Prevention of haemodialysis-induced hypotension by biofeedback control of ultrafiltration and infusion
Reinhard Schmidt1,,
Ottfried Roeher2,
Heiko Hickstein1 and
Steffen Korth3
1 Department of Internal Medicine, University of Rostock, Rostock,
2 Dresden
3 Erfurt, Germany
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Abstract
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Background. Haemodialysis-induced hypotension is still a severe complication in spite of all the progress in haemodialysis treatment. Because of its multifactorial causes, haemodialysis-induced hypotension cannot be reliably prevented by conventional ultrafiltration and sodium profiling in open-loop systems, as they are unable to adapt themselves to actual decreases in blood pressure.
Methods. A blood-pressure-guided closed-loop system, for prevention of haemodialysis-induced hypotension by biofeedback-driven computer control of both ultrafiltration and saline infusion was clinically tested in 237 treatments of seven patients prone to hypotension. As medical knowledge on multifactorial causes of hypotension is characterized by a lack in deterministic knowledge, fuzzy logic and linguistic variables were used to involve clinical experience on hypotension phenomena in terms of fuzzy knowledge. Biofeedback control is based on frequent measurements of blood pressure at 5 min intervals. Blood pressure behaviour is described by linguistic variables and fuzzy sets. Adaptive rule bases were used for the simultaneous fuzzy control of both the ultrafiltration and infusion of hypertonic saline (20% NaCl). Proper adaptation of control features to patient's conditions was provided by the critical borderline pressure, which was set by the physician individually at the beginning of each treatment. During the initial and medium phases of the sessions, ultrafiltration rates up to 150% of the average rates were applied as long as decreases in blood pressure could be compensated by saline infusion. The surplus of ultrafiltrate volume was used for blood pressure stabilization in the final phase in most instances by low ultrafiltration rates.
Results. The advantages of biofeedback-controlled haemodialysis were demonstrated by both decreasing the frequency of hypotonic episodes and by increasing or maintaining constant levels of systolic blood pressure during the final phase in 88% of treatments. As saline infusion was applied mainly in the initial and medium phases, blood sodium levels were not significantly higher at the end of the sessions, and interdialytic weight gain was not elevated.
Conclusion. The application of fuzzy logic in the blood-pressure-guided biofeedback control of ultrafiltration and sodium infusion during haemodialysis is able to minimize haemodialysis-induced hypotension.
Keywords: blood pressure; fuzzy control; haemodialysis; hypotension; infusion; ultrafiltration
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Introduction
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Symptomatic hypotension occurs in up to 30% of haemodialysis treatments and represents one of the most severe complications during haemodialysis treatment [1]. The causes of haemodialysis-induced hypotension are multifactorial. Inadaquate fluid removal, drug intake, complications of the primary disease, concomitant diseases, bloodmembrane interactions and others play a role [27]. These causes are known qualitatively but there is a lack in understanding as to what degree or quantity they contribute to hypotension [810]. Haemodialysis-induced hypovolaemia is supposed to be a key parameter and its changes are used to assess the cardiovascular situation of the haemodialysis patient and to predict hypotensive episodes. However, hypovolaemia is balanced by the refilling rate, peripheral vasoconstriction, increase of heart rate, augmentation of venous return and release of vasoacitve hormones. As hypovolaemia is compensated by different mechanisms, hypovolaemia alone cannot be the leading parameter in assessing the cardiovascular situation of the patient and it fails in predicting hypotensive episodes [11]. All influencing parameters and mechanisms result in the actual blood pressure and its course. The actual blood pressure represents comprehensively the actual caradiovascular situation of the patient. Therefore, the actual systolic blood pressure and its trend are the guiding parameters for a biofeedback-driven monitor for blood pressure stabilization during haemodialysis. Probabilistic reasoning and fuzzy logic were used to transfer medical knowledge into a closed-loop system providing blood pressure control in haemodialysis patients prone to hypotension.
The most frequently used therapeutic interventions in cases of haemodialysis-induced hypotension are infusion of volume, injection of hyperosmotic substances (sodium 20%, glucose 40%), interruption of ultrafiltration and application of vasoactive drugs. By giving the infusion of volume and interrupting ultrafiltration the goal of ultrafiltration might not be reached. Repeated injection of hypertonic saline could increase the exchangable sodium pool and may result in thirst and increased interdialytic weight gain. From former studies [11,12] and from routine haemodialysis programmes we know that the first half of the haemodialysis treatment shows much less hypotensive episodes in comparison with the second half of the treatment. In conclusion, it was obviously more beneficial to ultrafilter in the first part of haemodialysis as much as the actual systolic blood pressure allows with the consequence of low ultrafiltration rates at the end of haemodialysis. In order to maintain maximum ultrafiltration rates (MAX-rates) during the first part of haemodialysis, blood pressure can be additionally stabilized by the injection of 20% saline. Sodium application should be restricted to the first half of haemodialysis; a possible surplus of sodium can be eliminated during the second half of the treatment. The low ultrafiltration rates during the second half of haemodialysis result in blood pressure stabilization and the additional effect of sodium is unnecessary in this phase of treatment.
