Cerebral oximetry values in dialyzed surgical patients: a comparison between hemodialysis and peritoneal dialysis

Abstract

Background: Cerebral tissue regional oxygen saturation (rSO₂) through near-infrared spectroscopy (NIRS) is a method for non-invasive monitoring of cerebral tissue oxygenation. However, nowadays there is limited information on the level of cerebral tissue oxygenation in dialyzed end-stage renal disease (ESRD) surgical patients. The aim of this observational study was to evaluate the baseline cerebral rSO₂ values, to compare values between hemodialysis (HD) and peritoneal dialysis (PD) patients and identify risk factors that could predict cerebral tissue oxygenation in these patients. Patients/Methods: Thirty-two ESRD patients (≥18 years old), scheduled to undergo elective minor or major surgery, were enrolled. Patients were allocated in two groups according to dialysis modality. Twenty-three patients were treated with HD and nine were treated with PD. Demographic and clinical characteristics, comorbidities and arterial hemoglobin oxygen saturation (SpO₂) of the study population were recorded. Results: Patients who were treated with HD had significant lower baseline rSO₂ values compared with PD patients [median, 50% (28–63) vs. 63% (45–69), p = 0.002]. Hierarchical linear regression model analysis showed that preoperative Hb and SpO₂ were positive predictive variables (B = 0.353, p = 0.01 and B = 0.375, p = 0.009, respectively) for rSO₂. Moreover, dialysis modality was independent predictor for baseline rSO₂. The modality of dialysis remained an independent predictor for rSO₂ after controlling for the other significant variables (B = 0.291, p = 0.032) and PD was associated with higher baseline values of rSO₂. Conclusion: In conclusion, ESRD surgical patients undergoing PD treatment appear to have significantly higher baseline cerebral tissue saturation values compared with HD.

• Cerebral oximetry values
• end-stage renal disease
• hemodialysis
• peritoneal dialysis

Introduction

Cerebral oximetry is a user-friendly and non-invasive monitor that reflects the balance between cerebral oxygen supply and consumption in large parts of the brain tissue. The cerebral oxygen supply depends on cerebral blood flow and oxygen content of blood.1 It has been shown that decreased regional cerebral oxygen saturation (rSO₂) during surgery is associated with unjustified neurologic effects such as cognitive dysfunction, stroke and coma and moreover with a complicated postoperative course associated with higher morbidity rates.2,3 It is generally known that cerebral rSO₂ is influenced by age, carbon dioxide partial pressure, arterial hemoglobin oxygen saturation, cerebral blood flow and changes in cerebral metabolic rate, cardiac index, hemoglobin concentration and blood volume.4–10

However, limited information is currently available in the literature on the level of cerebral tissue oxygenation in end-stage renal disease (ESRD) surgical patients receiving dialysis. Although the two dialysis modalities, hemodialysis (HD) and peritoneal dialysis (PD), exhibit similar patient’s survival, important differences do exist with regard to technique and physiology. It has been shown that HD treatment itself contributes to transient hypotension, arterial hypoxemia, low cerebral perfusion, and finally cerebral atrophy.11

We conducted this observational study to evaluate the baseline cerebral rSO₂ values in dialyzed ESRD surgical patients, to compare values between HD and PD patients and identify risk factors that could predict cerebral tissue saturation in this population.

Patients and methods

This observational study was conducted at our tertiary care University Hospital between January and December 2011. Thirty-two ESRD dialyzed patients older than 18 years scheduled to undergo elective minor or major surgery were enrolled. Exclusion criteria were refusal to participate or inability to provide informed consent, emergency or urgent operation and trauma, major cardiovascular event during the last 3 months and time on dialysis less than 2 months. Patients were allocated in two groups according to dialysis modality (HD or PD). Twenty-three patients [aged 76 (21–85) years] were treated with HD through an arteriovenous fistula or a graft (Group A) and nine [aged 61 (51–80) years] were treated with PD (Group B). Demographic and clinical characteristics, including age, gender, time on dialysis, mean arterial pressure (MAP), preoperative hemoglobin (preop Hb) and arterial hemoglobin oxygen saturation (SpO₂) were recorded. Comorbidities namely, arterial hypertension (HT) and diabetes mellitus (DM) along with history of cardiovascular disease (CVD) (coronary heart disease, cerebrovascular disease and peripheral vascular disease) were recorder as well.

