FLUID BALANCE AND PARENTERAL NUTRITION

Transcription

1 2 FLUID BALANCE AND PARENTERAL NUTRITION Ian Nesbitt During your first ever night on call as a doctor, it is almost a certainty that you will be asked to write up intravenous (IV) fluids. Fluid balance is one of the most important subjects in medicine. Sadly, however, it is a subject that is poorly covered in most textbooks and so in this chapter we start from first principles to help you understand and remember the topic. BASIC FACTS The human body is composed of approximately two-thirds water. The contribution of water to body weight depends on how much fat you carry, because fat contains very little water. People gain fat as they grow older. Women also tend to have a greater proportion of fat and so females and the elderly will have a smaller proportion of total body water. If we assume body water is 60% of our weight, then a 70 kg man will carry 42 l water. Of this 42 l, two-thirds (28 l) will be intracellular fluid (ICF), and one-third (14 l) will be extracellular fluid (ECF). The ECF is subdivided into plasma (3 l), interstitial fluid (ISF an aquatic habitat for the cells, about 10 l) and transcellular fluids (CSF, ocular, peritoneal and synovial fluids, about 1 l). Osmotic pressure is the pressure needed to reverse osmosis (through a semipermeable membrane, i.e. a cell wall). Simply put, it is the ability of a solute to attract water. Oncotic pressure on the other hand is the pressure exerted by proteins to draw fluids back in. 13

2 14 Surgical Talk: Revision in Surgery Figure 2.1. Distribution of fluids within the body spaces. The osmolalities (reflecting the osmotic pressure) of the ICF and the ECF are similar, although the main cation in the ECF is sodium, whereas in the ICF it is potassium. Fluid distribution between the ECF and the ICF is governed only by changes in the osmotic pressure. This means that isotonic fluid (which has the same osmolality as plasma) administered into the plasma will not enter the ICF, since there is no difference in the osmolality. Within the ECF, fluid distribution between the plasma and the ISF is only governed by Starlings forces, i.e. hydrostatic pressure (pushing fluid out of the blood vessels) versus oncotic pressures (sucking fluid back in). Therefore, fluid administered into the plasma would increase the hydrostatic pressure and dilute the oncotic pressure until the fluid was evenly distributed throughout the ECF (Figure 2.1). TYPES OF FLUID Crystalloids are essentially electrolytes in water. Because they have no large molecules (and thus have no oncotic pressure), they are easily distributed to

3 Fluid Balance and Parenteral Nutrition 15 the extracellular spaces and so can be used as maintenance fluids. Examples of crystalloids are normal saline (which is isotonic, i.e. 0.9% saline) and 5% dextrose, which is hypotonic. There is also a solution called Hartmans, which contains lactate, potassium and calcium in addition to sodium chloride, and is therefore described as physiological. Colloids contain larger molecules which stay in the circulation for longer. They increase the oncotic pressure and thus can draw fluid back into the circulation. They are good for maintaining blood pressure, although they do not have oxygen-carrying capacity. Examples of colloids are Haemaccel, which contains gelatin, and Dextran, which is a solution of high molecular weight dextrose. Why is this important? Well, if for example one gave 1 litre of isotonic saline intravenously, it would initally only be distributed into the ECF (which includes the plasma). As plasma is only 3 litre of the whole ECF, which is 14 litre, only 3/14 or 214 ml would remain in the plasma (the distribution takes minutes). In contrast, 1 litre of 5% dextrose (which is hypotonic and so initially dilutes the ECF relative to the ICF) administered intravenously would be equally distributed throughout the body, and so 3/42 litre or 70 ml would remain in the plasma. However, of 500 ml of a colloid given intravenously, all of it will stay in the plasma initially as there is no change in osmotic pressure and so no distribution into the cells (ICF). It is clear that different situations require different types of fluid replacement, and you can see why crystalloid preparations are of little use in acute blood loss when colloids or blood may be more appropriate. THE FLUID BALANCE EQUATION The simplest way to think about fluid balance is that it is an equilibrium where input must equal output. In order to live we must excrete all of our waste products. The main route for this is via the kidneys. The minimal volume of urine we need to produce in order to be healthy is about 1 l a day (0.5 1 ml/kg/h). This is the minimal obligatory volume of urine (MOVU). If less urine than this is produced, the patient is oliguric, and if no urine is produced, anuric.

