The electrical heart axis and ST events in fetal monitoring: A post-hoc analysis following a multicentre randomised controlled trial

Fig 1. The fetal vectorcardiogram for different orientations of the electrical heart axis. In the top panels, the electrical currents within the heart during a cardiac cycle are depicted in terms of their vectorcardiogram; ventricular contraction (QRS-complex = large loop), relaxation phase (T-waves = small loop). From left to right, the entire vectorcardiogram has been rotated over 10 degrees to simulate a different orientation of the electrical heart axis. Note that these vectorcardiograms are 3-dimensional images and the 10 degree rotation was performed in 3-dimensional space. In the bottom panels, the fetal scalp ECG has been calculated by projecting the vectorcardiograms onto the scalp lead. Before rotation, the baseline T/QRS is 0.05 and the T/QRS rise resulting from hypoxia is 0.04, yielding no ST event. After rotation, the baseline T/QRS is 0.09 and the T/QRS rise resulting from the same level of hypoxia is 0.06, yielding a ST event.

Fig 2. Distribution of patient across the baseline T/QRS values. For each initial baseline T/QRS value encountered in our data set, we counted the number of patients with that particular baseline, showing a non-symmetric distribution with the most frequent encountered baseline T/QRS ratio at 0.12.

Fig 3. The number of ST events per 1000 T/QRS values as a function of the initial baseline T/QRS value. Cases with the same initial baseline T/QRS were grouped. The intensity of the black colour of the data points relates to the total number of T/QRS ratios that occurred in the group (right column in the graph). The red line represents a linear fit through the data points. There is an average increment of 1.42 ST events per 1000 T/QRS values for a rise of 0.1 of the initial baseline T/QRS. The correlation coefficient between data points and fit was 0.63 (p <0.001), as calculated with the linear regression model.

Fig 4. The pH of arterial cord blood and base deficit of the extracellular fluid as a function of the initial baseline T/QRS value. Cases with the same initial baseline T/QRS value were grouped. The intensity of the black colour of the data points relates to the total number of patients that were represented in the group (right column in the graph). The red line represents a linear fit through the data points. The fit suggests a reduction in the pH of 0.0009 and an increase in the base deficit of 0.03 for a rise in 0.1 of the initial baseline T/QRS. The respective correlation coefficients between data points and fit are -0.04 (p = 0.14) and 0.03 (p = 0.34), as calculated with the linear regression model. Abbreviations: BDecf = base deficit in the extracellular fluid, pHart = pH of the arterial cord blood.