Regional gas distributions and single-breath washout curves in head-down position

Vital capacity single-breath washouts using 90% O2-5% He-5% SF6 as a test gas mixture were performed with subjects sitting on a stool (upright) or recumbent on a stretcher (prone, supine, lateral left, lateral right, with or without rotation at end of inhalation). On the basis of the combinations of supine and prone maneuvers, gravity-dependent contributions to N2 phase III slope and N2 phase IV height in the supine posture were estimated at 18% and 68%, respectively. Whereas both He and SF6 slope decreased from supine to prone, the SF6-He slope difference actually increased (P = 0.015). N2 phase III slopes, phase IV heights, and cardiogenic oscillations were smallest in the prone posture, and we observed similarities between the modifications of He and SF6 slopes from upright to prone and from upright to short-term microgravity. These results suggest that phase III slope is partially due to emptying patterns of small units with different ventilation-to-volume ratios, corresponding to acini or groups of acini. Of all body postures under study, the prone position most reduces the inhomogeneities of ventilation during a vital capacity maneuver at both inter- and intraregional levels.

Section: Discussionsupporting

confidence: 90%

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

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Single-breath washouts in a rotating stretcher

Rodríguez-Nieto

Peces-Barba

Mangado

et al. 2001

“…Interestingly, if RV were reduced as a result of HDT and lowered PILV compared with upright, then Sn III might be expected to increase (36,37). Although we did not measure RV, Demedts et al (11), using 90°HDT, as well as other studies using water immersion (3) and inflation of antigravity suit (3), have not found significant changes in RV. Unlike the other lung volumes (e.g., FRC), RV appears to be the most stable and resistant to change.…”

Section: Discussionmentioning

confidence: 63%

Effect of 60° head-down tilt on peripheral gas mixing in the human lung

Olfert

Prisk

2004

Olfert, I. Mark, and G. Kim Prisk. Effect of 60°head-down tilt on peripheral gas mixing in the human lung. J Appl Physiol 97: 827-834, 2004. First published April 16, 2004 10.1152/ japplphysiol.01379.2003The phase III slope of sulfur hexafluoride (SF 6) in a single-breath washout (SBW) is greater than that of helium (He) under normal gravity (i.e., 1G), thus resulting in a positive SF 6-He slope difference. In microgravity (G), SF6-He slope difference is smaller because of a greater fall in the phase III slope of SF 6 than He. We sought to determine whether increasing thoracic fluid volume using 60°head-down tilt (HDT) in 1G would produce a similar effect to G on phase III slopes of SF 6 and He. Single-breath vital capacity (SBW) and multiple-breath washout (MBW) tests were performed before, during, and 60 min after 1 h of HDT. Compared with baseline (SF 6 1.050 Ϯ 0.182%/l, He 0.670 Ϯ 0.172%/l), the SBW phase III slopes for both SF 6 and He tended to decrease during HDT, reaching nadir at 30 min (SF 6 0.609 Ϯ 0.211%/l, He 0.248 Ϯ 0.138%/l; P ϭ 0.08 and P ϭ 0.06, respectively). In contrast to G, the magnitude of the phase III slope decrease was similar for both SF 6 and He; therefore, no change in SF 6-He slope difference was observed. MBW analysis revealed a decrease in normalized phase III slopes at all time points during HDT, for both SF 6 (P Ͻ 0.01) and He (P Ͻ 0.01). This decrease was due to changes in the acinar, and not the conductive, component of the normalized phase III slope. These findings support the notion that changes in thoracic fluid volume alter ventilation distribution in the lung periphery but also demonstrate that the effect during HDT does not wholly mimic that observed in G.phase III slope; single-breath washout; multiple-breath washout; helium; sulfur hexafluoride NONUNIFORMITY OF PULMONARY ventilation results from inhomogeneities in both convective and diffusive gas transport, sometimes referred to as convective-dependent inhomogeneity (CDI) and diffusion-convection-dependent inhomogeneity (DCDI) (7). CDI effects may be induced by either gravitational mechanisms, such as topographic differences in pulmonary ventilation resulting from regional differences in pleural pressure (2, 27), or nongravitational mechanisms, such as differing mechanical properties (e.g., lung compliance) among regional lung units (7,37). DCDI effects involve a complex interaction between both convection and diffusion and are believed to occur in the branching structure of the lung when convective and diffusive transport mechanisms are similar in magnitude, which in humans is reported to be at or near the level of the entrance of the acinus (32). The effects of DCDI can be thought of as occurring on a small scale, i.e., intraregional, whereas CDI principally occurs between regions of the lung, i.e., interregional. Studies performed in microgravity (G) have shown that a substantial portion (ϳ75%) of ventilatory inhomogeneity (as evidenced by the phase III slope) is due to DCDI effects (20, 38 -40). However, there is...

“…GRAVITY IS AN IMPORTANT CONTRIBUTOR to the distribution of ventilation (V A) and to ventilatory inhomogeneity (1,7,8,18,24). The commonly applied gravitational model of V A distribution predicts that, for a vital capacity (VC) maneuver in an upright subject, there will be preferential V A in the dependent zones of the lungs (1,13,16,19).…”

mentioning

confidence: 99%

Cardiogenic oscillation phase relationships during single-breath tests performed in microgravity

Lauzon

Elliott

Paiva

et al. 1998

We studied the phase relationships of the cardiogenic oscillations in the phase III portion of single-breath washouts (SBW) in normal gravity (1 G) and in sustained microgravity (microG). The SBW consisted of a vital capacity inspiration of 5% He-1.25% sulfurhexafluoride-balance O2, preceded at residual volume by a 150-ml Ar bolus. Pairs of gas signals, all of which still showed cardiogenic oscillations, were cross-correlated, and their phase difference was expressed as an angle. Phase relationships between inspired gases (e.g., He) and resident gas (n2) showed no change from 1 G (211 +/- 9 degrees) to microG (163 +/- 7 degrees). Ar bolus and He were unaltered between 1 G (173 +/- 15 degrees) and microG (211 +/- 25 degrees), showing that airway closure in microG remains in regions of high specific ventilation and suggesting that airway closure results from lung regions reaching low regional volume near residual volume. In contrast, CO2 reversed phase with He between 1 G (332 +/- 6 degrees) and microG (263 +/- 27 degrees), strongly suggesting that, in microG, areas of high ventilation are associated with high ventilation-perfusion ratio (VA/Q). This widening of the range of VA/Q in microG may explain previous measurements (G.K. Prisk, A.R. Elliott, H.J.B. Guy, J.M. Kosonen, and J.B. West J. Appl. Physiol. 79: 1290-1298, 1995) of an overall unaltered range of VA/Q in microG, despite more homogeneous distributions of both ventilation and perfusion.