PULMONARY AND CHEST WALL MECHANICAL FUNCTION
Principles of Gas Flow Measurement
Monitoring of inspiratory and expiratory flow can be useful for
a variety of reasons (see Chapter 26
).
First, measures of resistance (discussed later) require pressure and flow measurements.
Second, flow may be integrated to provide a monitor of inspired or expired volume.
Several types of flow meter exist. The most commonly used flow meter in anesthesia
probably is the rotameter, in which gas flow imparts movement to a series of vanes
connected to a wheel. The Wright spirometer integrates this movement, providing
the user with a measure of volume. The Fleisch pneumotachograph consists of a bundle
of small-diameter tubes in parallel. Flow within these capillary tubes is laminar,
and pressure drop is therefore linearly related to gas flow.[203]
Screen-type pneumotachographs work by measurement of the pressure drop across a
mesh screen. These devices are usually smaller and lighter than Fleisch-type pneumotachographs,
but they are inherently nonlinear. Moreover, their practical implementation is often
limited by their tendency to collect moisture and debris, which changes their operating
characteristics. Modern mechanical ventilators often incorporate vortex-type pneumotachographs,
which work by measurement of interruptions in an ultrasonic beam placed across a
tube in which struts disrupt the laminar flow, resulting in vortices.[203]
Ultrasonic flow meters measure the speed of ultrasonic waves propagated parallel
to the direction of flow.[204]
Hot-wire anemometers
consist of electrically heated wires placed in the gas stream, which tend to cool
the wire and change the electrical conductivity. Flow is measured by the amount
of additional electrical current generated by a feedback circuit that is necessary
to maintain the wire at constant temperature. These devices characteristically have
an extremely high-frequency response, but their behavior depends highly on gas temperature
and contamination of the gas stream with mucus and water droplets.[203]
 |
