body plethysmography · 2020. 9. 21. · body plethysmography • body plethysmography is used to...

Body Plethysmography

Prof. Ellie Oostveen, lung physiologist

Antwerp University Hospital

BVP 2020

ellie.oostveen@uza.be

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

4,0

4,5

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

3,5

4,0

4,5

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 #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)

3

4

5

6

7

8

9

3 4 5 6 7 8 9 10

Labadie & Van EenigeNTvG 1971; 115:938-940

Thank you!