Pressure/volume curves and lung computed tomography in acute respiratory distress syndrome

Abstract

Pressure/volume curves and lung computed tomography in acute respiratory distress syndrome. J-J. Rouby, Q. Lu, S. Vieira. #ERS Journals Ltd 2003. ABSTRACT: Pressure/volume (P/V) curves can be measured by static methods, constant or sinusoidal flow methods and the dynostatic method that allows a breath-tobreath determination of P/V curves. Recent ventilators are equipped with specific flow generators and software aimed at obtaining P/V curves without disconnecting the patient from the ventilator.The most recent generation of computed tomography scanners allows the quantitative determination of lung aeration, lung volumes (gas and tissue), alveolar recruitment and lung overinflation of the whole lung. In the supine position, the acute respiratory distress syndrome (ARDS) lung is characterised by an increase in lung tissue that predominates in upper lobes and a massive loss of aeration that predominates in lower lobes.In a minority of ARDS patients, the loss of aeration is homogeneously distributed. The overall lung volume of upper lobes is preserved suggesting an alveolar floodinginduced loss of aeration. In contrast, the overall lung volume of lower lobes is reduced because the heart and the abdomen exert an external compression that contributes to the loss of aeration. The P/V curve is a lung recruitment curve and the chord compliance indicates the potential for recruitment. In such patients, alveolar recruitment resulting from positive end-expiratory pressure is not accompanied by lung overinflation.In a majority of acute respiratory distress syndrome patients, upper lobes remain partially or totally aerated despite a marked regional increase in lung tissue. The upper lobes9 overall lung volume is either normal or increased, suggesting that the lung does not collapse under its own weight as generally believed. In lower lobes, the overall lung volume is reduced because the heart and the abdomen exert an external compression that contributes to the loss of aeration. The pressure/volume curve is influenced by the recruitment of poorly and nonaerated lung regions and by the mechanical properties of the part of the lung remaining aerated. In such patients, alveolar recruitment resulting from positive end-expiratory pressure w10 cmH 2 O is preceded and accompanied by lung overinflation. Eur Respir J 2003; 22: Suppl. 42, 27s- Respiratory pressure/volume (P/V) curves can be easily obtained at the bedside in critically ill patients whose lungs are mechanically ventilated. Many ventilators are now equipped with flow generators and software that allow the obtention and the analysis of the P/V curve without disconnecting the patient from mechanical ventilation. Hypothesis and concepts serving for interpreting the P/V curves of patients with acute respiratory distress syndrome (ARDS) have been deeply modified during the past 15 yrs. Initially believed to reflect the mechanical properties of the "baby" lung [1], the P/V curve is now rather considered as a lung recruitment curve [2] which is markedly influenced by the regiona...

Measurement of Alveolar Recruitment at the Bedside: The Beginning of a New Era in Respiratory Monitoring?

“…The first milliliters penetrating into the respiratory system at the beginning of the inflation maneuver are entering poorly aerated lung regions and contribute to their recruitment. 2,[7][8][9] In ARDS patients with normally aerated lung regions at zero PEEP (focal ARDS, characterized by radiological attenuations predominating in the lower quadrants), the static PV curve depends both on the elastance/compliance of aerated lung regions and on the recruitment of poorly and non-aerated pulmonary areas. 7 Lung recruitment at a given airway pressure can be defined as the difference in lung volume between PV curves starting at different end-expiratory lung volumes corresponding to different PEEP.…”

“…2,[7][8][9] In ARDS patients with normally aerated lung regions at zero PEEP (focal ARDS, characterized by radiological attenuations predominating in the lower quadrants), the static PV curve depends both on the elastance/compliance of aerated lung regions and on the recruitment of poorly and non-aerated pulmonary areas. 7 Lung recruitment at a given airway pressure can be defined as the difference in lung volume between PV curves starting at different end-expiratory lung volumes corresponding to different PEEP. This method was validated against the reference computed tomography scan method, 2 defining PEEP-induced alveolar recruitment as the amount of gas penetrating into poorly and non-aerated lung regions after PEEP.…”

“…This method was validated against the reference computed tomography scan method, 2 defining PEEP-induced alveolar recruitment as the amount of gas penetrating into poorly and non-aerated lung regions after PEEP. 7 Two methods based on PV curves and providing automatic measurement of PEEP-induced alveolar recruitment have been implemented on mechanical ventilators and are actually commercially available: the PV recruitment module on the eXtend ventilator (Air Liquide Medical Systems, Antony, France) and the PEEP Inview module of the Engström Carestation (GE Healthcare, Madison, Wisconsin).…”

Lung CT Scan

“…Overdistension may occur in regions that are normally aerated if alveolar flooding and/or interstitial oedema is present [3]. Despite these limitations, in ALI tidal hyperaeration has shown close correlation with overdistension, as evidenced from simultaneous assessment of respiratory mechanics [13][14][15]. Also, changes in aeration status of non-aerated and poorly aerated regions have been used to track tidal reaeation and/or recruitment in the lungs [10,15], which may promote lung injury [16].…”

“…The Pressure-Volume (P/V) curves of the respiratory system have been proposed to identify patients with higher potential of recruitment [14,49]. P/V curves can be measured by static, with constant or sinusoidal flow methods and/or dynostatic methods for a breath-to-breath determination.…”