Bohr's dead space (VD Bohr ) measurement is commonly calculated using end-tidal CO 2 instead of the true alveolar partial pressure of CO 2 (PACO 2 ). Tusman et al. [97] compared the measurement of VD Bohr using PACO 2 derived from either volumetric capnography or the standard alveolar air formula. The authors concluded that VD Bohr can be calculated with accuracy using volumetric capnography. In a correspondence letter, Graf [98] expressed concerns about using Bohr's formula for measuring physiological dead space. The measurement may refer to airway (VD aw ) but not alveolar (VD alv ) fraction. The letter also argued that using CO 2 as a tracer of overall ventilatory efficiency still requires the measurement of VCO 2 , expired minute ventilation and PaCO 2 . The value of PaCO 2 may be increased by shunt and/or V/Q inequalities, leading to physiological dead space overestimation. This is an interesting topic, and more studies and discussion are encouraged to clarify this issue. Despite all of the technical advances of recent years, auscultation provides both useful physiological information and close patient-physician interaction. Today, there are automated systems that are available to detect, analyse, and interpret lung sounds. Image-based techniques of lung sound analysis such as vibration response imaging have been introduced and tested clinically. However, studies are warranted to determine their potential role in clinical practice. Vena et al. [99] report an analysis of the spectral characteristics of lung sounds. They observed significant correlation between intratidal recruitment measured with dynamic CT and the degree of crackles in the sound analysis, suggesting that air passage through atelectasis as well as poorly aerated areas is crucial for generating lung crackle sounds. They concluded that the computer-based analysis of crackle sounds is more sensitive for differentiating between small differences in healthy and injured conditions, especially at higher positive end-expiratory pressure (PEEP) levels, compared to conventional clinical auscultation. This simple and noninvasive technique of computer-based lung sound analysis may help intensivists monitor mechanical ventilation and diagnostic or therapeutic procedures such as recruitment manoeuvres in real time at the bedside. Yet the validation of the technique, and the variation of frequent measurements and interpretation of complex respiratory signals to optimise ventilator settings or to detect alveolar recruitment, especially in injured lungs, remain to be further investigated at the bedside [100] . Endotracheal tubes at high volume and low-pressure cuffs may fail to protect the lower airway from leakage of potentially contaminated secretions below the longitudinal folds. Ouanes et al. [101] studied the effects of PEEP levels, inspiratory effort intensity, peak pressures, tracheal tube cuff materials and sizes on the leakage of fluids past the cuff in an in vitro model of tracheal intubation and mechanical ventilation. The authors observed that leakage occurred more frequently at lower levels of PEEP, higher inspiratory efforts were associated with higher leakage, and polyurethane cuffs performed better than polyvinylchloride cuffs. In theory, most aspiration during mechanical ventilation occurs when tracheal pressure falls below hydrostatic pressure. Thus, the most important dependent variables are PEEP, inspiratory efforts and duty cycles. More studies are required to examine the effects of mechanical ventilation on leakage across the cuff.