SECTION OUTLINE
- Nutritional Assessment and Techniques Used in Nutritional Assessment
- Pathophysiology of Malnutrition, Starvation, Metabolic Derangements in Diseased (Simple Starvation, Stress Starvation Malnutrition and the Response to Injury)
INTRODUCTION
The provision of nutrition support to the critically ill patient is a fundamental aspect of their care. Although the optimal strategy has not yet been agreed, undertaking a thorough nutrition assessment can greatly assist the clinician to make the safest and the most appropriate choice regarding the route, timing, and amount of nutrition support. Whilst most intensive care units (ICU) will have a feeding protocol in place to allow enteral feeding to commence as soon as hemodynamic stability has been achieved, it is now generally accepted that patients who are expected to stay for more than 72 hours undergo a full nutrition assessment to allow individualization of their nutrition prescription.1
An initial nutrition assessment consists of three elements:
- Nutrition screening to determine nutrition risk
- Nutrition assessment to determine the nutrition status of the patient
- Calculation of energy and protein targets
Following this, it is recommended that the patients are monitored frequently and adjustments made to their nutrition prescription accordingly. This chapter will outline the steps and techniques involved in undertaking a nutrition assessment of the critically ill patient.
NUTRITION SCREENING
Nutrition screening aims to identify those patients who may benefit the most from nutrition support, a concept defined as nutrition risk.2 This is an important consideration given many patients in the ICU will have short admissions. And therefore provision of scant resources in the healthcare setting, such as specialist staffing, can be directed where they may be most useful.
There are several tools available for use in the ICU, each with their own limitations. The Malnutrition Universal Screening Tool (MUST) is a five-step screening tool that requires knowledge of the patient's weight, height, and any history of weight loss. These are all factors that are difficult to obtain in the fluid overloaded, sedated ICU patient. In addition, obese patients score low on MUST which may not reflect their true nutrition risk in the ICU setting whereby, these patients are frequently underfed and may have higher protein needs than adequately nourished patients.
The Nutrition Risk Score (NRS) 2002 is recommended by both The European Society for Parenteral and Enteral Nutrition (ESPEN)3 and the American Society for Parenteral and 4Enteral Nutrition (ASPEN).1 NRS 2002 is a two-part screening tool that includes severity of illness as a variable, but is likely not sensitive enough for most critically ill patients as an APACHE II score more than 10 automatically classifies the patient as high risk. Similar to the MUST, it also requires height, weight, and previous dietary history making this a difficult tool to use in the ICU setting.
Finally, the Nutrition Risk in the Critically ill (NUTRIC) Score has been proposed as a more desirable screening tool in critical illness2, 4 and is also recommended by ASPEN.1 NUTRIC includes variables such as severity of illness and organ failure, body mass index (BMI), age, and length of stay in hospital prior to ICU admission. Whilst it does not include any nutrition specific variables, the last point is intended as a surrogate for malnutrition. NUTRIC has not yet been prospectively validated in the critically ill and its main limitation is the time taken to calculate the severity of illness and organ failure scores at the bedside making it a tool most likely to be used in the research setting at present.
As there is currently no ideal nutrition screening tool to determine the nutrition risk of critically ill patients, it is important to consider all variables included in current screening tools as well as additional factors which may influence the nutritional risk of the patient such as muscle mass, projected length of stay on the ICU and the specific injury or illness which may influence nutritional status (e.g., trauma or burns). Considering each of these factors together can help make an informed judgment of the nutrition risk of individual patients.
Whichever is chosen, a screening tool or clinical judgment, it is recommended that nutrition screening should be undertaken within 48 hours of admission to the ICU (Table 1).
NUTRITION ASSESSMENT
Following nutrition screening, a full nutrition assessment should be undertaken by a suitably qualified individual in those patients considered to be a high nutrition risk.
5However, it should be noted that a patient's nutritional status and hence nutrition risk may change over the course of their admission and therefore it is imperative that all patients are monitored frequently and care plans adapted accordingly. A low nutrition risk patient on admission is not necessarily a low risk patient for the entirety of their admission. In addition, patients staying in the ICU for more than 7 days should undergo a full nutrition assessment.
A nutrition assessment should consider each of the following aspects. These should be considered on initial nutrition assessment and when monitoring the patient and the nutrition care plan should be formulated and adapted accordingly (Table 1).
Anthropometry
Ideally, an accurate weight and height would contribute to a thorough nutrition assessment. However, due to the clinical condition of the patient, this is often not possible or may be highly inaccurate due to large fluid shifts. However, it may be possible to obtain a recent weight and height from medical notes or family members and attempts should be made to do this.
Muscle mass may be an important predictor of outcome and potentially the response to nutrition support.5,6 At present, tools to assess muscle mass at the bedside, such as muscle ultrasound or computed tomography, are reserved for the research setting only. However, utilizing techniques such as the Subjective Global Assessment (SGA) which include a visual assessment of muscle wasting, has been shown to predict outcome in ICU patients and should therefore be considered.7
Biochemistry
Biochemical indices should be utilized with caution in the presence of inflammation during critical illness. Measures such as albumin are unreliable indicators of nutritional status and those that may be more reliable, such as pre-albumin, are not routinely measured.
