Effects of sodium on measuring relative blood volume during hemodialysis differ by techniques

Estimation of Absolute Blood Volume Using Online Dialysate Dilution: When and How to Measure?

Absolute blood volume can be calculated from the increase in relative blood volume after an infusion of a well-defined volume bolus of ultrapure dialysate into the extracorporeal circulation. Several working groups have applied this method in research and clinical practice. A critical analysis of differing blood volume data between working groups revealed methodologic problems of the measurement procedure and some important technical aspects. This paper presents a statement to standardize the method.

Comparison of four different hematocrit assays and the effect of albumin on their measurements

Clinical decisions are influenced by hematocrit values. Centrifugation (reference standard), conductivity, optical and impedance methods are often used interchangeably to measure hematocrit. The effects of albumin, which are known to affect conductivity methods, have not been evaluated for limits of agreement (LOA) between hematocrit assays in small animals. Canine venous blood was collected from 74 clinical cases and measured by centrifugation (n = 72), conductivity (n = 73), impedance (n = 24) and optical (n = 50) methods. Bland-Altman analysis determined bias (± SD) and 95% LOA between methods. There was a statistically significant difference between centrifugation hematocrit values and values obtained via conductivity (p < 0.0001), optical (p < 0.0001), and impedance (p = 0.0082) methods. The conductivity method underestimated hematocrit by 2.1 ± 2.9% (95% LOA −3.54 to 7.88), the optical method by 3.1 ± 3.6% (95% LOA −4.0 to 10.2), and the impedance method by 2.3 ± 3.7% (95% LOA −5 to 9.6) when compared to centrifuged hematocrit values. The hematocrit difference between conductivity and centrifugation methods was statistically different for low (4%, 0–5%), within reference limits (3%, −5 to 8%), and high (2%, −2 to 5%) albumin values, respectively (p = 0.02), with post-hoc analysis demonstrating that the difference occurred between the low and high albumin groups. This study confirms that albumin values outside reference limits can affect the conductivity method and that hematocrit values obtained via conductivity, optical and impedance methods underestimate values obtained via centrifugation. Therefore, the hematocrit methods cannot be used interchangeably. The wide limits of agreement also demonstrates that care must be taken when making clinical decisions with different hematocrit methodologies.

A New Method for Continuous Relative Blood Volume and Plasma Sodium Concentration Estimation During Hemodialysis

OBJECTIVE

Non-invasive sensing and reliable estimation of physiological parameters are important features of hemodialysis machines, especially for therapy customization (biofeedback). In this paper, we present a new method for joint estimation of two important hemodialysis-related physiological parameters-relative blood volume and plasma sodium concentration.

METHODS

Our method makes use of a non-invasive sensor setup and a mathematical estimator. The estimator, based on the Kalman filter, allows merging data from multiple sensors, newly designed as well as onboard, with modeling knowledge about the hemodialysis process. The system was validated on in vitro hemodialysis sessions using bovine blood.

RESULTS

The estimation error we obtained (0.97 ± 0.73% on relative blood volume and 0.47 ± 0.19 mM on plasmatic sodium) proved to be comparable with that of the reference data for both parameters-the system is sufficiently accurate to be relevant in a clinical context.

CONCLUSION

Our system has the potential to provide accurate and important information on the state of a patient undergoing hemodialysis, while only low-cost modifications to the existing onboard sensors are required.

SIGNIFICANCE

Through improved knowledge of blood parameters during hemodialysis, our method will allow better patient monitoring and therapy customization in hemodialysis.

Effect of plasma sodium concentration on blood pressure regulators during hemodialysis: a randomized crossover study

Background

Intradialytic hypotension is a common complication of hemodialysis. The Hemocontrol biofeedback system, improving intradialytic hemodynamic stability, is associated with an initial transient increase in plasma sodium levels. Increases in sodium could affect blood pressure regulators.

Methods

We investigated whether Hemocontrol dialysis affects vasopressin and copeptin levels, endothelial function, and sympathetic activity in twenty-nine chronic hemodialysis patients. Each patient underwent one standard hemodialysis and one Hemocontrol hemodialysis. Plasma sodium, osmolality, nitrite and nitrate (NOx), endothelin-1, angiopoietins-1 and 2, and methemoglobin as measures of endothelial function, plasma catecholamines as indices of sympathetic activity and plasma vasopressin and copeptin levels were measured six times during each modality. Blood pressure, heart rate, blood volume, and heart rate variability were repeatedly monitored. Generalized Estimating Equations was used to compare the course of the parameters during the two treatment modalities.

Results

Plasma sodium and osmolality were significantly higher during the first two hours of Hemocontrol hemodialysis. Overall, mean arterial pressure (MAP) was higher during Hemocontrol dialysis. Neither the measures of endothelial function and sympathetic activity nor copeptin levels differed between the two dialysis modalities. In contrast, plasma vasopressin levels were significantly higher during the first half of Hemocontrol dialysis. The intradialytic course of vasopressin was associated with the course of MAP.

Conclusions

A transient intradialytic increase in plasma sodium did not affect indices of endothelial function or sympathetic activity compared with standard hemodialysis, but coincided with higher plasma vasopressin levels. The beneficial effect of higher intradialytic sodium levels on hemodynamic stability might be mediated by vasopressin.

