Low preoperative regional cerebral oxygen saturation in hemodialysis patients

Our study was approved by the ethics committee of Kyoto University (approval number: R0719), and the requirement for written informed consent was waived. We included adult (≥20 years of age) patients who underwent isolated CABG at Kyoto University Hospital between January 1, 2012 and December 31, 2015. Patients who concurrently underwent cardiac valvular or aortic surgery were excluded. Patients without available rSO₂ values taken before administration of oxygen or anesthetic drugs were also excluded.

Specific data from the electronic medical records of participants, including age, gender, body mass index, comorbidities, and preoperative examination findings, were retrospectively collected. Left ventricular ejection fraction (LVEF) was derived from preoperative transthoracic echocardiography records. For hemoglobin concentration values, the most recent value measured before the surgery was used.

Patients received no premedication. Upon entering the operating room, preoperative rSO₂ and arterial oxygen saturation by pulse oximetry (SpO₂) were obtained before administration of oxygen or anesthetic drugs. rSO₂ values were detected using the INVOS 5100 system (Somanetics, Troy, MI) with probes containing light sources, each providing two continuous wavelengths of near-infrared light (730 and 810 nm) that reach a brain area corresponding to the junction between the anterior and middle cerebral artery vascularization territory. Two probes were attached to the right and the left sides of the forehead; the preoperative rSO₂ values were defined as the mean of rSO₂ values obtained from these two probes.

Data were analyzed using the statistical program R (http://cran.r-project.org). Continuous data are presented as median (interquartile range), and categorical variables are expressed as a number (percentage). Differences between groups were compared using the Mann–Whitney U test for continuous variables. For categorical variables, Pearson’s chi-square or Fisher’s exact tests were used as appropriate. Because it has been shown that rSO₂ is affected by age, hemoglobin concentration, and cardiac function [3–7], we used multivariable linear regression analysis to assess the independent impact of HD on rSO₂ values after adjustment for these factors. We constructed a linear regression model with rSO₂ as a dependent variable and HD status, age, hemoglobin concentration, and LVEF as independent variables, and beta coefficients were calculated for all independent variables. All statistical tests were two-tailed, and the statistical significance was set at a P value of <0.05.

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 [7]. 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 [11] 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 [12]. (3) Cerebral atrophy seen in HD patients [13] 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 [5]. 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) [14]. 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.

In conclusion, this study demonstrated that HD showed strong associations with low preoperative rSO₂ values in patients undergoing CABG, even after adjusting for known factors that affect rSO₂ values, including age, hemoglobin concentration, and cardiac systolic function.