Measurement of Functional Residual Capacity of the by Nitrogen Washout, Carbon Dioxide Rebreathing and Body Plethysmography in Healthy Volunteers Lara Brewer, M.S. and Joseph Orr, Ph.D. Department of Bioengineering, University of Utah

Abstract ventilation management or as a way to follow Background: We measured Functional Residual disease progression for patients with acute lung Capacity (FRC) of the with three methods in 1,2 healthy volunteers. The three techniques included a injury and ARDS . While traditional methods of CO2 partial rebreathing technique, nitrogen washout FRC measurement are valuable for researching technique, and the reference technique for ambulatory patients, body plethysmography. disease progression and for monitoring Materials and Methods: After granting consent to an ambulatory patients, they have been of limited IRB-approved protocol, each of the 20 healthy volunteers participated in FRC measurement by three utility at the bedside due to difficulty of use, high methods, including body plethysmography, carbon expense, and impracticality during mechanical dioxide (CO2) rebreathing, and nitrogen washout. Gas concentration and volume data were collected from ventilation. FRC measurement at the bedside for the distal side of a mouthpiece during spontaneous ventilation for the washout and rebreathing mechanically and spontaneously ventilated measurements. The FRC was measured twice with a nitrogen washout measurement technique and then patients has recently been reported by several signals from five partial CO2 rebreathing researchers, who evaluated systems based on measurement cycles were collected. Finally, the nitrogen washout FRC measurements were repeated nitrogen washout3-7 and Electrical Impedance twice. We compared the average CO2 rebreathing 8,9 FRC measurements and the average nitrogen washout Tomography . FRC measurements to the body plethysmography FRC The nitrogen washout and CO2 measurements for each subject through statistical methods of linear regression analysis and Bland- rebreathing methods have previously been Altman Analysis. validated by our research group with the use of a Results: The squared correlation coefficient for the linear regression between nitrogen washout and body bench lung model. We also previously evaluated 2 plethysmography measurements was r = 0.91 (n = 10 35). The bias ± Standard Deviation was 0.054 ± 0.373 both methods in an animal study . The aim of L. this study was to evaluate the accuracy of the two Conclusion: These results indicate FRC measurement by nitrogen washout correlate well with the body FRC measurement systems compared to the plethysmography reference standard in ambulatory, clinical gold standard for ambulatory patients, spontaneously subjects. This method could possibly be used in space to monitor lung function. body plethysmography, in healthy, spontaneously breathing volunteers. Introduction Functional Residual Capacity (FRC) measurements may be useful as a guide for

1 Methods: manufacturer’s specifications for the Collins Testing protocol body plethysmograph (Model BP, Warren E. Twenty healthy volunteers (9 women and Collins Inc., Braintree, MA). In brief, the 11 men) consented to an IRB-approved measurement was performed as follows: the monitoring protocol which included measurement subjects were seated upright inside the chamber of FRC using each of three methods in a single with the door closed and a nose clip in place. The data collection session. The three methods subjects were instructed to breathe quietly included body plethysmography (the clinical gold through the mouthpiece until a stable end- standard), nitrogen washout and partial CO2 expiratory level was achieved (usually 3 to 10 rebreathing. The ambulatory volunteers qualified tidal breaths). When the patient was at or near for study inclusion if they were between the ages FRC, the shutter was closed at end-expiration for of 18 and 65. Exclusion criteria included known 2-3 seconds, and the subject was instructed to cardiac or pulmonary disease, including but not pant gently at a frequency between 0.5 and 1 Hz. limited to asthma; COPD; history of smoking; Next, the shutter was opened and the subject was and upper infection. first requested to fully expire to yield a measure Subjects were seated upright throughout of Expiratory Reserve Volume and then to the study period. For each washout measurement perform a slow Inspiratory session, a series of nitrogen washout maneuver. A series of 3-5 technically satisfactory measurements was completed, followed by a panting maneuvers was recorded. Three series of partial rebreathing measurements and measurements of FRC within 5% were obtained, ended with a second set of nitrogen washout in accordance with lung volume measurement measurements. The nitrogen measurements were guidelines11, 12. The mean of the individual body repeated in order to test whether they remained plethysmography measurements was recorded as stable both subsequent to the rebreathing the reference FRC for each volunteer. measurements and with time. Body Nitrogen Washout Method plethysmography measurements were Oxygen was analyzed using a side stream randomized to be taken before and after the paramagnetic O2 analyzer (Deltatrac, Datex, washout measurement session in order to prevent Helsinki, Finland). Carbon dioxide was measured bias introduced by the order of measurements. using an infrared analyzer and flow was Body Plethysmography Method measured using a differential pressure-type Body plethysmography FRC pneumotach, both of which are integrated in the measurement was conducted by trained staff in NICO2 mainstream sensor (Model 7300, Philips the Pulmonary Department of the University of Medical, Wallingford, CT). Each of the analyzers Utah Health Sciences Center according to the automatically re-zeroes periodically to avoid

