stikstofuitwas en heliumdilutie - vvlrvvlr.be/s/congres2012/helium en stikstofuitwas... ·...

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




• 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)


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%)


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


[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


• 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


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


Procedures and Calculations III


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


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


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


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


Switching Error

• Patient is not always switched exactly at end-expiratory level

• Correction should be made

LUNG

VOLUME

SPIROMETER

VOLUME


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



• Procedures and calculations

•Quality control


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





• 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

• 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


O2

Measurement of N2 fraction (over 25 msec)

N2 fraction = 40%

Measurement of flow


O2


N2 fraction = 40%

Measurement of flow


O2


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


• 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


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

•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





• 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



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


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

stikstofuitwas en heliumdilutie - vvlrvvlr.be/s/congres2012/helium en stikstofuitwas... ·...

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