Constant VT Research Articles

Self-control and external control of mechanical ventilation give equal air hunger relief.

Elevated end-tidal partial pressure of CO2 (PET(CO2)) causes air hunger; this sensation becomes intense with a relatively small rise in PET(CO2) if ventilation is held constant. Spontaneously breathing subjects increase ventilation in response to CO2, thereby greatly diminishing air hunger. In healthy subjects and ventilator-dependent patients, experimenter-induced increases in ventilator tidal volume (VT) relieve air hunger even if PET(CO2) is kept elevated. We addressed two questions: (1) Can paralyzed, ventilator-dependent patients use the sensation of air hunger to effectively control ventilator VT using nonrespiratory motor pathways; and (2) Do subjects obtain more relief when in control of their own ventilator? Four subjects were trained to increase ventilator VT using a mouth-operated switch. Subjects' ratings of air hunger intensity in response to elevated PET(CO2) were compared during three conditions: (1) constant VT; (2) subject-controlled VT; and (3) experimenter-controlled VT. When given control of their ventilator, all subjects increased VT in response to increased PET(CO2), thereby relieving air hunger. Air hunger relief was similar when the experimenter mimicked these VT changes. These results suggest that: (1) ventilator-dependent patients can use sensation, conscious decisions, and nonrespiratory motor pathways to achieve an appropriate respiratory response to increased PCO2 and (2) control of one's own ventilation is unimportant in these circumstances.

Tidal Volume Reduction in ARDS: Effect on Cardiac Output and Arterial Oxygenation

During continuous positive pressure ventilation (CPPV), mean airway pressure and lung volume will be influenced both by the tidal volume (VT) employed and the amount of positive end-expiratory pressure (PEEP). The effect of varying levels of CPPV on PaO2 and cardiac output (Q) has been previously assessed by adjusting the level of PEEP at constant VT. This study examined the influence of a 200-ml reduction in VT, at a constant PEEP of 15 cm H2O, on the PaO2 and Q of 21 patients with adult respiratory distress syndrome (ARDS). The relationship between change in Q and change in total respiratory system compliance (Cst) after VT reduction was also examined. VT reduction from 14.1 +/- 0.8 ml/kg to 11.2 +/- 0.9 ml/kg yielded an increase in Q (+ 15 +/- 12 percent, p less than 0.01) without a significant change in PaO2 (-6.3 +/- 15.0 mm Hg, p = 0.08). Cst increased with VT reduction (+ 3.1 +/- 1.8 ml/cm H2O). There was only a modest correlation (r = +0.42, p = 0.06) between delta Q percent and delta Cst following VT reduction. VT reduction at high level PEEP may yield a significant improvement in Q and net O2 delivery, but the degree of hemodynamic improvement is variable and is not reliably predicted noninvasively by measurement of Cst.

Effects of respiratory variables on regional gas transport during high-frequency ventilation.

The regional effects of tidal volume (VT), respiratory frequency, and expiratory-to-inspiratory time ratio (TE/TI) during high-frequency ventilation (HFV) were studied in anesthetized and paralyzed dogs. Regional ventilation per unit of lung volume (spVr) was assessed with a positron camera during the washout of the tracer isotope 13NN from the lungs of 12 supine dogs. From the washout data, functional images of the mean residence time (MRT) of 13NN were produced and spVr was estimated as the inverse of the regional MRT. We found that at a constant VT X f product (where f represents frequency), increasing VT resulted in higher overall lung spV through the local enhancement of the basal spVr and with little effect in the apical spVr. In contrast, increasing VT X f at constant VT increased overall ventilation without significantly affecting the distribution of spVr values. TE/TI had no substantial effect in regional spVr distribution. These findings suggest that the dependency of gas transport during HFV of the form VT2 X f is the result of a progressive regional transition in gas transport mechanism. It appears, therefore, that as VT increases, the gas transport mechanism changes from a relative inefficient dispersive mechanism, dependent on VT X f, to the more efficient mechanism of direct fresh gas convection to alveoli with high regional tidal volume-to-dead-space ratio. A mathematical model of gas transport in a nonhomogeneous lung that exhibits such behavior is presented.

Alterations in Ventilatory Pattern and Ratio of Dead-Space to Tidal Volume

The effects of alterations in ventilatory pattern on the simultaneously measured physiologic and anatomic dead-spaces (VDphys and VDan, respectively) and the dead-space to tidal volume ratio (VD-VT) were studied in 17 healthy normal subjects (13 men, four women, ages 21 to 36 years). There were no significant changes in VDan with increases in respiratory frequency (f) or tidal volume (VT). The VDphys increased (mean change +0.153 L, p less than 0.05) with increase in VT (mean increase +0.84 L, p less than 0.01), but did not alter significantly with a twofold increase in f, at control VT. Increase in VT significantly reduced VD/VT (mean change -10.4 percent, p less than 0.05), but increase in f, at control VT, did not significantly alter VD/VT. These results indicate that in normal subjects, increase in VT alters ventilation/perfusion matching in the lungs, whereas an increase in f, at constant VT, has no effect on ventilation/perfusion matching. Increases in VD/VT cannot, therefore, be ascribed to alterations in ventilatory pattern where either VT, or f, or both are increased.

