Continuous on-line optical absorbance recording of blood volume changes during hemodialysis

Feasibility of Dialysate Bolus-Based Absolute Blood Volume Estimation in Maintenance Hemodialysis Patients

BACKGROUND

Absolute blood volume (ABV) is a critical component of fluid status, which may inform target weight prescriptions and hemodynamic vulnerability of dialysis patients. Here, we utilized the changes in relative blood volume (RBV), monitored by ultrasound (BVM) upon intradialytic 240 mL dialysate fluid bolus-infusion 1 h after hemodialysis start, to calculate the session-specific ABV. With the main goal of assessing clinical feasibility, our sub-aims were to (i) standardize the BVM-data read-out; (ii) determine optimal time-points for ABV-calculation, "before-" and "after-bolus"; (iii) assess ABV-variation.

METHODS

We used high-level programming language and basic descriptive statistics in a retrospective study of routinely measured BVM-data from 274 hemodialysis sessions in 98 patients.

RESULTS

Regarding (i) and (ii), we automatized the processing of RBV-data, and determined an algorithm to select the adequate RBV-data points for ABV-calculations. Regarding (iii), we found in 144 BVM-curves from 75 patients, that the average ABV ± standard deviation was 5.2 ± 1.5 L and that among those 51 patients who still had ≥2 valid estimates, the average intra-patient standard deviation in ABV was 0.8 L. Twenty-seven of these patients had an average intra-patient standard deviation in ABV <0.5 L.

CONCLUSIONS

We demonstrate feasibility of ABV-calculation by an automated algorithm after dialysate bolus-administration, based on the BVM-curve. Based on our results from this simple "abridged" calculation approach with routine clinical measurements, we encourage the use of multi-compartment modeling and comparison with reference methods of ABV-determination. Hopes are high that clinicians will be able to use ABV to inform target weight prescription, improving hemodynamic stability.

Hemodialysis crossover study using a relative blood volume change-guided ultrafiltration control compared with standard hemodialysis: the BV-UFC study

Background

It has been difficult to sufficiently achieve body-fluid management using blood volume (BV) monitor during hemodialysis (HD) with constant ultrafiltration (UF) rate. Recently, a relative BV change-guided UF control (BV-UFC) system was developed by combining the concepts of an automatic feedback system that could control the UF rate and profile with real- time monitoring of relative changes in BV (%ΔBV). However, this system has limited application in the clinical setting. Therefore, in this study, we aimed to perform the crossover study on HD with BV-UFC compared to standard HD in terms of hemodynamic stability during HD.

Methods

Forty-eight patients entered an 8-week crossover period of standard HD or HD with BV-UFC. Prevalence of intradialytic hypotension (IDH) as a primary outcome and changes in blood pressure (BP), differences in %ΔBV, and achievement of the target ultrafiltration volume as secondary outcomes were compared. IDH was defined as a reduction in systolic BP ≥20 mmHg from the baseline value at 10 min after HD initiation.

Results

No significant differences were found in the prevalence of IDH, frequency of intervention for symptomatic IDH, and achievement of the target ultrafiltration volume between the groups. The %ΔBV was significantly fewer (-12.1 ± 4.8% vs. -14.4 ± 5.2%, p <0.001) in the HD with BV-UFC than that in the standard HD.

Conclusions

HD with BV-UFC did not reduce the prevalence of IDH compared with standard HD. The relief of a relative BV reduction at the end of HD may be beneficial in patients undergoing HD with BV-UFC.

Trial Registration

UMIN, UMIN000024670. Registered on December 1, 2016.

Hemoscan™: A Dialysis Machine-Integrated Blood Volume Monitor

We describe an opto-electronic device capable of measuring the hemoglobin concentration (Hgb) non-invasively and continuously, hence the percentage changes in blood volume (BV) during dialysis treatment by means of the optical absorption of monochromatic light by the blood in the arterial line. This method has been validated during several in vitro and in vivo tests, during which the system has shown a low sensitivity to all the common intra-dialytic interference factors, such as oxygen saturation (max. err.=1.6%), blood flow (max. err. =1.8%), osmotic pressure (max. err.=0.7°) and hydraulic pressure (max. err.=0.6°), a high precision (std. err.≤0.1 g/dl) and a good accordance (Hgb mean err.=0.1 g/dl; std. err.=0.38 g/dl; BV mean err.=0.1%; std. err.=1.6%) with the corresponding values derived from standard laboratory tests.

