Stikstofuitwas en Heliumdilutie Eric Derom Universitair Ziekenhuis Gent
Background Overview Helium-dilution technique Principle of Technique Equipment Procedure and Calculations Quality Control Nitrogen washout technique Principle of Technique Equipment Procedure and Calculations Quality Control Measurement of TLC and RV Conclusions
Static Lung Volumes and Capacities FRC = amount of air at end-expiration during tidal breathing
Static Lung Volumes and Capacities Correct measurement of IC and ERV is crucial!
Background Techniques Dilution Techniques Helium (He) Single breath Multiple breath Washout Techniques Compressible gas Nitrogen (N 2 ) Single breath Multiple breath Bodyplethysmography Imaging Chest X-ray and CT
Single Breath Technique Principle patient inhales from RV a gas mixture containing a tracer gas After breathhold, he expires the gas completely The dillution factor allows to recalculate the total volume in which the trace gas has been diluted
Single Breath Technique Principle Expiration RV = 2 L Inspiration TLC = 6 L He Expiration RV = 4 L
Single Breath Technique Practical Use In conjunction with a measurement of TL,CO Underestmates true lung volume in subjects with airflow limitation Not recommended (ERS 1993 recommentations on lung volumes) V A < TLC He < TLC Pleth
Overview Background Helium dilution technique Principle of Technique Equipment Procedure and Calculations Quality Control Nitrogen washout technique Principle of Technique Equipment Procedure and Calculations Quality Control Measurement of TLC and RV Conclusions
Helium Dilution Technique Principle To measure an unknown volume from the dilution of a known volume and concentration of a tracer gas, Helium (or another poorly soluble gas)
Helium Dilution Technique Principle volume? 500 mg
Helium Dilution Technique Principle patient is connected to a spirometer CO 2 is absorbed by soda lime O 2 is added to replace the subject's oxygen consumption Test gas air added O 2 (25-30%) He (10%)
Helium Dilution Technique Principle Initial amount of He = Final amount of He closed system [He] I x V s = [He] F x V F V F = V s + V TUB + V L V L = V F V s V TUB FRC = [He] I x V s V s V TUB [He] F
Helium Dilution Technique Add a known volume of gas to the spirometer containing concentration of Helium Open the breathing valve and allow the subject to inhale and exhale Gas from the spirometer will equilibrate with the air in the lungs Helium is now distributed equally in both the lungs and spirometer Measure final Helium concentration (< 5%) Measured volume is the lung volume at the instant the breathing valve was opened
Helium Dilution Technique Principle of technique Equipment Procedures and calculations Quality control
Helium Dilution Technique Procedures and Calculations I Check activity of the CO 2 absorber in spirometer Check activity of CO 2 and H 2 O absorber in Helium analyser Warm-up the Helium analyser Turn on the circulating fan
Helium Dilution Technique Procedures and Calculations III Ask about perforated eardrum ---) earplug Patient is seated, wearing a nose clip Patient is connected to mouthpiece breathing quietly for 30-60 sec Wait for stable end-tidal expiratory level Switch the valve and connect the patient to the closed system at the end of a normal expiration
Helium Dilution Technique Procedures and Calculations IV Add O 2 (constant flow or bolus) to keep volume of spirometer constant at FRC O 2 is added to compensate for O 2 consumption CO 2 is absorbed
Helium Dilution Technique Procedures and Calculations VI Criteria for end of testing the decrement in He concentration < 0.02% in 30 sec or change in FRC < 40 ml in 30 sec Procedure rarely exceeds 10 min
Helium Dilution Technique Procedures and Calculations VII After meeting the end of test criterium determine the stability of FRC (several breaths) instruct to exhale slowly and fully to RV instruct the patient to inhale completely to TLC at least one satisfactory test should be obtained in adults two or more satisfactory tests within 10% should be obtained in children / in research
Helium Dilution Technique Duplicate Analysis Coefficient of variation for FRC 5% in normal subjects 6% in COPD patients 95% C.I. ± 400 ml 85% within 200 ml Single measurement, unless special circumstances 3 measurements, if two tests do not agree within 200 ml 5 to 8 [10-20] min between measurements
Helium Loss ----) leads to overestimation of FRC absorption of Helium into the body no corrections excretion of N 2 no corrections equipment leaks -> check before test swallowing - leaks around nose clip/mouthpiece -> be attentive ruptured tympanic membranes -> ask the patient
Helium Dilution Technique Switching Error Patient is not always switched exactly at end-expiratory level LUNG VOLUME SPIROMETER VOLUME Correction should be made
Helium Dilution Technique Changes in Lung Volume Patient is sometimes encouraged to take intermittent deep inhalations not recommended in patients with airways obstruction, because: it may take several breaths to return to the original FRC it may cause errors in addition of O 2
Helium Dilution Technique Principle of technique Equipment Procedures and calculations Quality control
Helium Dilution Technique Quality Control After each measurement Inspection of volume-time tracings switch-in and switch-out volume > 300 ml suggests leaks Mention time for equilibration for each patient
Background Overview Helium dilution technique Principle of Technique Equipment Procedure and Calculations Quality Control Nitrogen washout technique Principle of Technique Equipment Procedure and Calculations Quality Control Measurement of TLC and RV Conclusions
Nitrogen Washout Technique Principle To measure an unknown volume from the washout of a known volume and concentration of a nitrogen
20% 80% How many balls?