2 baseline drift. The gas analyzers were calibrated The volume of alveolar ventilation and with calibration gas prior to the experiment. Gas the change in nitrogen concentration following for the side stream analyzer was sampled at both each change in FIO2 were used to calculate FRC in inspiratory and expiratory locations, while the a variation of the multiple compartment model mainstream sensor was placed between the side proposed by Hashimoto14: stream gas sampling adaptor and the mouthpiece. Compartment Ventilation = (1 / Number of Raw data of flow and gas concentrations compartments)* (iTV - VDaw - Apparatus were sampled with a frequency of 100 Hz and DS), [1] processed digitally using custom-written software where Number of compartments is assumed to be to generate end-tidal and volumetric O2 and CO2 3, iTV is inspiratory , VDaw is the measurements and tidal volumes as described measured airway deadspace, and Apparatus DS is previously3,10,13. Throughout the measurement the measured apparatus deadspace. period, inspired and expired volumes and concentrations of oxygen and carbon dioxide CompartmentN2(n) = CompartmentN2(n-1) * were recorded to a notebook computer. Nitrogen Compartment Volume / (Compartment was calculated as the balance of oxygen and Ventilation + Compartment Volume), [2] carbon dioxide. where Compartment Volume is initially estimated The subjects were instructed to wear a as 1000 mL. nose clip and breathe normally through a Average Compartment N2 = mouthpiece. One-way valves and a blender were CompartmentN2(n) * 1/Number of used to prevent rebreathing and achieve the step- compartments. [3] increase in oxygen. First, the inspired oxygen fraction (FIO2) was set to 0.3 and a period of 20 The Compartment Volume was adjusted minutes was allowed for stabilization. Then, the until the Average Compartment N matched the nitrogen washout FRC measurement was initiated 2 measured FetN . The compartment volumes were by switching the inspired oxygen fraction to 0.5. 2 summed to calculate the FRC. It should be noted After a period of up to five minutes was allowed that this calculation ignores the excretion of N for the washout to continue to completion, the 2 from the tissues. The effect of N excretion on the inspired oxygen fraction was increased to 1.0. 2 FRC measurement should be small (less than 100 Again, the washout was continued to completion ml)11. Both average and individual nitrogen for a period of up to five minutes. The inspired washout measurements were compared to the oxygen fraction was again set to 0.3 and the two body plethysmography reference value. step increases in oxygen were repeated.

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CO Rebreathing Method 2 FRC = 0.40 * (VCO2steady – VeCO2(n))/ The subjects were instructed to wear a (FCO2(n-1)-FCO2(n)), [4] nose clip and breathe normally through a where VCO2 is the volume of CO excreted mouthpiece. One-way valves and a blender were steady 2 in the last breath of rebreathing, VeCO2 is the used to prevent rebreathing and maintain an (n) CO2 excreted in the first breath following inspired oxygen fraction of 0.3. CO2 was rebreathing, FCO2(n-1) is the fraction of end-tidal analyzed via an infrared CO2 analyzer, while CO in the first breath following rebreathing, and airway flow was measured using a differential 2 FCO2 is the fraction of end-tidal CO in the last pressure-type pneumotach, both of which are (n) 2 breath of rebreathing. This equation assumes that integrated in the on-airway sensor of the NICO2 a steady state condition was achieved for the CO monitor (Philips Medical, Wallingford, CT). A 2 excretion rate during the rebreathing phase such partial rebreathing measurement was that the CO excretion rate was equal to the rate automatically activated once every three minutes 2 of CO2 elimination from the blood to the FRC. by the NICO2 monitor. The FRC measurement The partial rebreathing CO washout was based on a portion of the signal generated 2 FRC measurement was completed and recorded from the partial rebreathing sequence which is five times, the minimum and maximum values normally utilized for noninvasive pulmonary were removed from each session, and the average capillary blood flow (PCBF) measurement. The values of the remaining points were compared to partial rebreathing period lasted 35 seconds once the reference method of FRC measurement. every three minutes, and the parameters analyzed for the FRC measurement were obtained from the Statistical Analysis last breath of rebreathing and the first breath The mean FRC measurements recorded subsequent to the end of the rebreathing phase. from each the carbon dioxide washout method Thus, the FRC measurement was based on the and the nitrogen washout method were compared carbon dioxide washout of one breath. All data to those of the body plethysmography method by related to the flow and concentrations of the means of regression analysis and Bland-Altman breaths were automatically recorded to a statistics. The individual nitrogen washout computer for subsequent analysis. measurements were also statistically compared to The carbon dioxide washout FRC the reference value. measurement method was based on the system as previously reported10. The final calculation is:

4 Results 2

) 1.5 11 males and 9 females participated in L ( ) s

y 1 the study. Average age of the subjects was 31.3 h et l

P 0.5 years (range of 18 to 62). Average height was ody

B 0 - 2

68.5 inches (range of 59.5 to 76). Average weight N ( -0.5 e c n

was 156 pounds (range of 111 to 209 pounds). e r -1 e f f i Average measured Body Plethysmography FRC D -1.5

± SD was 3.55 ± 0.87 L with a minimum of 2.3 L -2 0123456 and a maximum of 5.6 L. Average Reference and N2 Washout (L)

Figure 2: Bland-Altman plot comparing the average Comparison of Nitrogen Washout and CO2 Rebreathing to Body Plethysmography FRC from the nitrogen washout method and that from body plethysmography. Linear regression analysis between the average nitrogen washout FRC measurements Linear regression analysis between the and the body plethysmography reference FRC for individual nitrogen washout FRC measurements each set of measurements yielded a squared and the body plethysmography reference FRC correlation coefficient of r2 = 0.91 (n = 35). The yielded a squared correlation coefficient of r2 = bias ± SD was -0.054 L ± 0.373 L. 0.86 (n = 73). The bias ± SD was -0.065 L ± 0.458 L. 7

7 6 ) L

C ( 6 5 2 )

R = 0.91 L ( FR C 4 5 R2 = 0.86 FR ashout 3 4 W n e ashout

g 3

o 2 r t Ni

1 ogen W 2 r t i N 0 1 01234567 Reference FRC (L) 0 01234567 Figure 1: Linear regression analysis of average Reference FRC (L) nitrogen washout FRC and body plethysmography Figure 3: Linear regression analysis of individual FRC measurements. nitrogen washout FRC and body plethysmography

FRC measurements.

5 Linear regression analysis between the individual measurements compared quite average CO2 rebreathing FRC and the body similarly to the average measurements with plethysmography FRC yielded a squared respect to r2, bias and standard deviation. Thus, correlation coefficient of r2 = 0.48 and a slope of the individual measurements may likely be 1.0 (n = 19). The bias ± SD was -0.43 L ± 0.91 L. accurate enough to be of use clinically and in

6 place of the measurement sets. The individual measurement requires approximately 6 minutes to

5 y = 1.00x - 0.39 complete, while a set of measurements would 2

) R = 0.48

L take 11-21 minutes. Therefore, it may be (

C 4 advantageous to employ a single FRC FR ng

hi measurement at the bedside rather than a set of 3 eat measurements in order to save time while not ebr

2 R 2 losing significant accuracy. O C The bias of the CO2 rebreathing method 1 was 12% of the average gold standard value, which was a factor of 10 larger than the bias for 0 0123456the nitrogen washout method. The signal Reference FRC (L) generated for the nitrogen washout method was Figure 4: Linear regression analysis of the CO2 significantly bigger than that for the carbon rebreathing FRC and the body plethysmography FRC dioxide method, and consequently the accuracy measurements. of the nitrogen method was better in this group of

spontaneously breathing subjects. Given the Discussion: underlying assumptions of the CO2 method and We observed acceptable accuracy for the the restriction of analyzing only one breath, the nitrogen washout FRC measurement method for CO2 method will likely be of limited use in this group of healthy, spontaneously breathing spontaneously breathing patients. The carbon volunteers, both when average measurements dioxide rebreathing system is especially were analyzed and when individual values were susceptible to noise caused by variations in breath considered. The ATS standard for lung volume size, so spontaneously breathing subjects are not measurements states11 that repeated nitrogen the best population for this method. Mechanically washout measurements should be within 10% of ventilated patients, in contrast, would be better each other. This nitrogen washout system was subjects for this method. The main advantage of within 1.5% of the mean gold standard reference the CO2 rebreathing method is that it is fully value, with a standard deviation of 10%. The automated and can be used for several hours at a

