Abstract
The potential interference of cutaneous circulation on muscle blood volume and oxygenation monitoring by near‐infrared spectroscopy (NIRS) remains an important limitation of this technique. Spatially resolved spectroscopy (SRS) was reported to minimize the contribution of superficial tissue layers in cerebral monitoring but this characteristic has never been documented in muscle tissue monitoring. This study aims to compare SRS with the standard Beer–Lambert (BL) technique in detecting blood volume changes selectively induced in muscle and skin. In 16 healthy subjects, the biceps brachii was investigated during isometric elbow flexion at 70% of the maximum voluntary contractions lasting 10 sec, performed before and after exposure of the upper arm to warm air flow. From probes applied over the muscle belly the following variables were recorded: total hemoglobin index (THI, SRS‐based), total hemoglobin concentration (tHb, BL‐based), tissue oxygenation index (TOI, SRS‐based), and skin blood flow (SBF), using laser Doppler flowmetry. Blood volume indices exhibited similar changes during muscle contraction but only tHb significantly increased during warming (+5.2 ± 0.7 μmol/L·cm, an effect comparable to the increase occurring in postcontraction hyperemia), accompanying a 10‐fold increase in SBF. Contraction‐induced changes in tHb and THI were not substantially affected by warming, although the tHb tracing was shifted upward by (5.2 ± 3.5 μmol/L·cm, P < 0.01). TOI was not affected by cutaneous warming. In conclusion, SRS appears to effectively reject interference by SBF in both muscle blood volume and oxygenation monitoring. Instead, BL‐based parameters should be interpreted with caution, whenever changes in cutaneous perfusion cannot be excluded.
Highlights
Near-infrared spectroscopy (NIRS) can provide potentially useful information about changes in blood volume and tissue oxygenation (Perrey 2008; Ferrari et al 2011; Hamaoka et al 2011; Ghosh et al 2012)
Blood volume indices exhibited similar changes during muscle contraction but only tHb significantly increased during warming (+5.2 Æ 0.7 lmol/LÁcm, an effect comparable to the increase occurring in postcontraction hyperemia), accompanying a 10-fold increase in skin blood flow (SBF)
This study showed that (1) cutaneous circulation differently affects Beer– Lambert (BL) and SRS blood volume measurements, to the extent that changes of opposite sign were detected for the same test in 31% to 46% of the subjects, depending on the test employed, (2) BL measurements better correlate with indicators of extracranial blood volume, as compared to SRS measurements (Canova et al 2011)
Summary
Near-infrared spectroscopy (NIRS) can provide potentially useful information about changes in blood volume and tissue oxygenation (Perrey 2008; Ferrari et al 2011; Hamaoka et al 2011; Ghosh et al 2012). Its employment and usefulness are still limited due to several reasons among which is the poorly defined influence of superficial tissue layers (skin) on NIRS variables, when aiming to assess changes occurring in deep tissues, such as brain and muscle. This is a relevant issue in cerebral monitoring as control of skin blood flow is considerably different from cerebral blood flow and can introduce important alterations in the measurement (Lam et al 1997; Smielewski et al 1997; Canova et al 2011).
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