FOR UNDERGRADUATE LEVEL

1. FLUID, ELECTROLYTE BALANCE &
ACID BASE DISORDER
DR SS KAMBLE

2. Learning objectives To understand:
• The causes and consequences of malnutrition in the surgical patient
• Fluid and electrolyte requirements in the pre- and postoperative
patient
• The nutritional requirements of surgical patients and the nutritional
consequences of intestinal resection
• The different methods of providing nutritional support and their
complications

3. INTRODUCTION
• Malnutrition is common.
• It occurs in about 30% of surgical patients with gastrointestinal
disease and in up to 60% of those in whom hospital stay has been
prolonged because of postoperative complications.
• It is frequently unrecognised and consequently patients often do not
receive appropriate support.

4. • Long-standing protein–calorie malnutrition as seen in cachexia or
general frailty is easy to recognise .
Short-term undernutrition, although less easily recognised, frequently
occurs in association with
• critical illness,
• major trauma,
• burns
• or surgery,
• and also impacts on patient recovery.

6. • The aim of nutritional support is to identify those patients at risk of
malnutrition and to ensure that their nutritional requirements are
met by the most appropriate route and in a way that minimises
complications.

7. PHYSIOLOGY
• Metabolic response to starvation After a short fast, lasting 12 hours
or less, most food from the last meal will have been absorbed.
• Plasma insulin levels fall and glucagon levels rise, which facilitates the
conversion of liver glycogen (approximately 200 g) into glucose.
• The liver, therefore, becomes an organ of glucose production under
fasting conditions.

8. Many organs, including
• brain tissue,
• red and white blood cells
• and the renal medulla,
• can initially utilise only glucose for their metabolic needs.

9. • Additional stores of glycogen exist in muscle (500 g), but these cannot
be utilised directly.
• Muscle glycogen is broken down (glycogenolysis) and converted to
lactate, which is then exported to the liver where it is converted to
glucose (Cori cycle).

11. Metabolic response to trauma and sepsis

12. NUTRITIONAL ASSESSMENT
Laboratory techniques
• There is no single biochemical measurement that reliably identifies
malnutrition.
• Albumin is not a measure of nutritional status, particularly in the
acute setting.
• Although a low serum albumin level (<30 g/L) is an indicator of poor
prognosis, hypoalbuminaemia invariably occurs because of alterations
in body fluid composition and because of increased capillary
permeability related to ongoing sepsis.

13. • Malnutrition is associated with defective immune function, and
measurement of lymphocyte count and skin testing for delayed
hypersensitivity frequently reveal abnormalities in malnourished
patients.
• Immunity is not, however, a precise or reliable indicator of nutritional
status, nor is it a practical method in routine clinical practice.

14. Body weight and anthropometry
• A simple method of assessing nutritional status is to estimate weight
loss.
• Measured body weight is compared with ideal body weight obtained
from tables or from the patient’s usual or premorbid weight.
• Unintentional weight loss of more than 10% of a patient’s weight in
the preceding 6 months is a good prognostic indicator of poor
outcome.

15. • Body weight is frequently corrected for height, allowing calculation of
the body mass index (BMI, defined as body weight in kilograms
divided by height in metres squared).
• A BMI of less than 18.5 indicates nutritional impairment and a BMI
below 15 is associated with significant hospital mortality.
• Major changes in fluid balance, which are common in critically ill
patients, may make body weight and BMI unreliable indicators of
nutritional status.

16. Anthropometric techniques incorporating measurements of skinfold
thicknesses and mid-arm circumference permit estimations of
• body fat
• and muscle mass,
• and these are indirect measures of energy and protein stores.

17. Clinical
• The possibility of malnutrition should form part of the workup of all
patients.
A clinical assessment of nutritional status involves
• a focused history and physical examination,
• an assessment of risk of malabsorption
• or inadequate dietary intake
• and selected laboratory tests aimed at detecting specific nutrient
deficiencies.

18. This is termed ‘subjective global assessment’ and encompasses
• historical,
• symptomatic
• and physical parameters.

19. Recently, the British Association of Parenteral and Enteral Nutrition
introduced a malnutrition universal screening tool (MUST), which is a five-
step screening tool to identify adults who are
• malnourished
• or at risk of undernutrition.

