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Published ahead of print on January 7, 2005, doi:10.1164/rccm.200409-1296OC

Am. J. Respir. Crit. Care Med., Volume 171, Number 7, April 2005, 714-721

A more recent version of this article appeared on April 1, 2005
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Submitted on October 4, 2004
Accepted on December 10, 2004

Topographic Basis of Bimodal Ventilation-Perfusion Distributions during Bronchoconstriction in Sheep

Marcos F Vidal Melo1*, R. Scott Harris2, J. Dominick H Layfield1, and Jose G Venegas1

1 Anesthesia and Critical Care, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA, 2 Internal Medicine, Pulmonary and Critical Care Unit, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA

* To whom correspondence should be addressed. E-mail: mvidalmelo{at}partners.org.

The distribution of ventilation-perfusion (VA/Q) ratios during bronchoconstriction measured with the Multiple Inert Gas Elimination Technique is frequently bimodal. However, the topographic basis and the cause of that bimodality remain unknown. In this paper, regional VA/Q is quantified by 3-dimensional positron emission tomography (PET) imaging of methacholine bronchoconstricted sheep. Regional VA/Q ratios were calculated from the imaged kinetics of IV injected 13NN-saline bolus, assembled into global VA/Q distributions, and used to estimate gas exchange. During bronchoconstriction, large regions with impaired tracer washout were observed adjacent to regions of normal ventilation. PET derived VA/Q distributions during bronchoconstriction were consistently bimodal with areas of low VA/Q receiving a large fraction of Q. The standard deviation of the VA/Q distribution was 38% lower if small-scale (sub-resolution) heterogeneity (<2.2 cm3 ) was ignored. Arterial blood gases predicted from PET data correlated well with measured values for PaO2 (r2=0.91, p<0.01) and PaCO2 (r2=0.90, p<0.01). We conclude that the bimodality of VA/Q distributions in bronchoconstriction reflects the involvement of large contiguous regions of hypoventilation with substantial sub-resolution intra-regional VA/Q heterogeneity. Assessment of the sub-resolution VA/Q heterogeneity is therefore essential to accurately quantify global gas exchange impairment during bronchoconstriction.
Key words: pulmonary gas exchange, lung imaging, positron emission tomography, theoretical models.




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