Effects of changes in frequency on guinea pig ventricular action potential duration and on QT interval under different experimental conditions.

Isolated perfused guinea pig hearts (Langendorff preparation) were arrested by carbachol (0.1-0.2 mg/l) and electrically stimulated in the region of the av-conducting system. The QT interval was determined by means of extracellular electrodes at different driving frequencies. Separate experiments were performed on papillary muscles from the right ventricle to measure the duration of the transmembrane action potential under comparable conditions. At 35 degrees C (Ke+ 5.4 mmol/l) increasing the frequency of stimulation (range 12-120/min) caused the action potential duration (APD) to decrease to a greater extent than the QT interval. Stepwise rising of the external K+ concentration up to 16.2 mmol/l produced a nearly parallel shift to the APD-frequency relation to lower values. Again, the QT interval was less affected by increasing the external K+ concentration than the APD. Stepwise reduction of the temperature down to 20 degrees C prolonged the APD as well as the QT interval, the effects being more pronounced at lower than at higher stimulation frequencies. Under all examined experimental conditions, the APD proved to be markedly shorter than the QT interval even when the latter is diminished by the duration of QRS. The results suggest that no close relation exists between the APD and the QT interval. The observed divergencies may be due to functional differences among various parts of the ventricles.

[Stereovectorcardiography. Analysis of congenital excitation-conduction anomalies using a new type of microelectronic equipment (author's transl)]. / Stereo-Vektorkardiographie. Analyse kongenitaler Reizleitungsanomalien mit einer neuen mikroelektronischen Technik.

The results of the first clinical trial of a stereo-VCG used for the quantitative and qualitative analysis of congenital anomalies of excitation-conduction are elaborated and discussed. Normal values for paediatric cardiological patients were calculated for the following parameters with the help of measurements from 78 patients: Pmax, Rmax, Tmax-stereovectors, their elevation and azimut, the stereo-angle between Rmax/Tmax, Pmax/Tmax and Pmax/Rmax as well as the periods from the beginning of the ventricular excitation until Rmax and Tmax. The patients were divided into 4 age groups. The group of normal individuals was compared with 22 patients having anomalies of conduction. The results show that this new technique allows more differentiated analyses of the vector loops registered according to Frank's orthogonal corrected registering system than the conventional measurements in cases of congenital anomalies.