This chapter is most relevant to Section F6(i) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "describe the concepts of global and regional ventilation and perfusion and the factors that affect these". No more detail is offered with regard to what exactly this means, and there are separate syllabus items which cover West's Zones, V/Q matching and various shunts and dead spaces, so one may be confident that "the concepts of global and regional ventilation and perfusion" don't cover any of that. The precise meaning is hard to tease out, because that specific phrase appears to be unique to the CICM syllabus and is not found anywhere else in the whole Interweb. If one were unwilling to descend into unfairly critical antiestablishment polemic, one would prefer to believe that the syllabus-writers were earnestly intending this item to cover all the foundation concepts required to understand the more specific topics like Wests' Zones. It is with this assumption that this chapter is constructed.
In summary:
- Global ventilation of the lungs is expressed as the minute volume , normally around 4L/min
- This is affected by multiple factors, most notably pregnancy, PaCO2, PaO2, pH, body temperature, exercise and blood pressure
- Global perfusion of the lungs is directly proportional to the cardiac output (normally 5L/min)
- Therefore, this is affected by all the factors which affect cardiac output, which include exercise, metabolic rate, volume-sensing reflexes, autonomic tone, etc.
- The global perfusion of the lungs is approximately 5L/min at rest
- Global V/Q mismatch occurs when:
- there is signficantly reduced ventilation with intact perfusion (shunt)
- there is reduced perfusion (increased physiological dead space)
- Regional differences in perfusion and ventilation develop because:
- The global perfusion of the lungs occurs at a low pressure, which means that the hydrostatic pressure of the column of blood blood therefore has a significant influence.
- Lung ventilation occurs predominantly because of the changes in the shape of the thoracic cavity which occur unevenly (i.e. the base expands more than the apex)
- Regional changes in pulmonary arterial resistance (eg. due to atelectasis and hypoxic vasoconstriction) change the distribution of blood flow in response to the distribution of ventilation
- Regional differences in perfusion and ventilation are affected by:
- Posture and gravity (which affects the pressure in the hydrostatic column)
- Factors which affect regional pulmonary blood flow:
- Lung volume (atelectasis increases pulmonary vascular resistance)
- Hypoxic pulmonary vasoconstriction
- Gravity (affects the direction of the hydrostatic gradient)
- Pulmonary vascular architecture (some lung units are structurally advantaged)
- Factors which affect regional ventilation:
- Gravity (the weight of the lung) which produces a vertical gradient in pleural pressure
- Posture, which changes the direction of this vertical gradient
- Anatomical expansion ptential (i.e. bases have more room to expand than apices)
- Lung compliance (more compliant lung regions, eg. lung bases, will be better ventilated at any given traspulmonary pressure
- Pattern of breathing
There is no single resource which covers this topic, but if one had to pick a single best article, at least for factors affecting regional ventilation/perfusion relationships (which is usually where the marks are), one would probably go with Galvin et al (2007).
In absence of any real scientific definition for this term, one could make something up; for example it would be relatively easy to say that global lung ventilation is the total volume of gas which passes through the lung per unit time. Though one could theoretically pick some random arbitrary volume units and time interval, the conventional way of expressing global lung ventilation is as the minute volume. Presumably for the purpose of making subsequent calculations neater, Nunn's gives 4L/min as the normal average minute volume, which corresponds to a tidal volume of 400ml with a respiratory rate of 10.
There are substantial differences in the rate of air supply to different parts of the lung. This unequal distribution of ventilation is the consequence of several factors:
Again, there is no credible scientific definition for "global lung perfusion" because it is probably completely unnecessary as a concept. If one had to invent a meaning for it, one could loosely define it as the total amount of blood circulating through the lung per unit time. Because the lungs receive a blood flow equal to the total volume of the blood pumped by the heart, it is convenient to express this as the cardiac output, in L/min. This global perfusion is split between the two lungs approximately equally, with some trivial variation. Cheng et al (2005) measured pulmonary blood flow in normal subjects using MRI and found the flow split 45%-55% at the most, with the right pulmonary artery getting a slightly larger amount of blood.
