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Saudi Journal of Kidney Diseases and Transplantation
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Year : 2001  |  Volume : 12  |  Issue : 3  |  Page : 337-344
Blood Pressure Guided Profiling of Ultrafiltration during Hemodialysis

1 Department of Internal Medicine, University of Rostock, Rostock, Germany
2 Dresden, Germany
3 Erfurt, Germany

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Hemodialysis-induced hypotension is still a common complication in spite of the progress achieved in hemodialysis (HD) treatment. Due to its multifactorial nature, dialysis-induced hypotension cannot be reliably prevented by conventional profiling of ultrafiltration in open-loop systems since they are unable to adapt themselves to actual decreases in blood pressure. A blood pressure guided closed-loop system for prevention of dialysis-induced hypotension by biofeedback-controlled profiling of ultrafiltration was clinically tested in 94 HD treatments of four patients prone to hypotension. Automatic profiling of ultrafiltration was based on frequent measurements of blood pressure at intervals of five minutes. Proper adaptation of control features to patients' conditions was provided by the lower limit of systolic pressure which was individually set by the physician at the beginning of each treatment. During the initial and medium phases of the HD sessions, ultrafiltration rates up to 200% of the average rates were applied as long as this was tolerated. The additional ultrafiltrate volume was used for blood pressure stabilization by lowering the ultrafiltration rates in the final phase of HD session. Biofeedback­controlled profiling of ultrafiltration provides reliable blood pressure stabilization in all phases of HD. During the first half of treatment, the frequency of hypotensive episodes remained below that with conventional therapy although ultrafiltration rates up to 200% were used. During the second half of treatment, blood pressure guided reduction of ultrafiltration rate provided a decreasing frequency of hypotensive episodes in contrast to the increasing trend during conventional therapy. Stable blood pressure trends during the last hour of HD were achieved in 91% of biofeedback-controlled treatments in comparison with only 32% of conventional treatments. Ultrafiltration rates of 150%-200% and blood pressure measurements at intervals of five minutes were well tolerated, since hypotension­prone patients were better monitored.

Keywords: Blood pressure, Hemodialysis, Hypotension, Profiling, Ultrafiltration.

How to cite this article:
Schmidt R, Roeher O, Hickstein H, Korth S. Blood Pressure Guided Profiling of Ultrafiltration during Hemodialysis. Saudi J Kidney Dis Transpl 2001;12:337-44

How to cite this URL:
Schmidt R, Roeher O, Hickstein H, Korth S. Blood Pressure Guided Profiling of Ultrafiltration during Hemodialysis. Saudi J Kidney Dis Transpl [serial online] 2001 [cited 2021 Apr 22];12:337-44. Available from: https://www.sjkdt.org/text.asp?2001/12/3/337/33557

   Introduction Top

Intra-dialytic hypotension continues to be the most common complication encoun­tered during hemodialysis (HD) therapy. Apart from causing discomfort, dialysis-induced hypotension results in a higher mortalitiy rate for patients suffering from this problem. [1] Since more and more elderly and cardiovascularly compromised patients require chronic HD, prevention of intra­dialytic hypotension remains the main task for optimizing HD therapy.

The causes of dialysis-induced hypotension are multifactorial. Hypovolemia, mainly caused by improper ultrafiltration, is con­sidered to be the leading factor. [2] Changes in osmolality of the serum result in fluid shifts between intra- and extra-vascular spaces. Concomitant diseases, drug intake, blood­membrane interactions, dialysate tempe­rature and others also contribute to intra­dialytic hypotension. [3],[4],[5],[6] With regard to hypovolemia, a certain success in preven­ting hypotensive episodes can be achieved by individual profiles of both dialysate sodium concentration [7],[8],[9] and ultrafiltration rate, [10],[11] which are pre-adjusted at the beginning of the treatment for each patient.

Further progress has been achieved with the usage of biofeedback-driven closed­loop systems which continuously adapt dialysate sodium concentration and ultra­filtration rate to the needs of the patient during HD therapy. [12],[13],[14]

The therapeutic effects essentially depend on the guiding parameter used for bio­feedback control. Changes of hematocrit reflecting blood volume was introduced as a guiding parameter to blood volume preser­vation by automatic profiling of ultrafil­tration rate. [15],[16],[17],[18] However, trials to predict intra-dialytic hypotension by individually defined hematocrit "thresholds" [19],[20] have failed in about 30% of patients [21],[22] since changes of blood volume do not reliably reflect the cardiovascular situation of the patient.

Considering the multifactorial causes of intra-dialytic hypotension as mentioned above, the actual cardiovascular situation can be comprehensively represented only by the actual blood pressure course of the patient.

In our earlier studies [23],[24] the systolic blood pressure and its short- and long-term trends have proven to be efficient guiding para­meters of biofeedback-controlled blood pressure stabilization in hypotension-prone patients. In the present study, blood pressure guided biofeedback control was aimed at achieving ultrafiltration rates as high as being allowed by the actual blood pressure during the first part of HD followed by low ultrafiltration rates at the end of treatment.