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Subjects and methods
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Patients
Seven patients with vascular instability were selected to take part in the study. Four were treated (97 treatments) with fuzzy-controlled infusion of sodium chloride 20% depending on the actual blood pressure and linear ultrafiltration. The same four patients underwent in the second part of the study fuzzy-controlled ultrafiltration profiling without sodium infusion (60 treatments). All seven patients were treated with fuzzy-controlled sodium infusion and fuzzy-controlled ultrafiltration profiling simultaneously in a third step (237 treatments). MAX-rates for ultrafiltration profiling were set to 140% (116 treatments) and finally to 150% (121 treatments) of the average rate as usual during conventional haemodialysis with linear ultrafiltration rate. Seventeen other patients out of the routine haemodialysis programme served as a control group and were treated with conventional haemodialysis.
The optimal weight of all patients is defined every 3 months by assessing chest X-ray, tissue bioimpedance measurement, blood pressure course and clinical signs. Myocardial function in all patients was sufficient (ejection fraction 43.6±8.2%). The interdialytic weight gain differed from 1.6 to 6.1 kg (average 3.4±1.2 kg). Comparisons of interdialytic weight gains after treatments with and without fuzzy-controlled infusion were focused individually on the same patient during the normal rhythm of 223 days weekly. For reliable results only 2-day intervals were considered. Haemodialysis was routinely performed with a high-flux dialyser. Bicarbonate dialysate was used with a sodium concentration of 138 mmol/l and a potassium concentration of 24 mmol/l. Dialysate temperature in all treatments was 36°C. Patients with vascular instability took their antihypertensive medication after haemodialysis. All patients were allowed to eat and to drink during haemodialysis.
Methods
The closed-loop system for biofeedback control is shown in Figure 1
. The high reliability of biofeedback control is maintained by non-invasive blood pressure measurements via an arm cuff (Dinamap 1846 SX, Critikon, Norderstedt), which are initiated automatically by the fuzzy controller at 5 min intervals. Three linguistic variables are calculated from the measuring values: (i) relative difference of systolic pressure and pre-adjusted set point pressure, (ii) short-term pressure trend (15 min), and (iii) long-term pressure trend (25 min). Each of the linguistic variables (i)(iii) 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 following steps:
- fuzzification of input data by matching of actual measuring values (i)(iii) and relevant fuzzy sets inclusive weighting of results by set operators;
- fuzzy inference by probabilistic reasoning extended to specific rule bases for both control of ultrafiltration rate and infusion rate of hypertonic saline solution; and
- defuzzyfication of conclusions obtained from fuzzy inference by conversion into crisp outputs for adaptation of ultrafiltration rate and infusion rate to patient's actual blood pressure behaviour.
Biofeedback control is provided by on-line transmission of the crisp outputs to both the haemodialysis machine (Dialog, B. Braun Melsungen) for ultrafiltration rate control and to the programmable infusion pump (Perfusor secura FT, B. Braun Melsungen) for infusion rate control.
As in most of the patients with vascular instability the frequency of hypotensive events is increasing with ongoing haemodialysis, 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 MAX-rates up to 150% 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 mmHg are used for patients having initial systolic pressures of 90 mmHg or higher. For patients with initial systolic pressures <90 mmHg the initial value itself is chosen as set point in most cases. If the actual systolic blood pressure comes close to the set point of 90 mmHg the automatic system reacts first with injection of 20% saline to stabilize blood pressure and in a second step if necessary with lowering ultrafiltration rate.
A MAX-rate of 150% of the average ultrafiltration rate predetermined individually for each treatment was used in 121 treatments in the seven patients prone to develop vascular instability during haemodialysis. Blood pressure decreases below the set point during the initial and medium phases of treatment were treated preferably by the biofeedback-controlled infusion of hypertonic saline (20% NaCl) to maintain the ultrafiltration rate as long as possible at the higher level.
