Distribution of blood flow and ventilation in the lung: gravity is not the only factor
Cambridge, UK
E-mail: drkrishnanmr{at}gmail.com
Editor—I read with interest the review article by Galvin and colleagues1 on the distribution of blood flow and ventilation in the lung. The authors should be congratulated for their extensive review on this important aspect in respiratory physiology. However, I would like to highlight one important statement which I feel needs to be clarified.
The authors state that in the left lateral position, the total blood flow, total ventilation, and V/Q ratio can be less in the dependent lung than in the non-dependent lung (which is against the traditional gravitational model). This concept has been explained well by the authors. This point has been highlighted again in the penultimate paragraph on the implications in left lateral position. However, in the same paragraph, the authors make a contradictory statement and conclude that the V/Q ratio for the whole lung was greater in the left as opposed to the right lateral position. Could the authors kindly clarify this point?
Manchester and Edinburgh, UK
* E-mail: m.nirmalan{at}manchester.ac.uk
Editor—Thank you very much for giving us the opportunity to respond to Dr Melarkode regarding our article.1 The traditional gravitational model would imply that in the lateral position both ventilation and perfusion will be greater in the dependent lung. However, the work by Chang and colleagues2 has demonstrated that this is not necessarily true. In fact, their work showed that in the left lateral position, both blood flow and ventilation were less in the dependent lung than in the non-dependent lung. Furthermore, the gravity-dependent vertical gradient in blood flow was greater in the right lateral than in the left lateral position. This observation was attributed to impaired blood flow in the dependent left lung in the left lateral position. These findings would strongly support the view that there are some underlying structural features (probably the weight of the heart and mediastinum) that impede blood flow and ventilation to the left lung in the left lateral position. The reduction in ventilation to some of the areas in the dependent left lung was sufficient to trigger hypoxic pulmonary vasoconstriction. Since hypoxic pulmonary vasoconstriction is a generalized phenomenon, it affects blood flow to the entire lung. This independent reduction in blood flow would lead to an increase in the ventilation/perfusion ratio (as perfusion is reduced due to hypoxic pulmonary vasoconstriction) as shown clearly by Chang and colleagues.2
References
1 Galvin I, Drummond GB, Nirmalan M. Distribution of blood flow and ventilation in the lung: gravity is not the only factor. Br J Anaesth (2007) 98:420–8.
2 Chang H, Lai-Fook SJ, Domino KB, et al. Spatial distribution of ventilation and perfusion in anaesthetised dogs in lateral postures. J Appl Physiol (2002) 92:745–62.
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