Thanks for that- it might be helpful to put the reference on the site that you have used in relaying this information. I think it is important to point out that the majority of what we know about ventilation and perfusion distribution is derived from animal studies and studies of normal lungs. Not a lot of research has been carried out looking at diseased lungs and situations where there are secretions resulting in hypoventilation.

The point I was making about postural drainage is that the MAIN AIM of this technique is the drainage of secretions. It is fortunate that in the spontaneously breathing patient you have the added advantage of preferentially ventilating the unaffected lung (when secretions are unilaterally distributed). This obviously needs to be taken into consideration when the patient is ventilated and the affected lung is placed uppermost for drainage of secretions as air will go into the affected lung preferentially and a degree of desaturation may be observed.

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The non-dependent lung, in the case of a patient with widespread secretions/infection etc would be breathing at a lower tidal volume. In this case I thin the ventilation is preferentially distributed to the non-dependent lung?

Regardless of whether or not there are secretions in the non- dependent lung by nature of the definition the non- dependent lung (in a spontaneously breathing person) is less preferentially ventilated than the dependent lung. So I am not quite sure of the point you are trying to make here?

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Also I read that "in the anaesthetised patient, however, irrespective of the mode of ventilation, the upper lung receives more gas flow." So one might actually have to consider the implications of the definition as it relates to the actual patient situation.

I think this statement supports my previous statement that ventilation in the ventilated person goes preferentially to the non- dependent lung (ie the uppermost lung).



I agree that we tend to take simplistic view of what is happening with ventilation and perfusion distribution in abnormal states but believe that is because we do not have the evidence to tell us. If you have any useful references of the effect of hypoventilation on V/ Q distribution I would be interested in reading them.