Biomechanics of the urinary bladder: spontaneous contraction activity and micromotions related to accommodation.

PURPOSE: Knowledge of the fundamental properties of the urinary bladder is required to better understand its pathological conditions. Research on the passive and active properties of the bladder during stretching and contraction is important. The bladder is not passive during the filling phase. Spontaneous contractions are observed as variations in pressure, which are mostly related to urgency and/or incontinence and sometimes to pelvic pain. The purpose of this study was to describe distributed spontaneous contractions and micromotions (MMs), which besides being related to symptoms, are crucial in the physiological process of accommodation, and to express accommodation in a concept. METHOD: After describing MMs in the bladder wall as the type of spontaneous activity that may not be reflected in detrusor pressure and as a source of afferent nerve activity, its biomechanical effects are considered. In a simple mechanical model, contractions and elongations are related to the plastic elongated state of the bladder. The changing distributed character of contractions and elongations in the bladder wall is represented in a modular scheme. RESULTS: Distributed transient contractions and MMs yield a balanced dynamic plastic state of the regions of the bladder wall. An almost constant detrusor pressure can be attributed to the active accommodation of detrusor pressure to changes in bladder volume. CONCLUSION: Localized contractile activity and MMs that change the plastic elongated state of varying bladder regions are biomechanically effective in the active accommodation of detrusor pressure to changes in bladder volume. According to this concept, autonomous bladder wall activity as a source of nerve activity, also is crucial for active accommodation.

Micromotions of bladder wall in chronic pelvic pain (CPP): a pilot study.

The aim of the study was to investigate the occurrence and duration of micromotions of the bladder wall. Thirty women with CPP and 7 healthy women underwent micromotion detection (MMD). A latex balloon provided with eight electrodes was placed within the bladder through the urethra and filled with saline up to 200 ml. Micromotions (MM), pressure within the balloon, abdominal pressure and respiratory excursions of the abdomen were registered simultaneously. A significant difference in duration as well as frequency of occurrence was found for MM activity between subjects with CPP and controls. For the occurrence of variations in detrusor presure, the difference between groups tended towards significance. We conclude that there are indications that the bladder is involved in CPP.

Simulation study of the intercompartmental fluid shifts during hemodialysis.

Hypotension is the most frequent complication during hemodialysis. An important cause of hypotension is a decrease in the intravascular volume. In addition, a decrease in plasma osmolality may be a contributing factor. Modeling of sodium and ultrafiltration (UF) may help in the understanding of underlying relationships. We therefore simulated, in a mathematical model, the intercompartmental fluid shifts during standard hemodialysis (SHD), diffusive hemodialysis (DHD), and isolated ultrafiltration (IU). We analyzed the relative theoretical effect of hydration status, dialysate sodium concentration, the initial plasma concentrations of sodium and urea, and tissue permeation to solutes on the magnitude and direction of intracellular and intravascular volume changes. This theoretical analysis shows that the transcellular fluid shifts taking place during hemodialysis treatment are, to a great part, due to inhomogeneous distribution of regional blood flow and tissue fluid volumes. During hemodialysis treatment, the cellular fluid shifts in tissue groups with relatively high perfusion and small volume occur from the intra- to the extracellular spaces. However, the fluid shift in tissue groups with a low perfusion and large volume takes place in the opposite direction. The UF volume and rates, and the size of the sodium (Na+) gradient between the dialysate and blood side of the dialyzer membrane are the most important factors influencing the fluid shifts. Higher UF volumes and flow rates cause an increasing decline in the plasma volume in both SHD and IU. High dialysate sodium concentration (150 mEq L(-1)) helps plasma refilling slightly when compared with a normal dialysate sodium concentration (140 mEq L(-1)). However, a high dialysate sodium concentration is associated with a high plasma sodium rebound, which in turn may lead to interdialytic water intake resulting from thirst and may cause increased weight gain and hypertension.

Continuous arterio-venous haemodiafiltration: hydraulic and diffusive permeability index of an AN-69 capillary haemofilter.

The dependence of uraemic solute clearance on the hydraulic and diffusive permeability index of an AN-69 capillary haemofilter is investigated during the treatment of patients with continuous arterio-venous haemodiafiltration (CAVHD). A mathematical model is presented to calculate solute clearance and the hydraulic and diffusive permeability index parameters from clinical data and to predict the blood flow rate entering the extra-corporeal circuit from the manufacturer's specifications and blood viscosity. By measuring the flow rates, the patient's mean arterio-venous pressure difference and uraemic solute clearance under different clinical and operational conditions, mathematical model equations are evaluated. During the average survival time of an AN-69 capillary haemofilter of about five days, it is found that both the hydraulic permeability index and the diffusive permeability index decline over treatment time, independent of the haemofilter resistance to blood flow. The measured haemofilter resistance to blood flow is three times higher than the haemofilter resistance predicted from the manufacturer's specifications and blood viscosity. Predicting the blood flow rate entering the extra-corporeal circuit from the arterial haematocrit, plasma protein concentration and temperature and the manufacturer's specifications is not reliable.

