Chapter 32: Nutritional Assessment

Nutritional assessment and nutritional support are important considerations during mechanical ventilation (Figure 32-1). Assessment of nutritional status and determination of nutritional requirements for mechanically ventilated patients requires the teamwork of physicians, dietitians, respiratory therapists, and nurses. Too few calories cause respiratory muscle catabolism and muscle weakness. Too many calories—particularly carbohydrate calories—increase metabolic rate and can result in respiratory muscle fatigue or hypercapnia due to increased CO₂ production (V̇co₂).

The relationship between metabolism, oxygen consumption (V̇o₂), and V̇co₂ is dependent on the substrate metabolized. V̇co₂ divided by V̇o₂ is the respiratory quotient (R). R is 1 for carbohydrate metabolism, 0.7 for fat metabolism, 0.8 for protein metabolism, 8.7 for lipogenesis, and 0.25 for ketogenesis. Whole-body R is normally 0.7 to 1. With balanced metabolism, R is 0.8, carbohydrate metabolism raises it toward 1, and fat metabolism lowers it toward 0.7. With lipogenesis, the overall R may be greater than 1, but it seldom exceeds 1.2. With ketogenesis, the overall R may be less than 0.7 but is seldom less than 0.65.

The principal function of the cardiopulmonary system is to provide the O₂ needed for energy production and to clear the CO₂ produced. An increase in metabolic rate increases V̇o₂ and V̇co₂, increases ventilation requirement, and forms the relationship between breathing (V̇o₂ and V̇co₂) and nutrition expenditure (energy as Kcal). Excessive caloric intake, particularly with carbohydrates, results in increased V̇co₂.

If mechanically ventilated patients receive inadequate nutritional support, they may suffer the effects of starvation. The initial response to starvation is an increase in glycogen and fat metabolism. Glycogenolysis provides glucose, which is necessary for cerebral metabolism. Glycogen stores are depleted after 4 to 5 days of fasting. Lipolysis of adipose tissue triglycerides produces ketones, which can also be metabolized by brain cells. Gluconeogenesis also occurs, primarily due to the breakdown of muscle and visceral proteins. By the third day of fasting, ketogenesis and gluconeogenesis are at maximal rates. There is also a decrease in metabolic rate with starvation that slows the rate at which nutritional stores are depleted. There are numerous effects of starvation on respiratory function (Table 32-1), the most serious of which is loss of respiratory muscle mass due to catabolism.

From height and weight, the basal energy expenditure (BEE) can be estimated using the Harris-Benedict equation:

where W is body mass (kg), H is height (cm), and A is age (years). The total daily caloric needs calculated from the Harris-Benedict equation are increased by an activity factor and an injury stress factor to determine the caloric needs of a patient. The activity factor is 20% if the patient is confined to bed and 30% if the patient is ambulatory. Typical stress factors are 10% to 30% for major trauma, 25% to 60% for sepsis, and 50% to 110% for burns.

The Ireton-Jones formula adjusts for obesity and mechanical ventilation. For the obese patient:

where G is gender (male = 1, female = 0), W is actual body weight (kg), A is age (years), and V is ventilator (present = 1, absent = 0). The Ireton-Jones formula for ventilated patients is:

where T is for trauma (present = 1, absent = 2) and B is for burn (present = 1; absent = 0). Energy needs may also be estimated with calories per kilogram of body weight (usually 25-35 kcal/kg) if other data are unavailable.

Biochemical data are also useful in the assessment of nutritional status (Table 32-2). Albumin levels correlate with the degree of malnutrition, and decreased levels are associated with increased risk of morbidity and mortality. Because its half-life is about 20 days, albumin levels reflect chronic rather than acute protein depletion. Albumin is not considered a specific indicator of visceral protein status in critically ill patients. Transferrin is a more sensitive indicator of acute changes in nutritional status than albumin because its half-life is about 8 to 10 days. Thyroxine-binding prealbumin (transthyretin) is a sensitive indicator of visceral protein status, especially in acute stages of protein-energy malnutrition. An advantage of prealbumin as an indicator of nutritional status is its short half-life (2-3 days). Retinol binding protein is highly sensitive to changes in nutritional status, with a 12-hour half-life. However, it has limited use as an assessment parameter in renal failure because it is filtered by the glomerulus and metabolized by the kidney. The total lymphocyte count is useful as a nutritional screening parameter with noncritically ill patients, and it correlates with albumin in reference to postsurgical mortality and morbidity.

