Measurement of Diaphragmatic Electrical Activity by Surface Electromyography in Intubated Subjects and Its Relationship With Inspiratory Effort.

BACKGROUND: Quantification of patient effort during spontaneous breathing is important to tailor ventilatory assistance. Because a correlation between inspiratory muscle pressure (Pmus) and electrical activity of the diaphragm (EAdi) has been described, we aimed to assess the reliability of surface electromyography (EMG) of the respiratory muscles for monitoring diaphragm electrical activity and subject effort during assisted ventilation. METHODS: At a general ICU of a single university-affiliated hospital, we enrolled subjects who were intubated and on pressure support ventilation (PSV) and were on mechanical ventilation for > 48 h. The subjects were studied at 3 levels of pressure support. Airway flow and pressure; esophageal pressure; EAdi; and surface EMG of the diaphragm (surface EAdi), intercostal, and sternocleidomastoid muscles were recorded. Respiratory cycles were sampled for off-line analysis. The Pmus/EAdi index (PEI) was calculated by relying on EAdi and surface EAdi (surface PEI) from an airway pressure drop during end-expiratory occlusions performed every minute. RESULTS: surface EAdi well correlated with EAdi and Pmus, in particular, after averaging breaths into deciles (R = 0.92 and R = 0.84). When surface PEI was used with surface EAdi, it provided a reliable estimation of Pmus (R = 0.94 in comparison with measured Pmus). CONCLUSIONS: During assisted mechanical ventilation, EAdi can be reliably monitored by both EAdi and surface EMG. The measurement of Pmus based on the calibration of EAdi was also feasible by the use of surface EMG.

A template subtraction method for the removal of cardiogenic oscillations on esophageal pressure signals.

Esophageal pressure (Pes) is usually measured in patients receiving mechanical ventilation and is used for the assessment of lung mechanics. However, its interpretation is complicated by the presence of cardiogenic oscillations (CGO). In this article we present a novel method for the reduction of CGO based on the identification of pressure templates. Similar approaches are known for the removal of electrocardiographic (ECG) artifacts from the electromyogram (EMG). The proposed method is tested on clinical recordings of patients under assisted spontaneous ventilation. Besides the improvement of the respiratory signals, the identified CGO templates can be used diagnostically when viewed in relation to corresponding ECG data. This approach is illustrated on a few sample datasets.

Convolutive blind source separation of surface EMG measurements of the respiratory muscles.

Electromyography (EMG) has long been used for the assessment of muscle function and activity and has recently been applied to the control of medical ventilation. For this application, the EMG signal is usually recorded invasively by means of electrodes on a nasogastric tube which is placed inside the esophagus in order to minimize noise and crosstalk from other muscles. Replacing these invasive measurements with an EMG signal obtained non-invasively on the body surface is difficult and requires techniques for signal separation in order to reconstruct the contributions of the individual respiratory muscles. In the case of muscles with small cross-sectional areas, or with muscles at large distances from the recording site, solutions to this problem have been proposed previously. The respiratory muscles, however, are large and distributed widely over the upper body volume. In this article, we describe an algorithm for convolutive blind source separation (BSS) that performs well even for large, distributed muscles such as the respiratory muscles, while using only a small number of electrodes. The algorithm is derived as a special case of the TRINICON general framework for BSS. To provide evidence that it shows potential for separating inspiratory, expiratory, and cardiac activities in practical applications, a joint numerical simulation of EMG and ECG activities was performed, and separation success was evaluated in a variety of noise settings. The results are promising.

Convolutive blind source separation on surface EMG signals for respiratory diagnostics and medical ventilation control.

The electromyogram (EMG) is an important tool for assessing the activity of a muscle and thus also a valuable measure for the diagnosis and control of respiratory support. In this article we propose convolutive blind source separation (BSS) as an effective tool to pre-process surface electromyogram (sEMG) data of the human respiratory muscles. Specifically, the problem of discriminating between inspiratory, expiratory and cardiac muscle activity is addressed, which currently poses a major obstacle for the clinical use of sEMG for adaptive ventilation control. It is shown that using the investigated broadband algorithm, a clear separation of these components can be achieved. The algorithm is based on a generic framework for BSS that utilizes multiple statistical signal characteristics. Apart from a four-channel FIR structure, there are no further restrictive assumptions on the demixing system.

Noninvasive method for measuring respiratory system compliance during pressure support ventilation.

PURPOSE: To date, few methods have been accepted for assessing the respiratory system compliance (C(rs)) in patients under assisted ventilation at the bedside. The aim of this study was to evaluate our adaptive time slice method (ATSM) to continuously calculate the C(rs). METHODS: One breath is divided into several time periods (slices). For each slice, a compliance value C(i) is calculated. The slice width is adapted according to the confidence interval of C(i). After all C(i) values are obtained and the outliers are eliminated, the C(rs) of this breath is calculated as the mean value of the remainder of C(i)'s. Seven patients with Chronic Obstructive Pulmonary Disease were evaluated during pressure support ventilation. The results are compared with the values calculated with the transdiaphragmatic pressure (P(di)). RESULTS: 95 ± 4% of the recorded data could be analyzed with ATSM. In 6 patients out of 7, the results delivered with ATSM and with P(di) had similar variation (standard deviation) and accuracy (difference<20%). They were strongly correlated (weighted correlation coefficient = 0.86, p<10(-5)) with a mean difference of 3.22 ml/mbar. CONCLUSIONS: The ATSM is a robust method and able to provide accurate C(rs) in spontaneously breathing patients during pressure support ventilation noninvasively without extra instrumentation or complicated maneuvers.