Modeling Indicator Dispersion in Extracorporeal Blood Lines

The measurement of indicators such as saline diluted by blood flow provides important information on transport characteristics during extracorporeal blood treatments. When saline is injected and measured using the extracorporeal system, the effects caused by dispersion within the extracorporeal system have to be taken into consideration in order to adequately identify intracorporeal transport characteristics. It was the aim of this study to quantify the extracorporeal contribution and to obtain a transport function for specified sections of the extracorporeal system.

The dispersion of saline following an impulsive input was measured in arterial and venous segments of customary extracorporeal blood lines with different distribution volumes (Vt=23–87 mL) using a range of different blood flows (Qb=200–450 ml/min). The dispersion was analyzed using a modified Gamma distribution function characterized by three parameters n, k, and τ, where n is real, positive, and n 1, where k=Q b/Vt*n2, and where trefers to the indicator appearance time at the sampling site. The parameters n, k, and t were identified by fitting the model function to experimental data.

The value of n was 2.3±0.5 and largely independent of the type of line segment, Qb, or Vt. τ showed a strong dependence on Vt and Qb which was described by τ= Vt/Qb*(n-1)/n.

Thus, with a given n, and when Vt and Qb are known, the transport function for saline in important sections of the extracorporeal circulation can be determined for specific experimental conditions. With this information indicator dilution curves measured in extracorporeal blood lines can be corrected for extracorporeal effects.

Randomized Crossover Trial of Blood Volume Monitoring-Guided Ultrafiltration Biofeedback to Reduce Intradialytic Hypotensive Episodes with Hemodialysis

BACKGROUND AND OBJECTIVES

Intradialytic hypotension (IDH) is associated with morbidity. The effect of blood volume-guided ultrafiltration biofeedback, which automatically adjusts fluid removal rate on the basis of blood volume parameters, on the reduction of IDH was tested in a randomized crossover trial.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We performed a 22-week, single blind, randomized crossover trial in patients receiving maintenance hemodialysis who had >30% of sessions complicated by symptomatic IDH in five centers in Calgary, Alberta, Canada. Participants underwent a 4-week run-in period to standardize dialysis prescription and dry weight on the basis of clinical examination. Those meeting inclusion criteria were randomized to best clinical practice hemodialysis (control) or best clinical practice plus blood volume-guided ultrafiltration biofeedback (intervention) for 8 weeks, followed by a 2-week washout and subsequent crossover for a second 8-week phase. The primary outcome was rate of symptomatic IDH.

RESULTS

Thirty-five participants entered, 32 were randomized, and 26 completed the study. The rate of symptomatic IDH with biofeedback was 0.10/h (95% confidence interval, 0.06 to 0.14) and 0.07/h (95% confidence interval, 0.05 to 0.10) during control (P=0.29). There were no differences in the rate or proportion of sessions with asymptomatic IDH or symptoms alone. Results remained consistent when adjusted for randomization order and study week. There were no differences between intervention and control in the last study week in interdialytic weight gain (difference [SD], -0.02 [0.8] kg), brain natriuretic peptide (1460 [19,052] ng/L), cardiac troponins (3 [86] ng/L), extracellular water-to-intracellular water ratio (0.05 [0.33]), ultrafiltration rate (1.1 [7.0] ml/kg per hour), and dialysis recovery time (0.43 [19.25] hours).

CONCLUSION

The use of blood volume monitoring-guided ultrafiltration biofeedback in patients prone to IDH did not reduce the rate of symptomatic IDH events.

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.

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.

Ultrafiltration biofeedback guided by blood volume monitoring to reduce intradialytic hypotensive episodes in hemodialysis: study protocol for a randomized controlled trial

BACKGROUND

Fluid removal during dialysis, also known as ultrafiltration (UF), leads to intradialytic hypotension (IDH) in a significant number of patients treated with hemodialysis (HD) and is associated with an increase in morbidity and mortality. At present, there are no accepted standards of practice for the prevention or treatment of IDH. Relative blood volume monitoring (BVM) is based on the concept that the hematocrit increases with UF, relative to the patient's baseline hematocrit. The use of BVM biofeedback, whereby the HD machine automatically adjusts the rate of UF based on the relative blood volume, has been proposed for the prevention of IDH.