The ultrafiltration rate was reduced only if the systolic blood pressure could not be stabilized sufficiently by saline infusion. Vice versa, the biofeedback-controlled decrease of the ultrafiltration rate was used preferably in the final phase of treatment to avoid saline infusion. Hypertonic saline infused during the initial and medium phases will be eliminated during the final phase by a half-life of only 1015 min by ongoing convective and diffusive mass tranfer via the dialyser membrane. For this reason, no considerable increase of post-dialytic blood sodium is to be expected from intradialytic saline infusion. Blood sodium levels were measured at the beginning and after the end of each session.
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Results
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Frequency of hypotonic intervals
In our previous studies [11,12] the frequency of hypotensive episodes was considerably reduced in patients prone to hypotension, if the biofeedback-driven infusion of hypertonic saline (20% NaCl) was used. However, decreasing trends of blood pressure in the final phase of treatment could not be avoided in cases where the fluid transfer from both the intracellular space as well as the interstitium into the intravascular space is limited by refilling rates which are too low. Therefore, in the recent study, the blood-pressure-guided fuzzy control of the ultrafiltration rate was applied simultaneously with the automatic infusion of 20% NaCl.
Figure 2
shows a typical run where infusion was necessary during the initial phase of the session only. As a result of severe decreases in blood pressure in the interval from 15 to 40 min the ultrafiltration rate was reduced simultaneously. However, after stabilizing the blood pressure the ultrafiltration rate automatically increases again up to the MAX-rate. The total surplus of ultrafiltrate volume achieved during the first 2 h of treatment enabled the ultrafiltration rate to be reduced stepwise to only 463 ml/h during the third hour. The systolic blood pressure was reliably stabilized during the last 2 h above the set point pressure (90 mmHg).

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Fig. 2. Simultaneous fuzzy control of ultrafiltration and infusion. BP, systolic blood pressure; SP, set point pressure; UFR, ultrafiltration rate; UFRav, average ultrafiltration rate; PR, infusion rate; PRmax, maximum infusion rate.
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All of the treatments where hypotensive episodes <90 mmHg occurred were analysed by recording the measuring intervals (5 min) with low systolic pressure (<90 mmHg). Each of the hypotonic intervals was classified by both its hour of occurrence and as the level of blood pressure decrease (<60; 60...<70; 70...<80; 80...<90 mmHg). The results were compared with regard to four types of treatment: - (i) conventional method of haemodialysis treatment without fuzzy control (69 treatments);
- (ii) fuzzy-controlled infusion of 20% NaCl and fixed ultrafiltration rate (97 treatments);
- (iii) fuzzy-controlled ultrafiltration profiling without infusion (60 treatments); and
- (iv) simultaneous fuzzy control of infusion and ultrafiltration (45 treatments).
Figure 3
shows the frequency distributions of measuring intervals (5 min each) with low systolic pressure versus the time of occurrence during haemodialysis treatment. The figure illustrates, that the frequency of low blood pressure situations under conventional haemodialysis (filled columns) increases almost linearly during the treatment time course. Using only the fuzzy-controlled infusion of 20% saline (crossed columns) the trend is similar but to a smaller extent. The fuzzy-controlled ultrafiltration profiling with a MAX-rate of 150% without infusion (hatched columns) shows an increase of hypotensive episodes in the second hour but a decrease of hypotension in the third and especially in the fourth hour of treatment. The combination of a fuzzy-controlled ultrafiltration with a MAX-rate of 150% and a fuzzy-controlled infusion of 20% NaCl (dotted columns), results in the lowest levels for the occurrence of hypotension. It can clearly be seen that there is a stabilization of blood pressure in the second half of haemodialysis (third and fourth hour).

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Fig. 3. Frequency distributions of intervals with low systolic blood pressure versus time of occurrence.
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Figure 4
illustrates the frequency of intervals with low systolic blood pressure versus the level of blood pressure decrease in the different types of treatment mentioned above. Conventional haemodialysis shows the highest frequency of hypotensive intervals with the lowest blood pressure levels, whereas the fuzzy-controlled infusion of 20% NaCl and the fuzzy-controlled ultrafiltration also, achieve less frequent hypotensive intervals and the extent of hypotension was less severe.
The fuzzy-controlled infusion of 20% NaCl and fuzzy-controlled ultrafiltration together show significantly better results at all levels of blood pressure.
