Is there utility in including insensible losses and endogenous water in calculating daily fluid balance in critically ill patients?

From the Guidelines

Including insensible losses and endogenous water in daily fluid balance calculations for critically ill patients has limited clinical utility in routine practice. Current guidelines and clinical evidence, such as the recent study by Ostermann et al. 1, suggest focusing on measurable inputs and outputs for practical fluid management, while acknowledging these physiological processes occur. Insensible losses (approximately 500-800 mL/day from respiration and skin) and endogenous water production (approximately 300-400 mL/day from metabolism) tend to roughly balance each other in many patients, creating a practical "wash" effect that simplifies calculations.

However, certain clinical scenarios warrant consideration of these factors, including patients with hyperthermia (increased insensible losses), those on mechanical ventilation with humidified circuits (reduced respiratory losses), extensive burn injuries, or prolonged hypermetabolic states. The study by Ostermann et al. 1 recommends intraoperative administration of an adequate volume of fluid, generally aiming for 1-2 L positive balance by the end of the case, which supports the idea of focusing on measurable fluid balance.

For routine critical care, clinicians should prioritize accurate measurement of intake (IV fluids, medications, nutrition) and output (urine, drains, nasogastric drainage, stool), while using clinical assessment parameters like vital signs, physical examination findings, laboratory values, and hemodynamic monitoring to guide fluid management decisions. This approach balances practical clinical utility with physiological understanding, recognizing that precise quantification of insensible losses and endogenous water remains challenging at the bedside. Key considerations in fluid management include:

• Accurate measurement of intake and output
• Clinical assessment parameters to guide fluid decisions
• Recognition of specific clinical scenarios where insensible losses and endogenous water may significantly impact fluid balance.

From the Research

Utility of Insensible Losses and Endogenous Water in Daily Fluid Balance

• The inclusion of insensible losses and endogenous water in calculating daily fluid balance in critically ill patients is a topic of discussion among clinicians, with some studies suggesting its importance in accurately estimating fluid status 2, 3.
• Insensible losses, such as those due to fever, sweating, and respiratory losses, can significantly impact fluid balance, and correcting for these losses may improve the accuracy of fluid balance calculations 2.
• However, other studies have found that correcting for insensible losses does not significantly improve the correlation between fluid balance and changes in body weight, suggesting that other factors may be more important in determining fluid status 3.
• The measurement of body weight is also an important aspect of estimating fluid status, but it can be challenging to obtain accurate and consistent measurements in critically ill patients 4.

Clinical Evidence and Recommendations

• A systematic review of 13 cohort studies found that the accuracy of daily fluid balance charting was affected by inaccuracies in seven studies, and that the measurement of body weight was hard to accomplish in five studies 4.
• The review suggested that a combination of fluid balance charting and body weight measurement may be more effective in estimating fluid status than reliance on one single approach 4.
• Other studies have highlighted the importance of considering individual patient factors, such as renal function and fluid overload, when estimating fluid status and making clinical decisions 5, 6.
• The use of diuretics, such as furosemide, may also be useful in achieving fluid balance and predicting worsening acute kidney injury in critically ill patients 5, 6.

Key Findings

• The accuracy of fluid balance calculations can be improved by considering insensible losses and endogenous water, but the impact of these factors may vary depending on individual patient characteristics 2, 3.
• The measurement of body weight is an important aspect of estimating fluid status, but it can be challenging to obtain accurate and consistent measurements in critically ill patients 4.
• A combination of fluid balance charting and body weight measurement, along with consideration of individual patient factors, may be the most effective approach to estimating fluid status and making clinical decisions 4, 5, 6.