Journal of applied physiology
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Clinical Trial
Response of ventilator-dependent patients to different levels of proportional assist.
Proportional-assist ventilation (PAV) is a form of ventilatory support in which airway pressure increases in proportion to patient effort. Because it effectively reduces the mechanical load to an adjustable extent, PAV permits the study of the pattern of breathing in patients with respiratory disease when unconstrained by abnormal respiratory mechanics. We studied 11 patients with assorted medical problems requiring ventilatory support. ⋯ The correlation between VT and VE at the highest assist was very high (r = 0.91), suggesting that ventilatory demand is the most important determinant of VT variability. There were no systematic changes in breathing pattern as the level of assist was altered; at the highest and lowest levels of support, VE, VT, and f were, respectively, 12.8 +/- 5.4 (SD) vs. 11.6 +/- 4.3 l/min, 517 +/- 217 vs. 459 +/- 175 ml, and 25.0 +/- 4.2 vs. 25.7 +/- 3.9 breaths/min. These results indicate that within each patient, in a given state, there exist unique values for a desired VE, VT, and f that are largely independent of the mechanical load; if assist is increased, patient effort is decreased to maintain the desired ventilatory targets.
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Clinical Trial
Contribution of PO2, P50, and Hb to changes in arteriovenous O2 content during exercise in heart failure.
Arteriovenous O2 content (a-vCO2) differences increase during exercise in normal subjects through several mechanisms including PO2, O2 pressure at which hemoglobin (Hb) is half saturated with O2 (P50), and Hb concentration changes. The present study was undertaken to evaluate how much these biochemical changes are relevant to a-vCO2 difference through exercise in patients with heart failure. Twenty-seven patients with congestive heart failure [10 patients in functional class A (peak exercise O2 uptake >20 ml x kg-1 x min-1), 9 in class B (20-15 ml x kg-1 x min-1), and 8 in class C (15-10 ml x kg-1 x min-1)] underwent a cardiopulmonary exercise test with once-per-minute simultaneous blood sampling from the pulmonary and systemic arteries for determination of Hb, PO2, PCO2, pH, O2 content (CO2), Hb saturation and lactic acid (pulmonary artery only), and calculation of P50. ⋯ Thus a-vCO2 difference increase during exercise is notable in patients with heart failure but unrelated to the severity of the syndrome. Hb, P50, and, to the greatest degree, PO2 changes participate in the increment of a-vCO2 difference. In class C patients, the lack of PO2 reduction in the second half of exercise suggests the achievement of a "whole body critical venous PO2."