22. FLUID AND ELECTROLYTES
Fluid intake is derived from both
• exogenous (consumed liquids)
• and endogenous (released during oxidation of solid foodstuffs) fluids.
The average daily water balance of a healthy adult is given below.

24. Fluid losses occur by four routes:
1 Lungs.
• About 400 mL of water is lost in expired air each 24 hours.
• This is increased in dry atmospheres or in patients with a
tracheostomy, emphasising the importance of humidification of
inspired air.

25. 2 Skin.
• In a temperate climate, skin (i.e. sweat) losses are between 600 and
1000 mL/day.
3 Faeces.
• Between 60 and 150 mL of water are lost daily in patients with
normal bowel function.

26. 4 Urine.
• The normal urine output is approximately 1500 mL/ day and,
provided that the kidneys are healthy, the specific gravity of urine
bears a direct relationship to volume.
• A minimum urine output of 400 mL/day is required to excrete the end
products of protein metabolism.

27. • Maintenance fluid requirements are calculated approximately from
an estimation of insensible and obligatory losses.
• Various formulae are available for calculating fluid replacement
based on a patient’s weight or surface area.
• For example, 30–40 mL/kg gives an estimate of daily requirements.

28. The following are the approximate daily requirements of some
electrolytes in adults:
• sodium: 50–90 mM/day;
• potassium: 50 mM/day;
• calcium: 5 mM/day;
• magnesium: 1 mM/day

29. • The nature and type of fluid replacement therapy will be determined
by individual patient needs.
• The composition of some commonly used solutions is shown in Table
19.2.
• Note that Hartmann’s solution also contains lactate 29 mmol/L.

30. • Dextrose solutions are also commonly employed.
• These provide water replacement without any electrolytes and with
modest calorie supplements
(1 litre of 5% dextrose contains 400 kcal).

32. • A typical daily maintenance fluid regimen would consist of a
combination of 5% dextrose with either Hartmann’s or normal saline
to a volume of 2 litres.
• There has been much controversy in the literature regarding the
respective merits of crystalloid versus colloid replacement.
• There is no consensus on this topic and the usual advice is to replace
like with like.

33. • If the haematocrit is below 21%, blood transfusion may be required.
• There is increasing recognition, however, that albumin infusions are
of little value.

34. In addition to maintenance requirements,
• ‘replacement’ fluids
are required to correct pre-existing deficiencies
• and ‘supplemental’ fluids
are required to compensate for anticipated additional intestinal or
other losses.

35. The nature and volumes of these fluids are determined
by:
A careful assessment of the patient including
• pulse,
• blood pressure
• and central venous pressure, if available.

36. Clinical examination to assess
• Hydration status
a.peripheries,
b.skin turgor,
c.urine output
d.specific gravity of urine,
• urine
• and serum electrolytes
• and haematocrit.

37. Estimation of losses already incurred and their nature:
for example,
• vomiting,
• ileus,
• diarrhoea,
• excessive sweating
• or fluid losses from burns
• or other serious inflammatory conditions.

38. Estimation of supplemental fluids likely to be required in view of
anticipated future losses from
drains,
fistulae,
nasogastric tubes
or abnormal urine or faecal losses.

39. • When an estimate of the volumes required has been made, the
appropriate replacement fluid can be determined from a
consideration of the electrolyte composition of gastrointestinal
secretions.
• Most intestinal losses are adequately replaced with normal saline
containing supplemental potassium (Table 19.3).

41. NUTRITIONAL REQUIREMENTS
Total enteral or parenteral nutrition necessitates the provision of the
macronutrients,
• carbohydrate,
• fat
• and protein,
• together with vitamins,
• trace elements,
• electrolytes
• and water

42. • When planning a feeding regime, the patient should be weighed and
an assessment made of daily energy and protein requirements.
• Standard tables are available to permit these calculations.
• Daily needs may change depending on the patient’s condition.
• Overfeeding is the most common cause of complications, regardless
of whether nutrition is provided enterally or parenterally.

43. • It is essential to monitor daily intake to provide an assessment of
tolerance.
• In addition, regular biochemical monitoring is mandatory (Table 19.4)

45. Macronutrient requirements
Energy
• The total energy requirement of a stable patient with a normal or
moderately increased need is approximately 20–30 kcal/kg per day.
• Very few patients require energy intakes in excess of 2000 kcal/day.
• Thus, in the majority of hospitalised patients in whom energy
demands from activity are minimal, total energy requirements are
approximately 1300–1800 kcal/day.

