A Ghost from the Past: Low Tidal Volume Mechanical Ventilation Revisited

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It has long been recognized that monotonous spontaneous or mechanical ventilation with a low VT (5 to 7 ml/kg) of normal or minimally damaged lungs allows the development of alveolar atelectasis. As a result, an increase in intrapulmonary shunting and a decrease in lung compliance and PaO2 occur.1Ferris BG Pollard DS Effect of deep and quiet breathing on pulmonary compliance in man.J Clin Invest. 1960; 39: 143-149Crossref PubMed Scopus (87) Google Scholar, 2Bendixen HH Hedley-Whyte J Laver MB Impaired oxygenation in surgical patients during general anesthesia with controlled ventilation-a concept of atelectasis.N Engl J Med. 1963; 269: 991-996Crossref PubMed Scopus (468) Google Scholar These effects can be reversed by inflation of the lungs with a larger VT1Ferris BG Pollard DS Effect of deep and quiet breathing on pulmonary compliance in man.J Clin Invest. 1960; 39: 143-149Crossref PubMed Scopus (87) Google Scholar, 2Bendixen HH Hedley-Whyte J Laver MB Impaired oxygenation in surgical patients during general anesthesia with controlled ventilation-a concept of atelectasis.N Engl J Med. 1963; 269: 991-996Crossref PubMed Scopus (468) Google Scholar presumably by reopening atelectatic alveoli. This led to the practice of providing periodic larger breaths, or sighs, while mechanically ventilating patients in respiratory failure with a low VT.3Egan DF Fundamentals of respiratory therapy. CV Mosby Co, St Louis1977: 415-422Google Scholar Subsequently, it was demonstrated that constant mechanical ventilation with a large VT (10 to 15 ml/kg) also forestalled the development of atelectasis and increased intrapulmonary shunting, and periodic sighing fell into disuse.4Fairley HB The mechanical ventilation sigh is a Dodo.Respir Care. 1976; 21: 1127-1230PubMed Google Scholar Nevertheless, the custom of using a large VT during mechanical ventilation continues today despite the application of positive end-expiratory pressure (PEEP), another modality that prevents alveolar atelectasis in most patients with acute respiratory failure. The concomitant use of a large VT and PEEP may be deleterious. Airway pressures are higher and can result in greater barotrauma. In patients with acute respiratory failure requiring high levels of PEEP, mechanical ventilation with a large VT reduces static lung thorax compliance in comparison with ventilation with a low VT.5Suter PM Fairley HB Isenberg MD Effect of tidal volume and positive end-expiratory pressure on compliance during mechanical ventilation.Chest. 1978; 73: 158-162Crossref PubMed Scopus (165) Google Scholar The reduction in compliance with the use of a large VT signifies overdistention of the lung at end-inspiration. As a result, pulmonary vascular resistance and physiologic dead space may increase. In addition, recent studies in experimental animals suggest that extremely high airway pressures resulting from use of a large VT actually produce lung injury.6Kolobow T Moretti MP Fumagalli R Mascheroni D Proato P Chen V et al.Severe impairment in lung function induced by high peak airway pressure during mechanical ventilation-an experimental study.Am Rev Respir Dis. 1987; 135: 312-315PubMed Google Scholar In this issue of Chest (see p 430), Lee and colleagues report the results of a study in which patients in a surgical intensive care unit (ICU) were randomized to receive mechanical ventilation with PEEP using a large VT (12 ml/kg) or low VT (6 ml/kg). They found no major adverse effects from using a low VT. Although arterial oxygenation was slightly worse with a low VT, it was of little clinical significance. Furthermore, trends were noted for a lower rate of pulmonary infections and shorter durations of intubation and ICU stay. These results potentially have important implications for the current management of patients receiving mechanical ventilation. Before these findings are applied to all mechanically ventilated patients, however, additional data should be obtained from other patient populations. Many of the patients studied by Lee et al were ventilated after having undergone elective thoracic or abdominal surgery and presumably did not have severe parenchymal lung disease. The average maximum peak airway pressure and PEEP used in the low VT group were only 28.9 and 7.2 mm Hg, respectively. Therefore, whether the observations of Lee et al will be applicable to patients with severe ARDS, obstructive airways diseases, or primarily unilateral disease processes is not clear. Inasmuch as there was no greater amount of barotrauma in the large VT group, it also is not obvious why use of a low VT should result in trends toward a lower rate of pulmonary infections and shorter durations of intubation and ICU stay. Furthermore, hyperventilation in head injury patients was not possible with a low VT using acceptable (<26 breaths/min) ventilator rates. Nevertheless, it appears that we have completed a full circle. In mechanically ventilated patients, if PEEP is used to prevent atelectasis in those with relatively normal lungs and to increase functional residual capacity in those with parenchymal lung disease, the ghost of mechanical ventilation with a low VT may be in the midst of a reincarnation.