Study population
Measurements were taken from 30 patients admitted to the ICU from November 2006 to September 2007. The study was approved by the institutional review board of our hospital, and written informed consent was obtained from conscious patients and delayed consent in unconscious patients. Inclusion criteria was the clinical need of a lung CT scan in patients already in mechanical ventilation. Exclusion criteria were age younger than 16 years, pregnancy, haemodynamic instability, documented barotrauma and the presence of chronic lung disease (e.g. chronic obstructive pulmonary disease (COPD)).
Data collection
The CT scan was performed at end expiration at 5 cmH2O positive end expiratory pressure (PEEP), which is the standard pressure at which CT scans are taken for clinical purposes. Thereafter, the patients underwent the measurement of the EELV with the nitrogen washout/washin technique performed with the Engstrom Carestation ventilator and with helium dilution at 5 cmH2O PEEP. The nitrogen washout/washin technique was performed by either decreasing or increasing the inspired oxygen fraction (FiO2). The average result of these two measurements allowed the accuracy of the Engstrom Carestation in relation to the CT scan to be assessed. Each sequence (decreasing or increasing the FiO2) was performed twice to assess the precision (repeatability) of the technique. The accuracy of the helium dilution technique was assessed relative to the CT scan measurement. Only one CT measurement was performed, as the precision of this method has previously been determined [17].
Quantitative computed tomography analysis
The CT scanner was set as follows: collimation 5 mm; interval 5 mm; bed speed 15 mm per second; voltage 140 kV; and current 240 mA. A whole lung CT scan was performed at a PEEP value of 5 cmH2O during an end-expiratory pause. Immediately before each CT scan was obtained, a recruitment manoeuvre was performed. Lungs profile was manually delineated in each cross-sectional lung image in order to identify the regions of interest. Each region of interest was then processed and analysed by a custom-designed software package (Soft-E-Film, University of Milan, Italy), as previously described [18].
We assumed that lung tissue has a density similar to that of water (Hounsefield unit number 0) and considered each voxel to be made of lung tissue and air (Hounsefield unit number -1000). Gas volume can be computed from the Hounsefield number of each voxel according to the following formula:
Gas volume = (CT number/-1000) × voxel volume
The EELV is the sum of gas volumes present in all the voxel included in the lung profiles.
Nitrogen washout/washin technique
The nitrogen washout/washin technique is based on the following principle: the gas lung volume, at baseline, includes a volume of nitrogen (V(1)N2) that is determined by the alveolar fraction of nitrogen (FAN2(1)) (which varies inversely to the alveolar oxygen fraction) and by the EELV accordingly to the following relation:
V(1)N2 = FAN2(1) × EELV
If the alveolar nitrogen fraction (FAN2(2)) is changed by changing the FiO2, a new nitrogen volume (V(2)N2) will be present in the lung after the equilibrium time:
V(2)N2 = FAN2(2) × EELV
Assuming that after changing the FiO2 the total EELV does not change until the new equilibrium in alveolar gas composition is reached, by subtracting term by term in the equation 1 and 2 the following relation holds true:
VN2(1) - VN2(2) = (FAN2(2) - FAN2(1)) × EELV
as the changes in FAN2 are specular to the changes in FiO2, i.e. ΔFAN2 = -(FiO2(1) - FiO2(2)), the EELV can be calculated as:
EELV = ΔN2 (ml)/ΔFiO2
where ΔN2 equals the nitrogen exhaled after the change of inspired FiO2 until the equilibration time is reached (about 20 breaths).
The algorithm of the nitrogen washout/washin technique employed by the Engstrom Carestation is detailed by Olegard and colleagues [14]. Nitrogen concentration in expired and inspired air is not directly measured but estimated from the end tidal concentrations of oxygen and carbon dioxide:
ETN2 (mmHg) = 713 - ETCO2 (mmHg) - ETO2 (mmHg)
The alveolar ventilation was calculated as:
Alveolar tidal volume expired = VCO2/ETCO2 × RR
Alveolar tidal volume inspired = Alveolar tidal volume inspired + ((VCO2/RQ + VCO2)/RR)
Inspired and expired nitrogen volumes were calculated as:
Expired nitrogen volume = ETN2/713 × Alveolar tidal volume expired
Inspired tidal volume = Inspired nitrogen fraction × alveolar tidal volume inspired.
Simplified helium dilution technique
A flexible tube was inserted between the Y-piece and the patient's endotracheal tube or tracheostomy. The operator clamped the tube during an end-expiratory pause at a PEEP level of 5 cmH2O and then connected it to a balloon filled with 1.5 L of a gas mixture of helium (13.07 ± 0.40%) in oxygen. After releasing the clamp, the same operator delivered 10 tidal volumes to the patient in order to dilute the helium gas mixture with the gas contained in the patient's lungs. At the end of this procedure, the balloon was clamped off the circuit, and the patient was reconnected to the ventilator. The concentration of helium in the balloon was then measured by a previously calibrated helium analyser (PK Morgan, Chatham, England). EELV was then calculated using the standard formula: EELV (ml) = (Vb × Ci/Cf) - Vb, where Ci is the helium concentration of the known gas mixture, Cf is the final helium concentration and Vc is the volume of gas in the balloon. The Vb was inflated with 1500 ml of helium-oxygen mixture at 25°C. The volume measured was corrected for body temperature (37°C) using the Gay-Lussac law.
We first performed the CT scan, then the helium dilution technique and at the end the modified nitrogen washout/washin technique. The helium dilution technique and the modified nitrogen washout/washin technique were performed at the CT scan facility without moving the patient from the bed, to maintain the patient's condition. The whole experimental procedure (nitrogen washout/washin technique, helium dilution and CT scan) was performed in a total time of about five minutes.
Lung mechanics
The total inspiratory resistance of the respiratory system was calculated by dividing the difference in peak inspiratory airway pressure and the plateau inspiratory pressure, measured during an end inspiratory pause, by the inspiratory flow preceding the occlusion. The compliance of the respiratory system was calculated by dividing the plateau inspiratory pressure measured during an end inspiratory pause by the tidal volume.
Statistical methods
Gas volumes measured with CT scan, nitrogen washout/washin technique and helium dilution technique were compared with the Bland-Altman technique [19] and using a linear regression model. The bias of the EELV measurement performed increasing or decreasing the FiO2 were compared using a Student's t-test. Statistical analysis was performed with the R-project software (R foundation for statistical computing, Vienna, Austria [20]).