Nitrogen balance determines the amount of nitrogen (protein) required to maintain nitrogen equilibrium and reflects anabolism/catabolism and distribution of protein. It is determined as:

where UUN is the urine urea nitrogen. The determination of nitrogen balance requires an accurate 24-hour urine collection, an accurate assessment of protein intake, and a creatinine clearance greater than 50 mL/min. Nitrogen balance is normally positive and becomes negative with inadequate caloric and/or protein intake and metabolic stress.

Indirect calorimetry is the calculation of energy expenditure by the measurement of V̇o₂ and/or V̇co₂, which are converted to energy expenditure (Kcal/day) by the Weir method:

Indirect calorimetry also allows calculation of the R. Indirect calorimeters operate by using an open- or a closed-circuit method.

Open-Circuit Method

The open-circuit method measures the concentrations and volumes of inspired and expired gases to determine V̇o₂ and V̇co₂. The principal components of an open-circuit calorimeter (metabolic cart) are the analyzers (O₂ and CO₂), a volume-measuring device, and a mixing chamber. The analyzers must be capable of measuring small changes in gas concentrations, and the volume monitor must be capable of accurately measuring volumes from 0.05 to 1 L. Exhaled gas from the patient is directed into a mixing chamber. At the outlet of the mixing chamber, a vacuum pump aspirates a small sample of gas for measurement of O₂ and CO₂. After analysis, this sample is returned to the mixing chamber. The entire volume of gas then exits through a volume monitor. Periodically, the analyzer also measures the inspired oxygen concentration. A microprocessor performs the necessary calculations. Meticulous attention to detail is required to obtain valid results using an open-circuit indirect calorimeter (Table 32-3).

Closed-Circuit Method

The key components of the closed circuit calorimeter are a volumetric spirometer, a mixing chamber, a CO₂ analyzer, and a CO₂ absorber. The spirometer is filled with a known volume of oxygen and connected to the patient. As the patient rebreathes from the spirometer, O₂ is removed and CO₂ is added. The CO₂ is removed from the system by a CO₂ absorber before the gas is returned to the spirometer. The decrease in the volume of the system equals V̇o₂. Gas from the patient flows into the mixing chamber, and a sample is aspirated for Fēco₂ analysis. From the mixing chamber, gas flows through a CO₂ absorber and then to the spirometer. The volume of the spirometer is electronically monitored to measure tidal volume. The difference between end-expiratory volumes is calculated by a microprocessor to determine V̇o₂. If the patient is mechanically ventilated, a bag-in-the-box system is used as a part of the inspiratory limb of the calorimeter. The bellows is pressurized by the ventilator to ventilate the patient. Measurement time is limited by Fio₂ and the volume of the spirometer. When the volume of the spirometer decreases to a critical level, the measurement is interrupted to refill the spirometer.

Leaks from the closed-circuit system will result in erroneously high V̇o₂ measurements (uncuffed airway, bronchopleural fistula, sidestream capnograph). Another problem with this technique is that system compressible volume is increased and trigger sensitivity is decreased. The advantage of the closed-circuit method over the open-circuit method is its ability to make measurements at a high Fio₂ (up to 1).

Other Approaches

In patients with a pulmonary artery catheter, V̇o₂ can be calculated from arterial oxygen content (Cao₂), mixed venous oxygen content (Cv̄o₂), and cardiac output (Q̇c):

Metabolic rate can then be calculated from the V̇o₂:

Metabolic rate can also be calculated from V̇co₂, which can be used in conjunction with volumetric capnography:

Normal V̇o₂ is 250 mL/min and normal V̇co₂ is 200 mL/min (2.6 mL/kg/min).

General Considerations with Indirect Calorimetry

Because indirect calorimetry is labor intensive and expensive, it should be reserved for selected patients (Table 32-4). When measuring resting energy expenditure (REE) using indirect calorimetry, one must consider both the duration of each measurement and the number of measurements required for a reliable 24-hour estimate. Ideally, continuous 24-hour indirect calorimetry produces the best estimate of REE. For most critically ill patients, it is impossible to obtain measurements for longer than 15 to 30 minutes once every several days. It is important, however, to recognize that shorter and less frequent measurements will less reliably estimate REE. When indirect calorimetry is performed, the patient should be resting, undisturbed, motionless, supine, and aware of the surroundings (unless comatose). The patient should either be on continuous nutritional support or fasting for several hours before the measurement. Before indirect calorimetry is performed, there should have been no changes in ventilation for at least 90 minutes, no changes that affect V̇o₂ for at least 60 minutes, and stable hemodynamics for at least 2 hours. Because REE is measured with the patient at rest, calories must be added due to patient activity. There may be considerable fluctuation in REE throughout the day and from day to day.

