Role of absolute lung volume to assess alveolar recruitment in acute respiratory distress syndrome patients.

OBJECTIVE: It is commonly assumed that lung volume at zero end-expiratory pressure (functional residual capacity) is not affected by positive end-expiratory pressure (PEEP) even in presence of alveolar recruitment, and it is often ignored when measuring lung recruitment by pressure-volume curves. Aim of the study was to investigate the effect of PEEP on functional residual capacity, and quantify the error of considering equal functional residual capacity in measuring alveolar recruitment. DESIGN: Interventional human study. SETTING: A 10-bed general intensive care unit in a university hospital. PATIENTS: Ten sedated, curarized, mechanically ventilated acute respiratory distress syndrome patients. INTERVENTIONS: Three levels of PEEP (5, 10, and 15 cm H2O), were randomly applied, for 1 hr each. MEASUREMENTS AND MAIN RESULTS: At each PEEP we obtained a pressure-volume curve, the volume expired from PEEP to zero pressure (PEEP related lung volume) and functional residual capacity by helium dilution method. Functional residual capacity increased at increasing PEEP levels. Functional residual capacity was 507 +/- 292, 607 +/- 311, and 681 +/- 312 ml (p < .05), respectively, at PEEP 5, 10, and 15 cm H2O. Pressure-volume curves were aligned starting from PEEP related lung volume (relative volume method) or from end-expiratory lung volume at PEEP (absolute volume method). Recruitment was measured as vertical distance of pressure-volume curves at 20 cm H2O airway pressure. The relative volume method led to underestimation of recruitment (43 +/- 28% and 35 +/- 18 %, respectively, at PEEP 10 and 15). CONCLUSIONS: Functional residual capacity is affected by PEEP. Ignoring this effect leads to relevant underestimation of alveolar recruitment as measure by pressure-volume curve displacement.

Measurement of pulmonary edema in patients with acute respiratory distress syndrome.

OBJECTIVE: We measured pulmonary edema by thermal indocyanine green-dye double-dilution technique and quantitative computed tomography (CT) in patients with acute respiratory distress syndrome and compared the two techniques. DESIGN AND SETTING: Prospective human study in a university hospital. PATIENTS: Fourteen mechanically ventilated patients with acute respiratory distress syndrome (nine primary; nine with intubation <7 days). INTERVENTIONS: All patients underwent a spiral CT of the thorax. We measured pulmonary thermal volume (PTV) and its components, extravascular lung water and pulmonary blood volume, with an integrated fiberoptic monitoring system (COLD Z-021). MEASUREMENTS AND RESULTS: PTV was tightly correlated with lung weight (LW) measured by CT (PTV = 0.6875 * LW(CT) + 292.77; correlation coefficient = 0.91; p < .0001; bias -11 +/- 8 %). Neither etiology of acute respiratory distress syndrome (primary vs. secondary) nor days of intubation affected the accuracy of thermal dye dilution in comparison with CT. There was no correlation between the extravascular lung water (12.3 +/- 3.4 mL/kg) and CT distribution of lung tissue compartments. Extravascular lung water and pulmonary blood volume showed good reproducibility in 32 pairs of thermal dye dilution measurements. CONCLUSIONS: Measurements of lung edema by thermal indocyanine green-dye double-dilution method show good correlation with those by quantitative computed tomography and good reproducibility in patients with acute respiratory distress syndrome.

Lung volume in mechanically ventilated patients: measurement by simplified helium dilution compared to quantitative CT scan.

OBJECTIVE: We describe a simplified helium dilution technique to measure end-expiratory lung volume (EELV) in mechanically ventilated patients. We assessed both its accuracy in comparison with quantitative computerized tomography (CT) and its precision. DESIGN AND SETTING: Prospective human study. PATIENTS: Twenty-one mechanically ventilated ALI/ARDS patients. INTERVENTIONS: All patients underwent a spiral CT scan of the thorax during an end-expiratory occlusion. From the CT scan we computed the gas volume of the lungs (EELVCT). Within a few minutes, a rebreathing bag, containing a known amount of helium, was connected to the endotracheal tube, and the gas mixture diluted in the patient's lungs by delivering at least ten large tidal volumes. From the final helium concentration, EELV could be calculated by a standard formula (EELVHe). MEASUREMENT AND RESULTS: The results obtained by the two techniques showed a good correlation (EELVHe=208+0.858xEELV(CT), r=0.941; P<0.001). Bias between the two techniques was 32.5+/-202.8 ml (95% limits of agreement were -373 ml and +438 ml), with a mean absolute difference of 15%. The amount of pathological tissue did not affect the difference between the two techniques, while the amount of hyperinflated tissue did. Bias between two repeated helium EELV measurements was -24+/-83 ml (95% limits of agreement were -191 ml and +141 ml), with a mean absolute difference of 6.3%. CONCLUSIONS: The proposed helium dilution technique is simple and reproducible. The negligible bias and the acceptable level of agreement support its use as a practical alternative to CT for measuring EELV in mechanically ventilated ARDS patients.

Head helmet versus face mask for non-invasive continuous positive airway pressure: a physiological study.

OBJECTIVE: To assess selected physiological effects of non-invasive continuous positive airway pressure delivered by head helmet (CPAPH), a special interface device designed to completely contain the head of the patient, compared to face mask (CPAPM). DESIGN: Randomized physiological study. SETTING: University research laboratory. PATIENTS: Eight healthy volunteers. INTERVENTION: Continuous positive airway pressure delivered by face mask and CPAPH in random order. Three gas flow rates (20-30-40 l/min and 30-45-60 l/min, respectively, for CPAPM and CPAPH) and four CPAP levels (0-5-10-15 cmH2O) were employed in a randomized sequence. MEASUREMENTS AND RESULTS: In each patient we monitored airway pressure, esophageal pressure, expiratory flow, and inspiratory and expiratory CO2 concentration. End-expiratory lung volume changes from CPAP 0 were measured by inductance plethysmography. The application of increased levels of CPAP resulted in a significant increase in end-expiratory lung volume, similar for CPAPH and CPAPM. Inspiratory changes of airway pressure were comparable for the two CPAP modes. Inspiratory CO2 concentration was higher during CPAPH (significantly decreased at increased gas flow rates), compared to CPAPM. CONCLUSIONS: Continuous positive airway pressure delivered by head helmet is as effective as CPAPM in increasing end-expiratory lung volume and in compensating for airway pressure changes without the need of a reservoir bag. Higher gas flow rates are necessary to maintain a relatively low inspiratory CO2 concentration.