Mechanisms of ventilation inhomogeneity during vital capacity breaths standing and supine

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Abstract

Overall inhomogeneity of ventilation distribution, as measured by single-breath vital capacity (VC) washout (SBW) is known to be greater supine vs. standing. To establish the underlying mechanisms 13 healthy males performed VC SBW of 4% SF6 and He, standing and supine, with or without a 10 sec breathhold (BH). Overall inhomogeneity, as indicated by normalized phase III slopes, was >50% greater supine (SF6 13.1×10−3; He 10.7×10−3 L−1) than standing (SF6 8.6×10−3; He 6.4×10−3 L−1; P<0.001). The (SF6–He) slope, an index of intraacinar inhomogeneity, did not change with posture. Breathholding, assumed to eliminate convective dependent inhomogeneity within and/or between small lung units, produced twice as great reduction of inhomogeneity when supine vs. standing. After BH inhomogeneity remained significantly greater supine vs. standing. In conclusion, at least two events seem to underlie the increased inhomogeneity when supine: (1) a substantially increased convection dependent non-uniformity between well-separated lung regions; and (2) a somewhat increased convection dependent non-uniformity within and/or between peripherally located lung units.

Introduction

Gravity influences regional static lung volumes and ventilation distribution significantly in man in all body positions (Amis et al., 1984, Kaneko et al., 1966, Rehder et al., 1977). Lung volumes are larger in non-dependent than in dependent regions, whilst ventilation is greater in dependent regions when breathing at functional residual capacity (Rehder et al., 1977). Because the gravity determined pleural pressure gradient, which to a large extent governs regional lung expansion and ventilation (Agostoni, 1986), is lesser in the supine than in the erect posture overall ventilation distribution would be expected to be more homogeneous when supine vs. upright. Nevertheless, overall ventilation distribution is in fact more inhomogeneous in the supine than in the erect posture, when assessed from the alveolar (phase III) slope generated by vital capacity (VC) single-breath washout (SBW) of an inert tracer gas (Cortese et al., 1976). Furthermore, experiments performed in transient or sustained weightlessness (microgravity; μG) indicate that the ventilation inhomogeneity caused by gravity, presumably secondary to inhomogeneity between widely separated lung regions, contributes only approximately 25–30% of the phase III slope measured (Guy et al., 1994, Michels and West, 1978). These observations would suggest that non-gravitational mechanisms may be responsible for the increased inhomogeneity found in the supine position.

In accordance with the Paiva–Engel theories of diffusive gas mixing in the lung (Paiva et al., 1988, Paiva and Engel, 1989), VC SBW tests using inert tracer gases with widely differing diffusivity allow assessment of inhomogeneities within the acinar region (intraacinar inhomogeneity). Surprisingly, evidence for a gravitational effect on such peripheral inhomogeneities has emerged from SF6 and He VC SBW studies performed in sustained μG (Prisk et al., 1996). In these experiments, undertaken in three males and one woman, overall ventilation inhomogeneity as indicated by the SF6 or He phase III slopes decreased significantly in weightlessness (Prisk et al., 1996). Furthermore, the positive (SF6–He) phase III slope was abolished in μG suggesting modified intraacinar gas mixing. After a 10 sec post-inspiratory breathhold (BH) in weightlessness, allowing diffusion to reduce convective dependent gas concentration differences within and/or between small lung units, the (SF6–He) phase III slope was even reversed (Prisk et al., 1996). In that paper Prisk and co-workers also presented some VC SBW results obtained in the standing and supine positions. A small numerical increase of the (SF6–He) phase III slope supine vs. standing was shown, but the results were not discussed. While the SF6, He and N2 phase III slopes in the standing position and in μG before and after BH were reported, no results from the recordings in the supine position were given. The study by Prisk et al. (1996) did not primarily aim at assessing the influence of body position on gas mixing between and/or within lung regions and included only four test subjects (Prisk et al., 1996). Their observations intrigued us as to the possible effects of the supine position on ventilation inhomogeneity within small lung units. We therefore designed this study to assess the gas distribution standing and supine in healthy male subjects using similar procedures.

