Commenting on the ACCP Consensus Conference: Mechanical Ventilation

Abstract

Communications for this section will be published as space and priorities permit. The comments should not exceed 350 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 350 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. The ACCP Consensus Conference on Mechanical Ventilation published in the December 1993 issue of Chest1Slutsky AS Brochard L Dellinger RP Downs JB Gallagher TJ Gattinoni L Et Al ACCP Consensus Conference: mechanical ventilation..Chest. 1993; 104: 1833-1859Abstract Full Text Full Text PDF PubMed Scopus (542) Google Scholar provides a useful overview of many important concepts. However, a few points merit further comment. First, as the authors note, alveolar overdistention is increasingly recognized as the fundamental mechanism underlying ventilator-induced lung injury. Since there is no way to assess alveolar distention directly, measurements of pressure are often substituted. Among the terms used to characterize the respiratory system during mechanical ventilation are peak airway pressure (PAP), peak alveolar pressure, transalveolar pressure, transpulmonary pressure, and transthoracic pressure. These terms are not synonymous, and unfortunately are sometimes used imprecisely, causing confusion about the relationship between pressure and ventilator-induced lung injury. For example, it is stated in the discussion of adult respiratory distress syndrome that “animal studies have clearly established the damaging effects of overdistention produced by the application of high peak transthoracic pressures to normal and injured lungs.” This is incorrect: animal studies clearly show that high peak transalveolar pressures cause overdistention and lung injury, but high transthoracic pressures cause no injury when transalveolar pressure is low, as occurs when the chest wall is bound.2Caldwell EJ Powell RD Mullooly JP Interstitial emphysema: a study of physiologic factors involved in experimental induction of the lesion..Am Rev Respir Dis. 1970; 102: 516-25, 14PubMed Google Scholar, 3Hernandez LA Peevy KJ Moise AA Parker JC Chest wall restriction limits high airway pressure-induced lung injury in young rabbits..J Appl Physiol. 1989; 66: 2364-2368PubMed Google Scholar, 4Dreyfuss D Soler P Basset G Saumon G High inflation pressure pulmonary edema: respective effects of high airway pressure, high tidal volume, and positive end-expiratory pressure..Am Rev Respir Dis. 1988; 137: 1159-1164Crossref PubMed Scopus (1323) Google Scholar Alveolar distention is a function of alveolar compliance and the transmural pressure applied to the alveolus. Hence, only the terms “static transpulmonary pressure” and “transalveolar pressure” reflect alveolar distention; the remaining terms bear no predictable relationship to alveolar distention. Second, the statement that in asthma “a high PAP may also predict hyperinflation” is not quite correct. Although in asthma a high PAP may sometimes be associated with hyperinflation, it is just as likely not to correlate at all. This comes as no surprise, since PAP may or may not correlate with transalveolar pressure. In their study of patients with severe airflow obstruction undergoing mechanical ventilation, Tuxen and Lane5Tuxen DV Lane S The effects of ventilatory pattern on hyperinflation, airway pressures, and circulation in mechanical ventilation of patients with severe air-flow obstruction..Am Rev Respir Dis. 1987; 136: 872-879Crossref PubMed Scopus (229) Google Scholar found that PAP did not predict the magnitude of hyperinflation; in fact, they found PAP correlated inversely with the degree of hyperinflation. Finally, the authors suggest the strategy of reducing VE and increasing expiratory time in the management of patients with obstructive airways disease. In deciding whether to reduce VT by reducing VT or by reducing frequency, it is worth noting that in patients with severe obstructive airways disease, one gains relatively little by prolonging expiratory time beyond 3 to 4 s because of the extremely low flow rates late in expiration. As a rough example, an additional second for expiration might allow the exhalation of an additional 50 mL of gas, whereas a 100 to 200 mL reduction in VT would have a much greater effect on minimizing hyperinflation. Commenting on the ACCP Consensus Conference: Mechanical VentilationCHESTVol. 106Issue 5PreviewWe thank Dr. Harold Manning for his careful reading of the ACCP Consensus Conference: Mechanical Ventilation and his helpful comments. Dr. Manning brings up three points that I will comment on. Full-Text PDF