Standard monitoring equipment, heart rate (HR), electrocardiography (Lead II), non-invasive arterial blood pressure, pulse oximetry was used during surgery. Baseline cerebral oximetry values were recorded upon arriving to the operating room, using the INVOS® (In Vivo Optical Spectroscopy) 5100C monitoring device (Somanetics®, Troy, MI), with sensors placed bilaterally on the patient forehead in accordance with manufacturer instructions. However, as the left hemisphere is considered to be the dominant hemisphere in the vast majority of the general population, we chose to use the left rSO₂ baseline values as the dependent variable in the analysis. Baseline rSO₂ values from the left hemisphere were recorded 3 min after the sensors were applied in all patients. Supplemental oxygen administration (40% by Venturi mask) started after recording of baseline rSO₂ was completed.

The Ethical committee approved this study and an informed consent was obtained by the patient.

Statistical analysis

Data were expressed as median (range) or absolute number and percentage, as appropriate. Kolmogorov–Smirnov test was performed to evaluate the normality of each parameter. Comparisons between two groups of continuous variables were performed by the Mann–Whitney U-test, while Fisher’s exact test was used for categorical variables. Multiple linear hierarchical stepwise regression analysis was performed to investigate any association between baseline rSO₂ values from the left cerebral hemisphere, and statistical significant variables as derived from the univariate regression analysis, and analyze the contribution of these variables to the observed baseline rSO₂ variability. Comparisons were considered statistically significant when p < 0.05. All analyses were carried out with SPSS® 17.0 statistical package for Windows® (SPSS Inc, Chicago, IL).

Results

Demographic characteristics, comorbidities and clinical data, including baseline rSO₂ values from the left cerebral hemisphere (baseline rSO₂ L) are presented in Table 1. Hemodialysis patients were experienced significant lower levels of Hb compared with patients on PD. No statistical significant differences were observed in age, MAP, SpO₂, duration of renal replacement therapy (RRT) and prevalence of HT, DM and history of CVD between the two study groups (Table 1).

Table 1. Demographic, clinical characteristics, comorbidities and baseline rSO₂ values in the two groups.

Variables	Group A n = 23	Group B n = 9	p Value
Age, median (range)	76 (21–85)	61 (51–80)	NS
Gender (male/female)	16/7	7/2	NS
Diabetes mellitus, n	12	4	NS
Hypertension, n	13	7	NS
History CVD, n	11	5	NS
Duration of RRT (months), median (range)	8.5 (3–168)	10.5 (2–64)	NS
Preoperative Hb, (mg/dl), median, (range)	10 (7–12)	11 (9–12)	0.04
SpO2 (%), median, (min–max)	96 (88–99)	99 (93–99)	NS
MAP (mmHg), median (min–max)	98 (66–135)	96 (73–120)	NS
Baseline rSO2 L (%) median (min–max)	50 (28–63)	63 (45–69)	0.002
Baseline rSO2 L < 50 (%), n	11 (48%)	1 (11%)	0.05

Note: Values are expressed as median (range) or number (proportion) as appropriate.

Patients who were treated with HD had significant lower rSO₂ values compared with PD patients [median, 50 (28–63) vs. 63 (45–69), p = 0.002, for group A and group B patients, respectively]. Furthermore, 11 (out of 23) versus 1 (out of nine) patients in group A and B, respectively, experienced levels of rSO₂ lower than 50% (Table 1).

Τhe results from univariate regression analysis are shown in Table 2. Age, MAP and duration of RRT as well as co-morbidities were not statistically significant predictors for cerebral tissue saturation as derived from univariate analysis (Table 2). Hierarchical linear regression model analysis (Table 3) was then performed taking into account the statistical significant variables as derived from an univariate analysis. The analysis demonstrated that preop Hb and SpO₂ were positive predictive variables (B = 0.353, p = 0.019 and B = 0.375, p = 0.009, respectively) for rSO₂. Moreover, dialysis modality was an independent predictor for baseline rSO₂ left (L) and remained independent predictor for rSO₂ after controlling for the other significant variables (B=0.291, p = 0.032), while PD (group B) was associated with higher baseline values of rSO₂ (Table 3).

