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Respiratory Cycle: Definition And Explanation

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Cool Physician OnlineInhalation - the first phase of a respiratory cycle. It is characterized by a positive flow in airways. While air is arriving in lungs, the respiratory volume increases. Inhalation ends upon the reduction of a positive flow to zero.
Inhalation - the first phase of a respiratory cycle. It is characterized by a positive flow in airways. While air is arriving in lungs, the respiratory volume increases. Inhalation ends upon the reduction of a positive flow to zero.

Let's define a respiratory cycle (RC) as a time interval between two consecutive inspirations. For the detailed analysis of respiratory curves let's determine following phases of a respiratory cycle:

Inhalation - the first phase of a respiratory cycle. It is characterized by a positive flow in airways. While air is arriving in lungs, the respiratory volume increases. Inhalation ends upon the reduction of a positive flow to zero.

Inspiratory pause (plateau) - the second phase of a respiratory cycle. This phase is present only at mandatory modes of artificial lung ventilation. Inspiratory pause is characterized by the period of a zero flow between the end of inhalation and the exhalation beginning. Air volume in lungs during inspiratory pause does not change.

Exhalation - the third phase of a respiratory cycle. It is characterized by a negative flow in respiratory paths, thus air leaves lungs while the respiratory volume is decreasing. Exhalation end is characterized by the termination of a negative flow. The respiratory volume thus should decrease to zero normally.

Period of rest - the fourth phase of a respiratory cycle, characterized by the absence of a flow in airways between the end of an exhalation and the beginning of inhalation of a following respiratory cycle. Duration of this period is very important, as it allows us to estimate respiration rate reserves.

For the detailed analysis of events during artificial lung ventilation and spontaneous breathing the graphic curves are used. These curves represent changing of the key parameters of a respiratory cycle. The majority of modern graphic monitors are able to display following dependences: airway flow /time, respiratory volume/time, airway pressure/time.

17th August, 2011