The aim of this study was to investigate local muscle O(2) consumption (muscV(O(2))) and forearm blood flow (FBF) in resting and exercising muscle by use of near-infrared spectroscopy (NIRS) and to compare the results with the global muscV(O(2)) and FBF derived from the well-established Fick method and plethysmography. muscV(O(2)) was derived from 1) NIRS using venous occlusion, 2) NIRS using arterial occlusion, and 3) the Fick method [muscV(O(2(Fick)))]. FBF was derived from 1) NIRS and 2) strain-gauge plethysmography. Twenty-six healthy subjects were tested at rest and during sustained isometric handgrip exercise. Local variations were investigated with two independent and simultaneously operating NIRS systems at two different muscles and two measurement depths. muscV(O(2)) increased more than fivefold in the active flexor digitorum superficialis muscle, and it increased 1.6 times in the brachioradialis muscle. The average increase in muscV(O(2(Fick))) was twofold. FBF increased 1.4 times independent of the muscle or the method. It is concluded that NIRS is an appropriate tool to provide information about local muscV(O(2)) and local FBF because both place and depth of the NIRS measurements reveal local differences that are not detectable by the more established, but also more global, Fick method.

A B S T R A C TThe influence of adipose tissue thickness (ATT) on near-IR spectroscopy (NIRS) measurements in vivo was studied in the human flexor digitorum superficialis muscle at rest and during sustained isometric handgrip exercise. NIRS was used for the quantitative measurement of muscle O 2 consumption (mV c O 2 ) and forearm blood flow (FBF) in 78 healthy subjects. Skinfold thickness ranged from 1.4 to 8.9 mm within the group. Resting mV c O 2 was 0.11p0.04 ml of O 2 :min − 1 :100 g − 1 , and FBF was 1.28p0.82 ml:min − 1 :100 ml − 1 . There was a negative correlation (r lk0.70, P 0.01), indicating a decrease in mV c O 2 with increasing ATT. mV c O 2 in the 10 leanest subjects appeared to be twice as high as that in the 10 subjects with the highest ATT. A poor correlation (r l 0.29, P 0.01) was found between ATT and FBF. The gender difference that we found for mV c O 2 was due to the difference in ATT between female and male subjects. No correlation was found between maximum voluntary contraction and mV c O 2 , nor between maximum voluntary contraction and ATT, indicating that the contraction force did not confound our results. These results show that ATT has a substantial confounding influence on in vivo NIRS measurements, and that it is essential to incorporate this factor into future NIRS muscle studies in order to justify comparisons between different groups. To facilitate such comparisons, upper and lower boundaries for normal values of mV c O 2 and FBF in relation to ATT are presented. 1 These authors contributed equally to this study.

The influence of adipose tissue thickness (ATT) on near-IR spectroscopy (NIRS) measurements in vivo was studied in the human flexor digitorum superficialis muscle at rest and during sustained isometric handgrip exercise. NIRS was used for the quantitative measurement of muscle O(2) consumption (mV.O(2)) and forearm blood flow (FBF) in 78 healthy subjects. Skinfold thickness ranged from 1.4 to 8.9 mm within the group. Resting mV.O(2) was 0.11+/-0.04 ml of O(2).min(-1).100 g(-1), and FBF was 1.28+/-0.82 ml.min(-1).100 ml(-1). There was a negative correlation (r=-0.70, P< or =0.01), indicating a decrease in mV.O(2) with increasing ATT. mV.O(2) in the 10 leanest subjects appeared to be twice as high as that in the 10 subjects with the highest ATT. A poor correlation (r=0.29, P< or =0.01) was found between ATT and FBF. The gender difference that we found for mV.O(2) was due to the difference in ATT between female and male subjects. No correlation was found between maximum voluntary contraction and mV.O(2), nor between maximum voluntary contraction and ATT, indicating that the contraction force did not confound our results. These results show that ATT has a substantial confounding influence on in vivo NIRS measurements, and that it is essential to incorporate this factor into future NIRS muscle studies in order to justify comparisons between different groups. To facilitate such comparisons, upper and lower boundaries for normal values of mV.O(2) and FBF in relation to ATT are presented.

