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The Relationship Between Dynamic Compliance and Inspiratory Flow

Khorasani, Arjang MD; Candido, Kenneth D. MD; Saatee, Simin MD; Khorasani, Arjang MD

doi: 10.1213/00000539-199902000-00045
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(Khorasani, Candido, Saatee, Khorasani) Department of Anesthesiology and Pain Management; Cook County Hospital.

(Khorasani, Candido, Saatee) Department of Anesthesiology; Rush-Presbyterian-St. Luke's Medical Center; Chicago, IL 60612.

To the Editor:

In their recent article, Chaney et al. [1] studied the pulmonary effects of methylprednisolone (MP) in patients undergoing coronary artery bypass grafting and early tracheal extubation. They found a significant postoperative decrease in dynamic lung compliance for both MP and control groups. We disagree with the authors' determination of these findings in concluding a postoperative decrease in the dynamic compliance.

The authors failed to include in their methods the effect of inspiratory flow time or inspiratory hold time (plateau period) in the calculation of dynamic compliance. There is absolutely no validity in comparing two calculated dynamic compliances and then formulating a medical judgement regarding the dynamic properties of the lung (based on changes in dynamic compliance) if one fails to assure a constant inspiratory flow time or inspiratory hold time along with other mechanical ventilation variables (as described by the authors) for the entire period of the study.

The dynamic compliance, an empirically determined measurement, is defined as the tidal volume divided by the peak inspiratory pressure change. Peak airway pressure is primarily a function of airway resistance and flow (rapidity of delivering a tidal volume). Increases in airway resistance may be one cause for increased peak airway pressure and, hence, decreased dynamic compliance. However, without changes in airway resistance, the calculated dynamic compliance varies during mechanical ventilation if one fails to maintain a constant flow.

The calculated inspiratory time in this study was 1.875 s. A portion of this inspiratory time is used to deliver the predetermined tidal volume (inspiratory flow time), and the rest is used to create a no-flow state (inspiratory hold time). Plotting the airway pressure versus time, the end-inspiratory flow time produces peak pressure, and the period of no flow produces plateau pressure. When the inspiratory time and tidal volume are constant, an increase in the inspiratory hold time results in a decrease in the inspiratory flow time. A decrease in inspiratory flow time requires a higher flow to deliver the same tidal volume in a shorter period of time. An increase in inspiratory flow produces a higher peak airway pressure, and the calculated dynamic compliance decreases. Under these circumstances, the reduction in dynamic compliance is not caused by an increase in airway resistance or any other lung pathology. The authors failed to take this important issue into consideration in their methods; therefore, the decrease in dynamic compliance in the postoperative period may or may not reflect changes in lung mechanical properties.

Arjang Khorasani, MD*[dagger]

Kenneth D. Candido, MD*[dagger]

Simin Saatee, MD*[dagger]

Arjang Khorasani, MD*

*Department of Anesthesiology and Pain Management; Cook County Hospital

[dagger]Department of Anesthesiology; Rush-Presbyterian-St. Luke's Medical Center; Chicago, IL 60612

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REFERENCES

1. Chaney MA, Nikolov MP, Blakeman B, et al. Pulmonary effects of methylprednisolone in patients undergoing coronary artery bypass grafting and early tracheal extubation. Anesth Analg 1998;87:27-33.
© 1999 International Anesthesia Research Society