Table 2.

Findings on confounding factors of the fNIRS signal.

Study	N	Anesthetics tested	Aim	Main findings
Kim et al. [42]	27	Sevoflurane and propofol	To determine rSO2 differences between propofol and sevoflurane during laparoscopic surgery in the Trendelenburg position	rSO2 values at Trendelenburg and after Trendelenburg were significantly higher in the sevoflurane group compared to propofol. In the propofol group, rSO2 after Trendelenburg was significantly lower than that before Trendelenburg. Cerebral desaturation occurred in 2 propofol patients
Kim et al. [43]	40	Desflurane and propofol	To determine the effect of desflurane and propofol on rSO2 in the sitting position during arthroscopic shoulder surgery	rSO2 was higher in the desflurane group compared to the propofol group at 3, 5, 7, and 9 min after the sitting position. However, rSO2 decreased significantly from the baseline at the same time points after the sitting position
Closhen et al. [44]	35	Sevoflurane	To investigate changes in cerebral rSO2 in the beach chair position with 2 different fNIRS devices	A significant decrease in rSO2 after beach chair position was measured, which was reversible after return to supine position. The decrease correlated with MAP during beach chair but not during supine position
Kitajima et al. [45]	12	Sevoflurane	To determine the influence of the head-up position on Hb, HbO2, HbTot, Cytaa3, propofol induction, and maintenance sevoflurane + 66% N2O	Significant decrease in HbO2 was measured in the head-up position after movement and 30 minutes after. HbO2 remained low after return to supine position. No significant changes were measured for Hb and Cytaa3. HbTot displayed the same trend as HbO2
Lovell et al. [46]	20	Propofol	To measure the changes in cerebral blood volume (CBV) caused by changes in posture in awake and anesthetized subjects	CBV decreased with 18° head-up tilt and increased with 18° head-down tilt in awake subjects. In the anesthetized group there were differences between head-up and head-down tilt. In the head-down, CBV was correlated with the degree of table tilt. There was an insignificant reduction in CBV in the head-up position
Owen-Reece et al. [47]	13	Thiopentone or propofol	To evaluate the extent and duration of the hemodynamic response to an alteration in PaCO2 in anesthetized and healthy volunteers	CBV decreases with lower PaCO2. This fall in CBV is slower and smaller during anesthesia when compared to conscious subjects
Alexander et al. [48]	26	Propofol-remifentanil and sevoflurane	To examine the role of hyperventilation and systemic hemodynamic changes on the cerebral tissue oxygen saturation	Hyperventilation led to significant decreases in rSO2 in both the propofol-remifentanil and the sevoflurane groups. Saturation correlated significantly with etCO2 in both groups. Saturation also correlated significantly with MAP and CO in propofol-remifentanil group but not in the sevoflurane group
Kim et al. [49]	60	Propofol-sufentanil and midazolam	To examine the effect of induction with midazolam and propofol on oxygen supply demand balance after 100% preoxygenation	rSO2 increased during the preoxygenation phase compared to the baseline values. No additional increase in rSO2 was measured after administration of midazolam or propofol with sufentanil
Yoshitani et al. [50]	42	Propofol and isoflurane with nitrous oxide	To compare changes in rSO2 to changes in venous bulb oxygenation after hemodilution during propofol and isoflurane/nitrous oxide anesthesia	Mean jugular bulb O2 saturation was lower in the propofol than in the sevoflurane group; no significant differences were found between anesthetics in the cerebral rSO2. During reduction of hemoglobin concentration, jugular O2 saturation remained unchanged, while cerebral O2 saturation decreased significantly in both anesthetic groups
Nissen et al. [51]	71	Propofol-fentanyl	To examine the effect of a reduction in MAP on the rSO2 in the PFC during propofol-fentanyl anesthesia induction	rSO2 increased significantly with decreasing MAP. After induction variables recovered and remained at preanesthetic levels. No correlation between MAP and rSO2 could be established