Blood pressure trend in the final phase of haemodialysis
The significant progress obtained from biofeedback-controlled ultrafiltration with respect to blood pressure behaviour in the final phase of haemodialysis is shown in Figure 5
. The blood pressure trend during the final phase of haemodialysis (total time 4 h) was analysed by comparing the mean systolic blood pressures during the last (fourth) hour (BP4) and mean systolic blood pressures during the third hour (BP3) for each individual treatment. Percental differences (BP4-BP3)/BP3 were calculated and classified by steps of 5% each. The results from conventional haemodialysis with linear ultrafiltration were compared with (i) fuzzy-controlled infusion and linear ultrafiltration, (ii) fuzzy-controlled ultrafiltration (MAX-rate 150%) without infusion, and (iii) simultaneous fuzzy control of both ultrafiltration (MAX-rate 150%) and infusion.

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Fig. 5. Frequency distribution of systolic blood pressure in the final phase of treatment. BP4 and BP3, mean systolic blood pressure during the fourth (BP4) and the third hour (BP3).
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As shown in Figure 5
, the frequency distributions of fuzzy-controlled groups (i)(iii) are characterized by significantly better locations in comparison with conventional haemodialysis. Only from 32.4% of conventional treatments (black columns) values (BP4-BP3)/ BP3
0% were obtained. That means, in only 32.4% of conventional treatments the mean systolic pressure BP4 was higher than the mean systolic pressure BP3 during the preceding hour, or at least equal to BP3. In group (i), fuzzy-controlled infusion and linear ultrafiltration (crossed columns), this number was elevated to 44.4% of treatments, which met this criterion. In group (ii), fuzzy-controlled ultrafiltration without infusion (hatched columns), a high majority of 81.7% of treatments complied with values (BP4-BP3)/BP3
0%. However, additional progress was obtained from group (iii), simultaneous fuzzy control of ultrafiltration and infusion (dotted columns), where 88.4% of treatments met this criterion.
Hypertonic saline infusion and post-haemodialysis blood sodium
The amounts of hypertonic saline infused during the treatments were analysed separately for each hour as shown in Figure 6
. The significant different trends of saline infusion in the last 2 h of treatment demonstrate clearly the essential effects of fuzzy-controlled ultrafiltration. As visible from the series of 97 treatments (four patients) with fixed ultrafiltration rates during the entire session, an increasing volume of hypertonic saline solution was necessary to stabilize the blood pressure under ongoing haemodialysis if fuzzy-controlled ultrafiltration was not available. The volume of saline infusion was elevated progressively from 8.4 ml during the first hour of treatment to 37.8 ml in the final hour, i.e. 450% of the first value.

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Fig. 6. Distribution of hypertonic saline infusion (20% NaCl) during haemodialysis treatment using simultaneous fuzzy control of ultrafiltration and infusion.
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Considering the group of 82 treatments (seven patients) with fuzzy-controlled ultrafiltration and infusion of 20% NaCl, the distribution of saline infusion shows during the first 2 h a very similar behaviour like the series mentioned above as in this phase the ultrafiltration rate is kept at the MAX-rate preferably. However, the new quality of fuzzy-controlled ultrafiltration is characterized by a systematic decrease of saline infusion after the second hour of the treatment. In this phase the ultrafiltration rate is reduced automatically by the biofeedback control system because of the surplus of ultrafiltrate volume accumulated by the MAX-rate during the first half of the session. The ultrafiltration rate is decreased stepwise to a very low level. In the final phase of the session <40% of the average ultrafiltration rate, as predetermined by the physician for the entire session, will be achieved (Fig 2
). With respect to the post-dialytic level of blood sodium it is highly important that saline infusion decreased from 12.0 ml during the second hour of the sessions to only 5.6 ml during the final hour. In contrast, an increase up to 37.8 ml was necessary if fuzzy-controlled ultrafiltration was not available. Thus, saline infusion in the final hour of treatment was reduced from 37.8 ml (100%) to <15% (5.6 ml) if fuzzy-controlled ultrafiltration was applied simultaneously. Blood sodium levels were measured at beginning and at the end of these sessions (Table 1
). The average levels of post-dialytic blood sodium after the simultaneous infusion of 20% saline for the individual patients and for seven patients altogether result in slightly higher values only (139.4 mmol/l) in comparison with the average levels of the pre-dialytic measurements (137.9 mmol/l). There were also small differences only in post-dialytic blood sodium between sessions with (139.4 mmol/l) and without (138.0 mmol/l) saline infusion.
These results agree with statistical results from correlation analysis and linear regression of treatments with simultaneous fuzzy control of both ultrafiltration and saline infusion. The correlation r between post-dialytic blood sodium and saline volume infused during the first to third hour of treatment has proven non-significant (r<0.1; P>0.05) as a result of the short saline infusion half-life of about 15 min during haemodialysis. Alternatively, a highly significant correlation r was found between the blood sodium and saline volume infused during the last (fourth) hour of treatment (r>0.5; P<0.001). However, the very low ultrafiltration rates in the final phase, as provided by fuzzy-controlled ultrafiltration, require extremely small volumes of saline infusion during the last (fourth) hour of treatment.
