Does the advent of (new) low tidal volumes bring the (old) sigh back to the intensive care unit?

Abstract

Division of Surgical Intensive Care, Department of Anesthesiology, Pharmacology, and Surgical Intensive Care (APSIC), University Hospital of Geneva, Geneva, Switzerland. peter.suter@hcuge.chACUTE lung injury (ALI) and adult respiratory distress syndrome (ARDS) are characterized by the tendency of peripheral air spaces to collapse, particularly in dependent lung areas. The application of a positive end-expiratory pressure (PEEP) and an appropriate tidal volume (VT) is thought to prevent and reverse the collapse of airways and respiratory units in such patients. Other means to recruit alveoli include frequent position changes, e.g . from supine to prone. The introduction of specific inflation maneuvers, using either an intermittent high-tidal volume, or an elevated inspiratory (plateau) pressure, allows significant increases in end-inspiratory and end-expiratory lung volume, and an associated improvement in gas exchange. Clinical results obtained with such recruitment strategies are reported in the current issue of Anesthesiology by two groups of well-known clinical investigators. 1,2Both articles describe the effects of lung inflation in patients with ARDS, studied in the early or late phases of their disease. The advent of ventilatory techniques using lower VTto decrease ventilator-induced lung injury and improve outcome in ALI and ARDS 3,4has renewed interest in the requirement for, and the potential benefit of, periodic inflation maneuvers. It has indeed been shown that VTparticipates to a significant extent in the opening of collapsed or hypoventilated areas in such patients. 5Therefore, it is not surprising that with the new low VTapproach, an old intermittent inflation method, called sigh, 6would regain some credibility in ventilatory management. In the first of the two studies discussed here, Salvatore Grasso et al . reports the effects of a continuous 40-cm H2O inflation pressure applied for 3–5 s in 22 patients undergoing controlled mechanical ventilation (MV) with a VTof 6 ml/kg and a mean PEEP of 9 cm H2O. Pulmonary gas exchange, respiratory mechanics, and hemodynamics were assessed. The investigation was done at two different times during the evolution of ARDS, i.e. in early (1 to 2 days after beginning MV) or later phases (5–10 days after onset of MV). The results are clearcut: only in the early phase of the disease was a marked positive response in the Pao2/ Fio2ratio observed. Patients in the later phases of ARDS had smaller increases in oxygenation, which were associated with a significant decrease in cardiac output during the inflation maneuver. This difference may simply be related to respiratory mechanics; in the early phase of ARDS, the elastance of the lung and chest wall were lower, which means higher compliance values, and hence, larger increases in volume for a given change in distending pressure.In the second study, 2Nicolo Patroniti et al. describes thirteen ARDS patients intubated and breathing spontaneously, assisted by an inspiratory pressure support of 12 cm H2O and a PEEP of 11 cm H2O, resulting in a VTof 420 ml. A majority of these patients can be considered to be in an early phase of the disease. The recruitment maneuver applied was similar to that used by Patroniti et al. ; airway pressure was increased to a mean of 38 cm H2O for 3.6 s, resulting in a VTof 1,150 ml. This sigh produced marked improvements in gas exchange and expiratory lung volume and a decrease in respiratory drive. What can we learn from these two elegant investigations? First, the findings confirm that sustained inflation using a pressure of 30 to 45 cm H2O results in a marked improvement in arterial oxygenation and has no significant side effects. 7Second, the reader will not be surprised to see that in early ARDS, functional respiratory units can be inflated and recruited more easily than in later phases of the disease, and that the gain in arterial oxygenation is more impressive. Similar results have been shown for prone positioning where improvement in gas exchange is greater in patients with pulmonary edema or with early ARDS, as compared with late ARDS or pulmonary fibrosis. 8Third, it should not be forgotten that alveolar recruitment and pulmonary gas exchange depend largely on the level of PEEP applied during protective ventilatory strategies. 9In conclusion, the two articles on recruitment maneuvers published in this issue add important information for the clinician; however, a number of questions remain. For instance, further studies should explore which single or combined therapy, including higher PEEP levels, regular position changes, and intermittent inflation strategies achieves the greatest sustained improvement in lung function. In addition, the effects on outcome in ARDS patients, including end-points such as duration of mechanical ventilation, duration of stay in the intensive care unit, and hospital mortality, must be assessed. It seems likely that understanding the role of factors such as the underlying morphologic features of lung injury, the stage of ARDS, hemodynamic status, and tolerance to increased intrapulmonary pressure will be essential to determining optimal treatment strategies for individual patients.