Visual resolution with retinal implants estimated from recordings in cat visual cortex.

We investigated cortical responses to electrical stimulation of the retina using epi- and sub-retinal electrodes of 20-100 microm diameter. Temporal and spatial resolutions were assessed by recordings from the visual cortex with arrays of microelectrodes and optical imaging. The estimated resolutions were approximately 40 ms and approximately 1 degrees of visual angle. This temporal resolution of 25 frames per second and spatial resolution of about 0.8 cm at about 1m and correspondingly 8 cm at 10 m distance seems sufficient for useful object recognition and visuo-motor behavior in many in- and out-door situations of daily life.

Retino-cortical information transmission achievable with a retina implant.

Blind subjects with photoreceptor degeneration perceive phosphenes when their intact retinal ganglion cells are stimulated electrically. Is this approach suitable for transmitting enough information to the visual cortex for partially restoring vision? We stimulated the retina of anesthetized cats electrically and visually while recording the responses in the visual cortex. Transmission of retino-cortical information T was quantified by information theory. T was 20-160 bit/s (per stimulation and recording site) with random electrical or visual impulse stimulation at rates between 20 and 40 s-1. While increasing spatial density of independent electrical stimulation channels T did not saturate with 7 electrodes/mm2 retina. With seven electrodes up to 500 bit/s was transmitted to 15 cortical recording sites. Electrical stimulation basically employs temporal stimulus patterns. They are intimately linked with intensity/contrast information coded by the spike density of retinal ganglion cells. From the cortical information spread we estimated the spatial resolution as 0.5mm cortex corresponding to 0.5-1.0 degrees visual angle. If the human cortex can receive and decode the information transmitted by a retina implant, our quantitative results measured in cats suggest that visuo-motor coordination and object recognition in many in- and out-door situations will be possible.

Visual resolution with epi-retinal electrical stimulation estimated from activation profiles in cat visual cortex.

Blinds with receptor degeneration can perceive localized phosphenes in response to focal electrical epi-retinal stimuli. To avoid extensive basic stimulation tests in human patients, we developed techniques for estimating visual spatial resolution in anesthetized cats. Electrical epi-retinal and visual stimulation was combined with multiple-site retinal and cortical microelectrode recordings of local field potentials (LFPs) from visual areas 17 and 18. Classical visual receptive fields were characterized for retinal and cortical recording sites using multifocal visual stimulation combined with stimulus-response cross-correlation. We estimated visual spatial resolution from the size of the cortical activation profiles in response to single focal stimuli. For comparison, we determined activation profiles in response to visual stimuli at the same retinal location. Activation profiles were single peaked or multipeaked. In multipeaked profiles, the peak locations coincided with discontinuities in cortical retinotopy. Location and width of cortical activation profiles were distinct for retinal stimulation sites. On average, the activation profiles had a size of 1.28 +/- 0.03 mm cortex. Projected to visual space this corresponds to a spatial resolution of 1.49 deg +/- 0.04 deg visual angle. Best resolutions were 0.5 deg at low and medium stimulation currents corresponding to a visus of 1/30. Higher stimulation currents caused lower spatial, but higher temporal resolution (up to 70 stimuli/s). In analogy to the receptive-field concept in visual space, we defined and characterized electrical receptive fields. As our estimates of visual resolutions are conservative, we assume that a visual prosthesis will induce phosphenes at least at this resolution. This would enable visuomotor coordinations and object recognition in many indoor and outdoor situations of daily life.

Quantification of sensory information transmission using timeseries decorrelation techniques.

To estimate the information transmitted across a neuronal sensory system one has to deal with serial dependence among consecutive samples of the stimulus and the response signal. Common methods usually require a huge amount of data, or are restricted to Gaussian stimuli. Here, we describe stimulus and response as stochastic processes, i.e. as sequences of random variables, in the same coordinate system. Stimulus-response pairs of these random variables must not be considered independently because otherwise the transinformation is overestimated. To account for the linear fraction of the serial dependence, we present two decorrelation techniques based on coordinate transformation. They provide a representation of the processes with uncorrelated random variables and yield a more precise estimate of the transinformation.

Activation zones in cat visual cortex evoked by electrical retina stimulation.

BACKGROUND: A retina implant for restoring simple basic visual perception in patients who are blind due to photoreceptor loss requires optimisation of stimulation parameters for obtaining high spatio-temporal resolution. We developed effective low-power epi-retinal stimulation and intracortical recording in semichronically prepared cats. METHODS: Individually driveable fibre electrodes were inserted through a small scleral incision and positioned at the area centralis. Polyimide-platinum film electrodes were inserted via a corneal incision and fixed by instillation of perfluorocarbon liquid on the internal limiting membrane. For electrical stimulation we used short charge-balanced current impulses of 100-400 micro s duration and amplitudes ranging from 1 to 100 micro A. During stimulation we recorded multiple single-cell and population activities from areas 17 and 18. Recordings were stored digitally. Stimulus-response relations including response strength, cortical activation zones, information transmission, and electrical receptive fields were analysed off-line. RESULTS: We found low-threshold activations with fibre electrodes and polyimide-platinum film electrodes in close mechanical contact to the retina. Retinal stimulation with bipolar charge-balanced impulses resulted in cortical activation zones corresponding to 1-5 degrees visual angle at paracentral locations dependent on the eccentricity of the retinal stimulation point. Retino-cortical transinformation analysis revealed 20-30 bits/s per electrode, corresponding to 10-15 four-level pictures/s. Electrical receptive fields had sizes of 1-3 degrees visual angle. CONCLUSIONS: Coarse visuomotor coordination and navigation seems possible with retina implants.