6 time. With automated measurement, it may be functional residual capacity assessment during spontaneous breathing. Anesth Analg. 2007 possible to track the size of the FRC with time in Mar;104(3):598-604. mechanically ventilated patients. 5. Wolfgang Eichler, Jan Schumacher, Angela Roth- Isigkeit, Jörg Braun, Hermann Kuppe and Karl- Future studies will examine whether the Friedrich Klotz. Automated Evaluation of Functional more stable signal from mechanically ventilated Residual Capacity by Oxygen Washout. J Clin Monit Comput. 2002 Apr-May;17(3-4):195-201. patients provides better input data for 6. H. Heinze, B. Sedemund-Adib, M. Heringlake, U. measurement stability of the carbon dioxide W. Gosch, H. Gehring, and W. Eichler. The Impact of Different Step Changes of Inspiratory Fraction of rebreathing method. Other planned analysis Oxygen on Functional Residual Capacity Measurements Using the Oxygen Washout Technique includes comparison of CO2 rebreathing and in Ventilated Patients. Anesth. Analg., May 1, 2008; nitrogen washout values with each other for these 106(5): 1491 - 1494. spontaneously breathing subjects as well as 7. Maisch S, Boehm SH, Weismann D, Reissmann H, Beckmann M, Fuellekrug B, Meyer A, Schulte Am measurement repeatability for each method. Esch J. Determination of functional residual capacity This study demonstrated that nitrogen by oxygen washin-washout: a validation study. Intensive Care Med. 2007 May;33(5):912-6. washout FRC measurements can be made 8. HOdenstedt, S Lindgren, C Olegård, K Erlandsson, accurately for spontaneously breathing subjects. S Lethvall, A Åneman, O Stenqvist and S Lundin. Slow moderate pressure recruitment maneuver minimizes Therefore, the method could likely be applied to negative circulatory and lung mechanic side effects: monitor an ambulatory astronaut’s lung volume. evaluation of recruitment maneuvers using electric impedance tomography, Intensive Care Med. 2005 Future studies will examine whether repeatable Dec;31(12):1706-14. nitrogen washout FRC measurements can reliably 9. Meier T, Luepschen H, Karsten J, Leibecke T, Grossherr M, Gehring H, Leonhardt S. Assessment of be made in mechanically ventilated patients. regional lung recruitment and derecruitment during a PEEP trial based on electrical impedance tomography. Intensive Care Med. 2008 Mar;34(3):543-50. References: 10. Brewer LM, Haryadi DG, Orr JA. Measurement of functional residual capacity of the lung by partial 1. Rylander C, Hogman M, Perchiazzi G, Magnusson CO2 rebreathing method during acute lung injury in A, Hedenstierna G. Functional residual capacity and animals. Respir Care. 2007 Nov;52(11):1480-9. respiratory mechanics as indicators of aeration and collapse in experimental lung injury. Anesth Analg 11. Wanger J et al., “Standardisation of the 2004;98(3):782-789. measurement of ,” Eur Respir J 26, no. 3 (2005): 511-522. 2. Hedenstierna G. The recording of FRC--is it of importance and can it be made simple?Intensive Care 12. Blonshine S, Foss C, Mottram C, Ruppel G, Med. 1993;19(7):365-366. Wanger J: AARC clinical practice guideline: Body plethysmography: 2001 revision and update. Respir 3. Olegard C, Sondergaard S, Houltz E, Lundin S, Care 2001; 46: 506–513. Stenqvist O. Estimation of functional residual capacity at the bedside using standard monitoring equipment: a 13. Fretschner R, Deusch H, Weitnauer A, Brunner modified nitrogen washout/washin technique requiring JX. A simple method to estimate functional residual a small change of the inspired oxygen fraction. Anesth capacity in mechanically ventilated patients.Intensive Analg. 2005;101(1):206-212. Care Med. 1993;19(7):372-376. 4. Heinze H, Schaaf B, Grefer J, Klotz K, Eichler W. 14. T. Hashimoto, A. C. Young, and C. J. Martin. The accuracy of the oxygen washout technique for Compartmental analysis of the distribution of gas in the lungs. J Appl Physiol 23: 203-209, 1967.

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