Dynamical control of the dialysis process. Part II: An improved algorithm for the solution of a tracking problem

An efficient algorithm for the optimization of process parameters during dialysis has been developed. By solving a tracking-problem for prescribed time courses of distinguished variables, it is possible to compute optimal concentrations of electrolytes in dialysate as well as an optimal rate of ultrafiltration. These variables are indirectly influencing the status of the patient and can be directly modelled. They are describing the important exchange processes between blood and dialysate as well as between the different distribution spaces within the patient during dialysis. Their time courses are determined by an individually identifiable patient model. The tracking problem was treated as a dynamic optimization problem, and a continuous descent procedure which is usually employed for solving unconstrained static optimization problems has been adapted in such a manner that it is applicable for the solution of this problem. The used method is characterized by its simple mode of application, short solution time and moderate storage need. Especially in cases of contradictional requirements for desired time courses of model outputs the used optimization method performs well.

Dynamical control of the dialysis process. Part I: Structural considerations and first mathematical approach

Individual optimization of the dialysis process requires the (open-loop or closed-loop) control of many different variables, e.g. plasma ion concentrations, acid base state, volemic state and hemodynamic quantities. For this purpose a general concept for multiple-input-multiple-output (MIMO) control of the dialysis process is presented. The controlled variables have been differentiated into variables which can be modeled mechanistically (primary controlled variables, PCVs) and (hemodynamic) variables for which no mechanistic model has been developed up to now (secondary controlled variables, SCVs). Accordingly the controller is decomposed into two stages. Stage 1 contains an expert system which links the PCVs to the SCVs and provides the generation of optimal profiles for the PCVs with respect to maximum hemodynamic stability of the patient. Stage 2 is a tracking controller for the PCVs. An algorithm for the multidimensional tracking problem at stage 2 has been developed. It can be used for open-loop and future closed-loop control. The algorithm has been tested for 4 controlled (plasma Na+, plasma K+, plasma volume and ratio between intra- and extracellular volume) and 3 control variables (dialysate Na+, dialysate K+, ultrafiltration rate) up to now. It renders possible the exact tracking of the prescribed trajectories as long as all points are reachable under consideration of all physical and physiological boundary conditions. If they are not, appropriate weighting of the conflicting optimization goals must be applied. An extension towards more than 4 controlled variables is possible on principle. Main advantages of the method are its mathematical simplicity and the applicability of standard optimization subroutines.

Diagnosis of congenital pericardial defects, including a pathognomic sign for dangerous apical ventricular herniation, on magnetic resonance imaging

OBJECTIVE

To establish criteria for the accurate diagnosis of different forms of left sided pericardial defects on magnetic resonance imaging. Early detection of a partial apical defect is essential as it is potentially fatal.

DESIGN

Examination of four children with congenital pericardial defects by magnetic resonance imaging, the results being compared with the features on conventional chest radiography and echocardiography and with published data.

RESULTS

Magnetic resonance imaging improved the ability to diagnose and distinguish between complete and partial left sided pericardial defects. A deep myocardial crease was visualised in a patient with apical pericardial defect, indicating the risk of a life threatening ventricular strangulation. A prominent left atrial appendage was, in contrast to many reports, not a reliable sign for partial left sided pericardial defect.

CONCLUSIONS

The various forms of congenital left sided pericardial defects cannot reliably be diagnosed in plain chest radiographs or on echocardiography. Their diagnosis and the distinction between partial and complete defects, however, is of clinical importance and can be accomplished more confidently by magnetic resonance imaging.

Development and critical evaluation of an improved comprehensive multicompartment model for the exchange processes during hemodialysis

An improved comprehensive multicompartment model for the simulation of the most important metabolic state variables in the patient during dialysis is presented. With this approach time courses of urea, creatinine, K+, Na+, Cl-, HCO3-, H+ and CO2 can be predicted. Additionally, osmotic water shifts as well as resting membrane potentials are calculated. The model contains the following extensions compared to classical approaches: For the calculation of osmotic water shifts, not only sodium, but also urea, potassium, chloride and unspecified indiffusable ions are taken into account. Furthermore, hemodynamic aspects are considered by assuming two tissue groups with different perfusion. Thus it is possible to estimate the influence of hemodynamic parameters (e.g. cardiac output or blood flow distribution) on the exchange processes. The model can be adjusted individually by several system parameters. This adjustment is performed by minimizing the sum of the quadratic differences between simulated and measured plasma concentrations of the considered substances. A first validation has been performed successfully with measured data from 18 dialysis patients. After the effective whole-body exchange area of the resting cell membranes for potassium, sodium and chloride had been estimated, rebound effects for those electrolytes could be simulated successfully.

Functional cardiac assessment before and after left ventricular anterior aneurysm repair, especially as related to work capacity

Clinical, angiographic and hemodynamic parameters were assessed preoperatively and 12-20 weeks (mean +/- SD: 14 +/- 2) after left ventricular anterior aneurysmectomy in 31 consecutive patients. In 18 patients, associated coronary artery bypass surgery was necessary (1.4 grafts/patient). One patient with mitral valve insufficiency postoperatively required mitral valve replacement. No other complications and no deaths arose. The working capacity of the patients, as measured by bicycle exercise testing, compared a stage of exercise that the patient reached to the expected level of exercise for a normal person of the same sex, age and body surface. Expressed as percent of normals it increased from 39.3 +/- 28.9% (mean +/- SD) to 60.8 +/- 15.5% (p less than 0.001), angina-pectoris-free working capacity increased from 52.6 +/- 41.4 to 89.2 +/- 33.2 W (p less than 0.001). Left ventricular end-diastolic volume was determined by simultaneous biplane angiography and decreased from 277.1 +/- 84.7 to 191.0 +/- 49.1 ml (p less than 0.001). Ejection fraction increased from 35.4 +/- 12.4 to 41.0 +/- 9.6% (p less than 0.05). Left ventricular end-diastolic pressure after angiography decreased from 24.6 +/- 9.2 to 21.1 +/- 11.3 mm Hg (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

