Charles University in Prague, 1st Medical Faculty, Lab. of biocybernetics & Computer-Aided Learning

Assessment of respiratory system function

Stanislav Matoušek

What is the function of lungs?

Alveolus• Ventilation – mechanical function of

the lung – get air in and out • Perfusion with blood – get blood in

and out • Diffusion – get gas molecules from air

to blood and back• Matching of ventilation and perfusion

Possible respiratory system disturbances

• // ventilation • // perfusion • // distribution of ventilation and per-

fusion = ventilation perfusion mismatch• // diffusion • Important: Ventilation, perfusion

and their distribution are feedback regulated processes.

• Disturbance: – 1. In the effector part (lungs, resp.

muscles for ventilation, heart for perfusion)

– 2. In the regulator part (sensors, CNS eg. in uremia, liver in hepatopulmonary syndrome)

The overall measure of respiratory system function

• pO2 & pCO2 in arterial blood - („Astrup“)

• O2 solubility in water is low => need of Hemoglobin

• pO2 = 13,3 kPa = 100 Torr• pCO2 = 5,3 kPa = 40 Torr (1 kPa = 10 cm H2O = 7,6 mmHg or

Torr)

Solubility of gases in liquids

Oxygen-hemoglobin dissociation curve

Ventilation

• Is carried out by respiratory muscles, that change volume of thorax.

• Volume changes cause changes of pressures

• Changes of pressure in alveoli cause air flow (

• ↑ transthoracic pressure – expiration;

• ↓ transthoracic ressure – inspiration)

• Flow behaves according to Ohm’s law

Spirometry - Measure of ventilation volumes (and air flow)

Spirometry

Spirometry – Volume-flow loop

Pressures in the lungs

Transthoracic pressure

Transpulmo-nary pressure

-12 mmHg

+1 mmHg

Normal lung

Lung compliance

• Is a measure of the pressures developed by the effect of the lung elasticity while the lung is at rest.

• C = Vbreath/ (pendin.- pendexp.) = ΔV /Δ p • The pressures are not pressures needed to make

the air flow (it is measured at rest/0 flow points• They are pressures needed to keep the lung inflated

!!• The more you want to have the lung inflated, the

more pressure you need• Decreased compliance = stiff lung = restrictive

disease• Increased compliance - loose lung – emphysema –

(increased compliance does not cause problems by itself. However, it causes increased resistance during expiration)

Restrictive disease

Lung resistence

• Depends on pressures needed to make the air flow (transthoracic in normal breathing)

• R = (penviron.- palv.)/ F = Δ p / F

• Increased resistance – obstruction in the lungs – obstructive desease

Obstructive disease

Measuring of the Compliance of the lungs

• C = Vbreath / (pendin.- pendexp.)• Direct – difficult, because

you need to measure the transpulmonary pressure difference

• Easy in artificial ventilation

• Indirect - decreased lung compliance = stiff lung - will cause ↓ decreased static lung volumes, especially ↓ in VC a FVC.

Measuring of the resistance of the airways

R = (penviron.- palv.)/ FDirect: Temporary

occlusion method (spirometry, whole body pletysmography)

Indirect: Increased airway resistance = blonchoconstriction will cause decrease of flow and „dynamic lung volumes“ FEV1, FEF25-75% and MEFs.

-12 mmHg

+1 mmHg

What type of lung disease?

a)

b)

c)

Normal spirogram

Obstruction- medium degree

Obstruction parameters

Restrictive disease

Real world situation

Whole body plethysmography

Other methods of measuring Residual volume and TLC

• Nitrogen washout method -person breathes in pure oxygen - concentration of N2 in the

expired air is measured• Helium dilution method

– Given amount of Helium

Ventilation disorders

• Lung impairment (mechanical properties change) – Obstructive disease - ↑ increased resistance R of

airways (R = “dynamic lung resistance”)– Restrictive disease – ↓decreased lung compliance C (‘↑

static resistance” `; C = 1/ static lung resistance)• Chest wall impairment

– ↓ decreased C of chest wall – severe scoliosis, extensive fibrosis, serial fractures

• Insufficient activity of respiratory muscles (// innervation or // muscle strength , // of CNS ) – E.g.. Respiratory centre suppression in barbiturate poisoning, myasthenia gravis

Perfusion

• All the blood volume flows through lungs

• Also behaves according to Ohm’s law

Disorders of perfusion

• Causes– Embolization to the pulmonary artery

(increased resistante to the blood flow)– Pulmonary hypotension (right heart failure)– Pulmonary hypertension

