Biomedical subjects

M Sammon

Publications and source records attributed to M Sammon.

7 recordsLinked to original sources

PubMed1997

Nonlinear systems identification: autocorrelation vs. autoskewness.

Autocorrelation function (C1) or autoregressive model parameters are often estimated for temporal analysis of physiological measurements. However, statistical approximations truncated at linear terms are unlikely to be of sufficient accuracy for patients whose homeostatic control systems cannot be presumed to be stable local to a single equilibrium. Thus a quadratic variant of C1 [autoskewness function (C2)] is introduced to detect nonlinearities in an output signal as a function of time delays. By use of simulations of nonlinear autoregressive models, C2 is shown to identify only those nonlinearities that "break" the symmetry of a system, altering the mean and skewness of its outputs. Case studies of patients with cardiopulmonary dysfunction demonstrate a range of ventilatory patterns seen in the clinical environment; whereas testing of C1 reveals their breath-by-breath minute ventilation to be significantly autocorrelated, the C2 test concludes that the correlation is nonlinear and asymmetrically distributed. Higher-order functionals [e.g., autokurtosis (C3)] are necessary for global analysis of metastable systems that continuously "switch" between multiple equilibrium states and unstable systems exhibiting nonequilibrium dynamics.

Algorithms↗

PubMed1994

Geometry of respiratory phase switching.

A second-order ordinary differential equation is outlined for the temporal dynamics of the respiratory central pattern generator (RCPG). Recurrent interactions between central excitation and inhibition confine the breathing cycle to the interior of a heteroclinic orbit between switching points (saddle equilibria) located at end expiration (E-I) and end inspiration (I-E). Dynamics depend on four eigenvalues that control inspiratory drive (lambda), excitability of inspiratory off switch (omega 1; stage 1 expiration), rate of central excitation disinhibition (omega 2; stage 2 expiration), and damping of the oscillator (epsilon). Ratios omega 2/lambda and omega 1/lambda determine local E-I and I-E phase switching, whereas inspiratory-to-expiratory balance varies as omega 2/(lambda omega 1). Stable apnea is seen when (lambda omega 2)/epsilon is near zero; inspiratory apneusis is seen when (lambda omega 1)/epsilon is low. The equations provide formalisms for discussing phase switching, apneas, apneuses, phase resetting and singularities, rapid shallow breathing, postinhibitory rebound excitation, redundancy, gating within the RCPG, and behavioral control of breathing. The model is offered as an explicit alternative to the harmonic oscillator models that have been used in the past to describe RCPG function.

Animals↗

PubMed1994

Symmetry, bifurcations, and chaos in a distributed respiratory control system.

A multivariate model is outlined for a distributed respiratory central pattern generator (RCPG) and its afferent control. Oscillatory behavior of the system depends on structure and symmetry of a matrix of phase-switching functions (F omega, phi) that control distribution of central excitation (CE) and inhibition (CI) within the circuit. The matrix diagonal (F omega) controls activation of CI variables as excitatory inputs are altered (e.g., central and afferent contributions to inspiratory off switch); off-diagonal terms (F phi) distribute excitations within the CI system and produce complex eigenvalues at the switching points between inspiration and expiration. For null F phi, phase switchings of saddle equilibria located at end expiration and end inspiration are overdamped all-or-nothing events; graded control of CI is seen for phi > 0. When coupling is significant (phi >> 0), CI dynamics become underdamped, admitting a domain of inputs where chaotic behavior is predictably observed. For the homogeneous RCPG (symmetric F omega, phi), CE oscillations are one-dimensional limit cycles (D = 1) or weakly chaotic (D approximately equal to 1). When perturbations from symmetry are significant, the distributed RCPG becomes partitioned where strongly chaotic oscillations (D > or = 2) and central apnea (D = 0) are seen more frequently. The equations provide means for mapping Silnikov bifurcations that alter the geometry and dimension of the breathing pattern and formalisms for discussing RCPG processing of afferent information.

Afferent Pathways↗

PubMed1993

Bifurcations of the respiratory pattern associated with reduced lung volume in the rat.

Rats with intact vagal reflexes exhibit patterns of breathing that contain greater degrees of freedom than those seen after vagotomy. To determine how alterations in end-expiratory volume modify the respiratory pattern, continuous positive (CPAP) and negative (CNAP) airway pressure was applied to tracheal openings of nine urethan-anesthetized vagi-intact rats (+3 to -9 cmH2O). Phase portraits (e.g., volume vs. flow curves), power spectra, correlation integral curves, and inspiratory-to-expiratory duration (TI/TE) ratios are used to interpret the vagal-dependent responses to changes in mean tracheal pressure (Ptr). With CPAP, respiratory oscillation was highly periodic and one dimensional, with TI/TE near 1.0. As Ptr was reduced in a stepwise manner, transient bursts of inspiratory airflow developed local to the expiratory-inspiratory transition, with amplitude increasing proportionally with the level of CNAP. These oscillatory "expiratory interrupts" (doubling TI/TE in five of nine cases) produced highly variable and asymmetric respiratory patterns. Progressive increases in correlation dimension (maximum = 1.8-3.0) and tendencies toward broadband power spectra were seen as Ptr was lowered. The irregular phase-switching dynamics seen with CNAP (which disappeared after vagotomy) are consistent with onset of low-dimensional chaos, probably correlated with activation of feedback mechanisms responsive to lung deflation.

