"Optimal" application of ventilatory assist in Cheyne-Stokes respiration: a simulation study.

Although a variety of ventilator therapies have been employed to treat Cheyne-Stokes respiration (CSR), these modalities do not completely eliminate CSR. As well, most current strategies require that ventilatory assist be provided continuously. We used a computer model of the respiratory control system to determine whether a ventilatory assist strategy could be found that would substantially reduce the severity of CSR while minimizing the application of positive airway pressure. We assessed the effects of different levels of ventilatory assist applied during breaths that fell below selected hypopneic thresholds. These could be applied during the descending, ascending, or both phases of the CSR cycle. We found that ventilatory augmentation equal to 30-40% of eupneic drive, applied whenever ventilation fell below 70% of the eupneic level during the ascending or descending-and-ascending phases of CSR led to the greatest regularization of breathing with minimal ventilator intervention. Application of ventilatory assist during the descending phase produced little effect.

Atrial defibrillation thresholds of electrode configurations available to an atrioventricular defibrillator.

INTRODUCTION: Little investigation has been conducted to assess the atrial defibrillation thresholds of electrode configurations using electrodes designed for internal ventricular defibrillation (right ventricle [RV], superior vena cava [SVC], and pulse generator housing [Can]) combined with coronary sinus (CS) electrodes. We hypothesized that a CS-->SVC+Can electrode configuration would have a lower atrial defibrillation threshold than a standard configuration for defibrillation, RV-->SVC+Can. We also tested the atrial defibrillation thresholds of five other configurations. METHODS AND RESULTS: In 12 closed chest sheep, we situated a two-coil (RV, SVC) defibrillation catheter, a left-pectoral subcutaneous Can, and a CS lead. Atrial fibrillation was burst induced and maintained with continuous infusion of intrapericardial acetyl-beta-methylcholine chloride. Using fixed-tilt biphasic shocks, we determined the atrial defibrillation thresholds of seven test configurations in random order according to a multiple-reversal protocol. The peak voltage and delivered energy atrial defibrillation thresholds of CS-->SVC+Can (168+/-67 V, 2.68+/-2.40 J) were significantly lower than those of RV-->SVC+Can (215+/-88 V, 4.46+/-3.40 J). The atrial defibrillation thresholds of the other test configurations were RV+CS-->SVC+Can: 146+/-59 V, 1.92+/-1.45 J; RV-->CS+SVC+Can: 191+/-89 V, 3.53+/-3.19 J; CS-->SVC: 188+/-98 V, 3.77+/-4.14 J; SVC-->CS+ Can: 265+/-145 V, 7.37+/-9.12 J; and SVC-->Can: 516+/-209 V, 24.5+/-15.0 J. CONCLUSIONS: The atrial defibrillation threshold of CS-->SVC+Can is significantly lower than that of RV-->SVC+Can. In addition, the low atrial defibrillation threshold of RV+CS-->SVC+Can merits further investigation. Based on corroboration of low atrial defibrillation thresholds of CS-based configurations in humans, physicians might consider using CS leads with atrioventricular defibrillators.

Right atrial septal electrode for reducing the atrial defibrillation threshold.

BACKGROUND: The atrial defibrillation threshold (ADFT) energy of the standard lead configuration, right atrial appendage (RAA) to coronary sinus (CS), was reduced by >50% with the addition of a third electrode traversing the atrial septum in a previous study. This study determined whether the ADFT would be lowered by a more clinically practical third electrode placed in the right atrium along the atrial septum (RSP). METHODS AND RESULTS: Sustained atrial fibrillation was induced in 8 closed-chest sheep with burst pacing and maintained with pericardial infusion of acetyl-beta-methylcholine chloride. A custom-made, dual-defibrillation catheter was placed with electrodes in the lateral RA, CS, and RSP. A separate defibrillation catheter was also placed in the RAA. ADFT characteristics of RAA-->CS and 6 other single- or sequential-shock configurations were determined in random order by using biphasic, truncated-exponential waveforms in a multiple-reversal protocol. The delivered-energy, peak-voltage, and peak-current ADFTs for the sequential-shock configuration CS-->RSP/RA-->RSP (0.53+/-0.31 J, 86+/-22 V, and 1.6+/-0.6 A, respectively) were significantly lower than those of RAA-->CS (1.14+/-0.64 J, 157+/-34 V, and 2.5+/-1.1 A, respectively). The ADFT characteristics of RAA-->CS and RA-->CS were not significantly different, nor were those of CS-->RSP/RA-->RSP and CS-->RSP/RAA--> RSP. CONCLUSIONS: The ADFT of the standard RAA-->CS configuration may be markedly reduced with an additional electrode situated at the RSP.

