Body Plethysmography · 2020. 9. 21. · Body plethysmography • Body plethysmography is used to...
Transcript of Body Plethysmography · 2020. 9. 21. · Body plethysmography • Body plethysmography is used to...
Body Plethysmography
Prof. Ellie Oostveen, lung physiologist
Antwerp University Hospital
BVP 2020
The origins."I do not have photographs of my lab because it was the broom closet of the basement of the medical school, and hard to take pictures there until Dr. Comroe, an excellent chief, moved me upstairs to the 8th floor of the Gates Pavilion in the hospital. He wanted me to make his body box work. Then, I could have my picture taken.
Now that I am emeritus I am back down in a basement again and underneath the parking lot.“
Courtesy by Arthur B. DuBois
A. B. DuBois, April 2007
Body plethysmography
• Body plethysmography is used to determine:
– Lung volumes
– Airway resistance (or conductance, the inverse of resistance)
• A body plethysmohraph is (usually) an airtightcabin with the size of an telephone booth(~900 Liter)
Body plethysmography
• Body plethysmography is used to determine:
– Lung volumes
– Airway resistance (or conductance, the inverse of resistance)
• A body plethysmohraph is (usually) an airtightcabin with the size of an telephone booth(~900 Liter)
Measurements are difficult/impossible in patients:
• With claustrophobia
• Who are immobile
A closed “telephone booth”…
Body plethysmography
• Body plethysmography is used to determinelongvolumes (L):
– RV: residual volume (amount of air in the lungsafter full deflation)
– FRC: functional residual capacity ((I)TGV, EELV, amount of air in the lungs at the end of a normalexpiration)
– TLC: total lung capacity (amount of air in the lungsafter full inflation)
Interpretation scheme ATS-ERS 2005
Measurement´s principle
Plethysmographic measurements are based on Boyle’s Law, which states that,
under isothermal conditions,
when a constant mass of gas is compressed or decompressed, the gas volume decreases or increases and gas pressure changes such that the product of volume and pressure at any given moment is constant.
P × V = constant
A constant mass of gas, i.e. a “closed” lung
Boyle’s Law: P×V = constant
• Shutter closes at FRC:
1) V= VL= FRC 2) P= (PB-PH2O)
• An inspiratory effort will lead to:
increase in VL, decrease in P
P×V = (P - DP) × (V + DV)
P×V = P×V+ P×DV - DP×V - DP×DV
P×V = P×V+ P×DV - DP×V - DP×DV
DP×V = P×DV
V = DV/ DP × P
Boyle’s Law: P×V = constant
V = DV/ DP × PThis equation states that:• If we know the initial pressure in the lung, and we
can measure the pressure swing in the lung inducedby the inspiratory and expiratory “breathing” manoeuvre and the corresponding volume change, we can calculate VL.
• Initial pressure: PB
• Pressure swing: DP measured at the mouth
• Volume change: DV is measured through the change in box pressure
Boyle’s Law: P×V = constant
Pm
Pb
FRCpleth= (DPb/DPm) × PB
FRCpleth= (DVL/DPA) × PB
Lung volume: P V = constant
DP=1kPa DV= 10 ml
Pm
Pb
1 L
DP=1kPa DV= 30 ml
Pb
Pm
3 L
VL = (DVL/DPA) PB
Body plethysmography
The bodybox is equiped with:
• Pneumotachograph to
measure airflow (V´)
• Shutter
• Pressure tranducer to record
Pmouth
• Pressure transducer to
record changes in box
pressure (Pbox)
V´ Pm
Pbox
Calibration
1) Leakage
control
2) Pbox
3) Pneumo…
ATS-ERS 2005
In Belgium practice:
• When performing bodybox: voluntary increasein breathing frequency to reliably measuresRaw (thus no “quiet” breathing!)
• Increase in frequency often changes “FRC”
• RV and TLC are not influenced (except whenVC- manoeuvre is submaximal)!
• Thus: determine RV and TLC reliably!
FRCplethdetermination
TLC = RV + VC
FRC (ITGV)
RV = FRC - ERV
1
2
3
FRC (ITGV)
TLC = FRC + IC
RV = TLC - VC
1
2
1
3
2
How are FRC, TLC and RV determined?
TLC = FRC + IC
FRC1
2
Or, in very dyspneuic patients: FRCpleth + IC TLC
TLC – VC RV
IC IVC
Body plethysmography
• The absolute gas volume in de lung is determined at the moment the shutter is closed (FRCpleth), this includes closed bullea.
• By subsequently exhaling completely (ERV-manoeuvre) and then inhaling maximally (IVC-manoeuvre) RV and TLC are determined.
ATS/ERS guidelines 2005
• How many measurements per patient?
At least 3 fully and technically acceptablemeasurements , i.e 3 times (TGV + VC).
• Calculation of the end-result?
FRC= average FRC
RV= average FRC – average ERV
TLC= RV + largest IVC
ATS/ERS guidelines 2005
An acceptable difference between 3 measurements?
1) (largest FRC - lowest FRC )
mean
2) VC: largest and second largest ≤ 150 mL
≤ 10%
Airway resistance: Raw
Vm
Pb
Raw-measurement: to inhale, alveolar pressure
needs to decrease below atmospheric pressure.
This will lead to a proportional increase in Pbox
.
Raw = PA/V’
Raw-measurement: Raw= DP/DV’
V’m
Pb
sRaw = DPb / DV’m
Body plethysmography is beingused to determine:
– Lung volumes
– sRaw-loops
sRaw-loops
(I)TGV-measur.
Raw-measurements
Advantages:
• Airway resistance is a measure of airwayobstruction which is independent of a special breathing maneuvre.
• Airway resistance is measured at FRC, the normal lung volume at which the patientbreathes
+0.5 L/s
-0.5 L/s
sReff sR0.5 sRmid sRtot
Different sRaw-calculations…
Reproducibility of bodybox measurements?
2,5
3,0
3,5
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5,0
5,5
6,0
6,5
7,0
0 10 20 30 40 50 60 70
FRC
(L)
TLC
(L)
weeknummer
FRC - TLC: Bio-ijk #1
Lab 1
Lab 2
Lab 3
Lab 4
2,5
3,0
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4,0
4,5
5,0
5,5
6,0
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7,0
0 10 20 30 40 50 60 70
FRC
(L)
TLC
(L)
weeknummer
FRC - TLC: Bio-ijk #2
Lab 1
Lab 2
Lab 3
Lab 4
Reproducibility of bodybox measurements?
Casus:
Mr KGD, 63 yrs, 1.67m, 101 kg
P mond (kPa)
Volume shift (ml)
ITGV (L): 3.87 2.77 2.96 3.76 3.15
ITGV & sRaw (breathing loop)
Comparisongas dilution vs. body plethysmography
• In healthy subjects: comparable results
• Severe airways obstruction: gas dilution techniques underestimates FRC in case of poorly or non-ventilated lung regions (or a too short wash-in or wash-out measurement)
• Body plethysmography: over-estimation of FRC attoo high panting frequencies (> 1 Hz)
• D (gasdilution and box): estimation of trapped air
TLCbodybox= 0.02+1.03×TLCHe
TLCHe verdunning (L)
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Labadie & Van EenigeNTvG 1971; 115:938-940
Thank you!