Algorithms↗

PubMed1993

Role of deflation-sensitive feedback in control of end-expiratory volume in rats.

Rats breathing from reduced end-expiratory volumes (EEV) exhibit transient bursting of inspiratory airflow local to the expiratory-inspiratory transition (E-I) accompanied by increases in inspiration-to-expiration duration ratio. Continuous positive (CPAP) and negative (CNAP) airway pressures (+3 to -9 cmH2O) were applied to tracheal openings of 17 unilaterally vagotomized rats (urethan anesthetized) to evaluate the feedback mechanisms associated with these oscillatory "expiratory interrupts." Whole nerve recordings of afferent vagus (Vag) were primarily inspiratory with CPAP. As tracheal pressure was reduced in a stepwise manner, progressive decreases in inspiratory peaks and increases in activity during late expiration were seen on Vag. Single-fiber recordings correlate Vag expiratory activity with slowly adapting receptors sensitive to lung deflation. With CNAP, 1) progressively earlier onsets of activity in inspiratory muscles (e.g., diaphragm, genioglossus, posterior cricoarytenoid) were observed (increased inspiratory duration, decreased expiratory duration); oscillations at E-I developed only when expiratory peaks on Vag were significant. 2) Thyroarytenoid muscle exhibited onset of electromyographic activity that immediately succeeded onset of Vag during late expiration. When the trachea is intact, these two actions might combine to produce a respiratory pattern similar to the "grunting" often seen in newborns breathing from reduced EEVs. In contrast to larger mammals, rats rely strongly on deflation reflexes for control of E-I phase switching, presumably to aid in maintenance of an elevated EEV.

Animals↗

PubMed1993

Bifurcations of the respiratory pattern produced with phasic vagal stimulation in the rat.

Geometric methods from nonlinear dynamics are employed to evaluate dynamic processing of vagal afferent information by the respiratory central pattern generator (RCPG). While measuring airflow and diaphragm EMG, we applied brief electrical stimuli (40- to 130-ms duration) to one afferent vagus of bilaterally vagotomized urethan-anesthetized rats during every breath at various phases of the respiratory cycle. Stimuli applied during early or late inspiration of every breath evoke highly predictable one-dimensional responses: reversible (graded inhibition) or irreversible (off-switching) inhibition of inspiratory activity, respectively. Stimulation during midinspiration produces higher-dimensional oscillations that wander unpredictably over a continuum of graded inhibition and off-switching; "spiral" attractors and "horseshoe" return maps at this phase are characteristic of Silnikov's bifurcation. Stimuli applied during early expiration always prolonged expiratory duration, but those delivered during midexpiration evoked unpredictable wandering between prolongations and shortening of expiratory duration. A narrow time window surrounds the expiratory-inspiratory (E-I) transition, where stimuli elicit either breaths of short duration and low amplitude (irreversible E-I transition, decreased total respiratory cycle duration) or transient bursts of inspiratory activity at the E-I transition followed by a prolonged breath (reversible E-I transition, increased total respiratory cycle duration). We conclude that RCPG "gating" of and adaptation to vagal feedback combine to produce complex breath-to-breath dynamics in the rat that are consistent with low-dimensional chaos.

Anesthesia↗

PubMedDate not supplied

Variational approaches toward nonlinear time-series analysis.

Linear autoregressive models (AR) have broad applications toward spectral analyses and digital filtering of signals, as well as identification, prediction, and control of dynamical systems. However, the symmetric Gaussian properties of linear AR presume independence between the first and second order moments of system outputs. Using nonlinear AR to quantify interactions between the mean and autocovariance of a signal, analysts can discriminate asymmetric frequency components, estimate the relative contribution of system nonlinearities toward mean output, evaluate relative structural stability of a system, and predict critical regions where bifurcations in its dynamics might occur. Quadratic analyses are performed on simulated outputs from a nonlinear model and various experimental time series (Canadian lynx population, respiratory volume dynamics in rat, Cheyne-Stokes respiration in patients with congestive heart failure, intracranial pressures in patient with cerebral hematoma). These case studies demonstrate the utility of the techniques for evaluating the qualitative behaviors of dynamical systems in the presence of slowly varying inputs.

Adult↗