How many balls? Red balls = 80% total amount
How many balls? Red balls = 80% total amount
Nitrogen Washout Technique Principle of Closed Circuit Method patient connected to a spirometer, inhales 100% O 2 during inspiration the expired volume of air is collected in a large bag or Tissot spirometer (± 120 L) lung volume is derived from expired volume and N 2 concentration in the large bag
100% O 2
Nitrogen Washout Technique Principle of Closed Circuit Method Initial amount of N 2 in lung = Final amount of N 2 in bag [N 2 ] I = 0.80 N 2 in lung = 0.80 x V L = [N 2 ] F x V b FRC = V L = [N 2 ] F x V b 0.80 Remarks: [N 2 ] F > 0 N 2 stores in tissue: ± 220 ml = ((BSA x 96.5) + 35)/08 ml) FRC = [N 2 ]F x V b 220 ml [N 2 ]I [N 2 ]F
Nitrogen Washout Technique Methodology Gas collection method [N 2 ] F might be very low Inaccuracy in determining [N 2 ] F Inaccuracy in determining bag volume Open circuit methods Immediate analysis of expired gas Frequent gas analysis and flow sampling Real time data processing Integration of data
O 2 Measurement of flow Integration of flow over time --> volume Continuous sampling On-line measurement of N 2 fraction 40 samples/sec 1 sample/25 msec
O 2 Measurement of flow Integration of flow over time --> volume Measurement of N 2 fraction
O 2 Measurement of flow Integration of flow over time --> volume Measurement of N 2 fraction (over 25 msec) N 2 fraction = 40%
O 2 Measurement of flow Integration of flow over time --> volume Measurement of N 2 fraction N 2 fraction = 40%
O 2 Measurement of flow Integration of flow over time --> volume Measurement of N 2 fraction N 2 fraction = 40%
O 2 Measurement of flow flow x time = volume e.g. volume = 5 ml V N 2 = [N 2 ] (t). flow. dt Measurement of N 2 fraction N 2 fraction = 40% Wash-out of N 2 5 ml x 40% = 2 ml N 2
Healthy subject
Nitrogen Washout Technique Principle of technique Equipment Procedures Quality control
Nitrogen Washout Technique Procedures 1. Adequate warm-up time with calibration 2. Ask for eardrum perforation 3. Patients sits comfortably 4. Breathing through mouthpiece for 30-60 sec - stable end-expiratory level 5. Switched into the circuit at FRC - inspiration of 100% O 2 breathing should generate enough negative pressure to open valves
Nitrogen Washout Technique Procedures 6. Monitoring of N 2 during washout 7. Measuring of exhaled volume 8. Stability of signal should be examined for noise and drift N 2 = 0 during inspiration configuration of end-expiratory N 2 curve 9. End of test: [N 2 ] < 1.5% for 3 breaths
Subject with early COPD
Nitrogen Washout Technique Procedures 10. Repeat test if leak is reported or results are not physiological 11. Document leaks eardrum perforation degree of effort during vital capacity maneuvers 12. 10-20 min between two tests 13. Take care of patients on O 2 therapy
Nitrogen Washout Technique Principle of technique Equipment Procedures Quality control
Nitrogen Washout Technique Quality Control 1. N 2 analyser [O 2 and CO 2 analyser] should be calibrated before each test at 0% N 2, 40% N 2 and room air 2. vacuum level, pneumotachograph temperature (38-40 C), flow (zero) and N 2 (79-81%) should be checked 3. volume calibration check of pneumotachograph every 24 h (both in- and exhalation)
Nitrogen Washout Technique Quality Control 4. Test of a technician with normal lung function at least once a week; error < 5% or < ± 3 SD of the mean of previous measurements 5. Volume of exhalations with room air and pure N 2 should be checked every month 6. Check the accuracy, drift and linearity characteristics of the O 2 and CO 2 analysers if measurements of N 2 concentration are made indirectly
Multiple-breath N 2 washout techniques Pittfalls Phase difference between N 2 measurement (O 2 and CO 2 measurement) and volume measurement ---> biologic calibration
Background Overview Helium dilution technique Principle of Technique Equipment Procedure and Calculations Quality Control Nitrogen washout technique Principle of Technique Equipment Procedure and Calculations Quality Control Measurement of TLC and RV Conclusions
MEASUREMENTS OF RV AND TLC Preferred Method of Calculation Sequence measure FRC then measure ERV (after FRC) thereafter a slow IVC maneuver to measure TLC all performed as linked maneuvers
MEASUREMENTS OF RV AND TLC Preferred Method of Calculation
MEASUREMENTS OF RV AND TLC Preferred Method of Calculation RV value RV = FRC - ERV ERV = mean of (3) technically acceptable ERV measurements TLC value TLC = FRC + IC IC = largest of (3) technically acceptable IC measurements
MEASUREMENTS OF RV AND TLC Second Preferred Method of Calculation Sequence measure first TLC by performing IC immediately after FRC often necessary in severe COPD or dyspnea allow them to come off the mouthpiece perform then an EVC maneuver, or an ERV maneuver followed by a IVC maneuver
Dillution Techniques Washout Techniques Background Techniques Helium (He) Dilution and washout techniques Single breath Multiple breath ---> communicating gas volume Nitrogen (N 2 ) Single breath Multiple breath Compressible gas Imaging Bodyplethysmography Chest X-ray and CT