Where is physiological dead space

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Your current browser may not support copying via this button. Subscriber sign in You could not be signed in, please check and try again. Weirdly, other authoritative sources eg. Presumably, because these people got published in Circulation , some senior peer reviewer somewhere agreed that this is a well-established definition for these terms.

In summary, there does not appear to be consensus on what alveolar dead space is, or how to represent it in a human-readable sentence. From the abovestated gibberish, fragments of reason can be extracted to make memorable soundbites for the vivas:.

The presence of a large enough shunt can give rise to the appearance of increased physiological dead space, provided one uses the Enghoff definition of dead space. In this thought experiment, the dead space fraction of the tidal volume is 7. However, one should note that in this one-alveolus model, there are no unperfused lung units.

Thus, where shunt is substantial, it can create the illusion of alveolar dead space because it increases the difference between the alveolar and arterial CO 2. The reader is warned: this is only an illusion, resulting from a limitation of the Bohr-Enghoff equation.

There's no unperfused alveoli in there. It does, however, behave very much like dead space, in the sense that increasing the ventilation will have little additional effect on decreasing the PaCO 2.

For this scenario to occur, the shunt has got to be pretty big in the scenario above, one third of all cardiac output is going through the shunt. In real life, shunts like this are occasionally seen in the setting of ARDS. The effect of airway equipment on changing the dead space is discussed in greater detail in the chapter on the effects of positive pressure ventilation.

In summary, the main reason for the change is that under some circumstances the aforementioned equipment will either decrease the anatomical dead space by bypassing the upper airway structures, or add to it by adding extra volume in the form of circuit components. Just as a reader rightly wonder how many more dead spaces they could possibly tolerate, this chapter confronts them with several more. References to these are seen throughout the literature, and there is probably some merit in discussing them.

In short, they are named after researchers who first demonstrated some novel way of measuring dead space, and the volumes they describe are dead space as measured by the eponymous method. As such, each dead space has some boundaries and inaccuracies which mirror the limitations of their specific measurement technique, discussed in greater detail in the chapter on dead space measurement.

Here, a brief summary will suffice:. Fowler, Ward S. The respiratory dead space. Postural changes in respiratory dead space and functional residual capacity. Respiratory dead space in old age and in pulmonary emphysema. Bohr, Christian. Haldane, John. Drummond, G. Astrom, E. Klocke, Robert A. Wagner, Peter D. Hedenstierna, G. A meaningful variable?. Numa, Andrew H. Gogniat, Emiliano, et al. Riley, R. Rea, H. Brewer, Lara M. Orr, and Nathan L. Nunn, J. Campbell, and B.

Kerr, A. Johns, David P. Thin, A. Steenblock, U. The last stages of the exhalation will be mainly the contents of gas-exchanging alveoli, and will therefore be relatively rich in nitrogen and stable in its concentration. This plateau is Fowler's Phase 3. In the middle, there is a phase where the exhaled nitrogen concentration rises rapidly, and this represents gas from the fast time-constant alveoli mixing with gas from the more distal airways. If one waits any longer, one can also see a Phase IV, where there is another increase in nitrogen concentration - this represents the closing capacity , as small airways in the more compliant regions of the lung close, and only poorly compliant alveoli continue to exhale their nitrogen-rich gas without it being diluted, its concentration rises.

However this has nothing to do with the measurement of dead space and it was not a part of Fowler's original experiment. Now, the nitrogen concentration over time curve is useful for explaining what happens during the procedure, but it is not useful for determining the dead space volume. In a viva scenario, it would be better to draw the curve of nitrogen concentration over volume, as below:.

Thus, one can measure the volume of the anatomical dead space. The initial stages where no nitrogen is coming out is definitely coming from the conducting airways. For the rapid upslope of nitrogen, the situation is somewhat more ambiguous, as some airway oxygen is now mixing with some alveolar nitrogen. To get some sort of cut-off point to call the end of dead space and the beginning of alveolar gas, Fowler decided to use a rough estimate it was "estimated visually with the help of a squared transparent ruler".

The midpoint of that rising nitrogen slope was selected by comparing two areas, above and below the curve. Fowler validated this technique by creating numerous artificial rubber dead spaces of known volume and then demonstrating that this visual estimation method is able to accurately predict that known volume. That is in fact what is generally done. The direct measurement of the alveolar dead space has been elusive, in the sense that we do not have a good technique which determines only this volume.

Fowler, Ward S. The respiratory dead space. Bohr, Christian. Haldane, John. Astrom, E. Klocke, Robert A. Wagner, Peter D.

Hedenstierna, G.