IEEE transactions on bio-medical engineering
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IEEE Trans Biomed Eng · Mar 2009
Time-varying surrogate data to assess nonlinearity in nonstationary time series: application to heart rate variability.
We propose a method to extend to time-varying (TV) systems the procedure for generating typical surrogate time series, in order to test the presence of nonlinear dynamics in potentially nonstationary signals. The method is based on fitting a TV autoregressive (AR) model to the original series and then regressing the model coefficients with random replacements of the model residuals to generate TV AR surrogate series. ⋯ Analysis of simulated time series showed that using TIV surrogates, linear nonstationary time series may be erroneously regarded as nonlinear and weak TV nonlinearities may remain unrevealed, while the use of TV AR surrogates markedly increases the probability of a correct interpretation. Application to short (500 beats) heart rate variability (HRV) time series recorded at rest (R), after head-up tilt (T), and during paced breathing (PB) showed: 1) modifications of the SE statistic that were well interpretable with the known cardiovascular physiology; 2) significant contribution of nonlinear dynamics to HRV in all conditions, with significant increase during PB at 0.2 Hz respiration rate; and 3) a disagreement between TV AR surrogates and TIV surrogates in about a quarter of the series, suggesting that nonstationarity may affect HRV recordings and bias the outcome of the traditional surrogate-based nonlinearity test.
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IEEE Trans Biomed Eng · Mar 2009
Noninvasive intracranial compliance from MRI-based measurements of transcranial blood and CSF flows: indirect versus direct approach.
Intracranial compliance (ICC) determines the ability of the intracranial compartment to accommodate an increase in volume without a large increase in intracranial pressure (ICP). The clinical utilization of ICC is limited by the invasiveness of current measurement. Several investigators attempted to estimate ICC noninvasively, from magnetic resonance imaging (MRI) measurements of cerebral blood and cerebral spinal fluid flows, either using indirect measures of ICC or directly by measuring the ratio of the changes in intracranial volume and pressure during the cardiac cycle. ⋯ In vivo volumetric blood and CSF flows measured by MRI phase contrast from healthy volunteers and from patients with elevated ICP were used for the comparison. An RLC circuit model of the craniospinal system was utilized to simulate the effect of a change in ICC on the CSF flow waveform. The simulations demonstrated that amplitude-based measures of ICC are considerably more sensitive than phase-based measures, and among the amplitude-based measures, the ICC index provides the most reliable estimate of ICC.
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IEEE Trans Biomed Eng · Mar 2009
Low-oxygen-saturation quantification in human arterial and venous circulation.
Conventional pulse oximetry has limited accuracy in measuring blood oxygen saturation in low-saturation and -perfusion scenarios. This limits the application of pulse oximetry in patients suffering from peripheral vascular afflictions. We present a novel pulse oximetry system that proposes solutions to these low-saturation and -perfusion scenarios by inducing an artificial pulse in the detected photoplethysmograph (PPG). ⋯ The arteriovenous hypothesis was validated, indicating that venous saturation can be extracted from the artificial PPG. The results indicate that the proposed system might be able to accurately monitor arterial and venous saturation in low- or no-perfusion scenarios. It is recommended that further studies into the system's performance are conducted.
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IEEE Trans Biomed Eng · Mar 2009
Instrumentation of the loss-of-resistance technique for epidural needle insertion.
Epidural anesthesia is the most common form of anesthesia in obstetrics. The loss-of-resistance to saline injection is used to confirm when the needle tip enters the epidural space. This procedure is highly dependent on skill and expertise, so it is useful to quantify the tissue resistance during insertion. ⋯ Significant differences in injection flow rate were also found for the muscle, interspinous ligament, and ligamentum flavum encountered in the two approaches. A small clinical study on human subjects was performed and again significant differences were found in flow rate for different tissues. These quantitative results improve the understanding of small differences in feel that have been previously known qualitatively and may help in the development of simulators.