   Patients Top

Four patients with vascular instability were selected to take part in the study. In the first part of the study (97 treatments), they were treated with linear ultrafiltration and fuzzy-controlled infusion of 20% sodium chloride depending on the actual blood pressure. The same four patients underwent, in the second part of the study, fuzzy­controlled ultrafiltration profiling without sodium chloride infusion (94 treatments). Maximum rates (MAX-rates) for ultra­filtration profiling were set to 150% (60 treatments) and finally to 200% (34 treat­ments) of the average rate achieved during conventional HD with linear ultra-filtration. A total of 17 other patients on routine HD program served as a control group and were treated with conventional HD.

Optimal weight of all patients was defined every three months by assessing chest X­ray, tissue bioimpedance measurement, blood pressure course and clinical signs. Myocardial function in all patients was sufficient (ejection fraction 41.5 ± 9.3%). The inter-dialytic weight gain varied from 1.9 kg to 5.7 kg (average 3.2 ± 1.3 kg). In order to obtain reliable results, only 2-day intervals were considered. HD was routinely performed with a high-flux dialyzer. Bicar­bonate dialysate was used with sodium concentration of 138 mmol/l, potassium 2-4 mmol/l. Dialysate temperature in all treatments was 36°C. Patients with vascular instability took their anti-hypertensive medication after HD sessions. All patients were allowed to eat and to drink during HD.

   Methods Top

Automatic profiling of ultrafiltration rate during HD was obtained from biofeedback control by a closed-loop system, which is directly guided by patient´s systolic blood pressure. High reliability of biofeedback control was maintained by non-invasive blood pressure measurements via arm cuff (Dinamap 1846 SX, Critikon, Norderstedt, Germany), which are initiated automatically by a fuzzy controller at intervals of five minutes. Three linguistic variables were calculated from the measured values: i) Relative difference of systolic pressure and pre-adjusted set point pressure, ii) Short­term pressure trend (15 min), and iii) Long­term pressure trend (25 min). Each of the above linguistic variables is defined by specific fuzzy sets which are described by trapezoid and triangular membership functions for the interesting ranges of the variable. Fuzzy logic is applied to the procedures of biofeedback control in the following stepwise manner: a) Fuzzification of input data by matching of actual measured values (i) - (iii) and relevant fuzzy sets inclusive of weighting of results by set operators

b) Fuzzy inference by probabilistic reasoning extended to specific rule bases for control of ultrafiltration rate

c) Defuzzyfication of conclusions obtained from fuzzy inference by conversion into a crisp output for adaptation of ultrafiltration rate to patient´s actual blood pressure.

Biofeedback control is provided by on-line transmission of the crisp output to the dialysis machine (Dialog, B. Braun Melsungen, Germany) for profiling of ultrafiltration rate.

Since, in most of the patients with vascular instability, the frequency of hypotensive events increases with ongoing HD, the biofeedback control is focused on ultra­filtration rates as low as possible during the final phase of the session. This goal can be achieved by applying maximum ultra­filtration rates (MAX-rates) up to 200% of the average ultrafiltration rate during the initial and medium phases of treatment as long as it is tolerated by the systolic blood pressure. In order to adapt the control characteristics properly to the individual requirements of each patient, the critical borderline of systolic pressure (set point) is selected by the physician before starting the treatment. Normally, set points of 90­100 mm Hg are used for patients having initial systolic pressures of 90 mm Hg or higher. For patients with initial systolic pressures lower than 90 mm Hg the initial value itself is chosen as set point in most cases.

   Results Top

Frequency of hypotensive episodes

[Figure - 1] shows a typical run where the MAX-rate of 200% (i. e. UFRmax = 1600 ml/h) was well tolerated without a fall in the blood pressure during the first 45 min of treatment. The blood pressure even increased despite the high MAX-rate and the rapidly decreasing blood volume (BV) (ABV = -15%, measured by Crit-Line Monitor III, In-Line Diagnostics Corp., USA). As clearly visible from the inverse courses of blood pressure and blood volume during the first 60 min, blood volume guided profiling of ultrafiltration could not ensure reliable blood pressure stabilization during HD.

Due to severe blood pressure decreases in the interval from 50 to 80 min, the ultra­filtration rate was reduced stepwise, and after stabilizing the blood pressure it was automatically reset up to the MAX-rate. The total surplus of ultrafiltrate volume achieved during the first two hours of treatment enabled the ultrafiltration rate to be reduced to only 489 ml/h in the final phase, i.e. 61% of the average rate (UFRave) of 800 ml/h as preset for the entire treatment. Thus, the systolic blood pressure was reliably stabilized during the last two hours above the set point pressure (systolic low limit (SLL) = 90 mm Hg).

All of the treatments where hypotensive episodes below 90 mm Hg occured were analyzed by recording the measuring intervals (5 min) with low systolic pressure (< 90 mm Hg). Each of the hypotonic intervals was classified according to its hour of occurence. The results were compared among the three types of treatment: (i) Conventional HD with linear ultrafil­tration (69 treatments), (ii) Fuzzy-controlled infusion of 20% NaCl and linear ultra­filtration (97 treatments), (iii) Fuzzy­controlled ultrafiltration MAX-rate 150% without infusion (60 treatments) and (iv) Fuzzy-controlled ultrafiltration MAX-rate 200% without infusion (34 treatments).