46. Carbohydrate
• There is an obligatory glucose requirement to meet the needs of the
central nervous system and certain haematopoietic cells, which is
equivalent to about 2 g/kg per day.
• In addition, there is a physiological maximum to the amount of
glucose that can be oxidised, which is approximately 4 mg/kg per
minute (equivalent to about 1500 kcal/day in a 70-kg person), with
the nonoxidised glucose being primarily converted to fat.

47. • However, optimal utilisation of energy during nutritional support is
ensured by avoiding the infusion of glucose at rates approximating
physiological maximums.

48. • Plasma glucose levels provide an indication of tolerance.
• Avoid hyperglycaemia.
• Provide energy as mixtures of glucose and fat.
• Glucose is the preferred carbohydrate source.

49. Fat
• Dietary fat is composed of triglycerides of predominantly four long-
chain fatty acids.
There are
• two saturated fatty acids (palmitic (C16) and stearic (C18))
• and two unsaturated fatty acids (oleic (C18 with one double bond)
• and linoleic (C18 with two double bonds)).

50. • In addition, smaller amounts of linolenic acid (C18 with three double
bonds) and medium-chain fatty acids (C6–C10) are contained in the
diet.
• The unsaturated fatty acids,
linoleic
and linolenic acid,
are considered essential because they cannot be synthesised in vivo
from non-dietary sources.

51. • Both soybean and sunflower oil emulsions are rich sources of linoleic
acid and provision of only 1 litre of emulsion per week avoids
deficiency.
• Soybean emulsions contain approximately 7% alpha-linolenic acid (an
omega-3 fatty acid).

52. • The provision of fat as a soybean oil-based emulsion on a regular
basis will obviate the risk of essential fatty acid deficiency.
• Safe and non-toxic fat emulsions based upon long-chain triglycerides
(LCTs) have been commercially available for over 30 years.
• These emulsions provide a calorically dense product (9 kcal/g) and are
now routinely used to supplement the provision of non-protein
calories during parenteral nutrition.

53. • Energy during parenteral nutrition should be given as a mixture of fat
together with glucose.
• There is no evidence to suggest that any particular ratio of glucose to
fat is optimal, as long as under all conditions the basal requirements
for glucose (100–200 g/day)
and essential fatty acids (100–200 g/week) are met.

54. This ‘dual energy’ supply
• minimises metabolic complications during parenteral nutrition,
• reduces fluid retention,
• enhances substrate utilisation (particularly in the septic patient)
• and is associated with reduced carbon dioxide production.

55. Protein
• The basic requirement for nitrogen in patients without pre-existing
malnutrition and without metabolic stress is 0.10–0.15 g/kg per day.
• In hypermetabolic patients the nitrogen requirements increase to
0.20–0.25 g/kg per day.
• Although there may be a minority of patients in whom the
requirements are higher, such as after acute weight loss when the
objective of therapy is long term repletion of lean body mass, there is
little evidence that the provision of nitrogen in excess of 14 g/day is
beneficial.

56. Vitamins, minerals and trace elements
• Whatever the method of feeding, these are all essential components
of nutritional regimes.
• The water-soluble vitamins B and C act as coenzymes in collagen
formation and wound healing.
• Postoperatively, the vitamin C requirement increases to 60–80
mg/day.

57. Supplemental vitamin B12 is often indicated in patients who have
undergone
• intestinal resection
• or gastric surgery
• and in those with a history of alcohol dependence.
Absorption of the fat-soluble vitamins A, D, E and K is reduced in
steatorrhoea and the absence of bile.

58. FLUID AND NUTRITIONAL CONSEQUENCES OF
INTESTINAL RESECTION
• Up to 50% of the small intestine can be surgically removed or
bypassed without permanent deleterious effects.
• With extensive resection (<150 cm of remaining small intestine),
metabolic and nutritional consequences arise, resulting in the disease
entity known as short bowel syndrome.
• The clinical presentation of patients with short bowel syndrome is
dependent upon the site and extent of intestinal resection.

