This chapter is most relevant to Section F3(ii) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "explain the ...effect of positioning", presumably where it relates to "the significance of the vertical gradient of pleural pressure". Or it could easily be related to Section F10(viii), "explain the effect of changes in posture on ventilatory function". Though this chapter has ended up in the respiratory mechanics section, regional distribution of blood flow is obviously also affected, and needs to be mentioned in any sweaty viva scenario where the examiner demands you explain the respiratory responses to different body positions.
Given that there are literally infinite possible positions, it would impossible to find data for every possible spatial orientation of the chest, even in such exhaustive experiments as those by Barnas et al (1993) who tested nine positions including "slouch" and "torso twist". Ergo, in the interest of sanity, only four cardinal orthogonal positions will be presented here. Each position influences several different respiratory parameters, and so one might conclude that this sort of data might be presented better in the form of a table. Thus:
Effects of Positioning on Respiratory Mechanics
Upright Supine Lateral Prone Respiratory compliance Highest Slightly decreased Lowest Low Chest wall resistance Lowest Low High Highest Lung
Low Highest Low Lowest FRC Highest Decreased Slightly decreased Slightly decreased
Thus, on going supine from an upright position:
- Compliance will decrease
- Chest wall resistance will increase slightly
- Lung resistance will increase
- FRC will decrease by ~ 30%
Upon turning the patient from supine to prone:
- Compliance will increase
- Chest wall resistance will increase
- Lung resistance will decrease
- Thus, total tissue resistance will remain unchanged
- FRC will increase
- The vertical pleural pressure gradient will decrease
Of the published peer-reviewed resources, the best and most easily accessible is probably Mezidi & Guérin (2018). Unfortunately it does not contain all of the information one might need for this topic; the rest needs to be pieced together from multiple sources.
Barnas et al (1993) recorded lung compliance of several healthy volunteers who allowed themselves to be insufflated forcefully with different (apparently they were specially trained to relax while this was happening). They did not put their subjects prone, so the prone data used here comes from a paper by Pelosi et al (1995):
The hacked y-axis graph fraud is completely intentional here, so one can better discriminate the differences in the bars, but the reader is warned: they are small. At least in healthy people, the highest compliance in this study was 82.6 ml/cm H2O, and the lowest was 73.0, i.e. there was only about 11% difference.
From this data, one can see that
One last important thing needs to be mentioned regarding the improvement in lung compliance which occurs in the prone position. This is a graph comparing prone position and supine position from dog data collected by D'Angelo et al (1970).
One may be able to recall that the vertical distribution of pleural pressure in the upright and (to a lesser extent) supine position creates a situation where the alveoli in the apex of the lung are perpetually overdistended, and the alveoli in the bases are perpetually collapsed. By decreasing this pleural pressure gradient, prone position ventilation has the effect of making the compliance of different lung units more homogeneous. and this reduces ventilator-associated lung injury from alveolar overdistension and cyclic atelectasis.
Barnas et al (1993) also made measurements of airway gas flow, airway pressure, oesophageal pressure and volume displacement in awake healthy subjects. With the abovementioned data in their hands, the investigators were able to calculate the various resistances of the respiratory system in nine different body positions. Their chest wall and lung resistance data were entered into a spreadsheet and presented graphically here, again with the prone data from Pelosi et al (1995):
So, in summary:
Harris (2005) presents an amazing set of diagrams which is reproduced below with only minor alteration (the grainy 1960s scan needed a 2019 makeover with some nice crisp vector art). These data are not directly from Harris, but their origin is obscured by a needlessly complicated quagmire of references.* These graphs illustrate the postural changes in the elastic recoil pressures exerted by the lungs and the chest wall, and they serve to illustrate an important change which occurs with supine position: the decrease in FRC.
As one can see, because of the changes in chest wall pressure (it becomes more positive, mainly because of some contribution from the abdomen), the lung volume at which the total respiratory elastic recoil pressure is zero (i.e the FRC) ends up being lower in the supine position. This has been demonstrated experimentally, by none other than Lumb & Nunn (1991). In fact, they collected a series of spirometry findings which looked at several different postures. Again, the original data were thrown into the Google Sheets cement mixer to generate these bar graphs:
On the basis of these data, all sorts of textbooks have subsequently stated that the FRC decreases from about 2.9 L to about 2.1L on average, when one goes from sitting to supine. In fact the FRC is never as good as it is in the upright position, but at least going prone has the effect of recovering this volume to some minor extent.
The slightly lower prone FRC in this data set conflicts with what was found during experiments by Moreno et al (1960), who found that in the prone position the FRC increases (by about 600ml), mainly because of a larger expiratory reserve volume. However this massive FRC improvement would not be replicated by other groups- eg. by Numa et al (1997) in children, or by Santini et al (2015) in adults - they all agreed that either the FRC does not change or it increases modestly when compared to the supine position. Moreover, Moreno et al did not go on to write the recommended college textbook for the CICM Part One exam, and so the Lumb & Nunn data should be viewed as the having a higher value in terms of marks.