A review of the patient's electrolyte status should be undertaken during a nutrition assessment as this can give clues as to the patient's nutritional status and response to nutrition therapy (e.g., refeeding syndrome). Although, like other biochemical indices, should be interpreted in the context of clinical condition as several factors, unrelated to the provision of nutrition support, may alter electrolyte status. Fluctuations in blood sugar levels should also be noted and monitored.
Clinical
The patient's clinical status and admission reason can provide valuable information during a nutrition assessment. For example, patients admitted with burns or trauma can experience a significant and rapid decline in their nutritional status. Factors such as temperature, rehabilitation, and certain medications (e.g., sedation) can provide details regarding fluctuations in energy expenditure or the need to adjust the nutrition prescription. It is also imperative to evaluate and monitor any extended periods of fasting and gastrointestinal intolerance as this may require alterations to the nutrition plan.
Nutritional
Previous nutrition intake should be considered where this information can be obtained from either recent medical notes or family members, however, inaccuracies associated with this 6should be noted. The cumulative energy and protein balance should be calculated at least twice per week during ICU admission, but ideally daily if this can be done via an electronic medical notes system. The aim should be to meet more than 80% of the prescribed target, otherwise plans should be put in place to achieve this. Non-nutritive energy sources such as propofol, citrate, and intravenous glucose should be included in this calculation as failure to do so may lead to overfeeding.
CALCULATION OF ENERGY AND PROTEIN TARGETS
The final step in a thorough nutrition assessment is to determine the individual energy and protein targets for the patient so that the most appropriate feeding regimen can be put in place. Optimal energy and protein targets are not yet known for critically ill patients, although guidelines are recommended adjusting energy targets according to the phase of critical illness. This is difficult given the lack of consensus on how to define the different phases. For this reason, energy and protein targets should be individualized and revised over the course of admission both when clinical condition changes and when the patient is discharged to the ward.
Energy
Studies investigating full feeding versus trophic or hypocaloric feeding over the first week of critical illness have not shown a benefit in terms of mortality or length of stay.8, 9 Despite this, some degree of hypocaloric feeding is currently advocated over the first week of ICU admission to reduce the risk of overfeeding that may be associated with the endogenous production of glucose. The currently accepted definition for hypocaloric feeding is 70% of measured energy expenditure or 80% using a predictive equation. Again, this is in relation to a reduction in mortality only, the effect of this feeding strategy on muscle wasting and recovery is not known.
The gold standard for determining energy targets is indirect calorimetry, although its use is not widespread due to the associated cost, labor, and exclusions. For this reason, predictive equations are most often used. There are several equations available for use in critically ill patients (Table 2), all with their limitations. Whichever equation is chosen, it is important to use the equation in exactly the way it was validated to obtain the most accurate result. This includes checking whether actual, ideal, or adjusted weight was used and the patient population in which the equation was derived (e. g., trauma).
Protein
Protein targets have been recently revised and are now higher than previous recommendations. There is a lack of robust evidence to support the higher targets and clinical judgement should be utilized in this case. A minimum of 1.2 g/kg/day is recommended with higher targets for specific patient groups (Table 3).7
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CONCLUSION
Nutrition screening to determine nutrition risk followed by a thorough nutrition assessment is recommended within 48 hours of admission to the ICU. Patients should be monitored frequently throughout their ICU admission and feeding regimens adjusted accordingly. Assessment and monitoring should not only include factors directly associated with nutritional status, biochemical and clinical factors that may provide information as to the patient's nutritional status or tolerance of their feeding regimen.
REFERENCES
- McClave SA, Taylor BE, Martindale RG, et al. Guidelines for the Provision and Assessment of Nutrition Support Therapy in the Adult Critically Ill Patient: Society of Critical Care Medicine (SCCM) and American Society for Parenteral and Enteral Nutrition (A.S.P.E.N.). JPEN J Parenter Enteral Nutr 2016;40(2):159–211.
- Heyland DK, Dhaliwal R, Jiang X, et al. Identifying critically ill patients who benefit the most from nutrition therapy: the development and initial validation of a novel risk assessment tool. Crit Care 2011;15(6):R268.
- Kondrup J. ESPEN Guidelines for Nutrition Screening 2002. Clin Nutr 2003;22(4):415–21.
- Rahman A, Hasan RM, Agarwala R, et al. Identifying critically-ill patients who will benefit most from nutritional therapy: Further validation of the “modified NUTRIC” nutritional risk assessment tool. Clin Nutr 2016;35(1):158–62.
- Moisey LL, Mourtzakis M, Cotton BA, et al. Skeletal muscle predicts ventilator-free days, ICU-free days, and mortality in elderly ICU patients. Crit Care 2013;17(5):R206.
- Weijs PJ, Looijaard WG, Dekker IM, et al. Low skeletal muscle area is a risk factor for mortality in mechanically ventilated critically ill patients. Crit Care 2014;18(2):R12.
- Bector S, Vagianos K, Suh M, et al. Does the Subjective Global Assessment Predict Outcome in Critically Ill Medical Patients? J Intensive Care Med 2016;31(7):485–9.
- Arabi YM, Aldawood AS, Haddad SH, et al. Permissive Underfeeding or Standard Enteral Feeding in Critically Ill Adults. N Engl J Med 2015;372(25):2398–408.
- Rice TW, Wheeler AP, Thompson BT, et al. Initial trophic vs full enteral feeding in patients with acute lung injury: the EDEN randomized trial. JAMA 2012;307(8):795–803.