Lung Mechanics: Theory and Practice IV Measuring Lung Mechanics in Mice

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Lung Mechanics Theory and Practice IVMeasuring
Lung Mechanics in Mice

• Jason H.T. Bates, PhD, DSc
• Research Professor of Medicine
• Vermont Lung Center
• University of Vermont College of Medicine

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Measuring lung volume by plethysmography
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Lundblad et al. Ann Biomed Eng 32 1420-1427,
2004.
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Lung mechanics (R and E) depend on

• frequency
• flow
• tidal volume
• mean lung volume
• volume history
• etc.

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These factors are all likely to change during an
intervention, so
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At One Extreme Controlling Everything

• anesthetized and paralyzed (eliminates
  spontaneous effort)
• tracheostomized (eliminates upper airways)
• mechanically ventilated (controls frequency,
  tidal volume, volume history)
• PEEP (controls mean lung volume)

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Forced oscillation technique in mice
The flexiVentTM
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The flexiVent
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The flexiVent can
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The forced oscillation technique in anesthetized,
tracheostomized, paralyzed animals gives great
data, but...
What physiological effects do these unnatural
conditions have?
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At The Other ExtremeControlling Nothing

• Conscious unrestrained animal free to move about
  inside a closed chamber

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Unfortunately
There is currently no completely noninvasive way
to assess lung mechanical function in mice (PenH
notwithstanding). The use and misuse of Penh in
animal models of lung disease. J.H.T. Bates and
21 co-authors. Am J Respir Cell Mol Biol 31
373-374, 2004.
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PenH (T1/T2) x (P1/P2)
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There are two reasons why chamber pressure varies
as a mouse breathes?
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Reason No. 1

• As air is inspired into the lungs it becomes
  heated and humidified to BTPS conditions, and so
  it expands. The amount of expansion is
  proportional to the volume inspired.

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Reason No. 2

• Inspiration requires a negative alveolar
  pressure. This requires expansion of alveolar gas
  by an amount proportional to flow.

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Only reason no. 2 pertains to lung mechanics.
Reason no. 1 reflects the pattern of breathing.
We can eliminate reason no. 2 by conditioning the
gas in the plethysmograph chamber before it is
inspired by the animal. (Lundblad et al. J Appl
Physiol 93 1198-1207, 2002)
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Pb(t)
Time
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If accurately tracking changes in mean lung
volume is a problem
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UnrestrainedVideoAssistedPlethysmography
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Vertical view
Tracking changes in lung volume
Horizontal view
Equivalent cuboidal mouse
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Latex mouse
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Real dead mouse
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live mouse
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The first measurements of Raw in a conscious,
unrestrained mouse
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UVAP is a work in progress.
We hope UVAP will eventually become a
user-friendly tool allowing researchers to assess
lung function in mice noninvasively.
In the meantime
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With unrestrained plethysmography, the subject is
happy, but...

• breathing pattern, lung volume, etc. may
  change with intervention, making it impossible to
  detect changes in lung mechanics separately from
  effects due to changes in the breathing pattern.

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The Phenotyping Uncertainty Principle
Most precise (forced oscillation technique)
Least invasive (unrestrained plethysmography)
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Summary

• The forced oscillation technique (e.g. flexiVent)
  gives very precise assessment of lung mechanics
  in mice, but at the expense of highly unnatural
  measurement conditions.
• Unrestrained plethysmography allows for natural
  and convenient measurement conditions, but at the
  almost complete expense of precision and
  specificity.