[Figure - 2] shows the frequency distributions of measuring intervals (5 min each) with low systolic pressure versus the time of occurence during HD treatment. The figure illustrates that the frequency of low blood pressure situations during conventional HD (black columns) increases almost linearly during the treatment time course. If only fuzzy-controlled infusion of 20% saline and linear ultrafiltration (crossed columns) are used, the progressive trend is similar but to a smaller extent. However, fuzzy-controlled ultrafiltration MAX-rate 150% (dotted columns) and 200% (hatched columns) show an increase of hypotensive episodes only in the second hour of HD, and a decrease of hypotensive episodes in the third hour and especially in the fourth hour of treatment.

Blood pressure trend in the final phase of HD

The significant progress obtained from biofeedback-controlled ultrafiltration with respect to blood pressure behavior in the final phase of HD is shown in [Figure - 3]. The blood pressure trend during the final phase of HD (total time 4 hours) was analyzed by comparing the mean systolic pressure during the last (fourth) hour (BP 4 ) and during the third hour (BP 3 ) for each individual treatment. Percental differences (BP4-BP 3 ) /BP3 were calculated and classified by steps of 5% each. The results from (i) conventional HD with linear ultrafiltration were compared with (ii) fuzzy-controlled infusion and linear ultrafiltration, (iii) fuzzy-controlled ultrafil­tration MAX-rate 150%, and (iv) fuzzy­controlled ultrafiltration MAX-rate 200%.

As shown in [Figure - 3], the frequency distribution of fuzzy-controlled groups (ii) to (iv) are characterized by significantly better locations in comparison with group (i), conventional HD. Only from 32.4% of conventional treatments (black columns), numerical values (BP 4 - BP 3 ) / BP 3 > 0% were obtained. That means, in only 32.4% of conventional treatments the mean systolic pressure BP 4 was higher than the mean systolic pressure BP3 during the preceding hour, or at least equal to BP3. By fuzzy-controlled infusion and linear ultra­filtration (crossed columns group ii) this number was elevated to 44.4% of treatments wherein this criteria was met. In group (iii), fuzzy-controlled ultrafiltration MAX-rate 150% (dotted columns), a high majority of 81.7% of treatments complied with values (BP 4 -BP3)/BP3 > 0%. However, additional benefit was obtained from group (iv), fuzzy-controlled ultrafiltration MAX­rate 200% (hatched columns), where 91.2% of treatments met this criteria.

Session goals regarding the total weight loss were achieved in all cases, although the ultrafiltration rates could be diminished by biofeedback control in the final phase below the average ultrafiltration rate due to the surplus accumulated during the preceding phases.

Ultrafiltration MAX-rates of 150%-200% and blood pressure measurements at intervals of five minutes were well tolerated. Hypotension-prone patients benefitted from automatic blood pressure stabilization and felt better monitored.

   Discussion Top

It is well known that most of the comp­lications that occur during HD, especially hypotensive episodes, are of multifactorial etiology and caused by different mechanisms.

Volume substitution, injection of osmotic substances, increase of dialysate conductivity, decrease of ultrafiltration to zero, reduction of speed of the blood pump and adminis­tration of vasoactive drugs are methods proven in clinical practice to treat acute hypotension during HD. In order to prevent hypotensive episodes, open-loop systems with pre-programmed ultrafiltration and sodium-profiles have been developed.

However, clinical experience shows that the methods mentioned above are of limited therapeutic effect in treating patients with vascular instability during HD, since actual changes in blood pressure behaviour are not considered continuously. Reliable prevention of dialysis-induced hypotension is achie­vable only from biofeedback driven closed­loop systems guided by frequent blood pressure measurements.

As verified by the results mentioned above, blood pressure guided ultrafiltration profiling has proven its superiority in all phases of treatment. During the first half of treatment, the frequency of hypotonic episodes remains below the relatively low level of conventional therapy although ultrafiltration MAX-rates up to 200% are used in comparison with linear ultrafil­tration. During the second half of treatment, the biofeedback-controlled diminution of ultrafiltration rate results in a decreasing frequency of hypotonic episodes in contrary to the increasing trend during conventional therapy. Thus, the closed-loop system for automatic profiling of ultrafiltration provides reliable blood pressure stabilization in hypotension-prone patients during all phases of HD treatment.

Since the blood pressure course is continuously monitored at intervals of five minutes, MAX-rates of 200% compared with the pre-adjusted average ultrafiltration rate should be preferred for routine clinical practice in order to provide low ultra­ filtration rates and stable blood pressure in the final phase of treatment.

   Acknowledgement Top

Authors express their gratitude to B. Braun Melsungen for supporting this work.

   References Top

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Correspondence Address:
Reinhard Schmidt
Klinik für Innere Medizin, Universität Rostock, Ernst-Heydemann-Str. 6, D-18055 Rostock
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PMID: 18209380

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