When analyzing transvalvular and venous flow velocity patterns, it is
important to relate them to respiration, An accurate recording of the
respiratory phase can be carried out with different methods, One of th
ese methods is the use of a thermistor, which reacts to the variation
in air temperature, placed in the nose of the patient, The thermistor
used has a diameter of 1.0 mm and is of standard bead type, Although s
mall, it has a considerable long time-constant and a long time-delay,
The high time-constant gives a low cutoff frequency, well below the re
spiratory frequency and thereby causing a large phase difference, The
thermistor was analyzed with the lumped heat capacity method, where it
was easy to study the influence from design parameters, time-dependen
t air temperature, and velocity, The analysis was extended using the f
inite element method and the temperature field in the thermistor and t
he probe was calculated as a function of space and time, These calcula
tions confirmed the result from the lumped model, The result showed th
at timing of respiration was not accurately obtained with the thermist
or analyzed, To improve the timing, it was necessary either to change
the measuring method or to use signal processing in order to achieve f
aster response.