Thus, patient's interdialytic weight gains were not significantly affected by fuzzy-controlled saline infusion during treatments with simultaneous fuzzy control of ultrafiltration.
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Discussion
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Patients on regular haemodialysis treatment are getting older, and simultaneously their time on a regular haemodialysis programme is increasing. In particular patients suffering from long-term hypertension or diabetes mellitus develop haemodialysis-induced hypotension during haemodialysis treatment. From fundamental publications [1316] it is known that most complicationsespecially hypotensive episodesduring haemodialysis are multifactorial and are caused by different mechanisms. Changes in plasma volume caused by ultrafiltration and time variance of fluid shifts between the compartments of the body are considered as important factors [17,18]. Autonomic neuropathy may be responsible for the limitation of vascular stability [19]. Depletion of vasoactive substances and acidbase changes are in discussion. Physical training, drug intake and food uptake can modify patients shape. Finally, the biocompatibility of haemodialysis membranes and blood temperature during haemodialysis play a certain role in initiating clinical complications.
All these factors can influence blood pressure behaviour during haemodialysis. If hypotension occurs the symptom collapse is treated symptomatically. Volume substitution, injection of osmotic substances, increasing of dialysate conductivity, decrease of ultrafiltration to zero, diminution of speed of the blood pump and administration of vasoactive drugs are methods proven in clinical practice to treat acute hypotension during haemodialysis. In order to prevent hypotensive episodes during haemodialysis, ultrafiltration and sodium profiles have been developed.
However, clinical experience shows that therapeutic methods as mentioned above are of limited therapeutical effect in treating patients with vascular instability during haemodialysis, as actual changes in blood pressure behaviour are not considered continuously. Reliable prevention of haemodialysis-induced hypotension is achievable only from biofeedback-driven closed-loop systems guided by frequent blood pressure measurements. As medical knowledge on the multifactorial causes of hypotension is characterized by a major lack in deterministic knowledge, the use of scientific approaches like fuzzy logic is necessary to involve clinical experience in terms of fuzzy knowledge. The advantages of preventing haemodialysis-induced hypotension by blood-pressure-guided closed-loop systems are proven by the results from the biofeedback-driven fuzzy control of ultrafiltration and saline infusion.
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Conclusions
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The simultaneous computer control of ultrafiltration and the infusion of hypertonic saline have proven the most effective means for automatic blood pressure stabilization during haemodialysis treatment.
Prevention of hypotensive events can be optimized obviously, if specific modes of biofeedback control are used in the initial, medium, and final phases of the sessions. To achieve a high surplus of ultrafiltrate volume in the initial and medium phases, decreases in blood pressure should be compensated during these phases by saline infusion. Because of remarkably reduced refilling rates in many patients during the final phase of treatment, automatic blood pressure stabilization should be dominated by ultrafiltration rate control in this phase.
As MAX-rates rates up to 150% of the average ultrafiltration rate were tolerated well by all patients involved in the clinical tests, MAX-rates of 150% are recommended for most haemodialysis patients in the interest of low ultrafiltration rates in the final phase, if the blood-pressure-guided biofeedback control of ultrafiltration is used.
Session goals regarding the total weight loss were achieved in all cases, although ultrafiltration rates could be diminished by biofeedback control in the final phase to <40% of the average ultrafiltration rate because of the surplus accumulated during the initial and medium phases.
Fuzzy-controlled ultrafiltration is preventing sodium- correlated complications during the interdialytic phase as saline infusion is minimized in the final phase of treatment. Saline infusions in the initial and medium phases obviously do not affect the post-dialytic blood sodium level because of the saline infusion half-life of about 15 min only during ongoing haemodialysis procedure.
Fuzzy control is reproducible at any time during haemodialysis treatment as fuzzy knowledge is processed by strictly defined fuzzy sets, operators and rules. Patients feel better monitored and staff appreciate biofeedback-controlled haemodialysis.
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Acknowledgments
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Authors express their gratitude to B. Braun Melsungen for supporting this work.
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Notes
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Correspondence and offprint requests to: Prof. Dr R. Schmidt, Klinik für Innere Medizin, Universität Rostock, Ernst-Heydemann-Str. 6, D-18055 Rostock, Germany. 
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Received for publication: 9. 4.99
Revision received 17.10.00.