Predictive value of non-invasive hemodynamic measurement by means of impedance cardiography in hypertensive subjects older than 50 years of age

Signal processing techniques applied to impedance cardiography ICG signals - a review

Over the last decade, Computer-Aided Diagnosis (CAD) systems have been provided significant research focus by researchers. CAD systems have been developed in order to minimise visual errors, to compensate manual interpretation, and to help medical staff to take decisions swiftly. These systems have been considered as powerful tools for a reliable, automatic, and low-cost monitoring and diagnosis. CAD systems are based on analysis and classification of several physiological signals for detecting and assessing different diseases related to the corresponding organ. The implementation of these systems requires the application of several advanced signal processing techniques. Specifically, in cardiology, CAD systems have achieved promising results in providing an accurate and rapid detection of cardiovascular diseases (CVDs). Particularly, the number of works on signal processing field for impedance cardiography (ICG) signals starts to grow slowly in recent years. This paper presents a review study of signal processing techniques applied to the ICG signal for the denoising, the analysis, the classification and the characterisation purposes. This review is intended to provide researchers with a broad overview of the currently used signal processing techniques for ICG signal analysis, as well as to improve future research by applying other recent advanced methods.

Hemodynamic Phenotypes of Hypertension Based on Cardiac Output and Systemic Vascular Resistance

BACKGROUND

Blood pressure is a physiologic measure that reflects cardiac output and systemic vascular resistance. Classification by these components could be useful in characterizing subtypes of hypertension, which may have a role in selecting treatment strategies. However, hemodynamic phenotypes of a large, stable, outpatient population with hypertension remain unknown.

METHODS

We included 34,238 people with systolic blood pressure of ≥130 mm Hg, who underwent impedance cardiography at 51 sites of iKang Health Checkup Centers throughout China between 2012 and 2018. Hemodynamic parameters measured included stroke volume, stroke volume index, heart rate, cardiac output, cardiac index, systemic vascular resistance, and systemic vascular resistance index. We characterized these by systolic blood pressure categories and assessed patient characteristics associated with the ratio of cardiac index to systemic vascular resistance index.

RESULTS

Among the study cohort (n = 33,414; mean age 52 ± 13 years; 36.6% female), 49%, 40%, and 11% had systolic blood pressure130-139, 140-159, and ≥160 mm Hg, respectively. Among patients with systolic blood pressure 140-159 mm Hg, 9353 (70%) had high systemic vascular resistance index but normal/low cardiac index, 1949 (15%) had high cardiac index but low/normal systemic vascular resistance index, and 2053 (15%) had low/normal cardiac index and systemic vascular resistance index. Using multivariable analysis, we found that cardiac index to systemic vascular resistance index ratio was negatively associated with age and body mass index (all P <0.05; R-square 0.16, 0.12, and 0.09 for systolic blood pressure 130-139, 140-159 and ≥160 mm Hg, respectively).

CONCLUSIONS

Different hemodynamic blood pressure phenotypes were identified across all hypertensive blood pressure categories. Although individual characteristics were associated with the cardiac index to systemic vascular resistance index ratio, they only weakly explained the variation.

Waveform analysis of differential graphs of reconstructed impedance cardiography from healthy individuals

Purpose

The aim is to measure and analyze the wave amplitudes and time intervals of differential graphs of reconstructed impedance cardiography (RICG).

Methods

180 adults with normal cardiac function between the ages of 18–78 were included in the study. Six mingled impedance changes on chest surface were simultaneously detected for each subject. The differential graphs of five impedance change components of RICG were obtained through waveform separation and software differentiation. The amplitudes of C, X, O, b waves and time intervals of Q‐b and Q‐C were measured and statistically analyzed.

Results

The amplitudes of C and X waves in PL, PR, AO, and that of C, O, b waves in LV and RV, all decrease as age increases. Wave amplitudes of the female group were bigger than those of the male group (p < .01), while the Q‐C intervals of the female group were shorter than that of the male group (p < .01). Among five impedance change components, the wave amplitude of AO was larger than those of PL and PR (p < .01), and wave amplitudes of PL and PR were bigger than those of LV and RV (p < .01). Q‐C intervals of LV and RV were longer than those of AO, PL and PR (p < .01), while the Q‐b intervals of LV and RV were shorter than the Q‐C intervals of AO, PL, and PR.

