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Hee Chan Kim, Ph.D.Department of Biomedical Engineering,

College of Medicine,Seoul National University

CHAPTER 9. MEASUREMENTS OF THE RESPIRATORY SYSTEM

○ Respiration : the process by which gas is exchanged across cell membranes in all living systems. - Inspiration : expansion of thorax, negative pressure in the lung - Expiration : return to resting state.

○ Assessment of respiratory function- pulmonary function test (PFT) : relatively long time interval (days/years) - patient monitoring : continuous or at intervals of min/hours.

9.1 MODELING THE RESPIRATORY SYSTEM

• Models of normal ventilatory mechanics – for small amplitude/low frequency breathing :

normal resting lung

PAWO : hydrostatic pressure at the airway opening PA : representative pressure within the lungs (alveolar pressure) PPL : interpleural pressure ΔPMUS : muscle pressure difference PBS : hydrostatic body surface pressure QAWO : volume flow of gas at airway opening VL : volume of the gas space within the lungs + airways

Fig. 9-1 Lung mechanical unit enclosed by chest wall

(1) mechanical unit enclosed by chest wall

Notation :① finite perturbations about anoperating point → low caseletter : y=Y-Ŷ② differences between twospatial points →Δ : Δ Y=Yi-Yj

(2) Equivalent Circuit Model

RAW : airway resistance CstL : pulmonary static compliance CstW : chest-wall static compliance

Fig. 9-2. Equivalent circuit for model in Fig. 9-1

- Measurable Variables ① volume flow of gas through the mouth and nose ② pressure near the mouse and nose, pressure at body surface ③ partial pressure and concentration of gas in gas mixtures passing the mouth and samples of blood (* all other variables should be inferred.)

9.2 MEASUREMENT OF PRESSURE

• Pressure Sensor

9.3 MEASUREMENT OF GAS-FLOW RATE

Fig. 9-3 Pneumotachometer for measurements at the mouth

ΔP=f(Q)

- differential pressure flowmeters (pneumotachometers)

Fleisch type pneumotachometer

an array of capillaries

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Lilly type pneumotachometer

a fine metal mesh

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Turbine flow meter

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Hot wire anemometer

- A chip is required to linearize the output of hot wire anemometers.

- As anemometers are insensitive to the direction of flow, two heated wires need to be placed in series; the flow direction can then be determined from the wire that cools first.

- A disadvantage of this type of meter is its sensitivity to gas composition and gas temperature; in addition these measuring systems are very vulnerable to damage and need to be handled with special care.

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Turbine flowmeter vs. Fleisch pneumotachometer: a comparative study for exercise testing.

Yeh MP, Adams TD, Gardner RM, Yanowitz FG.Fitness Institute, LDS Hospital, Salt Lake City 84143.

The purpose of this study was to investigate the characteristics of a newly developed turbine flowmeter (Alpha Technologies, model VMM-2) for use in an exercise testing system by comparing its measurement of expiratory flow (VE), O2 uptake (VO2), and CO2 output (VCO2) with the Fleisch pneumotachometer. An IBM PC/AT-based breath-by-breath system was developed, with turbine flowmeter and dual-Fleisch pneumotachometers connected in series. A normal subject was tested twice at rest, 100-W, and 175-W of exercise. Expired gas of 24-32 breaths was collected in a Douglas bag. VE was within 4% accuracy for both flowmeter systems. The Fleisch pneumotachometer system had 5% accuracy for VO2 and VCO2 at rest and exercise. The turbine flowmeter system had up to 20% error for VO2 and VCO2 at rest. Errors decreased as work load increased. Visual observations of the flow curves revealed the turbine signal always lagged the Fleisch signal at the beginning of inspiration or expiration. At the end of inspiration or expiration, the turbine signal continued after the Fleisch signal had returned to zero. The "lag-before-start" and "spin-after-stop" effects of the turbine flowmeter resulted in larger than acceptable error for the VO2 and VCO2 measurements at low flow rates. J Appl Physiol. 1987 Sep;63(3):1289-95

9.4 LUNG VOLUME

Fig. 9-4. A spirometer

- spirometer : light bellows to measure gas passingthrough the airway opening

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Fig. 9-5. Volume ranges of the intact ventilatory system

TLC : lung capacity FRC : functional residual

capacity RV : residual volume VC : vital capacity VT : tidal volume ERV : expiratory reserve

volume IC : inspiratory capacity IRV : inspiratory reserve

volume

- Volume ranges of the intact ventilatory system(with no external loads applied)

9.5 PLETHYSMOGRAPHY

Fig. 9-6. The body plethysmograph

- To detect TLC. (pneumotachograph cannot measure RV)

* Measuring Steps ① close the mouthpiece valve on the

patient sealed in the chamber ② ask the patient to make breathing

motions ③ read the change in pressure dPT on

meter 1. ④ read the change in pressure dPC in

the chamber on meter 2.

Theory : from ideal gas law( ) constant

( ) ( )( )

0

: ideal gas at constant temperature

P VOL kTP VOL P VOLd P VOL dP dVOL

P VOLVOL dP P dVOL

dP PdVOL VOL

= =∂ ⋅ ∂ ⋅

∴ ⋅ = ⋅ + ⋅∂ ∂

= ⋅ + ⋅ =

∴ = −

: lung

: chamber

: constant total volume

By approximation of

where is constant and is given from measureme

T T

C C

C C

C

CT C

T C

C T

CC

T

CC

T

dTLC TLCdP P

dVOL VOLdP P

dVOL dTLCVOLTLC dP dP

P PP P

dPTLC VOLdP

dPVOLdP

= −

= −

= −

∴ = −

≈

∴ ≈ −

nts.