Chest
Volume 73, Issue 2, February 1978, Pages 158-162
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Clinical Investigations
Effect of Tidal Volume and Positive End-Expiratory Pressure on Compliance during Mechanical Ventilation

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In 12 patients requiring therapy with mechanical ventilation for acute respiratory failure, total static compliance (Cst) increased from 29 ± 4 ml/cm H2O at a tidal volume (TV) of 5 ml/kg to 42 ± 7 ml/cm H2O at a TV of 15 ml/kg. Similarly, Cst increased from 42 ± 7 ml/cm H2O to 52 ± 8 ml/cm H2O between 0 and 6 cm H2O of positive end-expiratory pressure (PEEP). At high levels of pulmonary inflation (ie, high PEEP and large TV) compliance decreased. The changes of total respiratory compliance with TV were mainly due to changes in pulmonary compliance. With PEEP, the functional residual capacity increased, and specific compliance did not change. Two mechanisms may be responsible for the changes in compliance. First, varying TV or PEEP will alter the position of tidal ventilation on the pressure-volume curve, resulting in an increase in compliance with increasing TV and PEEP up to a point, where overdistention occurs and compliance decreases. Secondly, the function of the surface-lowering substance may be altered in acute pulmonary parenchymal disease, thus disturbing the regulation of surface tension over the range of pulmonary Inflation studied.

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Materials and Methods

Twelve patients requiring therapy with mechanical ventilation for various forms of pulmonary and ventilatory failure were studied in the supine position. Their ages ranged between 31 and 66 years (mean, 41 years). One patient had pure ventilatory failure secondary to injury of the central nervous system, six patients had pulmonary failure after multiple injuries, and in five patients, pulmonary failure was due to pneumonia or cardiac failure. Volumecontrolled ventilators delivering a constant

Results

Figure 2 illustrates the static pressure-volume points of the respiratory system in two forms of respiratory failure. One patient had neurogenic ventilatory failure without detectable pulmonary parenchymal disease. His pulmonary volumes were essentially normal. The second patient was treated for severe prolonged bilateral pneumonia. His FRC and inspiratory capacity were reduced to one third of predicted normal values. Figure 2 illustrates a marked difference in the slopes of the lines between

Discussion

The present study illustrates that in patients requiring therapy with mechanical ventilation for acute respiratory failure, Cst and Cdyn (determined from two-point pressure and volume differences) depend not only on the pulmonary pathologic abnormalities, but also vary markedly with the magnitude of TV and the level of end-expiratory pressure. The observed changes in compliance may be explained by the following mechanisms.

First, the abnormally low FRC present in most of the patients with acute

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Supported in part by grant GM 6300-14 from the National Institutes of Health and by SCOR grant HL-14201 from the National Heart and Lung Institute.

Médecin Responsable des Soins Intensifs Chirurgicaux, Hôpital Cantonal Universitaire. Dr. Suter was supported by grants from the Swiss Academy of Medical Sciences and the Swiss National Science Foundation (grant 3.564-0.75).

Professor and Vice Chairman, Department of Anesthesia, University of California.

Manuscript received February 4; revision accepted June 2.

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