Biomedical subjects

H Wijkstra

Publications and source records attributed to H Wijkstra.

7 recordsLinked to original sources

PubMed1992

The step response of left ventricular pressure to ejection flow: a system oriented approach.

Left ventricular pressure is dependent on both ventricular volume and ventricular ejection flow. These dependencies are usually expressed by ventricular elastance, and resistance, respectively. Resistance is a one-valued effect only, when ejection flow either is constant or increases. Decreasing ejection flow elicits a third effect: a decrease of elastance. The effects of elastance, resistance and elastance depression were modeled in a three-compartment model consisting of a dead-volume compartment, an elastance compartment, and a second series-elastance compartment connected to the elastance compartment by a resistance. This model was identified with the pressure response determined experimentally by imposing pumped constant-flow ejection epochs on isolated rabbit hearts. The experimental flow epochs consisted of two phases of constant flow separated by an increasing or decreasing flow step. It was found that elastance is not changed after the flow step if this is positive or zero. Negative flow steps induced a deactivation of elastance that is linearly dependent on the difference between isovolumic pressure that would be developed at the volume existing at the time of measurement and actual pressure. The parameters found from the identification procedure are ventricular active volume, nondepressed elastance, series-elastance, resistance, and the elastance deactivation factor. The first four parameter values were found in agreement with other results reported in literature. The elastance depression factor is a new parameter that could be of physiological or clinical significance since it may be related to the inability of the force generators in the heart muscle to be restored to their full number, after being inactivated or decoupled by filament sliding associated with ejection. On the basis of the results, an alinear state-model of the ventricle, for arbitrary, including physiological flow patterns is proposed.

Animals↗

PubMed1992

Motor evoked potentials from the bladder on magnetic stimulation of the cauda equina: a new technique for investigation of autonomic bladder innervation.

Although neurophysiological techniques have become widely accepted in the diagnostic evaluation of urogenital tract dysfunction, arguments have been forwarded to question their validity. Current techniques merely deal with somatosensory nerves, whereas micturition and sexual functions depend on autonomic motor nerve function as well. This study presents a new technique of motor evoked potentials obtained after magnetic stimulation of the cauda equina. Bladder and pelvic floor motor evoked potentials could be measured with concentric needle electrodes. Mean latency was 35.6 msec. for bladder motor evoked potentials and 10.6 msec. for pelvic floor motor evoked potentials. The technique was easy to perform, and provided consistent and objective data. No harmful side effects were noted.

Adult↗

PubMed1991

Comparison of the diagnostic value of pump and gravity cavernosometry in the evaluation of the cavernous veno-occlusive mechanism.

We performed cavernosometry in 96 patients with erectile dysfunction. Two different techniques were compared: pump cavernosometry using a roller pump as the inflow source and gravity cavernosometry using an infusion set as the inflow source. We conclude that the diagnostic value of both techniques is comparable. In pump cavernosometry maintenance flow is the most accurate parameter of veno-occlusive function. Gravity cavernosometry has the advantages of simplicity, cost-effectiveness and a lower complication rate. Incomplete cavernous smooth muscle relaxation appeared to be limiting the diagnostic value of both techniques.

Adult↗

PubMed1991

Left-ventricular dynamic model based on constant ejection flow periods.

Experiments with constant ejection flow periods on the rabbit left ventricle suggest that left ventricular pressure can be described by a time varying three-element model consisting of elastance Ee(t), resistance R(t), and series-elastance Es(t). Previous experiments demonstrated the existence of a "deactivation effect" after the cessation of a constant ejection flow period, which could be described by a decrease of elastance Ee(t). This paper presents a simulation model based on findings of constant ejection flow experiments, and tested on measured pressure and volume data. The results show that when the model is fitted on one single beat, left ventricular pressure can satisfactorily be described by a three-element model without deactivation. However, the model does not predict isovolumic pressure at end-ejection volume. When isovolumic pressure has to be described by the model as well, introduction of deactivation is necessary. The quality of the model was further tested by fitting it to two beats with different ejection parameters. Deactivation again was necessary for a good fit. Only with a deactivation effect in the model, the component values found are close to the normal range found with CFP experiments in the rabbit left ventricles. From the simulation results it can be concluded that (at least for constant ejection flow periods) elastance, resistance, series-elastance, and deactivation effects all are necessary in describing (and predicting) left ventricular pressure.

Animals↗

PubMed1991

Urethral sphincteric responses to sacral root stimulation.

Deafferentation and stimulation of sacral nerves in patients with complete spinal cord lesions is a good model to study the parasympathetic influence on urethral behavior. During intradural sacral deafferentation from S2 to S4/S5 in preparation for implantation of the Finetech-Brindley anterior sacral root stimulator, sacral roots were stimulated with 3 and 30 Hz and 3 and 10 V and the bladder and urethral responses noted. Stimulation of the posterior roots at 3 Hz gives a urethral relaxation without bladder contraction. At this frequency, anterior sacral root stimulation has no effect on bladder nor urethral pressure. Stimulation of the anterior roots at 30 Hz produces simultaneous bladder contraction and urethral relaxation. These findings support the presence of an afferent parasympathetic inhibitory mechanism in the spinal micturition center in the human.

Electric Stimulation↗

PubMed1991

Audex Medical, a new system for digital processing and analysis of ultrasonographic images of the prostate.

By transrectal ultrasonography the volume of the prostate is determined two times using the conventional planimetric method. Thereafter the US scanner is placed in a fixture which is retracted repeatedly. For this a motordrive has been developed. The images of each section are improved by digital processing, and the prostatic outline is determined interactively by developed image processing software. In this manner the total volume of the prostate is calculated. In a post-mortem study prostates of 9 cadavers were measured with the conventional planimetry and by means of digital processing and analysis. The results were compared with the results obtained after removal of the prostate. In a clinical study the prostatic volumes of 20 patients were measured in a similar way. The results of the two methods are compared. Both in the cadaver study and the patient study we found a good correlation between the conventional method and the computerized method. In the cadaver study we noticed a better correlation between the prostatic volume found with the computerized method and the volume obtained after prostatectomy. Also the reproducibility of this method turned out to be better.

Humans↗

PubMed1989

Deactivation in the rabbit left ventricle induced by constant ejection flow.

Pressure generated by the left ventricle after ejection with constant flow was studied for different values of the ejection flow, flow duration, time of flow arrest, and ventricular volume. It was found that pressure after ejection, normalized with respect to isovolumic pressure at the same volume, is regenerated according to a model consisting of an elastance, a resistance, a series elastance, and an additional deactivation component. Deactivation is defined as the difference between the value 1 and the plateau value of the normalized pressure after constant flow ejection. It is shown that this plateau value is constant after constant flow ejection until the minimum in isovolumic dP/dt, i.e., during physiological systole. The plateau value is uniquely related to the value of the normalized pressure at the end of a constant flow ejection. After the end of flow, the plateau is reached after an exponential increase of normalized pressure with a time constant of 10.44 +/- 0.09 ms which agrees with the series-elastance time constant of 10.35 +/- 0.26 ms. These results are discussed with respect to previously found results, and to the implication for predicting pressure generation during physiological flow patterns.

Animals↗