Impedance cardiography was introduced over 20 years ago as a noninvasive and unobtrusive technique for measuring systolic time intervals and cardiac output. Although our understanding of the physiological events reflected in the impedance cardiogram has become more refined, the technique's theoretical basis remains somewhat controversial and acceptance of its validity has relied heavily upon empirical validation. Largely as a consequence of this status, there have been inadequate grounds on which to develop sound methodological standardization. Currently, the methodological approaches that have been most frequently adopted may be viewed as representing the standard. The various aspects of impedance methodology are discussed, and alternative approaches described, with the objective of providing an informed basis for choosing among these methodological alternatives. It is recommended that studies utilizing impedance cardiography should be reported with clear and detailed methodological description. This should help clarify the extent to which methodological differences may underlie any discrepant research observations, as well as facilitate the emergence of improved methodological standards.

Methodological Guidelines for Impedance Cardiography

Responses to fear- and sadness-inducing films were assessed using a broad range of cardiovascular (heart rate, T-wave amplitude, low- and high-frequency heart rate variability, stroke volume, preejection period, left-ventricular ejection time, Heather index, blood pressure, pulse amplitude and transit time, and finger temperature), electrodermal (level, response rate, and response amplitude), and respiratory (rate, tidal volume and its variability, inspiratory flow rate, duty cycle, and end-tidal pCO(2)) measures. Subjective emotional experience and facial behavior (Corrugator Supercilii and Zygomaticus Major EMG) served as control measures. Results indicated robust differential physiological response patterns for fear, sadness, and neutral (mean classification accuracy 85%). Findings are discussed in terms of the fight-flight and conservation-withdrawal responses and possible limitations of a valence-arousal categorization of emotion in affective space.

Cardiovascular, electrodermal, and respiratory response patterns to fear- and sadness-inducing films.

Heart period, systolic time intervals, low and high frequency heart period variability, blood pressure, and respiration were measured in female subjects under three drug conditions (saline, atropine sulfate, metoprolol) while sitting and standing on three consecutive days. Following preinfusion baseline recordings, saline, metoprolol (14 mg), or atropine sulfate (2 mg) was infused for 15 min (by using a double-blind procedure). Recordings were taken during a postinfusion baseline and in response to an orthostatic stressor (standing versus sitting postures). At the end of the metoprolol session, atropine sulfate was infused and responses were monitored during the postinfusion (i.e., double blockade) baseline and during orthostatic stressor. Analyses of the blockade data revealed that the preejection period (PEP) reflected sympathetic but not vagal influences on the heart, and high frequency (HF, 0.12-0.40 Hz) heart rate variability (respiratory sinus arrhythmia) reflected vagal but not sympathetic influences on the heart. No other measure provided a specific index of the tonic sympathetic or vagal activation of the heart. Postinfusion PEP under saline predicted individual differences in postinfusion cardiac sympathetic activation, whereas postinfusion heart period (but not HF variability) under saline predicted individual differences in postinfusion cardiac vagal activation.

Autonomic cardiac control. II: Noninvasive indices and basal response as revealed by autonomic blockades

Research on rumination has demonstrated that compared with distraction, rumination intensifies and prolongs negative emotion. However, rumination and distraction differ both in what one thinks about and how one thinks about it. Do the negative outcomes of rumination result from how people think about negative events or simply that they think about them at all? To address this question, participants in 2 studies recalled a recent anger-provoking event and then thought about it in 1 of 2 ways: by ruminating or by reappraising. The authors examined the impact of these strategies on subsequent ratings of anger experience (Study 1) as well as on perseverative thinking and physiological responding over time (Study 2). Relative to reappraisal, rumination led to greater anger experience, more cognitive perseveration, and greater sympathetic nervous system activation. These findings provide compelling new evidence that how one thinks about an emotional event can shape the emotional response one has.

All in the Mind's Eye? Anger Rumination and Reappraisal

The growing need for more advanced ambulatory monitoring has led to the development of an ambulatory monitor for impedance cardiography (VU-AMD). This paper presents two studies addressing the validity of the VU-AMD. In the first study, the cardiovascular responses of 25 subjects during various conditions were simultaneously recorded with the VU-AMD and a standard laboratory impedance device. Correlations between the responses of the ambulatory and laboratory devices were high, both inter- and intraindividually, except for stroke volume and cardiac output during exercise. In the second study, 26 subjects underwent 24-hr monitoring with the VU-AMD. The values obtained with the VU-AMD were realistic and varied in a predictable way over activity and posture. It is concluded that the VU-AMD is a valid device for the measurement of systolic time intervals in real-life situations, but its applicability for absolute stroke volume and cardiac output determination remains to be established.

/pdf/ambulatory-monitoring-of-the-impedance-cardiogram-2wuhrpoqju.pdf

Ambulatory monitoring of the impedance cardiogram

We have modified impedance cardiography for monitoring cardiac output during stress tests. Employing an off-line microcomputer, our instrument ensemble averaged impedance signals to minimize the effect of motion artifacts. We proposed a new four-spot electrode array and replaced the usual encircling band electrode array with it. We tested ten normal subjects and compared the signal-to-noise ratio (SNR) from our spot electrode array to that from a typical band electrode array at rest and during four levels of exercise on a treadmill. The average of the sighal-to-noise ratios for ten subjects from our spot electrode array was 13.6-45.5 percent larger than that from the band electrode array at rest and during four levels of exercise. Thus, it is desirable to replace band electrodes with spot electrodes in impedance cardiography for exercising subjects.

Motion Artifact from Spot and Band Electrodes During Impedance Cardiography

We have modified impedance cardiography for monitoring cardiac output during stress tests. We adapted an ensemble-averaging technique for eliminating motion artifacts. We applied an array consisting of four spot ECG electrodes for impedance cardiogram (ICG) monitoring and compared it to conventional encircling band electrodes. We tested ten normal adults, compared the cardiac output obtained by our ICG monitoring system to that simultaneously obtained by the carbon dioxide (CO2) rebreathing method at rest and during three levels of treadmill exercise. The results show that the correlation coefficient between the spot electrode ICG and the CO2 rebreathing method is r = 0.90, and between the band electrode ICG and the CO2 rebreathing method is r = 0.96. If we use the peak-to-valley height of dz/dt instead of the peak height of dZ/dt in computing cardiac output, the correlation coefficient between the spot electrode ICG and the CO2 rebreathing method can be improved to r = 0.95.

Minghai Qu

Papers

Motion Artifact from Spot and Band Electrodes During Impedance Cardiography

Cardiac Output Monitoring by Impedance Cardiography During Treadmill Exercise