Minimally invasive surgery is at this moment one of the outstanding developments in surgery. In this type of surgery the actual operation is performed through a number of small incisions in the skin. In the operations special instruments are inserted via trocars, i. e. tubes which allow the surgeon to bring instruments or sensors into the body. The view at the operating field comes from a laparoscope, a camera presenting a two-dimensional image on a monitor. The minimally invasive surgical technique has many potentional benefits for the patients. However, compared to open surgery there are severe disadvantages for the surgeon, such as the loss of three-dimensional visual feed back and proprioception, the disturbed eye-hand coordination, and the poor ergonomic design of the surgical instrumentation, and of the working place. At this moment the differences beween open and minimally invasive surgery can mainly be ascribed to differences in the manual control task. In this paper, the man-machine aspects of the traditional open operation process will be compared with those of the minimally invasive surgery process. Especially the consequences of the restricted perception in minimally invasive surgery will be discussed. Some future developments will be discussed.@en

To perform an operation in the classical way, an incision is made in the skin and the underlying tissues. An alternative for this way of operating is minimally invasive surgery. Here only small incisions are made in the skin and operations are performed by using special instruments. Visual feedback is obtained with a camera producing a 2Ddisplay. Another important minimally invasive operating method is based on the use of catheters in arteries and venes. This research project aims at decreasing the negative effects and limitations which accompany minimally invasive surgery and interventional techniques, so that these can be used more widely, thus bringing less damage to the patient and decreasing the costs for society. @en

A functional distribution of coronary volume can be estimated from the response of arterio-venous O2 content difference (AVO2) to a flow step. However, the results depend on the assumed O2 exchange model. The previously used model consisted of a single mixed compartment with O2 exchange in series with an unmixed compartment without O2 exchange (reference model). The purpose of this study is to provide an estimate of the errors made in the volume estimations by not taking into account factors as flow heterogeneity, different mixing sites or Krogh-like O2 exchange. The approach is indirect: the response of the AVO2 to a flow step has been calculated with alternative O2 exchange models in which the factors mentioned are incorporated. These transients are fitted with the reference model. The resulting estimated volumes are different from the volumes assumed in the alternative models. Large differences are obtained with some of the alternative models, e.g. the model with Krogh characteristics. However, these models seem unrealistic because capillary pO2 is higher than venous pO2. Only small differences in volume are obtained with the more realistic models. Therefore, these results indicate that the coronary volumes are approximated well by the estimations obtained with the reference model. These volume estimations were 9.9 and 3.8 ml 100 g-1 for the O2 exchange vessels and the distal venous volume, respectively.@en

PLEXUS is a computer program which has been developed to provide recommendations for diagnosis and treatment planning of brachial plexus injuries. This computer program is meant for neurologists, neurosurgeons and orthopaedic surgeons who are not experienced in the field of brachial plexus injuries. The system detects the locations and severity of brachial plexus lesions. PLEXUS also indicates whether the patient may be referred to a specialist centre for nerve surgery. In order to determine whether the advice given by the system is of expert quality, a study of its recommendations is being carried out in cooperation with four international brachial plexus experts. To investigate if the system does indeed have the capability to assist physicians in the area of brachial plexus injuries, it is being tested clinically in four hospitals in The Netherlands.@en

In this study the response of driving pressure/flow ratio on an abrupt change in heart rate was analysed. The difference between the response obtained with constant pressure and constant flow perfusion was also studied. The responses show a fast initial reversed phase followed by a slow phase caused by regulation. To test whether the initial phase could be the result of mechanical changes in the coronary circulation, a model for regulation was extended by the addition of four different mechanical models originating from the literature. These extended models were able to explain the fast initial phase. However, the mechanical model consisting of an intramyocardial compliance (C=0·08 ml mm Hg-1 100 g-1) with a variable venous resistance, and the model consisting of a waterfall and a small compliance (C=0·007 ml mm Hg-1 100 g-1) both explained these responses best. The analysis showed that there is no direct relationship between rate of change of vascular tone and rate of change of pressure/flow ratio. However, on the basis of the two extended models, it can be predicted that the half-time for the response of regulation to be complete is about 9s with constant pressure perfusion and 15 s with constant flow perfusion.@en

We have previously shown that steady‐state coronary flow during auto‐regulation and metabolic rate changes is predicted by a mathematically expressed theory which assigns control of coronary vascular resistance to tissue PO2. Our present purpose was to test the applicability of this theory to the non‐steady state as exemplified by a sudden step change in heart rate. 2. The theory predicted that the response time of change of resistance in these circumstances would be slower with constant‐flow perfusion of the coronary bed than with constant‐pressure perfusion, and that with constant‐pressure perfusion only, the rate of adaption of resistance would be dependent on the level of pressure used. 3. These predictions were tested in open‐chest goats with cannulation of the left main coronary artery and perfusion with alternately constant pressure or constant flow. Sudden step changes in heart rate were induced by pacing to induce rapid transients in myocardial metabolic rate. 4. The half‐time of subsequent change in perfusion pressure‐flow ratio, which in the dynamical state is not equal to resistance, was 15.7 +/‐ 0.4 s (mean +/‐ S.E.M.), which was statistically shorter than for constant flow (22.2 +/‐ 0.5 s, P less than 0.001). 5. The half‐time of subsequent change in perfusion pressure‐flow ratio with constant‐pressure perfusion was 14.4 +/‐ 0.6 s at low pressure and 17.0 +/‐ 0.6 s at high pressure (P less than 0.001). 6. The results differed from those predicted by the theory, in that the changes described above were preceded by a rapid (5 s) step change in pressure‐flow ratio, up with an increase in heart rate and down with a decrease in heart rate. We postulated that this was a mechanical effect due to greater compression of the coronary microvasculature with more frequent contractions. 7. To test this hypothesis, we measured changes in coronary blood volume by integrating the difference between arterial inflow and venous outflow. These experiments showed a decrease in coronary blood volume with heart rate increase and vice versa. 8. Abolition of autoregulation and metabolic regulation was achieved with maximum vasodilatation of the coronary bed with adenosine. A sudden switch in heart rate then produced the initial step change in pressure‐flow ratio, but not the subsequent adaptation over 13‐25 s. This confirmed that the former effect is attributable to a passive mechanical mechanism.@en