Journal of applied physiology
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We studied the effect of catheter position and flow rate on gas exchange during constant-flow ventilation (CFV) in eight anesthetized, paralyzed dogs. The distal tips of the insufflation catheters were positioned 0.5, 2.0, 3.5, and 5.0 cm from the tracheal carina. Flow rates were varied between 10 and 55 l/min and steady-state arterial blood gases were measured. ⋯ Arterial O2 pressure (PaO2) was relatively constant at all flow rates and catheter positions. We conclude that, up to a point, CO2 elimination can be improved by positioning the catheters further into the lung; advancing the catheters further than 3.5 cm from the carina may cause over-ventilation of specific lung regions resulting in a relative plateau in CO2 elimination and relatively constant PaO2's. Positioning the catheters further into the lung permits the use of lower flow rates, thus potentially minimizing the risk of barotrauma.
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The esophageal balloon technique for measuring pleural surface pressure (Ppl) has recently been shown to be valid in recumbent positions. Questions remain regarding its validity at lung volumes higher and lower than normally observed in upright and horizontal postures, respectively. We therefore evaluated it further in 10 normal subjects, seated and supine, by measuring the ratio of esophageal to mouth pressure changes (delta Pes/delta Pm) during Mueller, Valsalva, and occlusion test maneuvers at FRC, 20, 40, 60, and 80% VC with the balloon placed 5, 10, and 15 cm above the cardia. ⋯ Qualitatively, observed delta Pes/delta Pm fit predicted corresponding values (based on lung and upper airway compliances). Quantitatively there were discrepancies probably due to lack of measurement of esophageal elastance and to inhomogeneities in delta Ppl. At every lung volume in both postures, there was at least one esophageal site where delta Pes/delta Pm was within 10% of unity.
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Constant-flow ventilation (CFV) is a ventilatory technique in which physiological blood gases can be maintained in dogs by a constant flow of fresh gas introduced via two catheters placed in the main-stem bronchi (J. Appl. Physiol. 53: 483-489, 1982). ⋯ In three other pigs, the rates of rise of PaCO2 were compared with the rates observed with apneic oxygenation (AO). At the maximum flow and deepest position, the rate of rise of PaCO2 was lower during CFV than during AO. These data suggest that flow through collateral channels might be important in producing adequate gas transport during CFV; however, other factors such as airway morphometry and the effects of cardiogenic oscillations may explain the differences between the results in pigs and dogs.
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Comparative Study
Effects of digoxin on diaphragmatic strength generation.
Contrary to hindlimb muscle, extracellular calcium plays an important role in diaphragmatic strength generation (J. Appl. Physiol. 58: 2054-61, 1985). ⋯ All measurements were performed during control and 30, 60, 90, and 120 min after digoxin administration. In group A, digoxin plasmatic level at 60 min reached a therapeutic range in all dogs (1.8 +/- 0.3 ng/ml), whereas in group B, digoxin plasmatic level was higher (8 +/- 1.3 ng/ml). No significant change in cardiac output and Qdi was noted after administration of digoxin, either in the dogs of group A or those of group B.(ABSTRACT TRUNCATED AT 250 WORDS)
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Carotid body-denervated (CBD) ponies have a less than normal increase in arterial PCO2 (PaCO2) when inspired CO2 (PICO2) is increased, even when pulmonary ventilation (VE) and breathing frequency (f) are normal. We studied six tracheostomized ponies to determine whether this change 1) might be due to increased alveolar ventilation (VA) secondary to a reduction in upper airway dead space (VD) or 2) is dependent on an upper airway sensory mechanism. Three normal and three chronic CBD ponies were studied while they were breathing room air and at 14, 28, and 42 Torr PICO2. ⋯ At each elevated PICO2 during both NBr and TBr, the increase in PaCO2 above control was always less in CBD ponies than in normal ponies (P less than 0.01). The VE-PaCO2, f-PaCO2, and tidal volume-PaCO2 relationships did not differ between NBr and TBr (P greater than 0.10) nor did they differ between normal and CBD ponies (P greater than 0.10). We conclude that the attenuated increase in PaCO2 during CO2 inhalation after CBD is not due to a relative increase in VA secondary to reducing upper airway VD.(ABSTRACT TRUNCATED AT 250 WORDS)