METHODS/DESIGN

This is a 22-week randomized crossover trial. Participants undergo a 4-week run-in phase to standardize medications and dialysis prescriptions. Subsequently, participants are randomized to standard HD or to BVM biofeedback for a period of 8 weeks followed by a 2-week washout phase before crossing over. The dialysis prescription remains identical for both arms. The primary outcome is the frequency of symptomatic IDH as defined by an abrupt drop in the systolic blood pressure of ≥ 20 mm Hg accompanied by headache, dizziness, loss of consciousness, thirst, dyspnea, angina, muscle cramps or vomiting. Secondary outcomes include the number of symptomatic IDH episodes and any reduction in IDH episodes, nursing interventions, dialysis adequacy, total body water, extra- and intracellular fluid volumes, brain natriuretic peptide and cardiac troponin levels, blood pressure, antihypertensive medication use, patient symptoms and quality of life.

DISCUSSION

Our study will determine the impact of using BVM biofeedback to prevent IDH and other serious adverse events in susceptible patients.

TRIAL REGISTRATION

Clinicaltrials.gov NCT01988181 (6 November 2013).

Techniques for the assessment of volume status in patients with end stage renal disease

While advances in assessing extracellular volume status have occurred, no methodology is sufficiently robust and accurate in all patients to recommend routine use in clinical practice. All of the methods described also perform best when measured serially in patients and when correlated with other ancillary methods of volume assessment such as body weight, physical examination and determination of vital signs and symptomatology. Perhaps, the best method for assessing and modifying dry weight is to utilize multiple complementary methods such as advocated by Ronco et in the “5B” approach (39). In this approach, the clinician utilizes data from: fluid balance (body weight changes), blood pressure, biomarkers (such as the natriuretic peptides), bioimpedance, and blood volume changes. Body (thoracic and IVC) ultrasound can also be included (Fig. 1). These combined data sources will likely lead to greater detection of subtle volume overload, a finding likely to contribute to excess mortality and morbidity. Clinical trials of such strategies are needed to better inform clinicians.

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

Recording the relative blood volume is a standard feature of modern dialysis devices, enabling feedback guidance of ultrafiltration and dialysate conductivity. Technically, the process is based on optical or ultrasonic methods. On the grounds of clinical evidence suggesting a malfunction of the optical hemoglobin (Hb)-dependent absorbance method in the presence of sodium changes, we compared the system with the ultrasonic method. Six patients underwent hemodialysis with a step sodium profile (140, 150, 130, and 140 mmol/L, hourly switch), with two dialysis devices featuring the optical and the ultrasonic blood volume detector, respectively. The ultrasonic system recorded a decreasing blood volume throughout the treatment. With the optical method, changes in dialysate sodium led to inverse deviations of the blood volume curve. In another treatment without profile administering, a bolus of hypertonic sodium led to the detection of a rapid 8.7% reduction in blood volume with the optical method, which was not observed with the ultrasonic device. Blood volume monitors using the optical absorbance device are influenced by osmotic changes. An increase in osmolality produces a paradox drop in the measured blood volume and vice versa rendering the monitor inappropriate for use in sodium profiling.

Significance of interdialytic weight gain versus chronic volume overload: consensus opinion

Predialysis volume overload is the sum of interdialytic weight gain (IDWG) and residual postdialysis volume overload. It results mostly from failure to achieve an adequate volume status at the end of the dialysis session. Recent developments in bioimpedance spectroscopy and possibly relative plasma volume monitoring permit noninvasive volume status assessment in hemodialysis patients. A large proportion of patients have previously been shown to be chronically volume overloaded predialysis (defined as >15% above 'normal' extracellular fluid volume, equivalent to >2.5 liters on average), and to exhibit a more than twofold increased mortality risk. By contrast, the magnitude of the mortality risk associated with IDWG is much smaller and only evident with very large weight gains. Here we review the available evidence on volume overload and IDWG, and question the use of IDWG as an indicator of 'nonadherence' by describing its association with postdialysis volume depletion. We also demonstrate the relationship between IDWG, volume overload and predialysis serum sodium concentration, and comment on salt intake. Discriminating between volume overload and IDWG will likely lead to a more appropriate management of fluid withdrawal during dialysis. Consensually, the present authors agree that this discrimination should be among the primary goals for dialysis caretakers today. In consequence, we recommend objective measures of volume status beyond mere evaluations of IDWG.

