Nutritional Assessment in ICU

 

Nutritional Assessment in the Intensive Care Unit: A Comprehensive Review

Dr Neeraj Manikath, Claude.ai

Introduction

Malnutrition is highly prevalent among critically ill patients, affecting 30-50% of patients admitted to the intensive care unit (ICU). Proper nutritional assessment is fundamental to identifying patients at nutritional risk and implementing appropriate nutritional support strategies. This review examines current evidence-based approaches to nutritional assessment in the ICU setting, with a focus on practical applications for critical care physicians.

Importance of Nutritional Assessment in Critical Care

Critically ill patients experience significant metabolic changes including hypermetabolism, increased protein catabolism, and altered substrate utilization. These changes, coupled with pre-existing malnutrition and prolonged inadequate intake, contribute to rapid nutritional deterioration. Malnutrition in ICU patients is associated with:

• Impaired immune function and increased infection risk
• Delayed wound healing
• Prolonged mechanical ventilation
• Extended ICU and hospital length of stay
• Increased mortality

Early identification of nutritional risk through systematic assessment allows for timely intervention and potentially improved outcomes.

Traditional Nutritional Assessment Methods

Anthropometric Measurements

While standard in stable patients, anthropometric measurements have significant limitations in the ICU:

• Weight measurements are confounded by fluid shifts, edema, and resuscitation
• Height may be difficult to measure in supine, unconscious patients
• Body mass index (BMI) fails to account for body composition changes
• Mid-arm circumference and skinfold thickness measurements require standardized techniques and may be affected by fluid status

Despite limitations, admission weight and height should be recorded when possible, with serial weight measurements interpreted cautiously in the context of fluid balance.

Biochemical Markers

Traditional biochemical markers include:

• Serum proteins (albumin, prealbumin, transferrin)
• Lymphocyte count
• Nitrogen balance

However, these markers are significantly affected by the acute phase response, making interpretation challenging in critical illness. Albumin and prealbumin primarily reflect inflammation rather than nutritional status in the ICU population.

Validated Nutritional Screening and Assessment Tools

Nutritional Risk Screening (NRS-2002)

The NRS-2002 incorporates disease severity and has been validated in hospitalized patients, including those in critical care. It evaluates:

• Nutritional status (weight loss, reduced intake, BMI)
• Disease severity
• Age adjustment

A score ≥3 indicates nutritional risk.

NUTRIC Score (Nutrition Risk in the Critically Ill)

The NUTRIC score is the first nutritional risk assessment tool developed specifically for critically ill patients. It incorporates:

• Age
• APACHE II score
• SOFA score
• Number of comorbidities
• Days from hospital to ICU admission
• IL-6 level (optional)

A high NUTRIC score (≥5 without IL-6, ≥6 with IL-6) identifies patients most likely to benefit from aggressive nutritional therapy.

Subjective Global Assessment (SGA)

The SGA evaluates:

• Medical history (weight change, dietary intake, gastrointestinal symptoms, functional capacity)
• Physical examination (muscle wasting, fat loss, edema)

While valuable, SGA requires training for consistent application and may be difficult to perform in unconscious patients.

Advanced Body Composition Assessment Techniques

Bioelectrical Impedance Analysis (BIA)

BIA estimates body composition by measuring tissue resistance to electrical current flow. In the ICU setting:

• Altered hydration status significantly impacts measurements
• Positioning requirements may be difficult to achieve
• Phase angle may provide prognostic information independent of fluid status

Computed Tomography (CT) and Magnetic Resonance Imaging (MRI)

CT and MRI scans performed for clinical purposes can be repurposed for body composition assessment:

• L3 vertebra level measurements correlate with whole-body muscle mass
• Sarcopenia at ICU admission (identified by CT) is associated with poorer outcomes
• Not practical for routine monitoring due to radiation exposure and cost

Ultrasound

Muscle ultrasound has emerged as a promising bedside tool:

• Measures muscle thickness, cross-sectional area, and echogenicity
• Can detect changes in muscle mass over time
• Quadriceps muscle thickness correlates with function and outcomes
• Not affected by fluid status to the same degree as other methods

Energy Expenditure Assessment

Predictive Equations

Numerous equations exist to estimate energy requirements, including:

• Harris-Benedict
• Penn State
• Ireton-Jones
• Faisy-Fagon

However, these equations have limited accuracy in critically ill patients due to the dynamic nature of metabolic stress.