Table 2. Univariate regression analysis.

	Unstandardized coefficients		Standardized coefficients
Parameter	B	Std. error	Beta	Significance
Age	−0.128	0.129	−0.178	0.330
Preoperative Hb	4.169	1.041	0.590	0.000
MAP	0.066	0.103	0.116	0.526
SpO2	1.829	0.396	0.645	0.000
CVD	−3.999	4.426	−0.175	0.382
Diabetes mellitus	−3.653	4.717	−0.159	0.447
Hypertension	−1.800	5.721	−0.065	0.756
Duration of RRT	−1.27	0.058	−0.461	0.065
Dialysis modality	−13.266	3.559	−0.563	0.001

Note: Dependent variable: baseline rSO₂.

Table 3. Multivariate hierarchical stepwise regression analysis with baseline rSO₂ as dependent variable: standardized and unstandardized coefficients.

		Unstandardized coefficients		Standardized coefficients
Model		B	Std. error	Beta	t	Significance
1	(Constant)	11.743	10.277		1.143	0.262
	Hb Preop	4.146	1.029	0.593	4.031	0.000
2	(Constant)	−103.171	34.843		−2.961	0.006
	Hb Preop	2.704	0.980	0.386	2.759	0.010
	SpO2	1.355	0.397	0.478	3.409	0.002
3	(Constant)	−60.860	35.475		−1.716	0.098
	Hb Preop	2.472	0.988	0.353	2.502	0.019
	SpO2	1.064	0.380	0.375	2.799	0.009
	Dialysis modality	−6.860	3.032	−0.291	−2.262	0.032

Note: Dependent variable: baseline rSO₂.

Preoperative Hb is responsible for 33% of the observed baseline rSO₂ variability (adjusted R² =0.330, p < 0.0001), while preop Hb and SpO₂ combined account for 50.5% of rSO₂ variability (adjusted R² = 0.505, p = 0.002).

Moreover, all independent variables combined (preop Hb, SpO₂ and dialysis modality) account for 58.9% of the observed variability (adjusted R² = 0.589, p = 0.032) (Table 4).

Table 4. Hierarchical stepwise regression, with baseline rSO₂ as dependent variable.

Model	R	R^2	Adjusted R^2	Std. error of estimate	R^2 change	F change	Sig. F. change
1	0.593^a	0.351	0.330	8.819	0.351	16.246	0.000
2	0.733^b	0.537	0.505	7.579	0.186	11.624	0.002
3	0.801^c	0.642	0.589	6.905	0.068	5.118	0.032

Notes: ^aPredictors: (Constant). Hb Preop.

^bPredictors: (Constant). Hb Preop, SpO₂.

^cPredictors: (Constant). Hb Preop, SpO₂, Dialysis modality.

Discussion

The results of this observational study showed that, in patients with ESRD on RRT undergoing elective minor or major surgery, preop Hb, SpO₂ and dialysis modality were predictors for the baseline rSO₂. Moreover, we found that PD, compared with HD, was associated with higher baseline levels of rSO₂ independently of patients’ levels of preop Hb and SpO₂.

Given the shortage of data coming from ESRD dialyzed surgical patients, we have compared our findings with the results from other investigators who studied rSO₂ in normal subjects and in cardiac surgical/neurosurgical patients. In line with our results, they have demonstrated that Hb and arterial Hb oxygen saturation could predict cerebral tissue saturation.6,9 A negative correlation between rSO₂ and age, and a positive correlation with Hb were observed by Kishi et al.7 Similarly, Yoshitani et al. reported a positive correlation of rSO₂ with Hb concentration and mean arterial pressure.9 On the other hand, Han et al.12 found that acute normovolemic hemodilution in patients undergoing orthopedic surgery does not influence rSO₂ values in the absence of hypotension, whilst Green5 reported a positive correlation of rSO₂ with Hb but a negative correlation with blood loss.