4 16 Surgical Talk: Revision in Surgery At rest, we also have other fluid losses that we are unaware of and these are called insensible water losses. Insensible losses occur from the lungs and in faeces, which amount to about 500 ml, and from the skin by sweating, which is also about 500 ml. If we add up MOVU to the insensible losses, then our minimal daily fluid replacement is therefore about l. This figure, however, relates to a healthy adult lying in bed; if we got up and moved around, then the requirements will go up. In view of this we usually estimate that the average adult will require about 3 l of fluids a day. As well as water, we also lose about 60 mm potassium and 100 mm sodium per day; these salts will also need replacing. In a normal person, large amounts of fluid are recycled in the body and must be accounted for in the so-called equilibrium. These include gastric juice (3 4 l), bile (about 1 l) and intestinal secretions (succus entericus, about 3 4 l). This enteric recycling accounts for about 8 l/day and is mostly reabsorbed further down the GI tract. It is common sense that any fluid and electrolyte losses must be replaced, if we are to remain in equilibrium. This is usually achieved by our daily dietary intake of food and drink, although a small proportion of water is derived as a by-product of metabolism (Figure 2.2). Figure 2.2. The balance between fluid input and fluid output.

5 Fluid Balance and Parenteral Nutrition 17 Figure 2.3. Typical daily requirement of intravenous fluids: two alternative regimens. When patients come to hospital they may be unable to have sufficient oral fluid intake either because they are nil by mouth (perioperatively or through illness) or because they are vomiting. Such patients require intravenous fluids and these can be any of the available crystalloid preparations. As highlighted above, the average adult will need about 3 l a day. This amount of dextrose saline would do (each litre bag contains 30 mm of sodium). Another method would be to give 1 l normal saline (containing 150 mm sodium) and 2 l of 5% dextrose. The dextrose is quickly metabolised, leaving water. In either of these two methods you have replaced 3 l of water with either 90 or 150 mm of sodium. If you add 20 mm of potassium to each litre bag (some bags come with this already added), then you will have also replaced the necessary 60 mm of potassium. If you request each bag to run over 8 h, then the 3 l will last the full day (Figure 2.3). In cases where patients are on fluids for several days, it is inadvisable to prescribe dextrose saline alone, because after a few days of replacing too little sodium, the patient could well become hyponatraemic. This is easily avoided by adding in normal saline in every third bag. These are the standard regimens given to most normal adults; there are, however, many exceptions to the rule, including the following. The Postoperative Period The metabolic response to the stresses of surgery involves a rise in various hormones, including circulating catecholamines, ADH and, through

6 18 Surgical Talk: Revision in Surgery stimulation of the hypothalamo-pituitary-adrenal axis, cortisol and aldosterone. The overall result of these is the renal conservation of salt and water, with somewhat increased losses of potassium and hydrogen ions. These effects usually last for about h. Despite the high potassium losses in the urine, the serum potassium is usually maintained or may even transiently rise, through release of cellular contents by damaged tissues. Therefore, unless serum potassium levels are very low, it is probably best to avoid potassium supplements in the first day or two postoperatively. In addition, since water is being retained it is usual to reduce the fluid replacements to about 2 l in the first postoperative day, especially in patients prone to heart failure. It is important to remember that a patient going to theatre is likely to have starved for several hours beforehand and may not have been given any fluids intraoperatively or whilst in recovery. This patient will probably need extra fluids to maintain the fluid balance equilibrium. It is easy to see why one cannot rely on standard regimens when calculating how much fluid to give someone and so urine output is the best indicator, aiming for greater than 50 ml/h. The nursing staff should document urine output on the charts by the patient s bed. Young, fit adults can usually tolerate excess fluids and so as long as the urine output is satisfactory, you are probably doing okay. The minimum urine output is about 30 ml/h (remember MOVU). Obviously, the urea and electrolytes (U & Es) can be checked, to help assess renal function. Third Space Losses As mentioned before, about 8 l of secretions per day are reabsorbed in the bowel. A patient who has undergone abdominal surgery is likely to have a transient ileus postoperatively, where the bowel temporarily stops working. In such patients this is usually due to mechanical handling of the bowel, although any patient can suffer a transient ileus, due to an electrolyte disturbance or even the effects of anaesthesia. When an ileus is present, the fluid secreted into the bowel simply lies there, and is not reabsorbed completely. These third space losses mean that the patient effectively has a reduced volume of the ECF, and hence is fluid depleted.