Blood volume-monitored regulation of ultrafiltration in fluid-overloaded hemodialysis patients: study protocol for a randomized controlled trial

BACKGROUND

Data generated with the body composition monitor (BCM, Fresenius) show, based on bioimpedance technology, that chronic fluid overload in hemodialysis patients is associated with poor survival. However, removing excess fluid by lowering dry weight can be accompanied by intradialytic and postdialytic complications. Here, we aim at testing the hypothesis that, in comparison to conventional hemodialysis, blood volume-monitored regulation of ultrafiltration and dialysate conductivity (UCR) and/or regulation of ultrafiltration and temperature (UTR) will decrease complications when ultrafiltration volumes are systematically increased in fluid-overloaded hemodialysis patients.

METHODS/DESIGN

BCM measurements yield results on fluid overload (in liters), relative to extracellular water (ECW). In this prospective, multicenter, triple-arm, parallel-group, crossover, randomized, controlled clinical trial, we use BCM measurements, routinely introduced in our three maintenance hemodialysis centers shortly prior to the start of the study, to recruit sixty hemodialysis patients with fluid overload (defined as ≥15% ECW). Patients are randomized 1:1:1 into UCR, UTR and conventional hemodialysis groups. BCM-determined, 'final' dry weight is set to normohydration weight -7% of ECW postdialysis, and reached by reducing the previous dry weight, in steps of 0.1 kg per 10 kg body weight, during 12 hemodialysis sessions (one study phase). In case of intradialytic complications, dry weight reduction is decreased, according to a prespecified algorithm. A comparison of intra- and post-dialytic complications among study groups constitutes the primary endpoint. In addition, we will assess relative weight reduction, changes in residual renal function, quality of life measures, and predialysis levels of various laboratory parameters including C-reactive protein, troponin T, and N-terminal pro-B-type natriuretic peptide, before and after the first study phase (secondary outcome parameters).

DISCUSSION

Patients are not requested to revert to their initial degree of fluid overload after each study phase. Therefore, the crossover design of the present study merely serves the purpose of secondary endpoint evaluation, for example to determine patient choice of treatment modality. Previous studies on blood volume monitoring have yielded inconsistent results. Since we include only patients with BCM-determined fluid overload, we expect a benefit for all study participants, due to strict fluid management, which decreases the mortality risk of hemodialysis patients.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT01416753.

Can a decreasing ultrafiltration profile affect the dialytic dose administered?

During hemodialysis, ultrafiltration (UF) seems to affect the dialytic dose because of convective removal of urea and contraction of its distribution volume. We aimed to assess whether the adoption of a decreasing UF profile could yield a different dialytic dose from that obtained with a constant UF mode. Ten patients were randomly assigned to undergo 12 sessions with a constant UF mode (phase A) followed by 12 sessions with a decreasing UF rate (phase B), or the reverse. Kt/V and urea reduction ratio (URR) were 1.77 +/- 0.26 and 70.02 +/- 8.26% in phase A vs. 1.81 +/- 0.36 and 71.02 +/- 6.48% in phase B, respectively, showing no significant difference. Measurement of the differences in volemic variations between the two phases showed a statistically significant difference at the second hour (P < 0.001, the volemic reduction being greater in phase B than A) and at the fourth hour (P < 0.001, being greater in phase A than B). In standard bicarbonate dialysis, the adoption of a decreasing UF profile rather than a constant one does not alter the efficiency of the dialytic treatment.

