Measurement of functional residual capacity by modified multiple breath nitrogen washout for spontaneously breathing and mechanically ventilated patients

• Published in: British journal of anaesthesia : BJA Vol. 107; no. 5; pp. 796 - 805
• Main Authors: Brewer, L.M., Orr, J.A., Sherman, M.R., Fulcher, E.H., Markewitz, B.A.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01-11-2011; Oxford University Press
• Subjects: Adult; Anesthesia; Anesthesia. Intensive care medicine. Transfusions. Cell therapy and gene therapy; Biological and medical sciences; Critical Care; Feasibility Studies; Female; Functional Residual Capacity; Humans; Lung - metabolism; lung, functional residual capacity; Male; measurement techniques; Medical sciences; model, mathematical; monitoring, intensive care; Monitoring, Physiologic - methods; Nitrogen - metabolism; Plethysmography, Whole Body; Reproducibility of Results; Respiration; Respiration, Artificial; Tidal Volume; measurement techniques; monitoring, intensive care; lung, functional residual capacity; model, mathematical; Human; lung; Nitrogen washout; Intensive care; Measurement technique; Anesthesia; model; Functional residual capacity; Monitoring; mathematical
• ISSN: 0007-0912; 1471-6771
• DOI: 10.1093/bja/aer220

There is a need for a bedside functional residual capacity (FRC) measurement method that performs well in intensive care patients during many modes of ventilation including controlled, assisted, spontaneous, and mixed. We developed a modified multiple breath nitrogen washout method for FRC measurement that relies on end-tidal gas fractions and alveolar tidal volume measurements as inputs but does not require the traditional measurements of volume of nitrogen or oxygen. Using end-tidal measurements, not volume, reduces errors from signal synchronization. This study was designed to assess the accuracy, precision, and repeatability of the proposed FRC system in subjects with variable ventilation patterns including some spontaneous effort.
The accuracy and precision of measurements were assessed by comparing the novel N2 washout FRC values to the gold standard, body plethysmography, in 20 spontaneously breathing volunteers. Repeatability was assessed by comparing subsequent measurements in 20 intensive care patients whose lungs were under controlled and assisted mechanical ventilation.
Compared with body plethysmography, the accuracy (mean bias) of the novel method was −0.004 litre and precision [1 standard deviation (sd)] was 0.209 litre [mean (sd)] [−0.1 (5.9)% of body plethysmography]. The difference between repeated measurements was 0.009 (0.15) litre [mean (sd)] [0.4 (6.4)%]. The coefficient of repeatability was 0.31 litre (12.7%).
The modified multiple breath nitrogen washout method for FRC measurement provides improved precision and equivalent accuracy and repeatability compared with existing methods during ventilation with variable ventilation patterns. Further study of the novel N2 washout method is needed.