Pulmonary Compliance and Effective Compliance in Patients Receiving Mechanical Ventilation

Abstract

Communications for this section will be published as space and priorities permit. The comments should not exceed 500 words in length, with a maximum of five references; one figure or table can be printed. Exceptions may occur under particular circumstances. Contributions may include comments on articles published in this periodical, or they may be reports of unique educational character. Specific permission to publish should be cited in a covering letter or appended as a postscript. Communications for this section will be published as space and priorities permit. The comments should not exceed 500 words in length, with a maximum of five references; one figure or table can be printed. Exceptions may occur under particular circumstances. Contributions may include comments on articles published in this periodical, or they may be reports of unique educational character. Specific permission to publish should be cited in a covering letter or appended as a postscript. To the Editor: We read with interest the article by Bone entitled ‘Diagnosis of Causes for Acute Respiratory Distress by Pressure-Volume Curves” (Chest 70:740-746, 1976). His results, obtained by noninvasive techniques, are of great importance to all workers in the field of intensive respiratory care. In his discussion, Bone stated that ‘The monitoring of lung compliance would be preferable to monitoring of compliance of the lungs and chest wall in all patients with rapidly changing respiratory status; however, in the absence of the capability to monitor lung compliance in all patients treated with mechanical ventilation, these measurements should be made” (p 745). For about one year, we have been engaged in a similar study in which both the pulmonary compliance (dynamic compliance [Cdyn]) and the compliance of the lung and chest wall (effective compliance) were measured. In ten patients with severe acute respiratory insufficiency from various causes who were receiving mechanical ventilation with a volume-constant ventilator, simultaneous measurements of both Cdyn and effective compliance were obtained. For this purpose an esophageal catheter with an inflatable balloon (filled with I ml of air) was inserted into the distal part of the esophagus. Dynamic pulmonary compliance was calculated from the esophageal pressure difference at zero inspiratory and expiratory flow and tidal volume (Vt), recorded by a pneumotachygraph. Effective compliance was calculated from Vt and plateau pressure (pressure measured in the inspiration tubes of the ventilator at the moment of zero flow at the end of inspiration). All measurements were performed with the patients in the recumbent position and not within 15 minutes after feeding or intratracheal suction. The frequency of measurements in each patient was different; some patients were measured only once, and others were measured several times, either continuously or intermittently. After calculation of effective compliance and Cdyn, we divided the patients into the following two groups according to the value of Cdyn: (1) group 1 consisted of four patients with Cdyn less than 150 ml/cm H2O (26 measurements); and (2) group 2 consisted of six patients with Cdyn greater than or equal to 150 ml/cm H2O (19 measurements). Effective compliance and Cdyn were compared (Fig 1) in both groups; in group 1 (Fig 1, left), a good correlation (r = 0.89; P < 0.001) was found, whereas the correlation in group 2 (Fig 1, right) was poor (r = 0.41; P > 0.05). Our preliminary results suggest that effective compliance may be used as an indicator in patients with low values of Cdyn. Absolute values of effective compliance can only be used as an approximation of the actual value of Cdyn. As a rule, the effective compliance was three times smaller than Cdyn.