Enteral nutrition should always be considered when a patient has a functioning gastrointestinal (GI) tract. Nutrients absorbed via the portal system with delivery to the liver may allow for better absorption and result in enhanced immune competence. The presence of nutrients in the gut prevents intestinal atrophy and maintains the absorptive capacity of the GI mucosa. Enteral nutrition also helps preserve normal gut flora and gastric pH, which may guard against bacterial overgrowth in the small intestine and development of pneumonia. If enteral nutrition is administered appropriately, it is safer and less expensive than parenteral nutrition.

The preferred method of nutritional support is the oral route. However, it is practically impossible for most mechanically ventilated patients. In the mechanically ventilated patient, nasogastric or orogastric tubes are often used initially. These are used for short-term feeding and may be contraindicated in patients who have severe reflux, delayed gastric emptying, and are at risk of aspiration. A feeding tube placed into the small intestine should be considered for uninterrupted duodenal or jejunal feeding. It is generally assumed that feeding distal to the stomach decreases the risk of aspiration. Because of the risk of aspiration with enteral feeding, patients should be placed into the semirecumbent position (head elevated 30 degrees). In patients receiving mechanical ventilation and early enteral nutrition, the absence of gastric volume monitoring has been shown not to be inferior to routine residual gastric volume monitoring in terms of development of ventilator-associated pneumonia. Even minimal amounts of enteral feedings, called trophic nutrition, may have beneficial effects such as preserving intestinal epithelium. A study by the ARDS Network reported no significant difference in clinical outcomes among patients with acute lung injury initially provided trophic versus full enteral feeding for the first 6 days of mechanical ventilation.

The more distal to the stomach the feeding is delivered, the less likely aspiration related to the feeding. Thus, the optimal postpyloric tube placement is past the ligament of Treitz, or in the fourth portion of the duodenum. A Dobhoff tube is a small-bore, flexible, nasogastric feeding tube that is inserted by use of a stylet and typically has a weighted end that helps guide it through the digestive system.

If long-term feeding is needed, tubes can be placed through the skin into the stomach or small intestine by surgical, endoscopic, radiologic, or laparoscopic techniques. The percutaneous endoscopic gastrostomy (PEG) tube is placed endoscopically. These tubes are generally more comfortable than the nasogastric or enteric tubes.

Parenteral nutrition, which bypasses the GI tract, may be necessary when the GI tract is not functioning or if stimulation of the GI or pancreatic systems would worsen the condition of the patient. Placement of a central or peripheral venous catheter is required for parenteral nutrition. Central venous access is usually preferred because solutions of greater than 600 to 900 mOsm/L may be infused, the volume of fluid is unrestricted, and support may continue for a long time. Parenteral nutrition leaves the GI tract unstimulated, which can lead to gut atrophy, mucosal compromise, weakening of the gut barrier, and increased risk of pneumonia. The results of a randomized controlled trial suggest the clinical usefulness of complementing the energy delivery of insufficient enteral nutrition with a parenteral booster between days 4 and 8 after intensive care unit (ICU) admission. This approach reduced the risk of development of nosocomial infections, the number of antibiotic days, and the duration of mechanical ventilation. However, this is in contrast to another randomized controlled trial, which reported no benefit for early parenteral nutrition.

The objectives of nutritional support in mechanically ventilated patients are to preserve lean body mass, maintain immune function, and avert metabolic complications. Early nutritional support using the enteral route is a proactive strategy that may reduce disease severity, diminish complications, decrease length of stay in the ICU, and improve survival. A variety of nutritional supplements are commercially available. However, omega-3 and antioxidant supplementation in mechanically ventilated patients has not been found to provide benefit in terms of important patient outcomes. Glycemic control is important, but current evidence does not support tight glucose control in terms of improved outcomes and is associated with a higher risk of hypoglycemia. Guidelines for nutritional support of the Society of Critical Care Medicine that applies to mechanically ventilated patients are listed in Table 32-5.