Noninvasive evaluation of skeletal muscle mitochondrial capacity with near-infrared spectroscopy: correcting for blood volume changes

Abstract

Near-infrared spectroscopy (NIRS) is a well-known method used to measure muscle oxygenation and hemodynamics in vivo. The application of arterial occlusions allows for the assessment of muscle oxygen consumption (mVo(2)) using NIRS. The aim of this study was to measure skeletal muscle mitochondrial capacity using blood volume-corrected NIRS signals that represent oxygenated hemoglobin/myoglobin (O(2)Hb) and deoxygenated hemoglobin/myoglobin (HHb). We also assessed the reliability and reproducibility of NIRS measurements of resting oxygen consumption and mitochondrial capacity. Twenty-four subjects, including four with chronic spinal cord injury, were tested using either the vastus lateralis or gastrocnemius muscles. Ten healthy, able-bodied subjects were tested on two occasions within a period of 7 days to assess the reliability and reproducibility. NIRS signals were corrected for blood volume changes using three different methods. Resting oxygen consumption had a mean coefficient of variation (CV) of 2.4% (range 1-32%). The recovery of oxygen consumption (mVo(2)) after electrical stimulation at 4 Hz was fit to an exponential curve, which represents mitochondrial capacity. The time constant for the recovery of mVo(2) was reproducible with a mean CV of 10% (range 1-22%) only when correcting for blood volume changes. We also examined the effects of adipose tissue thickness on measurements of mVo(2). We found the mVo(2) measurements using absolute units to be influenced by adipose tissue thickness (ATT), and this relationship was removed when an ischemic calibration was performed, supporting its use to compare mVo(2) between individuals of varying ATT. In conclusion, in vivo oxidative capacity can be assessed using blood volume-corrected NIRS signals with a high degree of reliability and reproducibility.

Figures

Fig. 1.

Experimental setup for measurements on the gastrocnemius muscle. A similar set up was used for measurements on the vastus lateralis muscle. BP, blood pressure; NIRS, near-infrared spectroscopy.

Fig. 2.

Muscle oxygenated hemoglobin/myoglobin (O2Hb; as a percentage of the ischemic calibration) during rest, resting arterial occlusions, and a 15-s electrical stimulation exercise followed by a series of transient arterial occlusions after exercise. The final 3–5 min are an ischemic calibration used to determine a relative concentration.

Fig. 3.

Postexercise arterial occlusion data for the oxygenated hemoglobin/myoglobin (O2Hb), deoxygenated hemoglobin/myoglobin (HHb), and the blood volume (tHb) signals for one individual uncorrected (A) and with a blood volume correction (B). Magnification of the final two cuffs for uncorrected (C) and corrected (D) signals are also shown. Monoexponential recovery curves are also shown for uncorrected (E) and corrected (F) data. The correction used was the individualized βi approach (see materials and methods for description).

Fig. 4.

Comparisons between the recovery time constants for the oxygenated hemoglobin/myoglobin (O2Hb) and deoxygenated hemoglobin/myoglobin (HHb) signals without (A) and with (B) the blood volume correction.

Fig. 5.

Blood volume correction factor (β) calculated for each individual data point over the course a resting arterial occlusion (A) and the first end-exercise arterial occlusion (B). Slope measurements of oxygen consumption were made over the first 3 s. Data presented are the mean changes in β for all individuals and all tests. Error bars were omitted for visual presentation, as β was variable between individuals. Despite individual differences the change in β over time was similar between individuals for resting cuffs (low metabolic rate) and end-exercise cuffs (high metabolic rate).

Fig. 6.

Reproducibility results for resting muscle oxygen consumption (mV̇o2) (A) and the postexercise recovery time constant of muscle oxygen consumption (B) for day 1 and day 2. Data are presented for each individual that completed two test sessions.

Fig. 7.

Sample recovery curves for a healthy, able-bodied subject from day 1 and day 2. Raw, blood volume-corrected data are represented by solid squares (day 1) and open triangles (day 2).

Fig. 8.

Comparisons between the NIRS interoptode distances for blood volume-corrected recovery time constants (Tc).

Fig. 9.

Relationship between resting mV̇o2 and adipose tissue thickness (ATT). A: calculation of resting mV̇o2 in absolute units using a differential pathlength factor (DPF) = 4. B: calculation of mV̇o2 as a percentage of the ischemic calibration. Data represent resting metabolic rate from all subjects tested and all test days.