Changes in Optical Path Length Reveal Significant Potential Errors of Muscle Oxygenation Evaluation during Exercise...

Area of Science:

• Physiology
• Biomedical Engineering
• Spectroscopy

Background:

• Near-infrared spectroscopy (NIRS) is a noninvasive tool for monitoring tissue oxygenation.
• The modified Beer-Lambert law (MBLL) is commonly used in NIRS, assuming a constant optical path length (PL).
• The impact of changing PL during dynamic conditions like exercise on NIRSMBLL accuracy is not fully understood.

Purpose of the Study:

• To investigate how optical properties, specifically PL, change during ramp incremental exercise.
• To compare muscle oxygenation dynamics derived from time-resolved NIRS with those calculated using NIRSMBLL with a constant PL assumption.

Main Methods:

• Twenty-two healthy men underwent ramp incremental cycling to exhaustion.
• Time-resolved NIRS measured optical properties and hemoglobin/myoglobin concentrations in the vastus lateralis.
• Muscle oxygenation values were recalculated assuming a constant PL for comparison.

Main Results:

• Optical path length significantly shortened during exercise, with a notable 9.8% reduction at 763 nm.
• Significant discrepancies were observed in muscle oxygenation kinetics between real-time PL measurement and the constant PL assumption.
• The constant PL assumption led to an underestimation of deoxygenation compared to real-time measurements.

Conclusions:

• Assuming a constant PL in NIRSMBLL significantly underestimates muscle deoxygenation during exercise.
• The degree of underestimation in oxygenation is greater than the observed changes in PL.
• Accurate NIRSMBLL analysis requires accounting for real-time variations in optical path length during exercise.