Dynamic relationship between cardiac imaging and physiological measurements

Abstract

Impedance cardiography (ICG) is a non-invasive technique to measure the dynamic changes in electrical impedance of the thorax. Photoplethymgraphy (PPG) is an optical- based non-invasive physiological measurement technique used to detect the blood volume pulses in the microvascular bed of tissue. These two physiological measurements have potential clinical importance to enable simple and cost-efficient ways to examine cardiovascular function and provide surrogate or additional clinical information to the measures from cardiac imaging. However, because the origins of the characteristic waveforms of the impedance and pulse are still not well understood, the clinical applications of these two techniques are limited. There were two main aims in this study: 1) to obtain a better understanding of the origins of the pulsatile impedance changes and peripheral pulse by linking their characteristic features beat-by-beat to those from simultaneous echocardiograms; 2) to validate the clinical indices from ICG and PPG with those well-established echocardiographic indices. Physiological signals, including ECGs, impedance, the first derivative impedance and finger and ear pulses, were simultaneously recorded with echocardiograms from 30 male healthy subjects at rest. The timing sequence of cardiovascular events in a single cardiac cycle was reconstructed with the feature times obtained from the physiological measurements and images. The relations of the time features from the impedance with corresponding features from images and pulses were investigated. The relations of the time features from peripheral pulses with corresponding features from images were also investigated. Furthermore, clinical time indices measured from the impedance and pulse were validated with the reference to the echocardiograms. Finally, the effects of age, heart rate and blood pressure on the image and physiological measurements were examined. According to the reconstructed timing sequence, it was evident that the systolic waves of the thoracic impedance and peripheral pulse occurred following left ventricular ejection. The impedance started to fall 26 ms and the pulse arrived at the fingertip 162 ms after the aortic valve opened. A diastolic wave was observed during the ventricular passive filling phase on the impedance and pulse. The impedance started to recover during the late ventricular ejection phase when the peripheral pulse was rising up. While the pulsatile impedance changes were mainly correlated with valve movement, the derivative impedance (velocity of impedance change) was more correlated with aortic flow (velocity of blood 2 flow). The foot of the finger pulse was significantly correlated with aortic valve open (R = 0.361, P < 0.05), while its systolic peak was strongly correlated with the aortic valve 2 closing (R = 0.579, P < 0.001). Although the pulse had similar waveform shapes to the inverted impedance waveform, the associations between the time features of these two signals were weak. During the validation of potential clinical indices from ICG, significant correlation was found between the overall duration of the derivative impedance systolic wave (359 ms) and the left ventricular ejection time (LVET) measured by aortic valve open duration from M- 2 mode images (329 ms) (R = 0.324, P < 0.001). The overall duration from the finger pulse foot to notch (348 ms) was also significantly correlated with the LVET from M-mode 2 images (R = 0.461, P < 0.001). Therefore, both ICG and PPG had the potential to provide surrogates to the LVET measurement. Age influenced the cardiovascular diastolic function more than systolic function on normal subjects. With age increasing, the reduction of the left ventricular passive filling was compensated by active filling. The ratio of the passive filling duration to the active 2 filling duration decreased with age (R = 0.143, P < 0.05). The influence of age on the diastolic wave of the impedance signals was striking. The impedance diastolic wave disappeared gradually with age. The effects of age on the peripheral pulse were mainly on the shortened pulse foot transit time (PPT) and prolonged pulse rise time. The large artery f stiffness index (SI) increased with age. Most time intervals were prolonged with heart rate slowing down. The effects of systolic blood pressure were evident on pulse transit time and pulse diastolic rising time. Driven by higher systolic blood pressure, both PPT and rising f time decreased significantly (P < 0.001). In conclusion, from the analysis based on simultaneous physiological measurements and echocardiograms, both the pulsatile impedance changes and peripheral volume pulse were initiated by left ventricular ejection. The thoracic impedance changes reflected volume changes in the central great vessels, while the first derivative impedance was associated with the velocity of blood flow. Both ICG and PPG had the potential to provide surrogates for the measures of cardiac mechanical functions from images. The PPG technique also enabled the assessment of changes in vascular function caused by age.