Hemodialysis monitoring in whole blood using transmission and diffuse reflection spectroscopy: A pilot study

Visible and near infrared transmission and diffuse reflection spectroscopy were used to monitor changes in whole blood resulting from hemodialysis treatment for end-stage renal disease. Blood samples from 8 patients on chronic hemodialysis therapy were measured in the 500- to 1700-nm wavelength range immediately before and after a single treatment. Principal component scores characteristic of each spectrum were derived, and mean pre- and posttreatment scores of the first principal component indicated a significant treatment-dependent change in both optical transmission (P = 0.004) and diffuse reflection (P < 0.001). Significant treatment-induced change persisted (P < 0.05) when the first four principal components were used to account for >97% of the treatment-dependent spectral variation. Some blood spectral changes expressed in terms of difference spectra (posttreatment - pretreatment) were consistent with standard clinical indicators of weight reduction, urea reduction, and potassium change, with probable origins at a molecular level. The results indicate the feasibility of using optical transmission and diffuse reflection spectroscopy to characterize clinically relevant blood changes for the future development of more comprehensive indicators of hemodialysis efficacy and long-term clinical outcomes. Moreover, the optical techniques employed are adaptable for potential online monitoring of blood changes during the hemodialysis treatment.

Inter-dialytic variations in blood volume and total body water in uraemic patients treated by dialysis

BACKGROUND

An optimal balance of sodium and water is one of the most important goals of haemodialysis (HD) therapy. However, while inter-dialytic variations in blood volume (BV) have been well described, very little is known about the dynamics of fluid accumulation and distribution in body compartments during the inter-dialysis period.

METHODS

We studied inter-dialysis variations in BV, measured as percent variation of plasma haemoglobin (Hb) concentrations (% triangle up BV) and percent variation of total body water (% triangle up TBW), in 24 uraemic patients treated by standard bicarbonate dialysis. These parameters were determined at the end of the last weekly dialysis (T0), after 24 h (T1), 48 h (T2), and at the beginning of the following dialysis session (T3). At each time point we measured Hb, haematocrit (Hct), serum albumin (sAlb), plasma sodium (Na), plasma potassium (K), blood urea nitrogen (BUN), plasma osmolality (Osm), body weight (BW), systolic blood pressure (SBP), diastolic blood pressure (DBP) and heart rate (HR). All patients were clinically stable and had no evidence of acute blood loss in the 3 weeks before the study.

RESULTS

During the inter-dialysis period, there were increases in BUN, K and Osm, but Na did not change. SBP and DBP also did not change. HR tended to decrease, and showed a significant reduction between T0 and T3. TBW increased in a linear fashion whereas BV increased exponentially, showing a slow rise during the first 24 h followed by a greater increase in the following time intervals. This was confirmed by concomitant but opposite percent variations in Hct and sAlb concentrations.

CONCLUSIONS

Despite the limitations of the current methodology, our data show that the increase in TBW is redistributed during the long inter-dialysis period and this may prevent the effects of a too premature expansion of the intra-vascular compartment. This is especially evident during the first 24 h after HD, during which % triangle up BV is lowest, indicating a preferential distribution of the fluid load towards the extra-vascular space. During the following time intervals, the extra-vascular compartment refills in conjunction with an exponential expansion of BV that reaches its maximum in the last 24 h before HD.

Model based sensitivity analysis of arterial pressure response to hemodialysis induced hypovolemia

The role of hemodynamic and regulatory factors in the arterial pressure response to hemodialysis induced hypovolemia was investigated by means of a computer model of the cardiovascular system, including the main short-term pressure regulatory mechanisms. The model mimics the arterial and venous systemic circulation, Starling's law and inotropic heart regulation, arterial and cardiopulmonary baroreflex controls of resistance, and capacitance vessels. All of the model parameters have a clear physiologic meaning: 10 represent the systemic circulation, 4 describe cardiac pump performance, and 3 characterize baroreflex regulation. Sensitivity analysis is performed to determine the effect of each parameter on the pressure response to mild hypovolemia (a 10% blood volume reduction after 4 hours). The results demonstrate that circulatory parameters, such as resistances and compliances, have no relevant effect upon the pressure response. Conversely, regulation of venous capacity seems to play a pivotal role in sustaining arterial pressure during hemodialysis induced hypovolemia. Regulation of systemic peripheral resistance exerts a compensatory action only as long as the blood volume reduction is < 5%, but it is inadequate to compensate for a larger blood volume reduction when venous capacity regulation is absent. A paradoxical arterial pressure increase during hypovolemia can be referred to a prevalence of cardiopulmonary afferences in the regulatory process.