Variations in foot breadth: effect on aspects of postural control during one-leg stance.

OBJECTIVE: To test the hypothesis that changing body weight toward the narrower rearfoot or the wider forefoot causes a decrease or increase in the amount of center of pressure (CoP) displacement per unit of foot tilt (EQ), and to examine the effect on postural control. DESIGN: Experimental, within-subject design with one controlled variable. SETTING: A biomechanics laboratory of a university department of biomedical physics and technology. SUBJECTS: Ten healthy, young subjects. MAIN OUTCOME MEASURES: The sagittal plane position of the CoP (Y) reflecting foot loading, EQ, and horizontal ground reaction forces (Fx) representing postural control. RESULTS: Shifting body weight to the rearfoot or forefoot decreases (d) or increases (i) Y (Pd = .002, Pi = .000) and EQ (Pd = .018, Pi = .064). In both cases Fx (Pd = .008, Pi = .098) increased and postural control deteriorated. CONCLUSIONS: Shifting body weight to the rearfoot or forefoot decreases or increases EQ. Besides the changes in EQ encountered here, other factors are held responsible for the deterioration of postural control. The absence or presence of changes in Y can be used to exclude or include the presence of an effect of EQ or these unknown factors on postural control.

An analytical solution to solute transport in continuous arterio-venous hemodiafiltration (CAVHD).

In conventional intermittent hemodialysis, the overall mass transfer coefficient (Kd) of a dialyser is mostly calculated at zero ultrafiltration and at relatively high dialysate flow rates. In continuous arterio-venous hemodiafiltration (CAVHD), the dialysate flow rates are low as comparable to the rates of ultrafiltration flows, making the dialysis treatment as slow as possible. Therefore the overall mass transfer coefficient (Kd) of a CAVHD hemofilter has to be calculated in the presence of ultrafiltration. A mathematical model of CAVHD is presented in order to calculate the diffusive mass transfer coefficient (Kd) for a solute when blood, filtrate and dialysate flow rates and solute concentrations are known. The ultrafiltration volume flux (Jv) is assumed to vary linearly along the axial direction of the hemofilter. The calculated mass transfer coefficient Kd shows that at high values of dialysate flow and low values of ultrafiltration, the overall mass transfer coefficient (Kd) of a CAVHD hemofilter equals mass transfer coefficient (Kd) of a dialyser in conventional intermittent hemodialysis. Also, the calculated mass transfer coefficient Kd shows no significant differences when the ultrafiltration volume flux is assumed to be constant along the length of the hemofilter if no backfiltration occurs in the hemofilter.

Comments on the interpretation of tissue impedance measurements during hemodialysis.

Comparison of electrical impedance measured in vivo with the in vitro impedance of several types of tissue shows that the former is mainly determined by the presence of striated muscle in the region between the electrodes. This means that fluid shifts occurring during hemodialysis, in the blood or any tissue other than skeletal muscle, do not affect tissue impedance. This implies that measurement of tissue impedance as a means of monitoring the process of hemodialysis lacks a model for a reliable interpretation in terms of fluid balance.

Drug clearance by continuous haemodiafiltration. Analysis of sieving coefficients and mass transfer coefficients of diffusion.

In patients who were treated with continuous arteriovenous haemodiafiltration (CAVHD), using the AN-69 capillary dialyser, we measured the clearance rates of uraemic solutes and drugs at dialysate flow rates of 0, 1 and 3 l/h. By using a mathematical model of CAVHD, we analysed the data in terms of sieving coefficients and diffusive mass transfer coefficients. These parameters proved to be related to drug protein binding and molecular weight. The parameter values may be used to calculate the expected drug clearance rate under different operating conditions.

New concepts in relation to urge and detrusor activity.

Investigations of micromotion characteristics of bladder wall strips and pressure wave phenomena in total bladders in vitro and in vivo indicate that micromotion phenomena occur in the bladder wall. Local contractions can occur without an increase in tension or pressure, because other parts are in antiphase. Local contractions stretch surrounding tissues, which can stimulate fast stretch receptors. Synchronisation of these micromotion phenomena appears to be possible. Hence, above threshold levels urge can theoretically occur, even in the absence of a pressure increase. This hypothesis could explain the weak relation between urge and pressure. The distinction between motor and sensory urge could be artifactual based on a misunderstanding of fundamental bladder wall processes.

Blood flow, ultrafiltration and solute transport rate in continuous arteriovenous haemodiafiltration: the AN-69 flat-plate haemofilter.

We measured blood flow, ultrafiltration rate and uraemic solute clearance at different dialysate flow rates during CAVHD using the AN-69 0.43 m2 flat plate haemofilter. As filter performance depends on clinical conditions and operational characteristics, data were analysed in terms of resistance to blood flow, membrane index of ultrafiltration, and diffusive mass transfer coefficients. An attempt was made to construct nomograms that may be used both to predict filter performance and to compare different haemofilters with each other.

A mathematical model of continuous arterio-venous hemodiafiltration (CAVHD).

