6. Lung Anatomy and Spirometers(2)

BG 3105 Biomedical Instrumentation

Lung Anatomy and Spirometers

Asst Prof Manojit Pramanik School of Chemical and Biomedical Engineering

Nanyang Technological University

[email protected] Office: N1.3-B2-11

Respiratory System and Measurements

1 Introduction 2 Lung volume 3 Respiratory system measurements 3.1 Flow measurement 3.2 Spirometer 3.3 Nitrogen-washout estimate of lung volume

Biomedical Instrumentation - wk 6 2

1. Introduction

• Respiration (= breathing) is the interchange of gases.

• The purpose is to deliver oxygen to the body and to take away carbon dioxide.

• The main organ of the respiratory system are the lungs.


• Lungs consists of 2 spongy organ.

• It contains 300 million alveoli (air sac).

• Each sac with 0.2 mm in diameter.


Lungs

Trachea

• The trachea filters the air we breathe and branches into the bronchi.


Bronchi

• The bronchi are two air tubes that branch of the trachea and carry air directly into the lungs.

Diaphragm

• The diaphragm is the main muscle involved in breathing.

• It is a dome-shaped muscle at the bottom of the lungs. • Breathing starts from diaphragm.


• When you breathe in, the diaphragm contracts. When it contracts it flattens out and pulls downward. This movement enlarges the space and pulls air into the lungs.

• When you breathe out, the diaphragm expands reducing the amount of space for the lungs and forcing air out.

• The red blood cells pick up the oxygen in the lungs (alveoli) and carry the oxygen to all the body.

• The red blood cells transport the carbon dioxide back to the lungs (alveoli).


Alveoli

• Capillaries are small blood vessel with thin walls, and are wrapped around these alveoli.

• The walls are so thin and close to each other that the air easily seeps through.

• Oxygen seeps through into the bloodstream and carbon dioxide, in the bloodstream, seeps through into the alveoli.


Capillaries

The effect of blood PCO2 and PO2 on the respiration rate

• An increase in PCO2 increases the breathing rate.

• An increase in PO2 slows down the breathing rate.


Internal respiration


• Internal respiration is the exchange of gases between the bloodstream and nearby cells.

External respiration


• External respiration is the exchange of gases between the lungs and bloodstream.

External respiration


• External respiration includes: Inspiration – intake of air

79% nitrogen (N) 20.96% oxygen (O2) 0.04% carbon dioxide (CO2)

Expiration – exhaust of waste gases

79% nitrogen (N) 17% oxygen (O2) 4% carbon dioxide (CO2)

2. Lung volume


TV: Tidal volume IRV: Inspiratory reserve volume ERV: Expiratory reserve volume VC: Vital capacity RV: Residual volume FRC: Functional residual capacity TLC: Total lung capacity

Lung Volume

• Dead Space (150 ml) – the volume of air that is not available for gas exchange with the blood. Air in air way Air in trachea Air in bronchi


Lung Volume

• Tidal volume (TV) (500 ml) – is the volume of gas inspired or expired during each normal respiratory cycle. At rest condition For normal adults


Lung Volume

• Residual volume (RV) (1200 ml) – is the amount of gas remaining in the lungs at the end of maximal expiration.


Lung Volume

• Functional residual capacity (FRC) (2400 ml) – is the amount of gas remaining in the lungs at the resting expiration level.


Lung Volume

• Vital capacity (VC) (4800 ml) – is the maximum amount of gas expelled from the lungs by forceful effort from maximal inspiration.


Lung Volume

• Total lung capacity (TLC) (6000 ml) – is the amount of gas contained in the lungs at the end of maximal inspiration.


Lung Volume – food for thoughts

Can this guy breathe comfortably?


3. Respiratory system and measurements

3.1 Air flow measurement A strain-gauge wire mesh is used to measure air

flow


Air flow measurement

• The strain-gauge is a component of a Wheatstone bridge


A circuit for measuring airflow rate and volume

Air flow measurement

• Here the change in resistance ∆𝑹 is proportional to the airflow rate 𝑭 = 𝒌∆𝑹, where 𝒌 is pneumotach coefficient.

