Mass/Volume Flow Meters (Urometer, Volumeter, Dilutional
Measurements)
We can actually measure the mass or volume of fluid flowing per
unit time by catching fluid in a container and either weighing it or measuring its
volume. The common urometer for measuring urine volume is an example. The volumeter
used in North American Drager anesthesia machines also measures aliquots of volume
integrated over time to measure tidal and minute volume ( Fig.
30-17
).[11]
Both volume and mass flow can be measured by dilutional techniques.
If some measurable indicator (a bolus of dye, a thermal pulse, oxygen consumption,
carbon dioxide production) is injected into a flow and its concentration is
Figure 30-17
Volumeter. Flow can be described as volume over time.
This design of flow meter allows gas to pass only in little aliquots, each of which
turns a counter to measure the amount flowing past. When divided by time, this method
results in flow measurement. (Adapted from Ehrenwerth J, Eisenkraft J:
Anesthesia Equipment: Principles and Applications. St Louis, Mosby-Year Book, 1993.)
measured downstream, the volume flow (
) can be calculated by integration.
The most common medical application is determination of cardiac output by the pulmonary
artery thermodilution method (see Appendix
6
). Errors associated with these methods involve using the wrong injectate
volume (too small a volume resulting in too large a flow) or an error in temperature
measurement (see the section on temperature).
"Continuous" cardiac output using an electric heating coil to
warm the blood removes errors associated with fluid injectate techniques but introduces
the need to average the smaller signal over a longer time base. Additionally, there
is an upper limit to warming the blood, so signal quality is reduced in febrile patients.
[12]
The mass flow of carbon dioxide or oxygen
can
be used to measure cardiac output by the Fick equation and modifications (see Appendix
6
). By using these variables, a change in metabolic rate can lead to errors
in measurement.
