The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts

A COMPARISON OF FUNCTIONAL RESIDUAL CAPACITY MEASUREMENT BETWEEN A MODIFIED NITROGEN WASHOUT/WASHIN TECHNIQUE VERSUS BODY PLETHYSMOGRAPHY IN HEALTHY VOLUNTEERS

Stanley M. Baldwin1, Michael H. Terry1, Jeanette M. Merrill-Henry1, Leo M. Langga1, Michael Lum1, Philip M. Gold1



Background: Bedside measurements of FRC might assist the clinician in setting optimal PEEP. The standard methods for measurement of functional residual capacity (FRC) include dilution with a low solubility gas, body plethysmography and a multiple breath washout technique. Recently, General Electric (GE) released the Engstrom Carestation ventilator including a modified nitrogen washout/washin FRC measurement technique. We sought to determine the accuracy and reliability of this measurement compared to the gold standard of FRC measurement at our institution, body plethysmography with a Sensor Medics V6200 Autobox.

Methods:
Four healthy subjects volunteered to have repeated measurements of their FRC by both methods; n = 45. The order of measurement was alternated to minimize bias. The subjects were connected to both systems using an appropriately sized Respironics PerformaTrakTM facemask with a Quest Medical Pulmogaurd PFT filter attached between the facemask and the circuit. A Fisher and Paykel MR850JHU humidifier was used when breathing on the GE Carestation ventilator; temperature was maintained at less than 30 degrees Celsius. Ventilator settings for all subjects were: CPAP 0, Pressure Support 0, Bias Flow 3LPM, Flow Trigger 1.5LPM, starting FIO2 0.25, step FIO2 0.35; breath scaling was set at 20 breaths. The washout/washin values had to differ by less than 25% to be included in the results, as recommended by the manufacturer, and washout & washin values were averaged for the GE-FRC measurement. Body plethysmography was performed using our standard protocol. Descriptive statistics and Bland-Altman analysis were used to compare methods.

Results:
Fifteen studies were excluded because of a greater than 25% difference between washout and washin FRC (33.3%). The mean difference in FRC for the GE washout/washin method versus body plethysmography was +837mL. The correlation coefficient (r) was 0.82 and R2 = 0.68. The coefficient of variance difference per individual ranged from -0.010 to 0.211. A Bland-Altman plot of accepted studies is presented below.

Conclusion:
The Bland-Altman plot demonstrates a positive bias and a proportional error at larger FRCs. The confidence interval for the data is very large compared to the desired treatment effect. The small number of subjects in this study greatly limits its generalizability. Further study is needed to understand the applicability of this new tool.