• Given Wheatstone bridge voltage 𝑽𝑩𝑩, we have

• The circuit may be designed so that 𝑹 ≫ ∆𝑹, then we have

• 𝑽𝑭 is proportional to flow 𝑭. Biomedical Instrumentation - wk 6 23

𝑉𝐴𝐴 =𝑅

𝑅 + ∆𝑅 + 𝑅−𝑅2𝑅

𝑉𝐴𝐴 =−𝑅∆𝑅

4𝑅2 + 2𝑅∆𝑅𝑉𝐴𝐴

𝑉𝐴𝐴 =−𝑅∆𝑅

4𝑅2 + 2𝑅∆𝑅𝑉𝐴𝐴 ≈

−𝑅∆𝑅4𝑅2

𝑉𝐴𝐴 =−∆𝑅4𝑅

𝑉𝐴𝐴

𝑉𝐹 = 𝐴𝐷𝑉𝐴𝐴 = −𝐴𝐷∆𝑅4𝑅

𝑉𝐴𝐴 =−𝐴𝐷𝑉𝐴𝐴

4𝑘𝑅𝐹

3.2 Spirometer

• Spirometer is used to measure lung volume under conditions Constant temperature Constant pressure


Spirometer


Mouthpiece & Spirometer

LCD Display graph and data

Spirometer

• The spirometer consists of An upright water filled cylinder An inverted floating drum An mechanical linkage

• How to operate?

The volume of gases inside spirometer will change as the patient breathes through the mouthpiece

This volume change is proportional to lung volume change


Spirometer

When no breathing

When inhaling

When exhaling This motion is recorded on a rotating drum through direct mechanical linkage


Spirometer

• The spirometer can only measure the gas volume inspired and expired, i.e., a change in volume, for example TV.

• It cannot measure gas volume remaining inside lungs, for example, FRC (Function Residual Capacity), RV (Residual Volume).


3.3 Nitrogen-washout estimate of lung volume


Nitrogen-washout estimate of lung volume

• Where 𝑽𝑳 is lung volume 𝑻𝑳 is lung temperature (in K) 𝑭𝑳𝑳𝟐 is 𝑳𝟐 molar fraction in lung 𝑽𝒔 is spirometer volume 𝑻𝒔 is spirometer temperature (in K) 𝑭𝑺𝑳𝟐 is 𝑳𝟐 molar fraction in spirometer



• It is a modified spirometer Two one-way valves are connected in air tube The mouthpiece is in between the valves A Nitrogen analyzer used to measure the fraction of

Nitrogen is installed

• So that When the patient starts breathing through the

mouthpiece, he can only inhale pure 𝑶𝟐 But, he exhales the mixture of 𝑶𝟐, 𝑳𝟐, and 𝑪𝑶𝟐, as

his lung initially contains 𝑶𝟐, 𝑳𝟐, and 𝑪𝑶𝟐 And the expired mixture enters into spirometer

through one-way valve Biomedical Instrumentation - wk 6 31


• What happens to 𝑳𝟐 in lungs after multiple-breathing? • The amount of 𝑳𝟐 is gradually decreasing



• What can it measure? Functional residual capacity (FRC) Residual volume (RV)

• At the beginning 𝒕𝟏, total number of 𝑳𝟐 moles in lungs is given by


𝑳𝟐 𝒎𝒎𝒎𝒎𝒎𝒍𝒍𝒍 = 𝑭𝑳𝑳𝟐 𝒕𝟏𝑽𝑳 𝒕𝟏𝑻𝑳

𝑷𝑹

Note: 𝑭𝑳𝑳𝟐 =𝑽𝑳𝑳𝟐𝑽𝑳

, 𝑽𝑳𝑳𝟐𝒊𝒔 𝒑𝒑𝒑𝒕𝒊𝒑𝒎 𝒍𝒊𝒕𝒑𝒎𝒍𝒎𝒍 𝒗𝒎𝒎𝒍𝒎𝒎,𝑽𝑳 𝒊𝒔 𝒕𝒎𝒕𝒑𝒎 𝒎𝒍𝒍𝒍 𝒗𝒎𝒎𝒍𝒎𝒎

𝑷𝑽 = 𝑳𝑹𝑻 ⇒ 𝑳 =𝑷𝑽𝑹𝑻

𝑳𝟐 𝒎𝒎𝒎𝒎𝒎𝒍𝒍𝒍 =𝑷𝑹𝑽𝑳𝑳𝟐𝑻𝑳

=𝑷𝑹𝑭𝑳𝑳𝟐 𝒕𝟏 𝑽𝑳 𝒕𝟏

𝑻𝑳


• At the beginning 𝒕𝟏, total number of 𝑳𝟐 moles in spirometer is assumed


𝑳𝟐 𝒎𝒎𝒎𝒎𝒔𝒑𝒊𝒑𝒎𝒎𝒎𝒕𝒎𝒑 = 𝑭𝒔𝑳𝟐 𝒕𝟏𝑽𝒔 𝒕𝟏𝑻𝒔

𝑷𝑹

= 𝟎

• After multiple-breathing from the mouthpiece, at time 𝒕𝟐, the number of 𝑳𝟐 moles in lungs become

𝑭𝑳𝑳𝟐 𝒕𝟐𝑽𝑳 𝒕𝟐𝑻𝑳

𝑷𝑹

Decreased!

