The Pathophysiology of Malnutrition in Critical Illness
Critical illness triggers a systemic inflammatory response, leading to severe catabolism and hypermetabolism that rapidly depletes nutrient stores. This occurs even in previously well-nourished individuals.
The Hypermetabolic and Catabolic State
Stress hormones like cortisol and glucagon mobilize energy reserves, breaking down muscle protein and leading to rapid lean body mass loss. This loss can impair muscle function, potentially prolonging ventilator dependence.
The Role of Inflammation
Pro-inflammatory cytokines further exacerbate catabolism, impairing nutrient use, causing insulin resistance, and making markers like albumin unreliable.
Methods for Assessing Nutritional Status
Assessing nutritional status in critically ill patients is challenging due to their unstable condition, fluid shifts, and limited mobility. A combination of tools and clinical judgment is vital.
Nutritional Screening Tools
Screening tools identify high-risk patients needing detailed assessment.
- NRS-2002: Evaluates nutritional status (BMI, weight loss, intake) and disease severity. A score of three or higher indicates risk.
- NUTRIC Score: Designed for ICU patients, it includes factors like age, disease severity (APACHE II, SOFA), and comorbidities to predict outcomes influenced by nutrition.
Anthropometric Measurements
Traditional measures like BMI are often unreliable in the ICU due to fluid shifts and difficulty in obtaining measurements. Muscle ultrasonography is being investigated as a bedside monitoring tool.
Biochemical Markers
Albumin and prealbumin levels are poor indicators of nutritional status in critical illness as they are affected by the inflammatory response. Use them cautiously alongside other methods.
Comparison of Key Nutritional Screening Tools
| Feature | NRS-2002 | NUTRIC Score (Modified) |
|---|---|---|
| Patient Population | General hospital patients, but adapted for ICU | Specifically designed and validated for critically ill patients |
| Assessment Parameters | Nutritional status (BMI, intake, weight loss) and disease severity | Age, APACHE II, SOFA score, comorbidities, days from admission |
| Primary Purpose | Identify general patients at risk of malnutrition for intervention | Identify critically ill patients who will benefit most from aggressive nutritional therapy |
| Use in ICU | High scores are common, often grouping most ICU patients as high-risk | Better discriminates between high- and low-risk patients in the ICU |
| Considered Factors | Weight loss, BMI, dietary intake, disease stress | Age, severity of illness, comorbidities, time to ICU admission |
The Importance of Early Nutritional Intervention
Early identification of malnutrition risk is critical. Initiate nutritional support within 24–48 hours for patients unable to meet needs orally. Early, adequate nutrition reduces hospital stay, complications, and costs. It helps attenuate hypermetabolism, preserve muscle mass, and modulate the immune system.
Nutritional Interventions and Support Strategies
Enteral vs. Parenteral Nutrition
For patients unable to eat, nutrition can be delivered enterally (EN) or parenterally (PN).
- Enteral Nutrition (EN): Preferred route via a tube to the GI tract. Associated with fewer complications, preserves gut integrity, and is cost-effective.
- Parenteral Nutrition (PN): Delivered intravenously. Used for patients with a non-functional GI tract or when EN is insufficient.
Protein and Calorie Requirements
Adequate protein and calorie targets are vital.
- Protein: 1.2-2.0 g/kg/day is typically needed to counteract catabolism and preserve muscle.
- Calories: 25-30 kcal/kg/day is generally recommended to provide energy without overfeeding.
The Challenge of Providing Adequate Nutrition
Balancing nutrient delivery is complex. Underfeeding is common due to interruptions and intolerance. Overfeeding can cause metabolic issues. A personalized, monitored approach is key, starting with early, low-dose feeding and increasing gradually while monitoring tolerance and responses. Automated systems can aid monitoring.
Conclusion
Malnutrition is a serious threat to critically ill patients, worsening outcomes. Understanding pathophysiology and using effective assessment strategies are vital. Integrating tools like NRS-2002 and NUTRIC with clinical observations allows for a personalized approach. Early, adequate nutritional intervention, ideally enteral, with careful monitoring is the cornerstone. A multidisciplinary team ensures optimal care and improves prognosis. The American Association for Respiratory Care provides a guide: Nutritional Assessment and Treatment of the Critically Ill Patient.
