Nitrogen washout/washin, helium dilution and computed tomography in the assessment of end expiratory lung volume

Abstract

Introduction: End expiratory lung volume (EELV) measurement in the clinical setting is routinely performed using the helium dilution technique. A ventilator that implements a simplified version of the nitrogen washout/washin technique is now available. We compared the EELV measured by spiral computed tomography (CT) taken as gold standard with the lung volume measured with the modified nitrogen washout/washin and with the helium dilution technique.

Methods: Patients admitted to the general intensive care unit of Ospedale Maggiore Policlinico Mangiagalli Regina Elena requiring ventilatory support and, for clinical reasons, thoracic CT scanning were enrolled in this study. We performed two EELV measurements with the modified nitrogen washout/washin technique (increasing and decreasing inspired oxygen fraction (FiO2) by 10%), one EELV measurement with the helium dilution technique and a CT scan. All measurements were taken at 5 cmH2O airway pressure. Each CT scan slice was manually delineated and gas volume was computed with custom-made software.

Results: Thirty patients were enrolled (age = 66 +/- 10 years, body mass index = 26 +/- 18 Kg/m2, male/female ratio = 21/9, partial arterial pressure of carbon dioxide (PaO2)/FiO2 = 190 +/- 71). The EELV measured with the modified nitrogen washout/washin technique showed a very good correlation (r2 = 0.89) with the data computed from the CT with a bias of 94 +/- 143 ml (15 +/- 18%, p = 0.001), within the limits of accuracy declared by the manufacturer (20%). The bias was shown to be highly reproducible, either decreasing or increasing the FiO2 being 117+/-170 and 70+/-160 ml (p = 0.27), respectively. The EELV measured with the helium dilution method showed a good correlation with the CT scan data (r2 = 0.91) with a negative bias of 136 +/- 133 ml, and appeared to be more correct at low lung volumes.

Conclusions: The EELV measurement with the helium dilution technique (at low volumes) and modified nitrogen washout/washin technique (at all lung volumes) correlates well with CT scanning and may be easily used in clinical practice.

Trial registration: Current Controlled Trials NCT00405002.

Figures

Figure 1

Comparison of end expiratory lung volume (EELV) measured by the Engstrom Carestation and the computed tomography (CT) scan. (a)The EELV measured by the Engstrom Carestation as a function of the EELV measured by the computed tomography (CT) scan (EELV carestation = 242 + 0.85 × EELV CT scan, r2 = 0.89, p < 0.00001). (b) The Bland-Altman plot of the EELV measured with the CT scan and the EELV measured with the Engstrom Carestation. The x axis shows the mean of the two measurement and the the y axis shows the difference between the EELV measured by the Engstrom Carestation and the EELV measured by the CT scan (average difference 93 ± 143 ml, limits of agreement -50 – 236 ml).

Figure 2

The relative error of the procedure performed by the Engstrom Carestation. This is expressed as (EELVGE – EELVCT SCAN)/EELVCT SCAN as a function of the ratio between tidal volume and the end expiratory lung volume (EELV) measured by computed tomography (CT) scan ((EELVGE – EELVCT SCAN)/EELVCT SCAN = 0.05 + 0.43 × (Tidal Volume/EELVCT SCAN, r2 = 0.58, p < 0.0001).

Figure 3

Accuracy of the nitrogen washin/washout technique. (a) The relation between the EELV measured by increasing the FiO2 as a function of the EELV obtained decreasing FiO2. The EELV obtained increasing the FiO2 was -56 + 1.0078 multiplied by the EELV obtained decreasing the FiO2 (r2 = 0.84, p < 0.0001). (b) The Bland-Altman plot of the EELV measurement obtained increasing the FiO2and the EELV obtained decreasing the FiO2. The x axis shows the mean of the two measurements and the difference between the EELV measured by increasing FiO2 and the y axis shows the EELV obtained decreasing FiO2 (average difference 48 ± 165 ml, limits of agreement -117–213 ml).

Figure 4

Comparison of end expiratory lung volume (EELV) measured by the helium dilution technique and the computed tomography (CT) scan. (a) The EELV measured by the helium dilution technique as a function of the EELV measured by the CT scan (EELV helium dilution = 20 + 0.84 × EELV CT scan, r2 = 0.91, p < 0.00001). (b) The Bland-Altman plot of the EELV measured with the CT scan and the EELV measured with the helium dilution method. The x axis shows the mean of the two measurements and the y axis shows the difference between the EELV measured by the helium dilution method and the EELV measured by the CT scan (average difference -136 ± 133 ml, limits of agreement -3 – 269 ml). The difference between the EELV measured with the helium dilution method and the EELV measured with CT scan was significantly correlated with the EELV, expressed as the average between the two measurements (Helium EELV - CT scan EELV = -15.52764 + -0.17034 × (helium EELV + CT scan EELV)/2, r2 = 0.21, p = 0.005838).

Figure 5

Comparison of end expiratory lung volume (EELV) measured by the helium dilution technique and the nitrogen washout/washin method. (a) The EELV measured by the helium dilution as a function of the EELV measured by nitrogen washout/washin method (EELV helium dilution = (290 + 0.92) × EELV GE, r2 = 0.82, p < 0.00001). (b) The Bland-Altman plot of the EELV measured with the nitrogen washout/washin technique and the EELV measured with the helium dilution method. The x axis shows the mean of the two measurements and the y axis shows the difference between the EELV measured by then helium dilution method and the nitrogen washoutwashin measured by the CT scan (average difference -229 ± 164 ml, limits of agreement -558 – 100 ml).