[Late complications following Björk-Shiley and Carpentier-Edwards heart valve replacements]

Valve related and valve induced, fatal or non fatal complications after Björk-Shiley valve replacement (n = 50) are compared with complications following Carpentier-Edwards valve replacement (n = 113). These patients had been operated upon between 1976 and 1982 and were followed up until 1985. Estimated actuarial cumulative survival following Carpentier-Edwards valve replacement (53 +/- 12% after a 100-month period) did not differ significantly from the estimate following Björk-Shiley valve replacement (80 +/- 6% after a 100-month-period), and subgroups of the cohort did not differ in survival after Carpentier-Edwards or Björk-Shiley valve replacement either. However, the estimate of actuarial cumulative event-free survival following Carpentier-Edwards valve replacement in patients under 40 years of age (46 +/- 14% after a 92-month period) was significantly worse (p less than 0.05) than with Björk-Shiley valves (100 +/- 0% after a 92-month period), (n = 30). The leading cause of clinical complications following Carpentier-Edwards valve replacement was a demonstrable degeneration of the bioprosthesis in 12 cases. The diagnosis of degeneration was established a mean of 73 months postoperatively (range 42-101 months).

[Surgical therapy of coronary disease: aortocoronary bypass. Risks and complications]

The aorto-coronary bypass surgery is a well established method for treating patients with coronary artery disease. In the last five years at the University Clinic in Innsbruck 417 operations have been performed. The operation letality decreased down to 0.66%, whereby the mean revascularisation rate increased to 2.8. The specific aim of this paper is to show the benefits and the risks of this operation to validate and to verify the benefits in regard to the risks, which could be kept very low.

[Intraoperative and postoperative course of enzyme activities in patients with angiographically closed and open aortocoronary bypass grafts]

30 patients who consecutively underwent cardiac surgery with extracorporeal circulation were investigated with regard to preoperative, intraoperative and postoperative activities of creatine kinase (CK) and CKMB isoenzyme (CK-MB), as well as myoglobin concentrations in the blood. 12 lead electrocardiograms were taken before and after the operation. At subsequent control angiography, of 18 patients with coronary heart disease 10 showed patent bypass grafts, whilst the remaining 8 patients had at least one occluded graft. 12 patients served as a control group, half of them undergoing mitral valve replacement and the other half aortic valve replacement. There was no significant difference in enzyme activity patterns between the groups. Specific determinations together with ECG monitoring can be of diagnostic help in perioperative control of these patients.

[Improvement of specificity in evaluation of aortocoronary bypass grafts by serial (dynamic) computed tomography (author's transl)]

Serial (dynamic) Computed Tomography (CT) with a non-periodic reconstruction of 21 images per minute improves specificity in evaluation of aortocoronary bypass grafts. In 35 bypass grafts, selective coronary arteriography was performed the day after CT: Sensitivity of CT-technique was 92%, specificity 89%. Without additional serial Ct, sensitivity was 85%, specificity 67%. Quantitative CT-measurements of flow equivalents showed no significant differences within two groups with angiographically displayed different "run off".

[Evaluation of coronary artery graft patency by computed tomography. Comparison with coronary arteriography (author's transl)]

In 14 patients with 19 coronary artery saphenous vein by-pass grafts (ACVB), computed transmission tomography has been performed the day before selective coronary angiography. Non-dynamic, contrast-enhanced sequential scans at 8-mm intervals were obtained starting at the cranial clips, ending at the left ventricle. CT scans were interpreted without knowledge of the catheterisation results. Sensitivity of this technique is 88%. Out of 17 ACVB patent by catheterisation, 15 ACVB are patent by CT. One occluded ACVB to the right coronary artery was thought to be patent by CT scan. This non-invasive technique shows promise as a method to follow up patients after ACVB-grafting.

[Severe hemolytic anemia after mitral valvuloplasty]

A patient with mitral insufficiency is described who developed severe hemolytic anemia six months after mitral valvuloplasty. Various laboratory tests suggested that the cause of the hemolysis was mechanical. A second heart operation was therefore performed which revealed intact valve function. In view of the mechanical cause of hemolysis a bioprosthesis was implanted but the patient died shortly after this operation. Since similar cases are not found in the literature the authors invite discussion of the course in this case.

[Secondary isophageal reconstruction after resection of a cardia carcinoma (author's transl)]

Following abdomino-thoracical total gastrectomy and partial resection of the esophagus performed because of the presence of a stomac cancer located in the upper part of the organ an attempt was made to execute from a left sided thoracotomy an esophago-jejunostomy. During which it was found that the previously prepared loop of the jejunum was too short. The execution of a colon interposition in this particular operational state would have been too demanding on the patient; therefore the operation was concluded with a blind closure of the distal end of the esophagus and a jejunostomy (discussion of the resultant problems included in paper). The reconstruction of the digestive tract by retrosternal colon interposition a few weeks after the resection of the cancer proceeded without complication and with consequently excellent functional effect.