Reduction of atrial defibrillation threshold with an interatrial septal electrode.

BACKGROUND: The standard lead configuration for internal atrial defibrillation consists of a shock between electrodes in the right atrial appendage (RAA) and coronary sinus (CS). We tested the hypothesis that the atrial defibrillation threshold (ADFT) of this RAA-->CS configuration would be lowered with use of an additional electrode at the atrial septum (SP). METHODS AND RESULTS: Sustained atrial fibrillation was induced in 8 closed-chest sheep with burst pacing and continuous pericardial infusion of acetyl-ss-methylcholine. Defibrillation electrodes were situated in the RAA, CS, pulmonary artery (PA), low right atrium (LRA), and across the SP. ADFTs of RAA-->CS and 4 other lead configurations were determined in random order by use of a multiple-reversal protocol. Biphasic waveforms of 3/1-ms duration were used for all single and sequential shocks. The ADFT delivered energies for the single-shock configurations were 1.27+/-0.67 J for RAA-->CS and 0. 86+/-0.59 J for RAA+CS-->SP; the ADFTs for the sequential-shock configurations were 0.39+/-0.18 J for RAA-->SP/CS-->SP, 1.16+/-0.72 J for CS-->SP/RAA-->SP, and 0.68+/-0.46 J for RAA-->CS/LRA-->PA. Except for CS-->SP/RAA-->SP versus RAA-->CS and RAA-->CS/LRA-->PA versus RAA+CS-->SP, the ADFT delivered energies of all of the configurations were significantly different from each other (P:<0. 05). CONCLUSIONS: The ADFT of the standard RAA-->CS configuration is markedly reduced with an additional electrode at the atrial SP.

Rapid, active hair bundle movements in hair cells from the bullfrog's sacculus.

Hair bundles, the mechanically sensitive organelles of hair cells in the auditory and vestibular systems, are elastic structures that are deflected by sound or acceleration. To examine rapid mechanical events associated with mechanoelectrical transduction, we stimulated individual hair bundles with flexible glass fibers and measured their responses with a temporal resolution of 400 microsec. When a hair bundle from the bullfrog's sacculus was abruptly deflected in the positive direction, the bundle's motion in the direction of stimulation was interrupted within the initial few milliseconds by an active movement, or twitch. This response was biphasic, with an initial component in the direction of the stimulus and a second component in the opposite direction. The amplitude and duration of the twitch depended on the bundle's initial position and the size and rise time of the stimulus; the twitch was largest over the range of bundle deflections in which transduction was most sensitive. Under displacement clamp conditions, in which a hair bundle's position was changed and then held constant with negative feedback, the twitch manifested itself as a biphasic force exerted by the bundle. Some hair bundles produced twitches in response to negatively directed stimuli, exhibited stimulus-evoked damped oscillations, or twitched spontaneously. The hair bundle's ability to perform work against an external load and to oscillate in response to stimulation indicates that the bundle could supply feedback for mechanical amplification in vertebrate auditory organs.

Hair-bundle stiffness dominates the elastic reactance to otolithic-membrane shear.

Efficient transduction by acousticolateralis organs requires that a stimulus force principally deflect hair bundles, rather than flex other structural elements. Hair bundles might therefore be expected to provide a large fraction of the impedence to shear motions of otolithic membranes and other accessory structures. We measured the stiffness for shear motions of the bullfrog's saccular otolithic membrane, and determined the stiffness due to a single hair bundle and its associated extracellular filaments; this component is termed the elemental stiffness. Stiffness measurements were made by displacing the base of a flexible probe whose tip was coupled to the otolithic membrane, and simultaneously measuring the flexion of the probe and the displacement of the membrane. The average elemental stiffness, about 1350 microN.m-1, only modestly exceeded the stiffness of individual hair bundles. The hair bundles therefore provide the dominant component of stiffness in the bullfrog's sacculus, and thus account for a significant component of impedance to otolithic-membrane shear. As a corollary, stiffness changes or active movements in hair bundles should influence the mechanical responses of this and other receptor organs.