59. Small bowel motility
• Small bowel motility is three times slower in the ileum than in the
jejunum.
• In addition, the ileocaecal valve may slow transit.
• The adult small bowel receives 5–6 litres of endogenous secretions
and 2–3 litres of exogenous fluids per day.

60. • Most of this is reabsorbed in the small bowel.
• In the jejunum, the cellular junctions are leaky and jejunal contents
are always isotonic.
• Fluid absorption in this region of bowel is inefficient compared with
the ileum.

61. • It has been estimated that the efficiency of water absorption is 44%
and 70% of the ingested load in the jejunum and ileum, respectively.
• The corresponding figures for sodium are 13% and 72%, respectively.
• It can be seen, therefore, that the ileum is critical in the conservation
of fluid and electrolytes.

62. Ileum
• The ileum is the only site of absorption of vitamin B12 and bile salts.
• Bile salts are essential for the absorption of fats and fat-soluble
vitamins.
• The enterohepatic circulation of bile salts is critical to maintain the
bile salt pool.
• Following resection of the ileum, the loss of bile salts increases and is
not met by an increase in synthesis.

63. • Depletion of the bile salt pool results in fat malabsorption.
• In addition, loss of bile salts into the colon affects colonic mucosa,
causing a reduction in salt and water absorption, which increases
stool losses.

64. Colon
• Transit times in the colon vary between 24 and 150 hours.
• The efficiency of water and salt absorption in the colon exceeds 90%.
• Another important colonic function is the fermentation of
carbohydrates to produce short-chain fatty acids.

65. These have two important functions:
• first, they enhance water and salt absorption from the colon
• and, second, they are trophic to the colonocyte.

66. Effects of resection
• Resection of proximal jejunum results in no significant alterations in
fluid and electrolyte levels as the ileum and colon can adapt to absorb
the increased fluid and electrolyte load.
• Absorption of nutrients occurs throughout the small bowel, and
resection of jejunum alone results in the ileum taking over this lost
function.

67. • In this situation, there is no malabsorption.
• Resection of ileum results in a significant enhancement of gastric
motility and acceleration of intestinal transit.
• Following ileal resection, the colon receives a much larger volume of
fluid and electrolytes and it also receives bile salts, which reduce its
ability to absorb salt and water, resulting in diarrhoea.

68. • Even the loss of 100 cm of ileum may cause steatorrhoea, which can
necessitate the administration of oral cholestyramine to bind bile
salts.
• With larger resections (>100 cm) dietary fat restriction may be
necessary.
• Regular parenteral vitamin B12 is required.

69. ARTIFICIAL NUTRITIONAL SUPPORT
• The indications for nutritional support are simple.
• Any patient who has sustained 5 days of inadequate intake or who is
anticipated to have no or inadequate intake for this period should be
considered for nutritional support.
• The periods may be less in patients with pre-existing malnutrition.
• This concept is important because it emphasises that the provision of
nutritional support is not specific to certain conditions or diseases.

70. Although patients with
• Crohn’s disease
• or pancreatitis,
• or those who have undergone gastrointestinal resections,
may frequently require nutritional support, it is the fact that they have
had inadequate intakes for defined periods that is the indication rather
than the specific disease process.

71. Enteral nutrition
• The term ‘enteral feeding’ means delivery of nutrients into the
gastrointestinal tract.
• The alimentary tract should be used whenever possible.
• This can be achieved with normal food, oral supplements (sip
feeding) or with a variety of tube feeding techniques delivering food
into the stomach, duodenum or jejunum.
• A variety of nutrient formulations are available for enteral feeding.

72. These vary with respect to
• energy content,
• osmolarity,
• fat and
• nitrogen content
• and nutrient complexity;
most contain up to 1–2 kcal/mL and up to 0.6 g/mL of protein.

73. • Polymeric feeds contain intact protein and hence require digestion,
whereas monomeric/elemental feeds contain nitrogen in the form of
either free amino acids or, in some cases, peptides.
• These are less palatable and are used much less frequently than in
previous years.

74. Newer feeding formulations are available that include
• glutamine
• and fibre
to optimise intestinal nutrition,
or immunonutrients such as
• arginine
• and fish oils,
but these are expensive and their use is controversial.