1858 DETERMINANTS OF TIDAL VOLUME (VT) DURING HIGH'S-FREQUENCY JET VENTILATION(HFJV)

The very short inspiratory time (TI) during HFJV may compromise the ability to deliver adequate VT. To document the airway pressures needed to maintain a constant VT (2.5 to 3.0cc/kg) (approx.dead space), TI was varied by changing frequency (f) over 120,240 and 480/min at I:E ratios of 1:1, 1:3, 1:5 and 1:9. A 16g cath. was inserted in a 3.0mm ET tube in 9 adult rabbits and used to monitor aΔP (peak minus end expiratory pressure). VT was measured via a pneumotachometer placed on the expiratory arm of the ventilator and validated with a body plethysmograph. The integrated flow signal was reset by the jet solenoid at onset and end of inspiration. At f of 240 and 480/min there was a significant inverse relationship between TI and the ΔP required to deliver constant VT (ANOVA p<.05). At TI <50ms there was at least a 3-fold increase in ΔP. Despite the longer TI (62ms) at f=480/min and I:E=1:1, ΔP was still increased because the shortened expiratory time produced marked air trapping. At a rate of 120/min, TI did not significantly influence ΔP. Net air flow at the pneumotach during the on phase of the jet cycle was in the outward direction indicating lack of gas entrainment. In summary: 1) as TI was shortened during HFJV higher ΔP was necessary to maintain VT and 2) in this model, gas entrainment did not contribute to the delivered VT. Duration of TI is therefore critical for optimal VT delivery if barotrauma is to be minimized. Furthermore, the delivery of humidification and O2 during HFJV cannot be dependent on gas entrainment. Supp. ALA-Ohio, ALAND

Circumstellar molecular emission of evolved stars and mass loss - IRC + 10216

view Abstract Citations (153) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Circumstellar molecular emission of evolved stars and mass loss : IRC+10216. Kwan, J. ; Linke, R. A. Abstract Using CO J=2→1 and 1→0 data, we revise an earlier model of the circumstellar envelope of IRC +10216. We determine that the mass loss rate is Mdot ≍ 4 × 10-5 (d/200 pc)2 Msun yr-1, and [CO]/[H2] ≍ 6 × l0-4. If we assume that Mdotυt ∝ L/c (where L is the stellar luminosity and υt the terminal outflow velocity) for a sample of evolved stars, we find that, as Mdot varies with L at constant Vt, the antenna temperature is roughly linearly proportional to the apparent luminosity or L/d2. Publication: The Astrophysical Journal Pub Date: March 1982 DOI: 10.1086/159769 Bibcode: 1982ApJ...254..587K Keywords: Emission Spectra; Infrared Stars; Molecular Spectroscopy; Stellar Envelopes; Stellar Evolution; Stellar Mass Ejection; Brightness Temperature; Carbon Monoxide; Flow Velocity; Mass Flow Rate; Molecular Spectra; Stellar Luminosity; Stellar Models; Astrophysics full text sources ADS | data products SIMBAD (1)

Effects of frequency, tidal volume, and lung volume on CO2 elimination in dogs by high frequency (2-30 Hz), low tidal volume ventilation.

Recent studies have shown that effective pulmonary ventilation is possible with tidal volumes (VT) less than the anatomic dead-space if the oscillatory frequency (f) is sufficiently large. We systematically studied the effect on pulmonary CO2 elimination (VCO2) of varying f (2-30 Hz) and VT (1-7 ml/kg) as well as lung volume (VL) in 13 anesthetized, paralyzed dogs in order to examine the contribution of those variables that are thought to be important in determining gas exchange by high frequency ventilation. All experiments were performed when the alveolar PCO2 was 40 +/- 1.5 mm Hg. In all studies, VCO2 increased monotonically with f at constant VT. We quantitated the effects of f and VT on VCO2 by using the dimensionless equation VCO2/VOSC = a(VT/VTo)b(f/fo)c where: VOSC = f X VT, VTo = mean VT, fo = mean f and a, b, c, are constants obtained by multiple regression. The mean values of a, b, and c for all dogs were 2.12 X 10(-3), 0.49, and 0.08, respectively. The most important variable in determining VCO2 was VOSC; however, there was considerable variability among dogs in the independent effect of VT and f on VCO2, with a doubling of VT at a constant VOSC causing changes in VCO2 ranging from -13 to +110% (mean = +35%). Increasing VL from functional residual capacity (FRC) to the lung volume at an airway opening minus body surface pressure of 25 cm H2O had no significant effect on VCO2.