Reversing Lung Collapse and Hypoxemia in Early ARDS
Abstract & Commentary
By David J. Pierson, MD, Editor, Professor, Pulmonary and Critical Care Medicine, Harborview Medical Center, University of Washington, Seattle, is Editor for Critical Care Alert.
Synopsis: Using a stepwise lung recruitment protocol using airway pressures as high as 60 cm H2O in patients with early ARDS, it was possible to achieve maximum reversal of atelectasis as determined by CT scan in 24 of 26 patients.
Source: Borges JB, et al. Reversibility of lung collapse and hypoxemia in early acute respiratory distress syndrome. Am J Respir Crit Care Med. 2006;174:268-278.
Borges and colleagues in Sao Palo, Brazil, studied 26 patients with acute lung injury or the acute respiratory distress syndrome (ARDS) to determine whether atelectatic lung areas could be fully recruited and oxygenation improved using a protocol employing progressively higher levels of positive end-expiratory pressure (PEEP). At the time of the study, the patients (median age 44 years, 46% women, with a variety of underlying pulmonary and non-pulmonary primary disease processes) had median PaO2/FIO2 ratios of 94 (range, 45-294) mm Hg, and had been receiving mechanical ventilation for a median of 2 (range, 1-7) days.
The patients were sedated and paralyzed, and ventilated with tidal volumes of approximately 6 mL/kg predicted body weight (PBW) using pressure control ventilation. A pressure-volume curve was constructed for each of the first 11 patients, who were studied in the computed tomography (CT) suite. After a 40-second recruitment maneuver at 40 cm H2O of continuous positive airway pressure, PEEP was set at 2 cm H2O above the lower inflection point. A protocol was then carried out which consisted of setting PEEP at 25 cm H2O and then sequentially increasing it by 5 cm H2O increments (with 15 cm H2O of additional inspiratory pressure) in 2-minute intervals with intervening "rest" periods on 25 cm H2O. This was continued until an airway pressure of 60 cm H2O was reached during inspiration, or the numerical sum of PaO2 + PaCO2 exceeded 400 mm Hg, or an adverse effect (mixed venous oxygen saturation < 80%, mean arterial pressure < 60 mm Hg, or overt barotrauma on CT) occurred. Arterial blood gases were assessed at each step, as was lung recruitment by CT scan. The last 15 patients had late recruitment evaluated in the ICU rather than in the CT suite.
With the stepwise recruitment protocol employed, all 26 patients were studied at step 1, 17 of them at step 2, 13 at step 3, 11 at step 4, and only 8 at step 5 (the highest airway pressure). Mean arterial PCO2 values at these steps were 70, 75, 81, 89, and 95 mm Hg, respectively; the mean arterial pH fell from 7.15 at baseline to 6.94 at step 5. No patient required discontinuation of the protocol because of hemodynamic compromise, although one patient developed subcutaneous emphysema.
Oxygenation improved and CT-determined lung collapse decreased in all patients. In 24 of the 26 patients, the authors' criteria for "maximum lung recruitment" were met. Maximum airway pressures required for this recruitment ranged from 40 to 60 cm H2O. There was a strong and inverse correlation between arterial oxygenation and the percentage of collapsed lung by CT (R = -0.91; P < 0.0001). By their technique for assessing lung over-inflation by CT, the authors did not detect this effect in the non-dependent areas of their patients' lungs.
Commentary
In managing patients with ARDS, using low tidal volumes (≤ 6 mL/kg PBW) and limiting lung distending pressures (keeping end-inspiratory plateau pressure ≤ 30 cm H2O) saves lives—in comparison with the use of higher tidal volumes (12 mL/kg PBW) and higher plateau pressures.1 A number of other interventions subjected to clinical trials—such as prone positioning, inhaled nitric oxide, high-frequency ventilation, and the use of high levels of PEEP—have been shown to improve various indices of pulmonary mechanics and gas exchange, at least in the short term, but not to affect survival. So far, interventions to maximally recruit the lung in ARDS have demonstrated short-term physiologic benefits but have not been shown to improve clinical outcomes.
The concept of lung recruitment holds that, particularly in early ARDS, much of the lung is collapsed, and that optimally opening the collapsed alveoli and keeping them open throughout the ventilatory cycle will improve not only lung compliance and gas exchange but also clinical outcomes. This study by Borges et al deals with this last approach to managing patients with ARDS, and uses an approach to lung recruitment that is more vigorous than those used in many studies.
In many patients with ARDS, when they are completely relaxed and making no respiratory efforts, the relationship between airway pressure and lung volume has the 3-part shape illustrated in the Figure. As distending pressure is initially increased from functional residual capacity in the absence of PEEP, much of the lung is atelectatic and little increase in volume occurs. As distal lung tissue opens with increasing pressure, an inflection point can often be identified, such that the slope of the curve of volume change over pressure change (compliance) increases. During tidal ventilation in this portion of the curve (using PEEP to maintain end-expiratory lung volume), alveoli are presumably open but not over-inflated.
Above a certain pressure, however, hyperinflation occurs, as shown by a decrease in the slope of the curve. Theoretically, ventilation with a given tidal volume that occurs on the steep portion of the curve (corresponding to the dotted line labeled B in the Figure) will yield better gas exchange and produce less shear injury than the same tidal volume delivered at a lower lung volume (breath A), and also less injury from overdistension than if delivered at a higher lung volume (breath C). Tidal ventilation at A would subject collapsed alveoli to repetitive opening and closing, while tidal ventilation at C would stretch their walls and risk both overdistension injury and overt barotrauma.
In the present study, Borges et al have shown that maximum lung recruitment can be achieved in patients with early ARDS (moving them from position A to position B on the curve in the Figure), and that this is accompanied by optimization of gas exchange. The investigators also present evidence to support their contention that severe hyperinflation (ventilation at position C) was generally avoided. They conclude that, using their protocol of sequential lung recruitment and very high airway pressures, the index of (PaO2 + PaCO2) ≥ 400 mm Hg, on 100% oxygen and with the patient paralyzed, 5% or less of collapsed lung units have been opened.
In the TV ads that show the latest luxury car swooping effortlessly through tight curves, there is always a disclaimer saying "Closed circuit with professional driver: do not attempt." While the results presented in this article are intriguing, the interventions involved are potentially dangerous, and the message here should be the same as in the TV ads. As the authors state, "It is often possible to reverse hypoxemia and fully recruit the lung in early ARDS. Due to transient side effects, the required maneuver still awaits further evaluation before routine clinical application." Also unknown at this point is whether the non-intuitive PaO2 + PaCO2 index will prove to be clinically relevant, and whether maximally recruiting the lung will be shown to benefit patients in terms of outcome.
Reference
- Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med. 2000;342:1301-1308.
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