75. Sip feeding
Commercially available supplementary sip feeds are used in patients
who can drink but whose
• appetites are impaired
• or in whom adequate intakes cannot be maintained with ad libitum
intakes.
These feeds typically provide 200 kcal and 2 g of nitrogen per 200 mL
carton.

76. There is good evidence to demonstrate that these sip-feeding
techniques are associated with a significant overall increase in
• calorie
• and nitrogen intakes
without detriment to spontaneous nutrition.

77. Tube-feeding techniques
Enteral nutrition can be achieved using
• conventional nasogastric tubes (Ryle’s),
• fine-bore feeding tubes inserted into the stomach,
• surgical or percutaneous endoscopic gastrostomy (PEG)
• or, finally, postpyloric feeding utilising nasojejunal tubes
• or various types of jejunostomy (Figure 19.3).

79. • The choice of method will be determined by local circumstances and
preference in many patients.
• Whichever method is adopted, it is important that tube feeding is
supervised by an experienced dietician
who will calculate the patient’s requirements and aim to achieve these
within 2–3 days of the instigation of feeds.

80. • Conventionally, 20–30 mL are administered per hour initially,
gradually increasing to goal rates within 48–72 hours.
• In most units, feeding is discontinued for 4–5 hours overnight to allow
gastric pH to return to normal.

81. Fine-bore tube insertion
• The patient should be semi-recumbent.
• The introducer wire is lubricated and inserted into the fine-bore tube
(Figure 19.4).
• The tube is passed through the nose and into the stomach via the
nasopharynx and oesophagus.
• The wire is withdrawn and the tube is taped to the patient.

83. • There is a small risk of malposition into a bronchus or of causing
pneumothorax.
• The position of the tube should be checked using plain abdominal
radiography (Figure 19.5).
• Confirmation of position by pH testing is possible but limited by the
difficulty of obtaining a fluid aspirate with narrow lumen tubes.

85. Gastrostomy
• The placement of a tube through the abdominal wall directly into the
stomach is termed ‘gastrostomy’.
• Historically, these were created surgically at the time of laparotomy.
• Today, the majority are performed by percutaneous insertion
underendoscopic control using local anaesthesia, known as PEG
(percutaneous endoscopic gastrostomy) tubes (Figure 19.6).

87. Jejunostomy
• In recent years, the use of jejunal feeding has become increasingly
popular.
• This can be achieved using nasojejunal tubes or by placement of
needle jejunostomy at the time of laparotomy.
• Some authorities advocate the use of jejunostomies on the basis that
postpyloric feeding may be associated with a reduction in aspiration
or enhanced tolerance of enteral nutrition.

88. • In particular, there are many advocates of jejunostomies in patients
with severe pancreatitis, in whom a degree of gastric outlet
obstruction may be present, related to the oedematous head of
pancreas.
• In most patients it is appropriate to commence with conventional
nasogastric feeding and progress to postpyloric feeding if the former
is unsuccessful.

89. • Nasojejunal tubes often necessitate the use of fluoroscopy or
endoscopy to achieve placement, which may delay commencement
of feeding.
• Surgical jejunostomies, even using commercially available needle-
insertion techniques, do involve creating a defect in the jejunum,
which can leak or be associated with tube displacement; both of
these complications result in peritonitis.

90. Complications
• Most complications of enteral nutrition can be avoided with careful
attention to detail and appropriate infusion rates.
• Patients should be nursed semi-recumbent to reduce the possibility
of aspiration.
• Complications can be divided into those resulting from intubation of
the gastrointestinal tract and those related to nutrient delivery.
• The former are more frequent with more invasive means of gaining
access to the intestinal tract.

91. The latter include
• diarrhoea,
• bloating
• and vomiting.
• Diarrhoea occurs in more than 30% of patients receiving enteral
nutrition and is particularly common in the critically ill.
• Up to 60% of patients in intensive care units may fail to receive their
targeted intakes.

92. • There is no evidence that the incidence of diarrhoea and bloating is
reduced by the use of half-strength feeds.
• It is important to introduce normal feeds at a reduced rate according
to patient tolerance.
• Metabolic complications associated with excessive feeding are
uncommon in enterally fed patients.