In our ESRD population, although the majority of our patients had a preop Hb level within the proposed range according to the KDIGO guidelines for anemia in chronic kidney disease,13 it continued to affect significantly the baseline rSO₂ values. In one of the few studies with ESRD population by Stefanidis et al., investigators found that during HD treatment, the percentual changes of mean blood flow velocity in the middle cerebral artery were interrelated among other parameters to the percentual changes of the hematocrit, of the arterial oxygen saturation and of the arterial blood oxygen content.14 In an another study in HD clinical stable patients, Hirakata et al. aimed at estimating an optimal hematocrit (Ht) for the maximum oxygen delivery to the brain by using both the actual values of cerebral blood flow (positron emission tomography) and the calculated arterial oxygen content (CaO₂).15 They showed that the oxygen delivery reached a maximum level of only 3.7 ml O₂/100 g/min at Ht of 35.3% in the cerebral hemisphere. They did not object to achieve higher levels of Ht, as normalization of Ht is known to cause a trend of higher mortality rates in HD patients with history of ischemic heart disease.16 Considering all the above, a low preop Hb level might negatively affect the rSO₂ of dialyzed ESRD patients during elective surgery, which could further result in a complicated postoperative course.

Interestingly, we did not find age and MAP to predict cerebral tissue saturation in our ESRD patients as showed by other investigators in non-uremic patients.7,9 Consistent with our results regarding the significant correlation between the percentage reduction rSO₂ and SpO₂, Germon et al. reached the same conclusion in healthy adults during hypoxia.17

In our specific population of dialyzed patients, the baseline cerebral oximetry measurements on the left hemisphere were significantly lower in HD compared with PD patients. The median level of rSO₂ in the HD group was 50% and almost half of rSO₂ values were less than 50%. As have been proposed from several studies in different studied populations, a critical threshold of rSO₂ value lower than 50% has been associated with specific adverse events, such as postoperative cognitive dysfunction,3,18 a higher rate of stroke or coma,19 increased postoperative morbidity from other nonneurological causes,20 and a prolonged stay in the high-dependency unit or the hospital.2 Although, we did not assess surgical complications and survival rates in the studied population, our results could probably support findings of another study which compared perioperative outcomes between HD and PD patients undergoing cardiothoracic surgery. They showed that HD patients had higher incidence of any postoperative complications than PD patients.21

Prohovnik et al. showed low oxygen saturation in stable HD patients.11 The mean level of rSO₂ measured before, during and after a dialysis session was 41% ± 13% compared with 70% ± 2% in their healthy control group. Transient hypotension, arterial hypoxemia and fluctuations in electrolytes during HD result in cerebral atrophy and possibly induce brain damage. The acute intravascular volume loss and fluid shifts that occur during dialysis induce cerebral edema and decreased intracerebral blood pressure, blood velocity and cerebral perfusion.22 Furthermore, the endothelial dysfunction because of the chronic inflammatory process and the oxidative stress exacerbate the pre-existent arteriosclerosis in patients with moderate or severe chronic kidney disease and in patients on HD. The above abnormalities are related to each other and potentially contribute to the lower cerebral saturation as well.23–25 On the other hand, ESRD patients undergoing PD exhibit more normal perfusion.11 This is in line with our finding and could justify the significant higher cerebral oximetry measurements in the PD group.

We recognize that our study exhibits some limitations. The sample size of our study is relatively small and therefore does not allow for definitive conclusions. The absence of a healthy control group to compare ESRD dialyzed patients is another limitation.

In conclusion, PD patients appear to have significant higher baseline cerebral tissue saturation values compared with HD population. The monitoring of cerebral oxygenation is a very useful tool in the course of some types of surgery. However, more studies are needed, potentially out of the operative room, in this group of patients, to whom dialysis modality, duration of dialysis and optimal control of renal anemia may play an important role in cerebral perfusion and in cerebral oxygenation.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.