Trial registration

ClinicalTrials.gov. Identifier: NCT03578510. Date of registration: July 5th, 2018. Retrospectively registered.

Effects of Osmotic Changes on Measuring Relative Blood Volume: Comparison of Three Hemodialysis Devices

Recording the relative blood volume is a standard feature of modern dialysis devices. Three different measurement systems are incorporated in currently available dialysis machines. The Gambro-Hospal group and Nikkiso feature blood volume monitors based on different optical methods. The Fresenius Medical Care machines perform blood volume monitoring with an ultrasonic method. On grounds of clinical evidence suggesting a malfunction of the optical methods in the presence of sodium changes, we compared these three systems. Under the tested conditions, both optical systems show opposite and nonplausible courses of blood volume changes. The ultrasonic system seems to be less susceptible to osmotic changes.

Measurement of Cardiac Output and Blood Volume During Hemodialysis with Fluorescent Dye Dilution Technique

Intradialytic hypotensive events (IDH) accompanied by deleterious decreases of the cardiac output complicate up to 25% of hemodialysis treatments. Monitoring options available to track hemodynamic changes during hemodialysis have been found ineffective to anticipate the occurrence of IDH. We have assembled opto-electronic instrumentation that uses the fluorescence of a small bolus of indocyanine green dye injected in the hemodialysis circuit to estimate cardiac output and blood volume based on indicator dilution principles in patients receiving hemodialysis. The instrument and technique were tested in 24 adult end-stage renal failure subjects during 64 hemodialysis sessions. A single calibration factor could be used across subjects and across time. Intra-subject variability of the measurements over time was <10%. Stroke volume index (SVI) (mean ± SEM = 34 ± 1 vs. 39 ± 2 mL m^-2) and central blood volume (CBV) index (783 ± 36 vs. 881 ± 33 mL m^-2) were lower at the beginning of the sessions in which IDH eventually occurred. Cardiac index, SVI, and CBV index decreased with hemodialysis in all treatment sessions but the decrease was more intense in the IDH sessions. We conclude that hemodynamic monitoring can be implemented in patients receiving hemodialysis with minimal disruption of the treatment and could help understand intradialytic hypotension.

Concordance of absolute and relative plasma volume changes and stability of Fcells in routine hemodialysis

Central hematocrit (H) measurements are currently used to track the degree of ultrafiltration-induced hemoconcentration with the aim to detect and prevent excessive intravascular fluid depletion during hemodialysis (HD). Failure to maintain hemodynamic stability is commonly attributed to the misinterpretation of H caused by an unaccountable increase in Fcells , the ratio of whole-body hematocrit to H. It was the aim to examine Fcells under everyday conditions in a group of stable HD patients. Absolute plasma volume (Vp ) and H were concomitantly measured during routine HD in the extracorporeal system in hourly intervals by noninvasive and continuous technology (CritLine-Instrument-III) and indocyanine green dye dilution to derive relative plasma volumes from Vp and H (RPVp , RPVH ), respectively, and to calculate Fcells . Thirteen patients were studied during two midweek treatments (n = 26). Both absolute Vp (P < 0.05) and relative plasma volumes RPVH (P < 0.001) decreased during HD. Vp at any time point was positively correlated to RPVH (r = 0.52). Moreover, relative plasma volumes RPVH and RPVp determined by independent techniques were identical and showed negligible bias (-0.2%) but considerable limits of agreement (-15.6% to +15.3%). Fcells was stable and in the range of 0.9 ± 0.05 throughout HD and not different from the value assumed at the beginning of HD. Although Fcells remains constant in patients on routine dialysis and relative plasma volumes (RPVH and RPVp ) determined by independent techniques are therefore comparable, the variability of experimental conditions during dialysis and the limited accuracy of absolute volume measurements using available technology continues to complicate the ultrafiltration control problem.

Sensitivity of Hematocrit to Osmotic Effects Induced by Changes in Dialysate Conductivity: Implications for Relative Blood Volume Measurement and Control

Hemodialysis is accompanied by osmotic perturbations with distinct effects on red blood cell, plasma, and blood volumes. A series of in vitro studies was done to analyze the separate effect on cell volume. Whole porcine blood was circulated through an extracorporeal circulation maintaining a constant blood volume. Hemoconcentration was continuously measured by established optical and ultrasonic online techniques. Osmotic perturbation was performed by variation of dialysate conductivity within the clinical range of 13-15 mS/cm. Blood samples were analyzed using a microcentrifuge and a standard cell counter. As dialysate conductivity increased, centrifuge hematocrit (in %) decreased with a slope of -1.91% per unit of conductivity in mS/cm (r2 = 0.98). At the same time, Coulter-Counter hematocrit slightly decreased only by -0.18% (r2 = 0.53), while optical and ultrasonic hematocrit showed a small increase by 0.44% (r2 = 0.97) and 0.69% (r2 = 0.94) per unit of conductivity in mS/cm. The sensitivity to osmotic perturbation is consistent with theory and with specific characteristics of measuring techniques used in this study. The differences, however, need to be considered when comparing measurements obtained by different techniques. Finally, devices used for relative blood volume measurement in hemodialysis should be insensitive to osmosis-induced changes in red blood cell volume.