Conclusions

The differential graphs of RICG could reflect indirectly the physiological activities and pathological changes of the heart and of the large blood vessels in thorax.

Comparing square root method of measuring the cardiac output by means of aortic impedance change component to Kubicek's method

PURPOSE

The aim of this study is to explore a calculated method used to measure the cardiac output using the aortic impedance change component of reconstructed impedance cardiography.

METHODS

Routine impedance cardiography was measured using Kubicek's method with four ring electrodes. The thoracic mixed impedance changes were measured by six leads, which consisted of 15 electrodes. The aortic impedance change component was separated from six thoracic mixed impedance changes through waveform reconstruction. The square root formula used to calculate the cardiac output was deduced based on the thoracic impedance change equation and the aortic volume change hypothesis during the systole period. The cardiac outputs of 180 normal adults and 72 patients with cardiac insufficiency who could still walk freely were contrastively computed with both Kubicek's formula and the square root formula.

RESULTS

For 180 normal adults, the cardiac index (CI) computed with the square root formula was 3.60 ± 0.45 L/min/m2 , with normal values ranging from 2.7 to 4.5 L/min/m2 . A total of 163 cases (90.6%) had a CI in the standard range (2.7-4.3 L/min/m2 ) adopted in clinical applications. The CI computed with Kubicek's formula was 3.61 ± 0.86 L/min/m2 , with normal values ranging from 1.9 to 5.3 L/min/m2 , and only 115 cases (63.9%) had a CI in the above standard range. Among the 72 patients with cardiac insufficiency, 20 (27.8%) patients had a CI < 2.0 L/min/m2 with Kubicek's formula. Of these 20 cases, 9 cases had a CI < 1.5 L/min/m2 , and 4 cases had a CI < 1.1 L/min/m2 . In contrast, none of the 72 patients had a CI < 2.0 L/min/m2 with the square root formula. In addition, the influence of the chest circumference on the CI was lower for the square root formula than for Kubicek's formula.

CONCLUSIONS

The CI calculated with the square root formula had a better normal value range, was more accurate for the patients with cardiac insufficiency, and was less affected by the chest circumference than the CI calculated with Kubicek's formula.

[Current status of noninvasive hemodynamics in hypertension]

Hypertension is a haemodynamic disorder resulting from a persistent mismatch between cardiac output and peripheral resistance. Hypertension undergoes haemodynamic progression during its natural history. Impedance cardiography is a method of evaluating the cardiovascular system that obtains haemodynamic information from beat to beat through the analysis of variations in the impedance of the thorax on the passage of an electric current. Impedance cardiography unmasks the haemodynamic deterioration underlying the increase in blood pressure as age and systolic blood pressure increases. This method may help to improve blood pressure control through individualized treatment with reduction of peripheral resistance, maintenance of cardiac output or its increase, improvement of arterial compliance and preservation of organ-tissue perfusion. It is useful in the management of patients with resistant hypertension, since a greater percentage of patients controlled with changes in the treatment in relation to the haemodynamic measurements are obtained. Impedance cardiography is important and has prognostic utility in relation to a haemodynamic deterioration pattern and increased cardiovascular events.

Comparison of atenolol versus bisoprolol with noninvasive hemodynamic and pulse wave assessment

We aimed to compare atenolol versus bisoprolol regarding general hemodynamics, central-peripheral blood pressure (BP), pulse wave parameters, and arterial stiffness. In this open-label, crossover study, we recruited 19 hypertensives, untreated or with stable monotherapy. Patients were randomized to receive atenolol (25-50 mg) or bisoprolol (2.5-5 mg), and then switched medications after 4 weeks. Studies were performed at baseline and after each drug period. In pulse wave analyses, both drugs significantly increased augmentation index (P < .01) and ejection duration (P < .02), and reduced heart rate (P < .001), brachial systolic BP (P ≤ .01), brachial diastolic BP (P ≤ .001), and central diastolic BP (P ≤ .001), but not central systolic BP (P ≥ .06). Impedance cardiographic assessment showed a significantly increased stroke volume (P ≤ .02). There were no significant differences in the effects between drugs. In conclusion, atenolol and bisoprolol show similar hemodynamic characteristics. Failure to decrease central systolic BP results from bradycardia with increased stroke volume and an earlier reflected aortic wave.