The pulmonary circulation is (vaguely) regulated, in the sense that individual vessels have some degree of control over their diameter and therefore over their resistance. The changes in pulmonary vascular resistance is affected by a large number of factors, such as blood flow, lung volume, alveolar oxygenation, as well as various paracrine hormonal and metabolic factors. One can imagine that it would be extremely unlikely for these factors to be acting on the whole lung in a totally homogeneous fashion, and therefore different lung regions will have different factors affecting their vascular resistance, changing the regional distribution of blood flow.
In summary, regional blood flow is affected by the following factors:
Having now discussed that the perfusion and ventilation differs across a lung, and that the differences are most pronounced between the bases and apices in an upright lung, we are now ready to represent this concept in the form of a graph:
Some version of this famous graph is repeated in virtually every physiology textbook and is usually attributed to West's textbook (1977). In actual fact, that is not where it is originally from. The ventilation slope probably dates back to Ball et al (1962) who measured the clearance of radioactive Xe133 from pulmonary blood at different levels in the chest, and from West & Dollery (1960), who did something very similar with radioactive CO2 (that's where that "3rd rib" thing seems to come from).
From the original graphs in these papers (shamelessly misappropriated here), one can see that the perfusion slope is much steeper than the ventilation slope. The graph known from modern textbooks seems to have been concocted from these data by West in his 1968 review for the Postgraduate Medical Journal. To represent the changing relationship of ventilation and perfusion along the span of the upright lung, West had graphed the ratio of these parameters on the same coordinate field.
Uren, Neal G., et al. "Reduction of mismatch of global ventilation and perfusion on exercise is related to exercise capacity in chronic heart failure." Heart 70.3 (1993): 241-246.
Cheng, Christopher P., et al. "Proximal pulmonary artery blood flow characteristics in healthy subjects measured in an upright posture using MRI: the effects of exercise and age." Journal of Magnetic Resonance Imaging: An Official Journal of the International Society for Magnetic Resonance in Medicine 21.6 (2005): 752-758.
Galvin, I., G. B. Drummond, and M. Nirmalan. "Distribution of blood flow and ventilation in the lung: gravity is not the only factor." British journal of anaesthesia 98.4 (2007): 420-428.
Roussos, CHARALAMBOS S., et al. "Influence of diaphragmatic contraction on ventilation distribution in horizontal man." Journal of applied physiology 40.3 (1976): 417-424.
Sampson, MICHAEL G., and GERALD C. Smaldone. "Voluntary induced alterations in regional ventilation in normal humans." Journal of Applied Physiology 56.1 (1984): 196-201.
Shykoff, Barbara E., Albertus Van Grondelle, and H. K. Chang. "Effects of unequal pressure swings and different waveforms on distribution of ventilation: a non-linear model simulation." Respiration physiology 48.1 (1982): 157-168.
Glenny, Robb W., Steven McKinney, and H. Thomas Robertson. "Spatial pattern of pulmonary blood flow distribution is stable over days." Journal of Applied Physiology 82.3 (1997): 902-907.
Shykoff, Barbara E., Albertus Van Grondelle, and H. K. Chang. "Effects of unequal pressure swings and different waveforms on distribution of ventilation: a non-linear model simulation." Respiration physiology 48.1 (1982): 157-168.
Wagner, Peter D., et al. "Continuous distributions of ventilation-perfusion ratios in normal subjects breathing air and 100% O 2." The Journal of clinical investigation 54.1 (1974): 54-68.
Wagner, Peter D. "Ventilation-perfusion relationships." Annual review of physiology 42.1 (1980): 235-247.
West, John B. "Ventilation-perfusion relationships." American review of respiratory disease 116.5 (1977): 919-943.
Wagner, P. D., et al. "Ventilation-perfusion inequality in chronic obstructive pulmonary disease." The Journal of clinical investigation 59.2 (1977): 203-216.
West, J. B. "Regional differences in the lung." Postgraduate medical journal 44.507 (1968): 120.
Ball, W. C. J., et al. "Regional pulmonary function studied with xenon 133." The Journal of clinical investigation 41.3 (1962): 519-531.
West, J. B., and C. T. Dollery. "Distribution of blood flow and ventilation-perfusion ratio in the lung, measured with radioactive CO2." Journal of Applied Physiology 15.3 (1960): 405-410.