Getting more out of the LCI with Scond and Sacin

The Lung Clearance Index (LCI) is a relatively simple test that provides a measure of ventilation inhomogeneity within the lung. This can be clinically useful information since several studies have shown that increases in LCI often precede decreases in FEV1 in cystic fibrosis and post-lung transplant. LCI results are only a general index into ventilation inhomegeneity however, and other than showing its presence, does not give any further information about its cause or location.

There is additional information that can be derived from an LCI test that can indicate the general anatomic location where ventilation inhomegeneity (or alternatively, ventilation heterogeneity) is occurring; specifically the conducting or acinar airways. This can be done because changes in the slope of the tidal N2 washout waveform during an LCI test are sensitive to the conduction-diffusion wavefront in the terminal bronchioles. Careful analysis of these slopes permits the derivation of two indexes; Scond, an index of the ventilation heterogeneity in the conducting airways; and Sacin, an index of ventilation heterogeneity the acinar airways.

To review, an LCI test is a multi-breath nitrogen washout test. An individual is switched into a breathing circuit with 100% O2.

Once this happens tidal volume is measured continuously and used to determine the cumulative exhaled volume. Exhaled nitrogen is also measured continuously and is used to determine the cumulative exhaled nitrogen volume. The LCI test continues until the end-tidal N2 concentration is 1/40th of what is was initially (nominally 2%). At that point FRC is calculated using the cumulative exhaled nitrogen volume:

FRC (L) = Exhaled N2 Volume / (Initial N2 Concentration – Final N2 concentration)

LCI is calculated by:

LCI = Cumulative Exhaled Volume (L) / FRC (L)

and is essentially a measure of how much ventilation is required to clear the FRC. When an individual tidal breath from the LCI test is graphed, it looks similar to a standard single-breath N2 washout:

and can be similarly subdivided into phase I (dead space washout), phase II (transition) and phase III (alveolar gas).

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Multi-breath Washout and the Lung Clearance Index

The March 1, 2014 issue of the American Journal of Respiratory and Critical Care Medicine had an article on the use of the Lung Clearance Index (LCI) with bronchiectasis. The study showed that the LCI was as good as high-resolution computed tomography and more sensitive than FEV1 when assessing changes in airway status. This is one of the few articles I’ve seen on the LCI that was specifically about adults and wasn’t about cystic fibrosis.

So what is the LCI and how is it measured?

When lung tissue and airways are normal, inhaled gas is distributed evenly throughout the lung and the mixing and turnover of alveolar gas is relatively rapid. When airway obstruction is present gas distribution tends to becomes more uneven and the mixing and turnover takes longer. The Lung Clearance Index (LCI) is a way to measure these ventilation inhomogeneities and is basically a description of how much ventilation is required to completely clear the FRC. It was first described by Margaret Becklake in 1952 but has languished for many years. It has been revived in the last decade or so, particularly because it requires only tidal breathing which allows it to be measured in infants and children.

The measurement process is called an Inert Gas Multi-Breath Washout. It uses an open circuit and requires a tracer gas that is both inert and relatively insoluble and for these reasons has been primarily limited to helium, nitrogen and sulfur hexafluoride (SF6) although methane and argon could potentially be used as well.

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