7 Fluid Balance and Parenteral Nutrition 19 In such patients, extra fluid needs to be given to allow for the third space losses. Unfortunately, you will not know how much extra fluid is needed and so must rely on urine output as an indicator. You will usually notice a sudden diuresis on day 2 or 3 postoperatively, explained by recovery of the ileus and reabsorption of the fluids from the bowel. Similarly, in pancreatitis, patients can lose several litres of fluids rich in electrolytes and plasma proteins into the peritoneal cavity. Really, the only way to effectively gauge these losses is by vigorous replacement to maintain their urine output and correcting any electrolyte disturbances according to daily U & Es. If after 2 days 10 l have been put in with only 3l of urine produced, then assuming 1 2 l of insensible losses, this equates to about 5 or 6 l of fluid sequestered into the peritoneal cavity. Other Losses If a patient has a nasogastric tube or a wound drain or is draining via a fistula, these losses need to be calculated daily and replaced (usually as normal saline) in addition to the standard losses. Ileostomy patients can have huge losses, especially several days postoperatively. It is advisable to assess the ileostomy output at least twice daily, replacing these fluids and electrolytes accordingly to prevent acute dehydration. Patients with pyrexia require more fluids. One can lose 3 l, maybe more, in certain circumstances. A rough estimate is to increase the fluid replacement by 10% for each degree of fever. Similarly, losses through vomiting or diarrhoea need replacing; remember that large-intestinal juices contain high concentrations of potassium and gastric juice contains a lot of hydrogen ions. Heart or Liver Failure Because the renin angiotensin system is already working overtime, conserving sodium, you should avoid giving fluids which contain sodium, hence you should mainly use 5% dextrose in these patients.

8 20 Surgical Talk: Revision in Surgery In heart failure, the patient is fluid overloaded, the usual cause in surgical patients being poor management of the fluid balance by the doctor. If you look at the fluid balance charts you will probably see a positive balance over the previous few days. Therefore, you will need to reduce their input, maybe even stop the fluids altogether and consider diuretics. Obviously, you should examine the patient regularly, measuring their JVP, listening to their chest and watching for oedema. Very sick patients on the ward usually will have a CVP line, and this makes the assessment of these patients a little easier. Ask the nursing staff to chart the patient s daily weight as this will help in monitoring progress. Acute Renal Failure This can be prerenal (e.g. hypovolaemia), renal (e.g. acute tubular necrosis) or postrenal (e.g. a blocked catheter). After surgery both pre- and postrenal causes are the commonest, and so should be looked for and treated first. If the patient is fluid depleted, you may respond simply by correcting the dehydration. Look over the fluid balance charts from the previous days to decide whether the patient is in negative fluid balance. If you think that a renal cause is likely, you should avoid potassium loads, stop any drugs that may affect renal function (such as NSAIDS, ACE inhibitors, etc.) and involve the renal team in the management early on. Usually, they advise replacing the previous days output plus 500 ml to cover insensible losses. Alternatively, one could measure hourly urine output and replace 100% of this every hour. In summary, input should equal output unless the above exceptions apply. Look over the fluid balance charts (remember to bear in mind that in practice these are often very inaccurate) and the daily weight charts. Assess the patient s state of hydration (dry lips, skin turgor, etc.) and check their blood results for renal function and haematocrit. Finally, do not forget the temperature, both the patient s and that of the room. We have deliberately gone into a lot of details on this subject, perhaps more than you need to know to pass the finals. This is because it is not really a topic that one can waffle about in the exam either you understand the principles or you do not.

9 Fluid Balance and Parenteral Nutrition 21 NUTRITION Patients who are malnourished are prone to many complications, such as delayed wound healing, muscle weakness and an increased tendency to infection. There is evidence that patients with poor nourishment prior to surgery will benefit from preoperative supplementation and do better after their operation. One caveat of this is that intervention must be for a reasonable period of time (more than 10 days) in order to be of significant benefit. There are a lot of reasons why hospital patients become malnourished. They may have a decreased appetite due to the illness itself. They may have increased nutritional demands or their digestion may be impaired. Another reason could be due to the hospital stay itself, i.e. dislike of hospital food, being rushed off for an X-ray or ultrasound at noon, or being nil by mouth. If oral intake is not anticipated within 7 10 days of surgery, then nutritional support is indicated (perhaps 5 days in a previously malnourished patient). The main indication for preoperative nutritional support is severe malnourishment (greater than 10% weight loss). Nutritional support can vary from mere supplementation of vitamins, or protein in a high-protein diet, to a complete replacement of all essential foodstuffs. In this section, we only cover the latter. Enteral versus Parenteral Nutrition Enteral diets are those given via the gut, including oral intake. Obviously, the ideal situation is one where the patient takes in all the required nutrition orally; if this is not possible, then enteral feeding is the next option. This involves passing the food into the gut, allowing it to be absorbed normally, either through a nasogastric tube or, if required for longer periods, via a gastrostomy or jejenostomy. The commonest indication for enteral feeding is where there is a problem with swallowing, caused by a stroke or oesophageal obstruction. Parenteral nutrition bypasses the gut and involves a specialised feed directly into the patient s bloodstream. Parenteral nutrition may be used as

