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Stikstofuitwas en Heliumdilutie
Eric Derom
Universitair Ziekenhuis Gent
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
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
• Washout Techniques
• Compressible gas
• Imaging
• Helium (He)
– Single breath
– Multiple breath
• Nitrogen (N2)
– Single breath
– Multiple breath
• Bodyplethysmography
• Chest X-ray and CT
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
• patient is connected to a spirometer
• CO2 is absorbed by soda lime
• O2 is added to replace the subject's oxygen consumption
• Test gas – air
– added O2 (25-30%)
–He (10%)
Helium Dilution Technique Principle
Initial amount of He = Final amount of He – closed system
[He]I x Vs = [He]F x VF
VF = Vs + VTUB + VL
VL = VF – Vs – VTUB
FRC = [He]I x Vs – Vs – VTUB
Helium Dilution Technique Principle
[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 CO2 absorber in spirometer
• Check activity of CO2 and H2O absorber in Helium analyser
• Warm-up the Helium analyser
• Turn on the circulating fan
Helium Dilution Technique
Procedures and Calculations II • Flush the spirometer with ambient air
• Add some O2, such that O2 will be about 25-
30% before connection of the patient
• Adjust Helium analyser to zero, once He reading is stable
• Add 2 to 3 L of room air and enough Helium to raise the concentration to 10%
• Note the Helium reading again
• 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 III
Helium Dilution Technique
Procedures and Calculations IV
• Add O2 (constant flow or bolus) to keep volume
of spirometer constant at FRC
• O2 is added to compensate for O2 consumption
• CO2 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 N2
– 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
• Correction should be made
LUNG
VOLUME
SPIROMETER
VOLUME
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 O2
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
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
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
• patient connected to a spirometer, inhales 100% O2 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 N2 concentration in the large bag
Nitrogen Washout Technique Principle of Closed Circuit Method
100%
O2
Initial amount of N2 in lung = Final amount of N2 in bag
[N2]I = 0.80 N2 in lung = 0.80 x VL = [N2]F x Vb
FRC = VL = [N2]F x Vb
0.80
Remarks:
– [N2]F > 0
– N2 stores in tissue: ± 220 ml = ((BSA x 96.5) + 35)/08 ml)
– FRC = [N2]F x Vb – 220 ml
[N2]I – [N2]F
Nitrogen Washout Technique Principle of Closed Circuit Method
• Gas collection method
– [N2]F might be very low
– Inaccuracy in determining [N2]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
Nitrogen Washout Technique Methodology
Continuous sampling
On-line measurement of N2 fraction
40 samples/sec
1 sample/25 msec
Measurement of flow
Integration of flow over time --> volume
O2
Measurement of N2 fraction
Measurement of flow
Integration of flow over time --> volume
O2
Measurement of N2 fraction (over 25 msec)
N2 fraction = 40%
Measurement of flow
Integration of flow over time --> volume
O2
Measurement of N2 fraction
N2 fraction = 40%
Measurement of flow
Integration of flow over time --> volume
O2
Measurement of N2 fraction
N2 fraction = 40%
Measurement of flow
Integration of flow over time --> volume
O2
Measurement of N2 fraction
N2 fraction = 40%
Measurement of flow
flow x time = volume
e.g. volume = 5 ml
Wash-out of N2
5 ml x 40% = 2 ml N2
O2
V’N2 =
∫[N2](t). flow . dt
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% O2
– breathing should generate enough negative
pressure to open valves
Nitrogen Washout Technique Procedures
6. Monitoring of N2 during washout
7. Measuring of exhaled volume
8. Stability of signal should be examined for noise
and drift
– N2 = 0 during inspiration
– configuration of end-expiratory N2 curve
9. End of test: [N2] < 1.5% for 3 breaths
Subject with early COPD
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 O2 therapy
Nitrogen Washout Technique Procedures
Nitrogen Washout Technique
• Principle of technique • Equipment
• Procedures
•Quality control
1. N2 analyser should be calibrated before each test at 0% N2, 40% N2 and room air
2. vacuum level, pneumotachograph temperature (38-40°C), flow (zero) and N2 (79-81%) should be checked
3. volume calibration check of pneumotachograph every 24 h (both in- and exhalation)
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
Nitrogen Washout Technique Quality Control
5. Linearity of N2 analyser should be tested every 6 months
6. Volume of exhalations with room air and pure should be checked every month
7. Check the accuracy, drift and linearity characteristics of the O2 and CO2 analysers if measurements of N2 concentration are made indirectly
8. Mass spectrometers should meet the previously outlined specifications for all three gases, have a molecular weight resolution of <1.0, and have <1% drift over 24 h
Nitrogen Washout Technique Quality Control
Multiple-breath N2 washout techniques Pittfalls
• Phase difference between N2 measurement
(O2 and CO2 measurement) and volume
measurement ---> biologic calibration
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
MEASUREMENTS OF RV AND TLC
Preferred Method of Calculation
• measure FRC
• then measure ERV (after FRC)
• thereafter a slow IVC maneuver to measure TLC
• all performed as linked maneuvers
Sequence
MEASUREMENTS OF RV AND TLC
Preferred Method of Calculation
MEASUREMENTS OF RV AND TLC
Preferred Method of Calculation
TLC value
• TLC = FRC + IC
• IC = largest of (3) technically acceptable IC
measurements
RV value
• RV = FRC - ERV
• ERV = mean of (3) technically acceptable
ERV measurements
MEASUREMENTS OF RV AND TLC
Second Preferred Method of Calculation
• 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
Sequence
Background Techniques
• Dillution Techniques
• Washout Techniques
• Compressible gas
• Imaging
• Helium (He)
– Single breath
– Multiple breath
• Nitrogen (N2)
– Single breath
– Multiple breath
• Bodyplethysmography
• Chest X-ray and CT
Dilution and washout techniques
---> communicating gas volume
Recommended