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LETTERS TO THE EDITOR

The Relationship Between Dynamic Compliance and Inspiratory Flow

Arjang Khorasani, MD^*,, Kenneth D. Candido, MD^*,, Simin Saatee, MD^*,, and Arjang Khorasani, MD^*

*Department of Anesthesiology and Pain Management Cook County Hospital Department of Anesthesiology Rush-Presbyterian-St. Luke's Medical Center Chicago, IL 60612

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.

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.[Abstract/Free Full Text]

Response

Department of Anesthesiology Loyola University Medical Center Maywood, IL 60153

We are well aware of the relationship between inspiratory flow and calculated dynamic lung compliance. In our clinical study, we controlled every pulmonary variable we could by standardizing respiratory rate, tidal volume, fraction of inspired oxygen, positive end-expiratory pressure, and inspiratory to expiratory ratio. We also standardized inspiratory flow, although this was not stated in the article. The inspiratory flow was adjusted in every patient so that the calculated tidal volume (10 mL/kg) was delivered over the entire inspiratory cycle (which resulted in the lowest peak inspiratory pressure for that tidal volume). Because all patients in both groups at each of the four data collection times were treated identically, we are confident that the changes in dynamic lung compliance over time were real and that the level of statistical significance (P < 0.000001 for each group) proves it. Furthermore, decreased lung compliance in patients after exposure to cardiopulmonary bypass has been documented by others (1,2).

References

1. Barnas GM, Watson RJ, Green MD, et al. Lung and chest wall mechanical properties before and after cardiac surgery with cardiopulmonary bypass. J Appl Physiol 1994;76:166–75.[Abstract/Free Full Text]
2. Coffin LH, Shinozaki T, DeMeules JE, et al. Ineffectiveness of methylprednisolone in the treatment of pulmonary dysfunction after cardiopulmonary bypass. Am J Surg 1975;130:555–9.[Web of Science][Medline]

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