Frequently Asked Questions
What is the difference between HCP and Impedance Cardio Graphy?
A principal difference between the HCP and Imedance Cardiography (ICG) or bioreactance-based techniques, is that the HCP performs a direct measurement of ventricular volume changes, instead of total heart volume changes. Therefore, the HCP, theoretically provides a better basis for assessment of stroke volume, because total heart volume variation is not necessarily a reliable indicator of ventricular stroke volume. The voltage signal measured in every ICG system is caused by the combined effect of volume changes in different intrathoracic compartments during the cardiac cycle, such as the intracardiac cavities, aorta, superior and inferior vena cava, and pulmonary circulation. From all the factors, ICG attemps to distill the stroke volume using a single input voltage stream. From a mathematical point of view, however, this single voltage input stream is insufficient to balance the many unknown variables inside the thorax. Some ICG-like techniques use multiple electrodes, but - in contrast to our method - still measure only one single voltage, because the leads of many of these electrodes are interconnected to form a single input channel. In contrast, with this technique (HCP), nine electrodes produce eight independent and simultaneous voltage input streams, connected to eight independent voltage demodulator units.
How does HCP manage the inter-individual thorax demensions, like thickness of sub-cutaneous fat and muscle layers, and overall thorax sizes?
HCP performs an auto-calibration procedure, the measuring electrodes are used temporarily for current injection. In an in-vitro study and in computer models, we have shown that, invoking the reciprocity theorem of electromagnetic fields, this procedure is capable of assessing the attenuation of sigles between the heart and the skin. This auto-calibration takes away the need for an initial calibration using ultrasound. While the auto-calibration cannot manage all thorax sizes with a single sized electrode patch, from the smallest new-born to the largest obese, further trials are expected to lead to 3 or 4 electrode slices that can cover the whole spectrum of the population's thorax sizes.
How has this technology been validated?
In animal research, an Ultrasonic Flow Probe (FP) around the aorta was used as the gold standard, while cardiac output was manipulated by decrease in venous return induced by a vena cava obstruction, increase in heart rate by external cardiac pacing, and inotropic stimulation by administration of dobutamine. The results are reported in http://ccforum.com/content/15/4/R165.
In order to cause changes in stroke volume in healthy volunteers, and at the same time let other (enviromental) parameters remain the same as much as possible, we used inflatable antishock trousers (known as 'G-suits' to airforce pilots). It has been shown that, in a standing position, sudden deflation of an inflated G-Suit can cause the stroke volume to drop suddenly. We performed a procedure on healthy volunteers, in which a G-suit was inflated gradually from 0 mmHg to 70 mmHg during 2 minutes and 30 seconds. Subsequently, a constant pressure of 70 mmHg pressure was maintained during 2 minutes, after which, finally, a sudden deflation back to 0 mm Hg took place within 5 seconds. Simultaneously, during the entire procedure, continuous HCP recordings, as well as Left Ventricular Outflow Tract (LVOT) Doppler recordings were performed. Results have been reported in: http://www.biomedical-engineering-online.com/content/11/1/51
The results indicate that the HCP was able to track the changes in ventricular stroke volume reliably. Furthermore, the HCP produced ventricular volume-time curves that were consistent with literature. Further validation to value HCP as a diagnostic tool, is ongoing with miniaturized devices and increased functionality, and will include different patient groups in a trial commencing in 2018.
How does HCP manage interference in measurements related to respiration?
In order to exclude artefacts in the measured voltage change pattern on the thoracic skin due to distortions caused by changing air volumes inside the lungs, we adopted a 'respiration-gated' approach. By monitoring the respiratory fluctiation in the series of absolute values of measurements from subsequent heart cycles, the HCP automatically selects a subset of cardiac cycles, out of the total set of cardiac cycles within a period of 20 seconds, that corresponds to a specific phase in the respiratory cycle as well as the same volume of air inside the lungs in this way.