General objective To assess the role of indirect calorimetry in measuring energy expenditure and assess guided nutritional support among critically ill patients in the ICU.

Specific Objectives

1. To assess the nutritional status of critically ill patients admitted to the ICU.
2. To measure energy requirements of critically ill patients using indirect calorimetry.
3. To compare measured energy requirements obtained by indirect calorimetry with those estimated by predictive equations.

Human beings continuously exchange energy with their environment. All energy is derived from chemical substrates as carbohydrates, lipids, and proteins, liberated through oxidative metabolism into carbon dioxide and water, with a portion captured as adenosine triphosphate (ATP) for cellular functions [1].

Critically ill patients are characterized by catabolic stress status and inflammatory response, which may contribute to a variety of complications, including multi-organ failure, increased infectious morbidity, and prolonged hospitalization. It is challenging for patients in the intensive care unit (ICU) to acquire their nutritional requirements due to their critical health conditions, making it crucial to pay attention to their nutritional status, which is the key factor to improve their health outcomes and avoid associated complications [2] In critically ill patients, optimizing nutrition remains challenging, particularly regarding the quantity and timing of energy delivery. Early aggressive feeding to meet estimated energy expenditure (EE) was historically advocated to prevent malnutrition and muscle loss. However, randomized trials have not consistently shown benefit and have reported harm, including hyperglycemia, hepatic steatosis, and increased mortality [3]. Undernutrition remains common due to prescription inadequacies and failure to meet targets [3] Negative energy balance is associated with prolonged mechanical ventilation, infections, organ failure, longer hospital stay, and increased mortality [4]. Energy needs in critical illness are highly variable, influenced by disease severity, inflammation, sedation, mechanical ventilation, and individual metabolic responses, highlighting the need for individualized, real-time nutrition therapy [5-8].

Predictive equations, often derived from healthy populations and adjusted with stress factors, remain widely used but frequently misestimate EE, resulting in under- or overfeeding [8-10]. Indirect calorimetry (IC) provides a precise, real-time assessment of resting energy expenditure (REE) and substrate utilization by measuring oxygen consumption (VO₂) and carbon dioxide production (VCO₂). The respiratory quotient (RQ), defined as VCO₂/VO₂, indicates substrate oxidation, with 1.0 reflecting glucose and 0.7 mixed substrate utilization [5]. Current guidelines recommend IC as the preferred method to guide energy delivery in critically ill patients [8,12 ,13].

Advances in clinical calorimetry now allow accurate, user-friendly metabolic monitoring, enabling individualized energy and protein prescriptions. Evidence demonstrates that IC-guided nutrition improves outcomes, including morbidity and mortality, compared to equation-based estimates [13]. Critically, the benefit derives not from measurement alone but from tailoring nutrition support within a precision medicine framework.

Previous study done by Hamdan et al[2], found that the energy and protein intake of ICU patients was markedly below their estimated nutritional requirements, even that feeding was initiated early and maintained for a substantial period, overall intake remained inadequate. These observations align with previous reports highlighting persistent gaps in achieving nutritional targets in critically ill patients in many other countries And on the opposite sides, over feeding to ICU patients cause refeeding syndrome, with proven complications up to mortality[2,14,15,16] Despite clear evidence that both under- and overfeeding adversely affect critically ill patients, significant gaps remain in the practical application of individualized nutrition therapy. Most ICUs continue to rely on predictive equations, which are freqthoughuently inaccurate and fail to capture dynamic metabolic changes during critical illness. Although indirect calorimetry is recommended by international guidelines, its routine use is limited due to perceived complexity, cost, and lack of implementation strategies in everyday clinical practice. Moreover, the optimal timing and dose of energy and protein delivery, particularly during different phases of critical illness and post-ICU recovery, remain poorly defined. There is also limited evidence on how integrating IC-guided nutrition into precision medicine approaches impacts morbidity, length of ICU stay, and long-term functional outcomes [2,8,11,12].

Inclusion Criteria:

• - Adult patients (≥18 years).
• Patients who are hemodynamically stable at the time of measurement.
• Patients receiving nutritional support (enteral and/or parenteral).

Exclusion Criteria:

• - Patients with air leaks or high oxygen requirements interfering with indirect calorimetry measurements.
• Pregnant patients.

Critically ill patients