I’ve been interested in ventilation inhomogeneity for a while and as ways to measure it I have looked at VA/TLC ratios, the Lung Clearance Index (LCI) and the phase III slope of the single-breath N2 washout (SIIIN2). All of these tests are able to provide some information about ventilation inhomogeneity but each has their own limitations and just as importantly although their results have a relatively clear relationship with ventilation inhomogeneity it’s not quite as clear what exactly it is they are measuring. A friend recently pointed me to an on-line article in Chest that discusses the dual-tracer single-breath washout test in patients with COPD. The apparent advantage of this test is that it is able to provide information about the site of the ventilation inhomogeneity. Although dual tracer gases have been used to study airway function for over 50 years the limitation of this technique has been the need to use a mass spectrometer. Some recent advances in technology have made it possible for this type of testing to be performed with a significantly less expensive gas analyzer and this has revived an interest in the dual-tracer gas single-breath washout (DTG-SBW).
The two tracer gases in question are Helium and Sulfur Hexaflouride (SF6). Helium has a density of 4 gm/mol and the density of SF6 is 146 gm/mol, and it is the difference in densities between these two inert and insoluble gases that make this test useful. In order to understand why we need to revisit to the anatomy of the terminal airways.
From Osborne S. Airway resistance and airflow through the tracheobronchial tree. www.SallyOsborne.com.
Turbine spirometers have been around in one form or another for well over a hundred years. The accuracy of the early versions of this type of spirometer was poor, partly because of the turbine designs weren’t terribly efficient and partly because these devices were mechanical in nature and the gear trains or other mechanical linkages added a lot of friction and resistance.
From: Nouveaux éléments de pathologie générale, de séméiologie et de diagnostic. by Eugène Bouchut, 1875, page 865. ]
The first electronic turbine spirometer was the Marion Labs Spirostat which came to market in the early 1970’s. It used a disposable in-line turbine where the turbine blades were directly in the stream of inspiratory and expiratory flow and rotated accordingly. There was an optical pickup (a light beam passing through a hole in the turbine) and the rotations were converted to inspiratory and expiratory volumes. The passageways through the Spirostat’s turbine sensor were quite narrow and the resistance to flow was high. The turbine also had a fair amount of mass, was perceptibly slow to start moving and slow to stop and would not have met the current ATS/ERS standards.
Spirostat sensor drawing from US patent number 3680378
There are at least a half dozen companies that use an ultrasonic flowmeter in their spirometer. The first patent for an ultrasonic flowmeter was made in the 1970’s but it wasn’t until the 1990’s that the first ultrasonic spirometers came to market. The basic idea is fairly simple and that is to measure the transit time of ultrasonic pulses through flowing gas. Pulses that travel in the same direction the gas is flowing will take less time to travel a given distance, while pulses traveling against the direction of gas flow take a longer time.
This particular measurement process is called time-of-flight (as opposed to doppler shift) and has a relatively flat flow/signal curve and frequency response. An early design of this kind of flowmeter had the ultrasonic transducers sitting in the flow of gas, but this both impedes the flow of gas and is hard to clean. A transverse design was developed that put the transducers outside the path of gas flow and this configuration has been used in all ultrasonic spirometers.
The first reasonably accurate flow-measuring device was the Fleisch pneumotachograph which was developed in 1925. Originally the Fleisch pneumotach bounced a light beam off a mirror mounted on a diaphragm and from there onto photographic film, all of which made it difficult to use. World War II saw the development of sensitive pressure transducers, amplifiers and recorders and by 1950 the pneumotachograph went totally electronic and began to be commonly used in routine pulmonary research.
The first spirometers that used a flow sensor came onto the marketplace around 1970. Since that time, flow sensors of one kind or another have made steady inroads and now the majority of test systems use flow sensors and there are only a small handful of volume-displacement spirometer systems still being manufactured.
There are a variety of difficulties involved with measuring gas flow rates and this has driven the development of a number of different flow measurement techniques. As well as the pneumotachograph, there are now Pitot tube, hot wire, turbine and ultrasonic flow sensors. Some of these techniques are more linear than others but none of them are perfectly linear.
The pneumotachograph however, is inherently the most linear method for measuring gas flow rates and has been more completely characterized than all other techniques. For this reason it is probably used in pulmonary function equipment more frequently than any of the other type of flow sensor and is also far more likely to be used in research.
Gas flow through a pneumotach is measured from the difference in pressure across a resistance. There are a variety of ways of creating this resistance but there are only two methods that are relatively linear, the Fleisch and the screen (aka Lilly) pneumotach.
The resistance in the Fleisch pneumotach consists of a set of narrow capillary tubes, parallel to the direction of flow.