The aim of the present study was to determine the influence of body position on inhomogeneity of ventilation distribution between widely separated lung regions as well as between and/or within small lung units when VC breaths are taken. Comparisons of simultaneously obtained SF6 and He phase III slopes allow diffusion dependent inhomogeneity to be separated from convection-dependent inhomogeneity (cdi), which influences the distribution of both gases to the same degree (Piiper and Scheid, 1987, van Muylem et al., 1992). BH over 10 sec following inspiration is assumed to allow continued diffusion to eliminate most if not all inhomogeneity within and among groups of acini (Crawford et al., 1986, Paiva et al., 1984, Paiva et al., 1988), functionally important units when considering the gas exchange properties of the lung. The degree of the remaining convection dependent inhomogeneity between well-separated lung regions can then be compared to the results after the no-BH maneuver in the two body positions. In the latter situation all mechanisms of ventilation inhomogeneity are present (Paiva and Engel, 1989). Our study presents evidence of a moderate increase of convection dependent inhomogeneity between and/or within small lung units (intraregional cdi) supine vs. standing in addition to a large increase of the inhomogeneity between widely separated lung regions (interregional cdi) when VC breaths are taken.

Section snippets

Subjects

The study was approved by the Ethics Committee for Human Research at the University of Gothenburg, Sweden.

Thirteen normal male non-smokers with normal dynamic and static lung volumes, as measured in a body plethysmograph (Jaeger Masterscreen; Erich Jaeger GmbH, Würzburg, Germany), were engaged in the study (Table 1).

Test equipment

The test apparatus allowed administration of two different gas mixtures and recording of inspired and expired gas concentrations and flows at the mouth. It consisted of a mouthpiece

Results

Expiratory VC was 5% greater standing than supine with or without BH (P<0.05; Table 2). The inspiratory and expiratory flow rates did not differ standing vs. supine, with or without BH. End-inspiratory pause time was not significantly different standing vs. supine with or without BH.

Phase III slopes were significantly greater supine than standing both without or with BH for SF6 (P<0.001) and for He (P<0.001) (Fig. 2). The (SF6–He) slopes did not differ significantly between body positions

Discussion

Body position has previously been shown to influence ventilation distribution in the human lung (Amis et al., 1984, Cortese et al., 1976, Kaneko et al., 1966, Milic-Emili et al., 1966) due to gravitational and non-gravitational factors (Engel, 1986). More than two decades ago Cortese et al. demonstrated that overall ventilation inhomogeneity is greater supine than standing (Cortese et al., 1976), but the methods utilized (VC N2 SBW) did not allow for the responsible mechanisms to be identified.

Conclusions

The results of this study support those of previous investigations where non-uniformity of ventilation distribution was found to be greater supine than standing when VC breaths are taken. Our study conveys evidence that inhomogeneity that exists between joined lung regions that cannot mix by diffusion is the major contributor to overall inhomogeneity in both body positions during VC breathing, whereas non-uniformities within acinar regions are minor components. The study shows that at least two

Acknowledgements

We express our gratitude to Mr. Eddie Bergsten for excellent technical assistance.

References (33)

  • A.B.H. Crawford et al.

    Convection- and diffusion-dependent ventilation maldistribution in normal subjects

    J. Appl. Physiol.

    (1985)
  • A.B.H. Crawford et al.

    Effect of breath holding on ventilation maldistribution during tidal breathing in normal subjects

    J. Appl. Physiol.

    (1986)
  • G.O. Dahlbäck

    Influence of intrathoracic blood pooling on pulmonary air-trapping during immersion

    Undersea Biomed. Eng.

    (1975)
  • G.O. Dahlbäck et al.

    Influence of hydrostatic compression of the chest and intrathoracic blood pooling on static lung mechanics during head-out immersion

    Undersea Biomed. Eng.

    (1978)
  • L.A. Engel

    Gas mixing within the acinus of the lung

    J. Appl. Physiol.

    (1983)
  • L.A. Engel

    Intraregional gas mixing and distribution

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