Whole-body Electric Bioimpedance Measurement in the Evaluation of Vascular Function

Background: Two pathologies affecting the arterial wall, atherosclerosis and arterial stiffening, are strong predictors of cardiovascular diseases and mortality. The identification of these at the sub-clinical, asymptomatic stages is potentially useful for the prevention of cardiovascular risk. Arterial stiffness can be evaluated locally by measuring carotid artery elasticity or segmentally by measuring pulse wave velocity (PWV), and of these two methods, PWV is considered the gold standard for assessing arterial stiffness. Whole-body impedance cardiography (ICGWB) has previously been shown to be a fast and operator-independent method to measure PWV, but the lack of reference values has limited its use in clinical practice. Moreover, the applicability of the ICGWB method in measuring PWV in large-scale epidemiological studies has not been tested previously.

Carotid artery intima-media thickness (IMT) and brachial artery flowmediated dilation (FMD) are well-known non-invasive markers of early atherosclerosis. Although the pathophysiology of atherosclerosis involves many features similar to arterial stiffness, whether IMT and FMD reflect similar or different aspects of vascular damage in comparison to PWV is not known. In addition, PWV and indices of carotid artery elasticity are often used interchangeably, but the relationship between these has received little interest to date.

Aims: The objective of the present study was to establish reference values for PWV as measured by ICGWB and gain more insight into the association of PWV with the markers of sub-clinical atherosclerosis (IMT, FMD) and local arterial elasticity. In addition, the aim of the current study was to study the applicability of the ICGWB method for measuring PWV in an epidemiological study. Furthermore, the objective of the present study was to develop a new integrated cardiovascular parameter reflecting several aspects of the cardiovascular system – i.e. arterial stiffness, arterial wall structure and cardiac pump function.

Subjects and Methods: The study population was combined from three distinct studies: 455 subjects from the Health 2000 Survey (supplemental study), 1872 subjects from the Cardiovascular Risk in Young Finns Study and 87 subjects from the Tampere Ambulatory Blood Pressure Study. Pulse wave velocity, stroke volume and systemic vascular resistance were measured from all subjects using the commercially available ICGWB monitor (CircMon™). Indices of carotid artery elasticity and carotid artery IMT were measured by ultrasound in the Health 2000 Survey (supplemental study) and the Cardiovascular Risk in Young Finns Study. Moreover, brachial FMD was measured by ultrasound in the Cardiovascular Risk in Young Finns Study.

Results: In subjects aged 46–76 years, IMT was directly and independently associated with PWV, but in younger subjects, IMT and PWV were not independently correlated. Carotid artery distensibility was inversely and independently associated with PWV, whereas FMD and PWV were not independently related. Metabolic syndrome and several other cardiovascular risk factors were found to associate with increased PWV, a finding which is in line with previous epidemiological studies using different methods to measure PWV.

The present thesis introduces and evaluates a new ICGWB-based hemodynamic parameter known as arterial tension time (ATT), which is defined as the time difference between the stroke-volume-introduced arterial distension and maximal integrated arterial distension. Decreased ATT was associated with increased arterial stiffness, increased subclinical atherosclerosis and decreased stroke volume. The current study also reports reference values for PWV measured by ICGWB for males and females in different age groups with no evidence of cardiovascular disease and a low burden of risk factors.

Conclusion: The present study has four main findings. Firstly, the current study establishes reference values for ICGWB-based PWV in an adult Finnish population. Reference values can be useful in the clinical management of patients in future studies. Secondly, PWV was not found to associate with IMT or FMD in young adults, but in older individuals, PWV and IMT were directly and independently correlated. Therefore, the current findings suggest that PWV may reflect a different aspect of vascular damage than FMD or IMT in young adults, whereas in older adults, the information provided by PWV and IMT may be, to some extent, similar. The present findings encourage the use of a combination of complementary non-invasive methods to evaluate arterial wall alterations, particularly in young adults. Thirdly, ICGWB provides a convenient and reliable tool for evaluating arterial stiffness in epidemiological studies. Fourthly, ATT developed in this study could potentially include information on several aspects of cardiovascular structure and function, and possibly serve as a new integrated parameter of cardiovascular health.