94. Parenteral nutrition
• Total parenteral nutrition (TPN) is defined as the provision of all
nutritional requirements by means of the intravenous route and
without the use of the gastrointestinal tract.
• Parenteral nutrition is indicated when energy and protein needs
cannot be met by the enteral administration of these substrates.

95. The most frequent clinical indications relate to those patients who have
undergone
• massive resection of the small intestine,
• who have intestinal fistula
• or who have prolonged intestinal failure for other reasons.

96. Route of delivery:
Peripheral or central venous access
• TPN can be administered either by a catheter inserted in the central
vein or via a peripheral line.
• In the early days of parenteral nutrition, the only energy source
available was hypertonic glucose, which, being hypertonic, had to be
given into a central vein to avoid thrombophlebitis.

97. Peripheral
• Peripheral feeding is appropriate for short-term feeding of up to 2
weeks.
• Access can be achieved either by means of a dedicated catheter
inserted into a peripheral vein and manoeuvred into the central
venous system (peripherally inserted central venous catheter (PICC)
line) or by using a conventional short cannula in the wrist veins.
• The former method has the advantage of minimising inconvenience
to the patient and clinician.

98. • PICC lines have a mean duration of survival of 7 days.
• The disadvantage is that when thrombophlebitis occurs, the vein is
irrevocably destroyed.

100. Central
• When the central venous route is chosen, the catheter can be
inserted via the
subclavian
or internal
or external jugular vein.

101. • There is good evidence to show that the safest means of establishing
central venous access is by insertion of lines under ultrasound
guidance; however, this will not be practicable for all cases.
Most intensive care physicians and anaesthetists favour cannulation
of
• internal
• or external jugular veins
as these vessels are easily accessible.

102. • They suffer the disadvantage that the exit site is situated
inconveniently on the side of the neck, where repeated movements
result in disruption of the dressing with the attendant risk of sepsis.
• The infraclavicular subclavian approach is more suitable for feeding as
the catheter then lies flat on the chest wall, which optimises nursing
care (Figure 19.8).

104. Complications of parenteral nutrition
• The commencement of TPN may precipitate or accentuate underlying
nutrient deficiency by encouraging anabolism.

106. Refeeding syndrome
• This syndrome is characterised by severe fluid and electrolyte shifts in
malnourished patients undergoing refeeding.
• It can occur with either enteral or parenteral nutrition, but is more
common with the latter.
• It results in
hypophosphataemia,
hypocalcaemia
and hypomagnesaemia.

107. These electrolyte disorders can result in
• altered myocardial function,
• arrhythmias,
• deteriorating respiratory function,
• liver dysfunction,
• seizures,
• confusion,
• coma,
• tetany
• and death.

108. Patients at risk include those with
• alcohol dependency,
• those suffering severe malnutrition,
• anorexics
• and those who have undergone prolonged periods of fasting.

109. Treatment
• involves matching intakes with requirements and assiduously avoiding
overfeeding.
• Calorie delivery should be increased slowly and vitamins
administered regularly.
• Hypophosphataemia and hypomagnesaemia require treatment.

110. Nutrition support teams
• Multidisciplinary nutrition teams ensure cost-effective and safe
nutritional support, irrespective of how this is administered.
• The incidence of catheter-related sepsis is significantly reduced.

111. SUMMARY
• Fluid therapy and nutritional support are fundamental to good
surgical practice.
• Accurate fluid administration demands an understanding of
maintenance requirements and an appreciation of the consequences
of surgical disease on fluid losses.
• This requires knowledge of the consequences of surgical intervention
and, in particular, intestinal resection.

112. • Malnutrition is common in hospital patients.
• All patients who have sustained or who are likely to sustain 5 days of
inadequate oral intake should be considered for nutritional support.
This may be
• dietetic advice alone,
• sip feeding
• or enteral
• or parenteral nutrition.

113. • These are not mutually exclusive.
The success or otherwise of nutritional support should be
determined by
• tolerance to nutrients provided
• and nutritional end points, such as weight.

114. • It is unrealistic to expect nutritional support to alter the natural
history of disease.
• It is imperative that nutrition-related morbidity is kept to a minimum.

115. This necessitates the
• appropriate selection of feeding method,
• careful assessment of fluid, energy and protein requirements, which
are regularly monitored,
• and the avoidance of overfeeding.