A meta-analysis of published studies concerning the validity of thoracic impedance cardiography

Our aim was to provide a meta-analysis of the literature concerning the validation of thoracic impedance cardiography (TIC) and to explain variations in reported results from differences in the studies. One hundred fifty-four studies (164 Fisher's Z-transformed correlation coefficients) comparing measurements of cardiac output or related parameters from TIC and a reference method were analyzed. Papers were classified according to differences in TIC methodology, reference method, and subject characteristics. Pooling using the random-effects method yielded an overall correlation of r = 0.82 (95% confidence interval: 0.80-0.84). ANOVA revealed a significant influence of the reference method and the subject characteristics on the correlation coefficient. In cardiac patients, the correlation was significantly decreased. No influence of the applied TIC methodology was found. CONCLUSION: TIC might be useful for trend analysis of different groups of patients. However, since the reference method was of significant influence, differences between TIC and the reference method are incorrectly attributed to TIC alon

Assessing fluid responses after coronary surgery: role of mathematical coupling of global end-diastolic volume to cardiac output measured by transpulmonary thermodilution

Background Mathematical coupling may explain in part why cardiac filling volumes obtained by transpulmonary thermodilution may better predict and monitor responses of cardiac output to fluid loading than pressures obtained by pulmonary artery catheters (PACs). Methods Eleven consecutive patients with hypovolaemia after coronary surgery and a PAC, allowing central venous pressure (CVP) and continuous cardiac index (CCIp) measurements, received a femoral artery catheter for transpulmonary thermodilution measurements of global end-diastolic blood volume index (GEDVI) and cardiac index (CItp). One to five colloid fluid-loading steps of 250 ml were done in each patient (n=48 total). Results Fluid responses were predicted and monitored similarly by CItp and CCIp, whereas CItp and CCIp correlated at r=0.70 (P<0.001) with a biasof 0.40 l mi

A study of the brain's resting state based on alpha band power, heart rate and fMRI

Considering that there are several theoretical reasons why fMRI data is correlated to variations in heart rate, these correlations are explored using experimental resting state data. In particular, the possibility is discussed that the "default network", being a brain area that deactivates during non-specific general tasks, is a hemodynamic effect caused by heart rate variations. Of fifteen healthy controls ECG, EEG and fMRI were co-registered. Slice time dependent heart rate regressors were derived from the ECG data and correlated to fMRI using a linear correlation analysis where the impulse response is estimated from the data. It was found that in most subjects substantial correlations between heart rate variations and fMRI exist, both within the brain and at the ventricles. The brain areas with high correlation to heart rate are different from the "default network" and the response functions deviate from the canonical hemodynamic response function. Furthermore, a general negative correlation was found between heart beat intervals (reverse of heart rate) and alpha power. We interpret this finding by assuming that subject's state varies between drowsiness and wakefulness. Finally, given this large correlation, we re-examined the contribution of heart rate variations to earlier reported fMRI/alpha band correlations, by adding heart rate regressors as confounders. It was found that inclusion of these confounders most often had a negligible effect. From its strong correlation to alpha power, we conclude that the heart rate variations contain important physiological information about subject's resting state. However, it does not provide a full explanation of the behaviour of the "default network". Its application as confounder in fMRI experiments is a relatively small computational effort, but may have a substantial impact in paradigms where heart rate is controlled by the stimulus

Monitoring of recruitment and derecruitment by electrical impedance tomtgraphy in a model of acute lung injury

OBJECTIVE: To evaluate a noninvasive system for obtaining information about alveolar recruitment and derecruitment in a model of acute lung injury. DESIGN: Prospective experimental study. SETTING: Animal research laboratory. SUBJECTS: Nine anesthetized pigs. INTERVENTIONS: Electrical impedance tomography measurements were performed. Electrical impedance tomography is an imaging technique that can register the ventilation-induced impedance changes in different parts of the lung. In nine anesthetized pigs, repeated lung lavages were performed until a PaO2 of <80 mm Hg was reached. Thereafter, the lungs were recruited according to two different recruitment protocols: the open lung approach and the open lung concept. Five time points for measurements were chosen: healthy (reference), lavage (atelectasis), recruitment, derecruitment, and maintain recruited (final). MEASUREMENTS AND MAIN RESULTS: After lavage, there was a significant increase in the impedance ratio, defined as the ventilation-induced impedance changes of the anterior part of the lung divided by that of the posterior part (from 1.75 +/- 0.63 to 4.51 +/- 2.22; p <.05). The impedance ratio decreased significantly after performing the recruitment protocol (from 4.51 +/- 2.22 to 1.18 +/- 0.51). During both recruitment procedures, a steep increase in baseline impedance change was seen. Furthermore, during derecruitment, a decrease in the slope in baseline impedance change was seen in the posterior part of the lung, whereas the anterior part showed no change. CONCLUSION: Electrical impedance tomography is a technique that can show impedance changes resembling recruitment and derecruitment of alveoli in the anterior and posterior parts of the lung. Therefore, electrical impedance tomography may help in determining the optimal mechanical ventilation in a patient with acute lung injur

Validity and repronducibility of electrical impedance tomography for measurement of calf blood flow in healthy subjects

The Sheffield electrical impedance tomography; (EIT) system produces images of changes in the distribution of resistivity within tissue. The paper reports on the application of electrical impedance tomography in monitoring volume changes in the limb during venous occlusion. The aim of the study is to assess the feasibility, reproducibility and validity of calf blood flow measurements by EIT. In 14 healthy volunteers calf blood flow is compared, as determined in a calf segment by strain-gauge plethysmography (SGP), with the impedance changes measured by EIT during rest and post-ischaemic hyperaemia. The measurements are repeated to assess reproducibility. The reproducibility for the EIT, assessed from the repeated measurements and expressed as a reproducibility coefficient, is 0.88 during rest and 0.89 during hyperaemia. The reproducibility coefficient for SGP data is 0.83 at rest and 0.67 during hyperaemia. Flow measurements, assessed by means of two methods, correlate well at rest (r = 0.89), but only moderately during hyperaemia (r = 0.51). The correlation coefficient for the pooled flow measurements is 0.98. It is concluded that EIT is a valid and reliable method for assessing blood flow in the limb. Possible applications of EIT in localising fluid changes are discusse