What is the difference between "open circuit" and "closed circuit" technique?
The term “open circuit” spirometry refers to the method of conducting spirometry where the subject takes a maximal inspiration, inserts the mouthpiece into the mouth, and then blows out either slowly (SVC) or fast (FVC) until the end-of-test criterion is met. Open circuit technique is commonly used with volume sensing spirometers based on wedge bellows, bell, water seal or rolling seal cylinder/piston. The term “closed circuit” spirometry refers to the method of conducting spirometry, where the subject inserts the flow transducer into the mouth and breathes for 2-3 tidal breaths (rest breathing), takes a maximal inspiration (or expiration) through the flow transducer, and then blows out (or breathes in) either slowly (VC) or fast (FEVC, FVC) until the end-of-test criterion is met, and after this returns to tidal breathing (rest breathing). Most bi-directional spirometers based on measurement of a pressure drop across a fixed resistance are using "closed circuit" technique. With Medikro spirometers one can use both techniques. In diagnostic spirometry "closed circuit" technique is recommended.
The advantages of closed circuit techniques?
Capability to measure also inspiratory variables. Forced inspiratory manoeuvres are useful in diagnosing and monitoring upper airway obstruction. If upper airway obstruction is suspected, flow-volume curve with particular emphasis on inspiration is the best test. Forced inspiratory manoeuvres are usually performed in the FVC+FIVC manoeuvre after the forced expiratory manoeuvre but may be performed separately by using the FIVC measurement button. Elderly or ill patients often have difficulty performing forced inspiratory and expiratory manoeuvres in the same measurement.
Allows technicians to better monitor the entire manoeuvre. This feature is somewhat a quality feature. Expiratory manoeuvres must be started from fully filled lungs. How can we be sure that this requirement fulfils? Patient effort can be monitored in real-time with spirometers using advanced "closed circuit" technique.
Better test quality. Although the "open circuit" technique works well for some subjects, others have difficulty maintaining a maximum inspiration while trying to position the mouthpiece correctly in the mouth. These subjects may lose some of their vital capacity due to leakage prior to the expiratory manoeuvre. The "closed circuit" technique allows the subject to obtain a tight seal with the flow transducer during the whole spirometry measurement.
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