Monthly Archives: August 2017

Getting more out of the LCI with Scond and Sacin

Posted on by
2

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).

Continue reading →

Oscillometry

Posted on by
2

A month or two ago in the AARC Diagnostics forum several members noted that their labs had acquired Impulse Oscillometry systems a number of years ago but that their physicians had since stopped ordering oscillometry tests, mostly because nobody understood what it was measuring and didn’t know how to interpret the results. There are a number of reasons why this is probably not an uncommon scenario and why, despite being first described in 1956, oscillometry is not used more widely.

But first, what is oscillometry, and what’s the best way to understand it?

Oscillometry refers to a closely related group of techniques for measuring respiratory impedance by superimposing small pressure waves on top of normal tidal breathing.

There are three main approaches: the Forced Oscillation Technique (FOT), which is sometimes used a blanket term for all oscillometry techniques but more often refers to a single frequency technique, Impulse Oscillometry (IOS) and Pseudo-Random Noise (PRN). Most commercial oscillometry systems use either PRN or IOS because each approach uses multiple oscillation frequencies more or less simultaneously which allows testing to performed relatively quickly. The mono-frequency technique is used mostly in research because although it is slow to scan all frequencies, it is able to resolve rapid changes occurring at a single frequency.

All techniques share a similar equipment configuration:

The oscillatory pressure is usually generated by a loudspeaker, although the actual waveform and the frequency it produces differ for each technique. The peak pressures are usually on the order of +/- 1 to 5 cm H2O (+/- 0.1 to 0.5 kPa). Because patients have to breathe during testing, the system provides a steady flow of fresh air in one manner or another but this has to include a low pass filter of some kind so that the pressure waveform is not significantly diverted or blunted. The key measurements are flow and the pressure at the mouth.
Continue reading →