Blood volume regulation during hemodialysis

Hemodialysis (HD)-induced hypotension may be precipitated by severe hypovolemia. To avoid the appearance of destabilizing hypovolemias, we have developed a biofeedback control system for intradialytic blood volume (BV)-changes modeling. The system, incorporated in a dialysis machine, is based on a multivariable closed-loop control with a dependent output variable, the BV changes, and two independent control variables, the ultrafiltration rate (Qf) and dialysate conductivity (DC). The relative BV changes occurring during HD are measured by an optical device. The Qf and DC are continuously adjusted by the control model during the treatment to minimize any discrepancies between the ideal targets for the BV, the patient's body weight reductions, and the experimentally obtained results. The system manages three kinds of errors: in BV changes, the total weight loss, and the sodium balance. The latter is controlled by a dedicated kinetic model that continuously calculates the equivalent DC and, by the end of the session, tends to make the sodium balance the same as the one obtained in conventional HD with constant DC. This system's capacity to improve intradialytic hemodynamic tolerance has been assessed in a crossover study of eight highly symptomatic patients. Conventional HD (CHD; period A) was compared with blood volume-controlled dialysis sessions (BV-CHD; period B) following a protocol with an A1-B-A2 sequence, with each period lasting 1 month. A lower decrease in BV (-10.6%) was obtained during BV-CHD (period B) compared with CHD (-12.3% in period A1 and -12.5% in period A2). The predialysis to postdialysis systolic arterial pressure changes were lower in period B (-12.4%) than in period A (-20% in A1 and -17.5% in A2; P < 0.05) despite similar total Qf and mean treatment times. A significant reduction in the number of severe hypotensive episodes (three in period B v 26 in period A1 and 16 in period A2; P < 0.05) and the overall incidence of complaints, especially of muscular cramps, was found in BV-CHD. These results were reflected in a reduced need for therapeutically administered isotonic saline in each session (60 mL in B v160 mL in A1 and 95 mL in A2; P < 0.05). In conclusion, the proposed biofeedback system for intradialytic BV control may be useful to avoid severe hypovolemic states, to stabilize BV by modeling its trend, and to avoid reaching individual critical BV thresholds in hypotension-prone patients.

Continuous measurement of hematocrit using an intravascular catheter equipped with a fiberoptic transmission cell

PURPOSE

The purpose of this study was to measure intravascular hematocrit values continuously by using a fiberoptic probe based on near-infrared photometry.

METHODS

We produced a catheter 1.5 mm in diameter that use a pair of plastic fibers. One of the fibers, the measuring fiber, was used to measure the optical density of blood, and the other, the reference fiber, was used to decrease the signal-to-noise ratio. We employed an 805-nm laser diode as the light source. Two photodiodes were used to measure the intensity of the light transmitted through the two fibers, and the output signals were amplified and sent to a personal computer through an analog-to-digital converter.

RESULTS

The hematocrit values obtained by this fiberoptic continuous measurement agreed well with those obtained by microcentrifugation within physiological ranges.

CONCLUSIONS

This method is effective for monitoring the rapid changes in hematocrit.

Ultrafiltration and backfiltration during hemodialysis

Ultrafiltration is the pressure-driven process by which hemodialysis removes excess fluid from renal failure patients. Despite substantial improvements in hemodialysis technology, three significant problems related to ultrafiltration remain: ultrafiltration volume control, ultrafiltration rate control, and backfiltration. Ultrafiltration volume control is complicated by the effects of plasma protein adsorption, hematocrit, and coagulation parameters on membrane performance. Furthermore, previously developed equations relating the ultrafiltration rate and the transmembrane pressure are not applicable to high-flux dialyzers, high blood flow rates, and erythropoietin therapy. Regulation of the ultrafiltration rate to avoid hypotension, cramps and other intradialytic complications is complicated by inaccurate estimates of dry weight and patient-to-patient differences in vascular refilling rates. Continuous monitoring of circulating blood volume during hemodialysis may enable a better understanding of the role of blood volume in triggering intradialytic symptoms and allow determination of optimal ultrafiltration rate profiles for hemodialysis. Backfiltration can occur as a direct result of ultrafiltration control and results in transport of bacterial products from dialysate to blood. By examining these problems from an engineering perspective, the authors hope to clarify what can and cannot be prevented by understanding and manipulating the fluid dynamics of ultrafiltration.