The aim of this study was to investigate the performance of in vivo quantitative near-infrared spectroscopy (NIRS) in skeletal muscle at various workloads. NIRS was used for the quantitative measurement of O2 consumption (mVO2) in the human flexor digitorum superficialis muscle at rest and during rhythmic isometric handgrip exercise in a broad range of work intensities (10-90% MVC=maximum voluntary contraction force). Six subjects were tested on three separate days. No significant differences were found in mVO2 measured over different days with the exception of the highest workload. The within-subject variability for each workload measured over the three measurements days ranged from 15.7 to 25.6% and did not increase at the high workloads. The mVO2 was 0.14 +/- 0.01 mlO2 min-1 100 g-1 at rest and increased roughly 19 times to 2.68 +/- 0.58 mlO2 min-1 100 g-1 at 72% MVC. These results show that local muscle oxygen consumption at rest as well as during exercise at a broad range of work intensities can be measured reliably by NIRS, applied to a uniform selected subject population. This is of great importance as direct local measurement of mVO2 during exercise is not possible with the conventional techniques. The method is robust enough to measure over separate days and at various workloads and can therefore contribute to a better understanding of human physiology in both the normal and pathological state of the muscle.

The authors investigated the effect of acclimatization to high altitude on cerebral blood flow and oxidative metabolism at rest and during exercise. Nine healthy, native sea-level residents were studied 3 weeks after arrival at Chacaltaya, Bolivia (5,260 m) and after reacclimatization to sea level. Global cerebral blood flow at rest and during exercise on a bicycle ergometer was measured by the Kety-Schmidt technique. Cerebral metabolic rates of oxygen, glucose, and lactate were calculated by the Fick principle. Cerebral function was assessed by a computer-based measurement of reaction time. At high altitude at rest, arterial carbon dioxide tension, oxygen saturation, and oxygen tension were significantly reduced, and arterial oxygen content was increased because of an increase in hemoglobin concentration. Global cerebral blood flow was similar in the four conditions. Cerebral oxygen delivery and cerebral metabolic rates of oxygen and glucose also remained unchanged, whereas cerebral metabolic rates of lactate increased slightly but nonsignificantly at high altitude during exercise compared with high altitude at rest. Reaction time was unchanged. The data indicate that cerebral blood flow and oxidative metabolism are unaltered after high-altitude acclimatization from sea level, despite marked changes in breathing and other organ functions.

Background and Purpose-Patients with orthostatic hypotension due to sympathetic failure become symptomatic when standing, although their capability to maintain cerebral blood flow is reported to be preserved. We tested the hypothesis that in patients with sympathetic failure, orthostatic symptoms reflect reduced cerebral perfusion with insufficient oxygen supply. Methods-This

Assuming the upright position evokes an asymptomatic decrease in frontal cortical oxygenation in healthy elderly subjects but not in healthy young subjects. Cortical [O(2)Hb] changes are affected by diastolic BP changes. These findings may indicate that regulation of cerebral oxygenation alters with increasing age.

Near infrared spectroscopy (NIRS) and functional magnetic resonance imaging (fMRI) both allow non-invasive monitoring of cerebral cortical oxygenation responses to various stimuli. To compare these methods in elderly subjects and to determine the effect of age on cortical oxygenation responses, we determined motor-task-related changes in deoxyhemoglobin concentration ([HHb]) over the left motor cortex in six healthy young subjects (age 35 +/- 9 years, mean +/- SD) and five healthy elderly subjects (age 73 +/- 3 years) by NIRS and blood-oxygen-level-dependent (BOLD) fMRI simultaneously. The motor-task consisted of seven cycles of 20-sec periods of contralateral finger-tapping at a rate as fast as possible alternated with 40-sec periods of rest. Time-locked averages over the seven cycles were used for further analysis. Task-related decreases in [HHb] over the motor cortex were measured by NIRS, with maximum changes of -0.83 +/- 0.38 mumol/L (P < 0.01) for the young and -0.32 +/- 0.17 mumol/L (P < 0.05) for the elderly subjects. The BOLD-fMRI signal increased over the cortex volume under investigation with NIRS, with maximum changes of 2.11 +/- 0.72% (P < 0.01) for the young and 1.75 +/- 0.71% (P < 0.01) for the elderly subjects. NIRS and BOLD-fMRI measurements showed good correlation in the young (r = -0.70, r(2) = 0.48, P < 0.001) and elderly subjects (r = -0.82, r(2) = 0.67, P < 0.001). Additionally, NIRS measurements demonstrated age-dependent decreases in task-related cerebral oxygenation responses (P < 0.05), whereas fMRI measurements demonstrated smaller areas of cortical activation in the elderly subjects (P < 0.05). These findings demonstrate that NIRS and fMRI similarly assess cortical oxygenation changes in young subjects and also in elderly subjects. In addition, cortical oxygenation responses to brain activation alter with aging.