High VT Research Articles

Frequency, Tidal Volume, and Mean Airway Pressure Combinations that Provide Adequate Gas Exchange and Low Alveolar Pressure during High Frequency Oscillatory Ventilation in Rabbits

We studied healthy and saline lavaged rabbits during high frequency oscillatory ventilation to determine what combination of frequency (f), tidal volume (Vt), and mean airway pressure (Paw) produced the lowest peak-to-peak alveolar pressure amplitude (Palv) and physiologic blood gas tensions. Sinusoidal volume changes were delivered through a tracheostomy by a piston pump driven by a linear motor. Tracheal pressure amplitude (Ptr) was measured through a tracheal catheter and alveolar pressure amplitude was measured in a capsule glued to the right lower lobe. PaO2, PaCO2, Ptr, and Palv were measured at the following settings: FiO2 = 0.5, frequency 2-28 Hz, Vt 1-3 mL/kg (50 150% dead space) and Paw 5-15 cm H2O. Many combinations of frequency and Vt resulted in the same PaO2 and PaCO2. Paw had a large effect on Palv and minimal effect on blood gas tensions. In lavaged rabbits, the composite variable f x Vt2 described the trends in Palv and blood gas tensions. As the product of f x Vt2 increased, PaO2 initially increased and then decreased, whereas PaCO2 decreased and Palv increased. No single combination of frequency, Vt and Paw simultaneously provided the lowest Palv and physiologic blood gas tensions. Adequate blood gas tensions and low Palv were obtained at frequencies less than 12 Hz, a Vt of 2 mL/kg and a Paw of 10 cm H2O. In healthy and lavaged rabbits PaO2 increased and PaCO2 decreased as frequency increased at lower Vt.PaO2 decreased as frequency increased at higher Vt in lavaged rabbits only. Palv tended to be greater in lavaged rabbits.

Acecainide Stabilizes Rhythm in High Risk VT patients

Acecainide Stabilizes Rhythm in High Risk VT patients

High-performance WN-gate MISFETs fabricated from MOVPE wafers

WN-gate heterojunction MISFETs with an AlGaAs layer as insulator have been fabricated on MOVPE wafers; they exhibit high transconductance (up to 460 mS/mm) and high Vt uniformity.

Radial and longitudinal compartmental analysis of gas transport during high-frequency ventilation

A model of gas exchange by low-tidal-volume (VT), high-frequency ventilation (HFV) is presented, based on the physical principles of dispersion. These are the nonuniformity of the velocity profile and the nonreversible mixing of fluid components in a diffusive manner. A numerical method was used to incorporate these principles into a quantitative model. The airways of a symmetrically bifurcating bronchial-tree model were partitioned in the radial direction into two concentric layers representing the kinematic dispersion by nonuniformity of the velocity profile. Mixing between the layers was invoked in proportion to the diffusivity and local dimensions. The effects of frequency (f), VT, shape of the velocity profile, and bronchial-model configuration were tested in the model, with favorable comparison to available experimental data. The model predicts that for a frequency-dependent velocity profile, the rate of tracer exchange is proportional to the square root of f and to the square of VT-V0, where V0 is a constant small volume under which gas exchange was nil. Intracycle asymmetric mixing is predicted to have a stronger effect on gas exchange than asymmetric velocity profile. Gas exchange when turbulent-flow regime is assumed is predicted to be less for the higher VT values than with laminar flow and with mixing by molecular diffusivity. This model was found to be didactic, flexible, and capable of modeling combinations of factors affecting either one of the two fundamental processes of dispersion.

1826 TRANSTHORACIC IMPEDANCE AS A METHOD OF MEASURING TIDAL VOLUME DURING HIGH FREQUENCY VENTILATION

During conventional ventilation, tidal volume (VT) can be measured from variations in the transthoracic impedance (TTI) signal. The purpose of this study was to determine if TTI could be used to measure VT during high frequency ventilation (HFV), thus providing a non-invasive technique which can be applied continuously during HFV of human infants. VT was measured in 2 lambs (3-4 weeks old) placed in a whole-body, pressure-compensated, integrated flow plethysmograph during HFV provided by a flow-interrupter (Bird VDR). The frequency response of the plethysmograph was flat up to 20 Hz. TTI was measured from a tetrapolar disk electrode system (SAAB Respimeter). The impedance signal and the calibrated plethysmograph volume signal were recorded. A computer averaging technique was used to extract the high frequency VT component of each signal from background volume changes associated with spontaneous breathing. Magnitude in ohms of the extracted TTI waveform (VTZ) was compared with the corresponding plethysmographic volume change (VTp1). 54 simultaneous measurements of VTp1 and VTZ were made at frequencies varied between 300-900 breaths per minute and at VT between 2.8 and 90 ml. VTZ had a close linear correlation with VTp1 (r=0.99). These results indicate that the frequency response of VTZ is sufficient to apply the technique to HFV and that measurement of absolute VT at high frequency can be provided by calibrating the impedance signal during low-rate conventional ventilation.

Breathing pattern and occlusion pressure during moderate and heavy exercise.

We studied changes in breathing pattern and mouth occlusion pressure (P0.1) in 11 healthy subjects performing graded steady-state exercise on a cycle ergometer up to the maximal load sustainable for 4 min. With increasing work intensity both the tidal volume (VT) and end-inspiratory volume relations to inspiratory (TI) and expiratory (TE) durations were linear in the moderate work load range; in the high load range VT and end-inspiratory volume tended to plateau with further decreases in TI and TE. The ratio of TI to total breath duration (TI/Ttot) increased with work intensity. Intraindividual coefficients of variation for VT, breathing frequency (f), mean inspiratory flow (VT/TI), and other respiratory variables decreased with increasing work intensity, indicating that breath-to-breath variations in breathing pattern became smaller as the level of ventilation increased. P0.1 rose with VT/TI as a power function with an exponent averaging 1.5 (range 1.3-1.9), indicating that the ratio P0.1/(VT/TI), an index of respiratory system impedance, increased with VT/TI and work intensity. We conclude that in moderate and heavy exercise the work of inspiration at a given ventilation is reduced because of the increase in TI/Ttot, the impedance of the respiratory system increases with work intensity because of both an increase in f and a flow-dependent rise in airway resistance, and the neuromuscular inspiratory activity is reflexly augmented because of internal flow-resistive loading.