Relative ventricular volume measurements by intracardiac impedance.

The purpose of this study was to investigate whether changes of the right ventricular end-diastolic and end-systolic volumes can be detected by the DC-coupled electrical intracardiac impedance signal. Measurements were conducted in 13 patients with an implanted or external pacemaker, testing various measurement configurations. Volume changes were induced by incremental overpacing and by postural changes. The pulse contour systolic integral from the peripheral blood pressure was used as a marker of stroke volume changes. The results show, that DC-coupled impedance can detect volume changes of the right ventricle. The quadrupolar impedance measurement configurations offer better amplitude resolution, but the tripolar and intraventricular bipolar configurations are better hemodynamic markers.

On the correlation of mechanical and optical properties of cataractous eye lens nuclei.

It is investigated whether mechanical properties of human cataractous eye lens nuclei can be assessed by their optical ones. This question is of importance as the success of phacoemulsification surgery among other criteria depends on the mechanical properties of the lens nucleus. The mechanical and optical properties of senile cataractous human eye lens nuclei were tested in vitro after extracapsular cataract extraction. A new measurement system was developed to determine their mechanical response. Hereby, a force is applied to the specimen and the resultant displacement is measured or vice versa. Stiffness and ultimate strength of the material are determined. Optical transmission is detected in dependence of light wavelength by a spectrophotometer. In addition the nuclear colour is determined. The correlation of mechanical stiffness and strength via optical transmission at 550 nm and nuclear colour is analyzed. Human eye lens nuclei show a viscoelastic mechanical behaviour. Measurement of stiffness is reproducible after 5-6 loading cycles when a steady-state is reached. Mechanical and optical properties correlate weakly but significantly. Stiffness and strength increase with increasing nuclear coloration. Alterations of mechanical and optical properties are caused by the same structural changes of the molecular components of the ageing lens. Especially lens crystallines are responsible for this process. Thus, it is concluded that optical properties can be useful to assess the mechanical response of cataractous eye lens nuclei.

Mechanical testing of isolated senile human eye lens nuclei.

Phacoemulsification is a cataract surgery technique during which the eye lens nucleus is carefully dissected by an oscillating hollow needle simultaneously serving as a suction line for lens fragments. As the success of this procedure depends greatly on the mechanical properties of the nucleus, the basic mechanical properties of extracted senile human eye lens nuclei were studied. For this purpose a novel measuring system had to be developed enabling the study of the biomaterial's responses to any chosen excitation function. Either force- or displacement-controlled testing signals can be selected serving as a one-dimensional input. The resultant quantity can be recorded in the time or frequency domain. An almost linear viscoelastic behavior of lens nuclei is observed. Typical phenomena such as hysteresis, creep, relaxation, and a frequency-dependent compliance are evaluated. The ultimate strength of the specimen is studied by penetration tests. Finally the possible correlation between mechanical and optical parameters of lens nuclei is discussed. If correlated, a surgeon could use this information when deciding whether an extracapsular extraction or a phacoemulsification should be the preferred approach.