Neurovascular Laboratory, Istituto Di Ricovero e Cura a Carattere Scientifico, National Neurological Institute, C. Mondino Foundation, Pavia, [corrected] Italy.
J Appl Physiol (1985). 2011 Jun;110(6):1646-55. doi: 10.1152/japplphysiol.00003.2011. Epub 2011 Apr 7.
The attractive possibility of near infrared spectroscopy (NIRS) to noninvasively assess cerebral blood volume and oxygenation is challenged by the possible interference from extracranial tissues. However, to what extent this may affect cerebral NIRS monitoring during standard clinical tests is ignored. To address this issue, 29 healthy subjects underwent a randomized sequence of three maneuvers that differently affect intra- and extracranial circulation: Valsalva maneuver (VM), hyperventilation (HV), and head-up tilt (HUT). Putative intracranial ("i") and extracranial ("e") NIRS signals were collected from the forehead and from the cheek, respectively, and acquired together with cutaneous plethysmography at the forehead (PPG), cerebral blood velocity from the middle cerebral artery, and arterial blood pressure. Extracranial contribution to cerebral NIRS monitoring was investigated by comparing Beer-Lambert (BL) and spatially resolved spectroscopy (SRS) blood volume indicators [the total hemoglobin concentration (tHb) and the total hemoglobin index, (THI)] and by correlating their changes with changes in extracranial circulation. While THIe and tHbe generally provided concordant indications, tHbi and THIi exhibited opposite-sign changes in a high percentage of cases (VM: 46%; HV: 31%; HUT: 40%). Moreover, tHbi was correlated with THIi only during HV (P < 0.05), not during VM and HUT, while it correlated with PPG in all three maneuvers (P < 0.01). These results evidence that extracranial circulation may markedly affect BL parameters in a high percentage of cases, even during standard clinical tests. Surface plethysmography at the forehead is suggested as complementary monitoring helpful in the interpretation of cerebral NIRS parameters.
近红外光谱（NIRS）具有无创评估脑血容量和氧合的诱人可能性，但受到颅外组织可能干扰的挑战。然而，在标准临床测试中，这种干扰会在多大程度上影响脑 NIRS 监测尚未得到关注。为了解决这个问题，29 名健康受试者接受了三种不同影响颅内和颅外循环的随机顺序操作：瓦尔萨尔瓦动作（VM）、过度通气（HV）和头高位倾斜（HUT）。分别从前额和脸颊采集推定的颅内（“i”）和颅外（“e”）NIRS 信号，并与前额皮肤容积描记法（PPG）、大脑中动脉的脑血流速度和动脉血压一起采集。通过比较 Beer-Lambert（BL）和空间分辨光谱（SRS）血液体积指标[总血红蛋白浓度（tHb）和总血红蛋白指数（THI）]，并通过将它们的变化与颅外循环的变化进行相关，研究了颅外对脑 NIRS 监测的贡献。虽然 THIe 和 tHbe 通常提供一致的指示，但在高百分比的情况下，tHbi 和 THIi 表现出相反的变化（VM：46%；HV：31%；HUT：40%）。此外，仅在 HV 期间，tHbi 与 THIi 相关（P < 0.05），而在 VM 和 HUT 期间则不相关，而在所有三种操作中，tHbi 与 PPG 相关（P < 0.01）。这些结果表明，即使在标准临床测试中，颅外循环也可能在高百分比的情况下显著影响 BL 参数。建议在前额进行表面容积描记法作为补充监测，有助于解释脑 NIRS 参数。