10 22 Surgical Talk: Revision in Surgery a supplement to enteral feeding when it is usually given through a cannula in a peripheral vein. Alternatively, total parenteral nutrition (TPN) can be used to deliver the complete nutritional requirements. As TPN has a high osmolality it is toxic to veins and is usually given via a central line. The buzz words would be to insert a small cannulae into a large vein with a high rate of blood flow. Hence, a central venous pressure (CVP) line is usually used. For longer-term use, a Hickman line is preferred, which is a modified CVP line usually tunneled under the skin to make it more secure and has a Dacron cuff to prevent infection from entering. Unfortunately, parenteral feeding has some complications, including an increased risk of infection: It is not uncommon for a house officer to get called to see a patient with parenteral feeding who has recently spiked a temperature. Obviously, your management would be as for any pyrexia (see page 36); however, if you suspect that the feed is the likely source of the infection, the correct thing to do is to stop the feed. If indeed this is the cause, then the temperature usually settles quickly, despite the fact that the CVP line is still in situ and may be infected. It appears that the running feed may be responsible for introducing the bugs from the infected line into the bloodstream. The CVP line will, however, ultimately need to be removed and replaced. Another complication of parenteral feeding is villous atrophy in the gut. Since the gut luminal cells (enterocytes) derive their nutrition from the lumen, long periods of rest can lead to atrophy. This makes the gut wall more permeable to bacterial flora and there is evidence that this can increase the risk of translocation of bacteria into the bloodstream. Electrolyte imbalances are likely and, therefore, the urea and electrolytes should be checked daily and adjusted accordingly. Hyperglycaemia is another problem and the patient may need to be given insulin temporarily whilst on TPN. Other disturbances of liver function are common (possibly because of fatty infiltration of the liver) and a cholestatic picture may be seen with raised alkaline phosphatase, and hence LFTs should be measured every few days.

11 Fluid Balance and Parenteral Nutrition 23 The take-home message must be that parenteral feeding should be reserved for patients in whom enteral feeding is impossible, such as patients with short gut syndrome, where large pieces of their gut have been surgically removed. Otherwise, enteral feeding should be your first choice. Requirements 1. Water. See section on fluid balance; roughly 2 3 l per day. 2. Energy. About 1800 calories per day. This is given as a mixture of carbohydrate and fats: roughly, in a ratio of two-thirds to one-third, respectively (but can be up to 50 50). 3. Nitrogen. About 14 g/day in protein, but the requirement may change (8 20 g/day) according to the metabolic state. 4. Vitamins. The fat-soluble vitamins are stored and so the levels are carefully adjusted to avoid overdose. Water-soluble vitamins being excreted are therefore given more generously. 5. Minerals. Sodium, potassium, calcium, magnesium, phosphate, etc. 6. Trace elements. Zinc, copper, iron, selenium, iodide, etc. The management of nutrition should be multidisciplinary, including the surgeon, the dietician, the pharmacist, who makes up the feed, and the nurses, who actually administer it. Nowadays, the feeds are usually made up into one complete sterile 3 l bag (even if it contains only 2 l) in the pharmacy department according to the specific requirements of the individual patient, hopefully with the dietician s advice. The most important step is the connection of the feed to the patient as this is when infection is likely to occur, and hence it should be a sterile procedure. Monitoring assessment of nutritional status is best done on a clinical basis. The patient s appearance and weight are the best indicators. Other anthropometric measurements, such as skin fold thickness, are not ideal but may be of benefit in monitoring progress. Daily measurement of albumin is pointless, since its half-life is long (about 21 days) and its level can be altered for many other reasons, although it is helpful in long-term monitoring.

12 24 Surgical Talk: Revision in Surgery Other biochemical tests are available, such as transferrin, which is better than albumin in the short term. But probably the best day-to-day biochemical measurement is prealbumin (a liver protein), which is a good marker of nutritional status. Obviously, electrolytes should be measured daily and LFTs should be checked every few days. Finally, of much amusement on ward rounds are the other markers of nutrition, such as grip strength and stool length but as to who does these, let alone how, I leave to your imagination.