This chapter is most relevant to Section F6(ix) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "outline the methods used to measure ventilation-perfusion mismatch". Though V/Q matching has been explored in multiple past papers, the methods of measuring it have never appeared, and one might be forgiven for skipping this topic entirely. The salient points are included in the grey box; everything else is essentially SEO-killing ballast.
In summary:
- For measurement of V/Q distribution, there are functional techniques and imaging techniques
- Functional techniques include MIGET and the three-compartment model.
- MIGET (Multuiple Inert Gas Elimination Technique) uses six dissolved gases infused intravenously
- The arteriovenous difference in gas concentration and the known blood:gas partion coefficient of each gas is used to determine the distribution of V/Q
- The three-compartment model assumes there are only three gase exchange units: one completely composed of shunt, one completely composed of dead space, and one which has
- This technique only requires the measurement of PaO2, PaCO2 and the estimation of alveolar O2 and CO2 partial pressures
- Imaging techniques include
- Radionuclide imaging:
- SPECT V/Q scans
- PET scans
- MRI using IV gadolinium and 3He or 129Xe
There is no single best article to give as the "one true reference" for this issue. For the MIGET method, the best article would probably be the retrospective by its creator (Wagner, 2008) but one would have to pay Springer 35 Euros for it. The Riley three-compartment model is well described by Riley and Cournard (1949. For different modalities of tomographic and radionuclide V/Q imaging, one really needs to scour the internet and look for individual articles, a substantial effort which will probably go unrewarded by primary exam marks.
Wagner Saltzman and West (1974) described this technique for the first time, and then published dozens of papers on the results of using it in different circumstances, producing countless graphs with which junior anaesthesia and ICU trainees have subsequently been tortured. Wagner wrote a review of this same technique in 2008, from the elevated vantage point of an extremely long and illustrious career. It is called MIGET, the Multiple Inert Gas Elimination Technique. For the purposes of understanding V/Q mismatch and even for the purpose of the CICM Part One exam (well known for its cruelty) it is not essential to know this in any great detail. It will suffice to describe it in the following point-form way:
Using this technique, Wagner and coworkers were able to measure all of the curves described in the chapter on the effects of V/Q mismatch on gas exchange.
It is named after Riley because Riley and Cournard (1949) were the first to publish and popularise this approach, even though it probably originated with Fenn Rahh and Otis in 1946. In essence, its genius rests in being able to take the bell curve of V/Q scatter and average it in a way that the lung is seen to consist of only three units:
Because gas exchange can only occur in the ideally matched unit, all changes in arterial and alveolar gas mixtures are due to events taking place in this unit. Thus, the only measurement you really need to make is alveolar O2, alveolar CO2, arterial O2 and arterial CO2. From these, it is possible to determine:
Shunt and dead space are discussed in greater detail elsewhere.
Various useful imaging techniques can be used to determine V/Q ratio. The basic premise here appears to be the use of tracer. It is relatively easy to determine the perfusion of the lung (just give them some sort of IV tracer) and the main challenge appears to be finding some way of imaging the gas inside the lungs while still allowing the patient to breathe normally and comfortably (i.e. ideally the presence of that gas should still allow enough air into the lungs to permit normal gas exchange).
These methods are mentioned in Nunn's (Ch.8, p. 129 of the 8th edition) and are therefore fair game for the examiners:
Wagner, Peter D., H. A. Saltzman, and J. B. West. "Measurement of continuous distributions of ventilation-perfusion ratios: theory." Journal of applied physiology 36.5 (1974): 588-599.
Wagner, Peter D. "The multiple inert gas elimination technique (MIGET)." Intensive care medicine 34.6 (2008): 994.
Riley, R. L., and A. Cournand. "‘Ideal’alveolar air and the analysis of ventilation-perfusion relationships in the lungs." Journal of Applied Physiology 1.12 (1949): 825-847.
Fenn, Wallace O., Hermann Rahn, and Arthur B. Otis. "A theoretical study of the composition of the alveolar air at altitude." American Journal of Physiology-Legacy Content 146.5 (1946): 637-653.
Freeman, Leonard M. "Don't bury the V/Q scan: it's as good as multidetector CT angiograms with a lot less radiation exposure." Journal of Nuclear Medicine 49.1 (2008): 5-8.
Bajc, Marika, et al. "EANM guidelines for ventilation/perfusion scintigraphy." European journal of nuclear medicine and molecular imaging 36.8 (2009): 1356-1370.
Melo, Marcos F. Vidal, et al. "Quantification of regional ventilation-perfusion ratios with PET." Journal of Nuclear Medicine 44.12 (2003): 1982-1991.