This study demonstrated that HD patients had significantly lower preoperative rSO2 values, even after adjusting for known factors that affect rSO2, including age, hemoglobin concentration, and LVEF.
Several reports have suggested that patients undergoing HD have low rSO2 values, reflecting low cerebral perfusion [9, 10]; however, they did not assess cardiac function, which is known to affect rSO2 values . In our study, we were able to compare rSO2 values in HD and non-HD patients after adjusting for LVEF because most patients underwent preoperative transthoracic echocardiography. Although HD patients had significantly lower hemoglobin concentrations than non-HD patients, and LVEF in HD patients tended to be lower, the estimated coefficient for HD in the multivariable linear regression analysis, adjusting for age, hemoglobin concentration, and LVEF, was −20.4. This means that HD patients had 20.4% lower rSO2 values than non-HD patients after adjusting for these factors, indicating that low rSO2 values in HD patients are, at least in part, because of factors other than age, anemia, or cardiac systolic function. Although it is not possible to determine the specific cause of low rSO2 values in HD patients in our study, possible explanations are as follows: (1) metabolic acidosis frequently seen in HD patients decreases affinity between hemoglobin and oxygen  and decreases microcirculatory oxygen saturation. (2) The acute intravascular volume loss and fluid shifts that occur during dialysis induce cerebral edema and decrease intracerebral blood pressure, blood velocity, and cerebral perfusion . (3) Cerebral atrophy seen in HD patients  might increase the thickness of the cerebrospinal fluid layer, which decreases the intensity of near-infrared light that the detector can receive, thereby, decreasing rSO2 values . Further research is required to elucidate the mechanisms by which the rSO2 values are decreased in HD patients.
It is important to determine adequate target values of rSO2 to guide intraoperative management in HD patients. rSO2 monitoring may be used to guide hemodynamics and CPB management during cardiac surgery by adjusting therapy based on relative changes from the preoperative baseline (i.e., to maintain relative rSO2 > 80% of baseline) . However, this concept might allow too low rSO2 values and be harmful for majority of HD patients, who frequently have abnormally low baseline rSO2 values. Future research should be conducted to establish the appropriate target value of rSO2 in HD patients.
This study had certain limitations, primarily based on its retrospective design. We used only preoperative LVEF to evaluate cardiac function because data on diastolic function or cardiac output were not available in most patients. In addition, we could not adjust for the influence of some factors, which are suggested to be related to rSO2, including partial pressure of carbon dioxide in arterial blood, central venous pressure, skull thickness, and area of cerebrospinal fluid layer [4, 5]. The number of patients was small; however, it was still sufficient to support our hypothesis.