Indirect Calorimetry

Indirect calorimetry measures oxygen consumption and carbon dioxide production to calculate resting energy expenditure:

• Gold standard for determining energy requirements
• Accounts for individual metabolic variations
• Can be repeated to track changing requirements
• Limited by technical challenges and availability

Functional Assessment

Handgrip Strength

Handgrip strength measurement:

• Correlates with overall muscle strength
• Predicts hospital outcomes
• May detect functional decline before visible muscle wasting
• Limited applicability in sedated patients

Physical Function in ICU Test (PFIT)

The PFIT evaluates:

• Shoulder strength
• Knee extension strength
• Ability to stand from sitting
• Step cadence

This functional assessment can guide nutritional therapy and rehabilitation efforts.

Comprehensive Approach to Nutritional Assessment

A comprehensive approach integrates multiple methods:

1. Initial Screening: Apply NRS-2002 or NUTRIC score within 24-48 hours of admission
2. Detailed Assessment: Evaluate body composition, functional status when possible
3. Energy Requirement Determination: Preferably by indirect calorimetry, or using predictive equations with caution
4. Protein Requirement Estimation: Based on severity of illness and comorbidities
5. Ongoing Monitoring: Regular reassessment of nutritional status and adjustment of therapy

Integration with Clinical Practice

Nutritional assessment should inform a comprehensive nutrition care plan:

• Early enteral nutrition when appropriate
• Supplemental parenteral nutrition when indicated
• Monitoring of nutrition delivery versus targets
• Prevention of refeeding syndrome
• Regular reassessment and plan modification

Emerging Approaches

Biomarkers

Novel biomarkers under investigation include:

• Citrulline (marker of enterocyte mass and function)
• MicroRNAs related to muscle metabolism
• Myostatin and growth differentiation factor-15

Metabolomics and Proteomics

These technologies may enable:

• Personalized nutritional assessment
• Early detection of metabolic derangements
• Monitoring of response to nutritional therapy

Conclusion

Nutritional assessment in critically ill patients requires a multifaceted approach that acknowledges the limitations of traditional methods in the context of critical illness. Integration of validated screening tools, body composition analysis, and functional assessment provides the most comprehensive evaluation. Future research should focus on developing ICU-specific assessment methods that account for the unique metabolic alterations of critical illness and can guide personalized nutritional support strategies.

References

1. Singer P, Blaser AR, Berger MM, et al. ESPEN guideline on clinical nutrition in the intensive care unit. Clin Nutr. 2019;38(1):48-79.

2. 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.

3. Heyland DK, Dhaliwal R, Jiang X, Day AG. 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.

4. Paris MT, Mourtzakis M. Assessment of skeletal muscle mass in critically ill patients: considerations for the utility of computed tomography imaging and ultrasonography. Curr Opin Clin Nutr Metab Care. 2016;19(2):125-130.

5. Thibault R, Pichard C. Nutrition and clinical outcome in intensive care patients. Curr Opin Clin Nutr Metab Care. 2010;13(2):177-183.

6. Zusman O, Theilla M, Cohen J, et al. Resting energy expenditure, calorie and protein consumption in critically ill patients: a retrospective cohort study. Crit Care. 2016;20(1):367.

7. Ferrie S, Allman-Farinelli M. Commonly used "nutrition" indicators do not predict outcome in the critically ill: a systematic review. Nutr Clin Pract. 2013;28(4):463-484.

8. Sheean PM, Peterson SJ, Gomez Perez S, et al. The prevalence of sarcopenia in patients with respiratory failure classified as normally nourished using computed tomography and subjective global assessment. JPEN J Parenter Enteral Nutr. 2014;38(7):873-879.

9. Chapple LS, Deane AM, Heyland DK, et al. Energy and protein deficits throughout hospitalization in patients admitted with a traumatic brain injury. Clin Nutr. 2016;35(6):1315-1322.

10. 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.

11. Dinglas VD, Aronson Friedman L, Colantuoni E, et al. Muscle weakness and 5-year survival in acute respiratory distress syndrome survivors. Crit Care Med. 2017;45(3):446-453.

12. Preiser JC, van Zanten AR, Berger MM, et al. Metabolic and nutritional support of critically ill patients: consensus and controversies. Crit Care. 2015;19:35.

13. Ridley EJ, Parke RL, Davies AR, et al. What happens to nutrition intake in the post-intensive care unit hospitalization period? An observational cohort study in critically ill adults. JPEN J Parenter Enteral Nutr. 2019;43(1):88-95.

14. Arabi YM, Casaer MP, Chapman M, et al. The intensive care medicine research agenda in nutrition and metabolism. Intensive Care Med. 2017;43(9):1239-1256.

15. Mogensen KM, Robinson MK, Casey JD, et al. Nutritional status and mortality in the critically ill. Crit Care Med. 2015;43(12):2605-2615.