Journal of applied physiology
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Comparative Study Clinical Trial
Patient-ventilator interaction during acute hypercapnia: pressure-support vs. proportional-assist ventilation.
The objective of this study was to compare patient-ventilator interaction during pressure-support ventilation (PSV) and proportional-assist ventilation (PAV) in the course of increased ventilatory requirement obtained by adding a dead space in 12 patients on weaning from mechanical ventilation. With PSV, the level of unloading was provided by setting the inspiratory pressure at 20 and 10 cmH2O, whereas with PAV the level of unloading was at 80 and 40% of the elastic and resistive load. Hypercapnia increased (P < 0.001) tidal swing of esophageal pressure and pressure-time product per breath at both levels of PSV and PAV. ⋯ With PAV, the increase in VE consequent to hypercapnia (27 +/- 3 and 64 +/- 4% during 80 and 40% PAV, respectively, P < 0.001) was related to the increase in VT (32 +/- 1 and 66 +/- 2% during 80 and 40% PAV, respectively, P < 0.001), respiratory rate remaining unchanged. The increase in pressure-time product per minute and per liter consequent to acute hypercapnia and the sense of breathlessness were significantly (P < 0.001) higher during PSV than during PAV. Our data show that, after hypercapnic stimulation of the respiratory drive, the capability to increase VE through changes in VT modulated by variations in inspiratory muscle effort is preserved only during PAV; the compensatory strategy used to increase VE during PSV requires greater muscle effort and causes more pronounced patient discomfort than during PAV.
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Forced expiratory maneuvers generated by rapid thoracic compression have been used to assess airway function in infants. It remains unclear whether flow limitation can be achieved in healthy infants because low pressure transmission across the chest wall and inspiratory effort may limit the maximum transpulmonary pressure developed during the maneuver. We have found that several rapid inflations to a lung volume set at an airway pressure of 30 cmH2O (V80) briefly inhibit respiratory effort and allow forced expiration to proceed from V80 to residual volume. ⋯ Pressure transmission between the compression jacket and the esophagus decreased with decreasing lung volume and averaged 60 and 37% at 50 and 75% of expired FVC, respectively. Subjects demonstrated plateaus in their isovolume pressure-flow curves at 50% of expired FVC and lower lung volumes. We conclude that this new methodology enables forced expiratory maneuvers to achieve flow limitation in healthy infants over at least the lower portion of their lung volume.
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Isolated perfused lung systems are commonly used to assess lung function in experimental studies. Assessment of hemodynamics and gas-exchange function in these systems is limited by the availability of venous blood. This study describes and validates a rat lung perfusion circuit in which a double-lung block ventilated with a hypoxic gas mixture [inspired O2 fraction (FIO2) 0.04; inspired CO2 fraction 0.08; deoxygenator (Deoxy) block] is used to provide blood with blood gases that are similar to mixed venous values to perfuse a study lung (FIO2 0.21; left lung only). ⋯ Finally, in protocol 4, perfusion of a damaged study lung did not impair the function of the system. We conclude that this model permits reliable assessment of pulmonary function in rats under controlled ventilation and perfusion conditions. The use of a Deoxy double-lung block simplifies the perfusion apparatus and eliminates the main cause of instability of other systems that use an anesthetized host animal to provide venous blood.
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The study's objective was to determine whether estradiol (E2) deficiency alters the functional relationship of muscle to bone and causes a differential increase in injury susceptibility. Ovariectomized 6-wk-old mice were administered E2 (40 micrograms. day-1. kg-1; n = 8) or the oil vehicle (n = 8) for 21 days. The anterior crural muscles of the left hindlimb were then stimulated to produce 150 maximal in vivo eccentric contractions. ⋯ In fact, the decrement in P(0) was only 36.9 +/- 3.8% in exercised EDL muscles from E2-deficient mice compared with 50.6 +/- 4.2% in exercised muscles from E2-treated mice (P = 0.03). Tibia stiffness was 3.9% higher in bones from exercised legs than in bones from unexercised legs (72.64 +/- 2.77 vs. 69.95 +/- 2.66 N/mm; P = 0.05) with ultimate load showing a similar trend (P = 0.07); no effect of E2 status was observed on these differences (P > or = 0.53). In conclusion, the functional relationship of bone to muscle and the susceptibility to injury in bone are not altered by the presence of E2 in ovariectomized mice; however, E2 does increase injury susceptibility in the EDL muscle.
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Together the abdominal muscles contribute significantly to ventilation under some conditions, but there is little information regarding individual recruitment and timing of activation of the four abdominal muscles in humans. Fine-wire electrodes were inserted under direct vision guided by high-resolution ultra-sound into the rectus abdominis (Rectus), external oblique (Extern), internal oblique (Intern), and transversus abdominis (Transv) in nine awake healthy subjects. Airflow, end-tidal CO2, and moving-average EMG signals were recorded during 1) supine resting and CO2-stimulated ventilation and 2) resting ventilation in the standing position. ⋯ EMG activities in the expiratory muscles after cessation of expiratory flow (postexpiratory expiratory activity) and in expiratory muscle activity preceding expiratory flow were observed consistently during supine stimulated ventilation and standing resting ventilation. These activities before and after expiratory airflow were prominent with stimulated ventilation during a substantial portion of inspiration, suggesting dual control of inspiratory pump action by both inspiratory and expiratory muscles, which provide acceleration and braking actions, respectively. These results suggest that in awake humans 1) during resting ventilation, expiration is an active process; 2) abdominal muscles are activated differentially; 3) Transv is the most active, Intern and Extern are intermediate, and Rectus is the least active expiratory muscle; and 4) during stimulated ventilation, inspiratory and expiratory muscles contribute dually to inspiratory pump action.