Where the left side is change of 𝑳𝟐 mole in lungs and right is change of 𝑳𝟐 in spirometer

𝑭𝑳𝑳𝟐 𝒕𝟏𝑽𝑳 𝒕𝟏𝑻𝑳

𝑷𝑹− 𝑭𝑳𝑳𝟐 𝒕𝟐

𝑽𝑳 𝒕𝟐𝑻𝑳

𝑷𝑹

= 𝑭𝒔𝑳𝟐 𝒕𝟐𝑽𝒔 𝒕𝟐𝑻𝒔

𝑷𝑹

Mass balance


• Suppose 𝒕𝟏 and 𝒕𝟐 are as follows


Then, we have 𝑽𝑳 𝒕𝟏 ≠ 𝑭𝑹𝑪,𝑽𝑳 𝒕𝟐 = 𝑭𝑹𝑪


• If the beginning time 𝒕𝟏 is shifted


Then, we have 𝑽𝑳 𝒕𝟏 = 𝑭𝑹𝑪,𝑽𝑳 𝒕𝟏 = 𝑽𝑳 𝒕𝟐 = 𝑭𝑹𝑪


• Therefore,


𝑭𝑳𝑳𝟐 𝒕𝟏𝑭𝑹𝑪𝑻𝑳

− 𝑭𝑳𝑳𝟐 𝒕𝟐𝑭𝑹𝑪𝑻𝑳

= 𝑭𝒔𝑳𝟐 𝒕𝟐𝑽𝒔 𝒕𝟐𝑻𝒔

𝑭𝑹𝑪 =𝑻𝑳𝑻𝒔

𝑭𝒔𝑳𝟐 𝒕𝟐 𝑽𝒔 𝒕𝟐𝑭𝑳𝑳𝟐 𝒕𝟏 − 𝑭𝑳𝑳𝟐 𝒕𝟐

FRC can be measured using the above formulation


• Procedure: At the beginning, measure initial 𝑭𝑳𝑳𝟐 𝒕𝟏 using

Nitrogen analyzer (sensor). Start at 𝑽𝑳 𝒕𝟏 = 𝑭𝑹𝑪

At the end, measure 𝑭𝑳𝑳𝟐 𝒕𝟐 and 𝑭𝒔𝑳𝟐 𝒕𝟐 . End at 𝑽𝑳 𝒕𝟐 = 𝑭𝑹𝑪

Measure 𝑻𝑳 and 𝑻𝒔 (both are constant).



• Similarly, we can measure RV, by setting 𝑽𝑳 𝒕𝟏 =𝑽𝑳 𝒕𝟐 = 𝑹𝑽


𝑹𝑽 =𝑻𝑳𝑻𝒔



• Example: a 𝑳𝟐–washout experiment is carried out. At beginning, 𝑽𝒔 𝒕𝟏 = 𝟕 𝒎𝒊𝒕𝒎𝒑𝒔, 𝑭𝒔𝑳𝟐 𝒕𝟏 = 𝟎. At the end 𝑽𝒔 𝒕𝟐 = 𝟏𝟐 𝒎𝒊𝒕𝒎𝒑𝒔, 𝑭𝒔𝑳𝟐 𝒕𝟐 = 𝟎.𝟎𝟐𝟎 and fraction of 𝑳𝟐 for the patient has decreased by 𝟎.𝟏. What is the lung volume at which the patient is

breathing?


• Solution: At the beginning of experiment 𝑻𝑳 = 𝟑𝟕 + 𝟐𝟕𝟑 = 𝟑𝟏𝟎 𝑲,

At the end of experiment 𝑽𝒔 𝒕𝟐 = 𝟏𝟐 𝒎𝒊𝒕𝒎𝒑𝒔

𝑭𝒔𝑳𝟐 𝒕𝟐 = 𝟎.𝟎𝟐𝟎 𝑭𝑳𝑳𝟐 𝒕𝟏 − 𝑭𝑳𝑳𝟐 𝒕𝟐 = 𝟎.𝟏 𝑻𝒔 = 𝟑𝟎𝟑 𝑲


With assumption of 𝑽𝑳 𝒕𝟏 = 𝑽𝑳 𝒕𝟐 = 𝑽𝑳,


𝑽𝑳 =𝑻𝑳𝑻𝒔


=𝟑𝟏𝟎𝟑𝟎𝟑

×𝟎.𝟎𝟐𝟎 × 𝟏𝟐

𝟎.𝟏= 𝟑.𝟏𝟏 𝒎𝒊𝒕𝒎𝒑𝒔

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6. Lung Anatomy and Spirometers(2)