Continuous arterio-venous hemodiafiltration (CAVHD) differs from conventional hemofiltration and dialysis by the interaction of convection and diffusion, the use of very low dialysate flow rates and by the deterioration of membrane conditions during the treatment. In order to study the impact of these phenomena on diffusive transport, we developed a mathematical model of the kinetics of CAVHD solute transport from plasma water to dialysate. The model yields an expression of the diffusive mass transfer coefficient, Kd, as a function of blood, filtrate and dialysate flow rates and solute concentrations, which can be measured in the clinical setting. This paper gives a description of the model derivation. Kd is demonstrated to vary depending on dialysate flow and duration of treatment.

Solute transport in continuous arteriovenous hemodiafiltration: a new mathematical model applied to clinical data.

A mathematical model of continuous arteriovenous hemodiafiltration is presented, by which the diffusive mass transfer coefficient (Kd) for a solute may be calculated from blood, filtrate and dialysate flow rates and solute concentrations. The model was applied to clinical data obtained with 0.6-m2 AN69 capillary dialyzers that had been used for up to 5 days. The diffusive mass transfer coefficient proved to depend on dialysate flow rate. Furthermore, it was related to the membrane index of ultrafiltration, as measured in the clinic, and to the filter resistance to blood flow. Measurement of these filter characteristics allowed a reasonable prediction of solute clearance.

Determinants of blood flow and ultrafiltration in continuous arteriovenous haemodiafiltration: theoretical predictions and laboratory and clinical observations.

In continuous arteriovenous haemofiltration (CAVH) or haemodiafiltration (CAVHD), it is important to obtain an adequate blood flow through the haemofilter to minimise the risk of excessive haemoconcentration and clotting. In this study we determined the resistance to blood flow of the extracorporeal device as well as the hydraulic permeability of the filter membrane is intensive care patients treated with CAVHD. Data were obtained for CAVH catheters and Scribner shunts and for a polyacrylonitrile (AN-69) plate filter, an AN-69 capillary filter and a polysulphone (PS) capillary filter. In accordance with recent literature we also predicted the resistance to flow by using Poiseuille's law and a formula for the estimation of blood viscosity. Although with all three filters an adequate blood flow was usually obtained, the resistance to blood flow was 2-3 times greater than the predicted value. With continued use of the filter, resistance to blood flow remained largely unchanged. When, in the laboratory, the AN-69 capillary filter was perfused with saline and with a viscous sucrose solution, the resistance to flow was only 1.4 time the predicted value, a difference that might result from small deviations of the capillary diameter. When perfused with blood, the resistance was 2.6 times greater than the predicted value. This was largely explained by gross underestimation of blood viscosity in these patients. By combining laboratory data on filter resistance during saline perfusion and a more accurate estimation of blood viscosity, a reasonably accurate prediction of blood flow rate would be feasible. In the clinic the hydraulic permeability of the filters decreased with time.(ABSTRACT TRUNCATED AT 250 WORDS)

Theory of propagation of peristaltic waves along ureter and their simulation in electronic model.

Various aspects of the propagation of peristaltic waves along the ureter, in particular the effect of the duration of the interperistaltic intervals on the propagation velocity of these waves and on the electromyographic response, are explained on the assumption that local tissue refractoriness is the underlying cause of these effects. The concept of local reaction time being determined by the slope of the local myogenic stimulus pulse is also introduced. An electronic model of the propagation process is described. The operation of this model suggests that a gradient in refractory period along the ureter is responsible for stopping the peristaltic waves. It is further concluded that the minimum interstimulus interval at which terminating peristaltic waves are generated is a measure of the local conductivity of the ureter.

Stress relaxation of the human cervix: a new tool for diagnosis of cervical incompetence.

A method is described for determination of the stress-relaxation characteristics of the cervix uteri of non-gravid women after almost stepwise dilatation of the cervical canal by a water-filled balloon. The stress relaxation is followed by measurement of the pressure decay in the same balloon. Application of this atraumatic technique to women with a normal cervix and women known to have an incompetent cervix yields diagnostic ranges of various parameters, in particular a time constant and the elastic ratio. The technique seems promising for easy detection of cervical incompetence, allowing suitable measures to be taken for prevention of immature deliveries. A mechanical model of the tissue of the ostium internum has further been developed on the basis of the measured characteristics.

Potassium-induced contractions in pig ureter strips. Consequences for helical muscle-fiber model of ureter.

Contractions were chemically induced in vitro in pig uterus segments by a high potassium concentration in a metabolic solution. Slow contractions were found mainly in the transverse direction. The near absence of contractions in the longitudinal direction is ascribed to a nearly circular orientation of the muscle fibers. (In terms of the helical muscle-fiber model of the ureter, our findings would correspond to a pitch of 3 to 5 degrees for the helix.) It is further postulated that the coupling structure of fibers in the transverse direction, determining an annular contraction mode, differs from that in the longitudinal direction, determining the longitudinal propagation of the annular contraction mode. The over-all conclusions on the basis of our results is that the helical muscle-fiber model now generally assumed for the ureter must be greatly modified to correlate with the experimental facts that its continued use is misleading and undesirable.