSAID) may be important in preventing arachidonic acid mediated tissue damage, as may nitric oxide inhibition and antioxidants in limiting free oxygen radical damage. These await further evaluation in clinically relevant models.

The current trend toward minimally invasive surgical interventions has led, in many cases, to early recovery from surgery and faster return to work. When these techniques are combined with other modulators, the improvements in post-operative outcome are likely to be quite profound.

Results of nutritional intervention

Short-term pre-operative nutritional intervention in severely compromised patients decreases postoperative complications. The effect is not nearly as apparent in patients with mild to moderate malnutrition. Post-operative nutritional support is one of the most important developments in modern surgery and has allowed surgeons much greater leeway in the management of surgical complications such as fistulas and bowel obstruction.

Recent work has demonstrated that outcome is improved by growth hormone, glutamine-enriched nutrition and epidural anaesthesia. Growth hormone improves post-operative fatigue, and enhances recovery in children with burns. Similarly, minimally invasive surgery results in a faster recovery from surgery and a marked improvement in post-operative fatigue.

In combination, minimally invasive surgery, early mobilisation, NSAID, epidural anaesthesia and early enteral feeding produce dramatic post-operative recovery and return to useful activity.
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