Low-tidal-volume ventilation should be implemented in the context of a broader strategy of critical care management in a patient with acute lung injury or ARDS. An initial tidal volume of 6 ml per kilogram of predicted, not actual, body weight should be used, as in the ARDSNet trial.19
The predicted body weight (PBW) is calculated as follows: for men, PBW = 50.0 + 0.91 (height in centimeters – 152.4); and for women, PBW = 45.5 + 0.91 (height in centimeters – 152.4).
The concept underlying this approach is that it normalizes the tidal volume to lung size, since lung size has been shown to depend most strongly on height and sex. For example, a person who ideally weighs 70 kg and who then gains 35 kg has essentially the same lung size as he or she did when at a weight of 70 kg and should not receive ventilation with a higher tidal volume just because of the weight gain.
The initial respiratory rate should be set in the range of 18 to 22 breaths per minute. This is a somewhat higher rate than is used in other ventilatory schemes; it is intended to maintain a minute ventilation that is high enough to avoid marked hypercapnia. However, some degree of hypercapnia is to be expected with low-tidal-volume ventilation. Ideally, the partial pressure of carbon dioxide should rise gradually to prevent acute acidemia and to ensure hemodynamic stability. Specific target values of partial pressure of carbon dioxide and pH are debatable, although some clinicians would argue to keep the current guidelines of a partial pressure of carbon dioxide of less than 80 mm Hg and a pH of greater than 7.20. Although the administration of sodium bicarbonate has sometimes been advocated to maintain an acceptable pH, this is controversial in theory and rarely necessary in practice. In fact, mean partial pressures of carbon dioxide below 50 mm Hg were usually achieved in the ARDSNet study in the low-stretch group.
The response to low-tidal-volume ventilation should be assessed initially on the basis of plateau airway pressure. The goal should be to maintain a plateau airway pressure (i.e., the pressure during an end-inspiratory pause) of 30 cm of water or less; if this target is exceeded, the tidal volume should be further reduced to a minimum of 4 ml per kilogram of predicted body weight. An important caveat relates to patients who have stiff chest walls (for example, those with massive ascites). In such patients, it is reasonable to allow the plateau pressure to increase to values greater than 30 cm of water, since the pleural pressures are elevated and hence the transpulmonary pressures are not elevated (i.e., there is not necessarily alveolar over-distention). Whether the tidal volume should be increased in the patient with a plateau pressure substantially lower than 30 cm of water is less clear; given the lack of evidence of a safe threshold, some experts would argue that the lower the plateau pressure, the better, provided that the patient is comfortable and that gas-exchange goals are reached.
The optimal FiO2
also requires consideration in the context of low-stretch ventilation. Since severe hypoxemia is a characteristic feature of ARDS, efforts to improve oxygenation and to achieve a target arterial oxyhemoglobin saturation of about 90% may initially require high FiO2
levels. However, the prolonged use of high FiO2
levels can theoretically increase the risk of oxygen toxicity, which may actually increase injury to the lung parenchyma. Therefore, other adjustments may be necessary to improve oxygenation while reducing the FiO2
. One approach is to use PEEP to increase oxygenation, although this should be done while plateau airway pressure is monitored. In the ARDSNet trial,19
combinations of FiO2
and PEEP values were specified for both study groups according to predefined settings (). However, the level of oxygenation is a poor predictor of outcome. In the ARDSNet trial, oxygenation was worse in the low-stretch group, despite a reduced mortality rate. Therefore, some experts recommend the application of PEEP based on lung mechanics rather than gas exchange (see below).
Settings for Positive End-Expiratory Pressure (PEEP), According to the Required Fraction of Inspired Oxygen (FiO2).*
Alternatives to low-tidal-volume ventilation either have been unsuccessful (e.g., partial liquid ventilation26
) or are unproven (e.g., high-frequency oscillation27,28
). However, many unproven strategies, such as open-lung protective ventilation or prone positioning, may be useful in combination with low-tidal-volume ventilation29,30
and thus should not be considered to be competing therapies.