The effects of body position and respiratory frequency (f) on regional gas transport during eucapnic conventional ventilation (CV) and high-frequency ventilation (HFV) were assessed from the washout of nitrogen 13 (13NN) using positron-emission tomography. In one protocol, six dogs were ventilated with CV or HFV at f = 6 Hz and tidal volume (VT) selected supine for eucapnia. A coronal cross section of the lung base was studied in the supine, prone, and right and left lateral decubitus positions. In a second protocol, six dogs were studied prone: apical and basal cross sections were studied in CV and in HFV with f = 3 and 9 Hz at eucapnic VT. Regional alveolar ventilation per unit of lung volume (spVr) was calculated for selected regions and analyzed for gravity-dependent cephalocaudal and right-to-left gradients. In both CV and HFV, nonuniformity in spVr was highest supine and lowest prone. In CV there were vertical gradients of spVr in all body positions: nondependent less ventilated than dependent regions, particularly in the supine position. In HFV there was a moderate vertical gradient in spVr in addition to a preferentially ventilated central region in all body positions. Overall lung spV was unaffected by body position in CV but in HFV was highest supine and lowest prone. Nonuniformity in eucapnic prone HFV was unaffected by f and always higher than in CV.(ABSTRACT TRUNCATED AT 250 WORDS)
The effects of changing tidal volume (VT) and frequency (f) on the distribution of ventilation during high-frequency ventilation (HFV) were assessed from the washout of nitrogen-13 by positron emission tomography. Six dogs, anesthetized and paralyzed, were studied in the supine position during conventional ventilation (CV) and during HFV at f of 3, 6, and 9 Hz. In CV and HFV at 6 Hz, VT was selected to achieve eucapnic arterial partial pressure of CO2 (37 +/- 3 Torr). At 3 and 9 Hz, VT was proportionally changed so that the product of VT and f remained constant and equal to that at 6 Hz. Mean residence time (MRT) of nitrogen-13 during washout was calculated for apical, midheart, and basal transverse sections of the lung and further analyzed for gravity-dependent, cephalocaudal and radial gradients. An index of local alveolar ventilation per unit of lung volume, or specific ventilation (spV), was calculated as the reciprocal of MRT. During CV vertical gradients of regional spV were seen in all sections with ventral (nondependent) regions less ventilated than dorsal (dependent) regions. Regional nonuniformity in gas transport was greatest for HFV at 3 and 6 Hz and lowest at 9 Hz and during CV. During HFV, a central region at the base of the lungs was preferentially ventilated, resulting in a regional time-averaged tracer concentration equivalent to that of the main bronchi. Because the main bronchi were certainly receiving fresh gas, the presence of this preferentially ventilated area, whose ventilation increased with VT, strongly supports the hypothesis that direct convection of fresh gas is an important mechanism of gas transport during eucapnic HFV. Aside from the local effect of increasing overall lung ventilation, this central area probably served as an intermediate shuttle station for the transport of gas between mouth and deeper alveoli when VT was less than the anatomic dead space.
The volume of lung at residual volume (RV) which fails to receive an inhaled tracer bolus (EXV) was quantitated in 13 normals by comparison of a scintigram of the distribution of a tracer bolus inhaled from RV (BORV) with a scintigram at RV of lung equilibrated with the tracer (EQRV). EXV was found in the dependent lung in the erect position in all subjects but also occurred to a lesser degree at the apex in 11 of 13 subjects. Basal EXV ranged from 1 to 7% of TLC, and unlike apical EXV increased with age (r= 0.91, P less than 0.01). EXV in the decubitus position shifted largely to the dependent lung with none remaining in the original erect apical and basal locations, demonstrating that gravity determined EXV location. Nitrous oxide, which is highly diffusible, failed in four subjects to carry the tracer to basal EXV even though perfusion was documented to persist to this area, implying basal EXV airways were closed, not narrowed. In one of the four subjects apical EXV was readily definable. Nitrous oxide carried tracer into this region, implying patent apical EXV airways.
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