• Manifestation– Pulmonary hypertension causing ever right

heart failure in massive embolism – Decrease in pO2 (increase in pCO2),

• Increase of shunting • Blood flows fast through a small part of the lungs

only – the rest functions as dead space

Measuring perfusion

• a) Ventilation - perfusion scan - diagnosis of pulmonary embolism and parenchymal lung disease should be performed in all clinically stable patients with the suspicion of pulmonary embolismshould be performed in all clinically stable patients with the suspicion of pulmonary embolism- Ventilation scan - 133Xe gas- Perfusion scan – microspheres of albumin (50-100 mm labeled with gamma emitting isotope 99mTc- “Mismatch” in ventilation and perfusion is characteristic for PTE

Lung scintigraphy -perfusion

Lung scintigraphy - ventilation

West’s weir

Physiological ventilation distribution

Distribution of ventilation and perfusion

• In healthy lung, the most perfused part is at the base… this part is at the same time the most ventilated one (No mismatch )

• Various pathologies can cause ventilation perfusion mismatch

• Every lung region (size is on us to decide) has its ventilation perfusion ratio VA/Q - number from O to ∞ … norm 0,8 - 1

Pulmonary shunt• Extreme case of ventilation perfusion

mismatch• Zero local ventilation VA/Q = OCauses:

– Atelectasis– Lung edema (alveolar)– Lung inflammation (inflammatory exsudate)– Collapsed lung (pneumothorax)

• Blood leaving the defect area has low ↓ pO2 and high ↑ pCO2 (no gas exchange)

• After mixing with blood from healthy regions, ↓ pO2 stays low, but pCO2 normalizes. Why?

Dead space

• Opposite extreme case of ventilation-perfusion mismatch. No perfusion.

• VA/Q = ∞• Increases of dead space:

– Embolism– Emphysema– Bronchiectasia

• Ventilation of dead space has by itself no influence on blood gases, but it is wasted respiratory work! =>

• Excessive ventilation of dead space can lead to insufficient ventilation of healthy alveoli.

Physiological dead space

Ventilation perfusion mismatch

Ventilation perfusion mismatch

• Is a very common cause of hypoxemia - ↓ pO2 (low ↑ pCO2 might probably not occur)

• Etiological factor of dyspnea onset in:– ARDS– COPD, especially chronic bronchitis

(smoker)– Asthma

Diffusion

Diffusion in lungs

Rate of diffusion

Diffusion impairments

Decrease ↓ of diffusion surface S:• Emphysema• Pneumothorax

Increase ↑ in diffusion distance d:• Lung fibrosis • Lung edema• Interstitial pneumonia

Measuring „diffusion“

• Transfer factor / Lung diffusion capacity

Respiratory insufficiency!!

• Respiratory insufficiency type I (partial, hypoxemic)– pO2 is ↓ low, but pCO2 is normal or

even also ↓ lower

• Respiratory insufficiency type II (global, hypoventilation)– pO2 is ↓ low and pCO2 is ↑ high

(respiratory a….)

Partial respiratory insufficiency(Type I)

• Impaired // distribution – Ventilation perfusion mismatch -

uneven VA/Q in different lung regions

– True shunting (right-left)

• Impaired // diffusion • Through water O2 diffuses about 20x slower

than CO2

Global respiratory insufficiency(type II)

• Impaired // ventilation - overall alveolar hypoventilation

ARDS

• Adult/acute respiratory distress syndrom • Cause: Sudden damage to alveolo-capillary

membrane – interstice and alveoli get infiltrated by plasma and proteins

• Ventilation-perfusion mismatch appears, in some parts of lungs to the degree of shunting

• Consequence: Partial respiratory insufficiency in serious cases evolving into global respiratory insufficiency

• With edema, lung compliance decreases ↓C , where only interstitial edema => // diffusion

ARDS

Signs of ARDS

• Dyspnea – decrease ↓ pO2• Low ↓ lung compliance – breathing

faster and more shallow, increased breathing effort

• Dry irritating cough (sometimes pink sputum)

• CO2 often decreased ↓ !! = hypocapnia – respiratory acidosis

<

Osmotic pressure

C1 C2 > = P2P1 > =

Membrane permeable to water, but NOT to solutes

H2O

H2O

H2O

H2O

[H2O]1 [H2O] 2 =

Cell

Isotonic environment

290 ± 10 mmol/l

Vessel

Interstice

H 20

Gradient of

hydraulic pressuresGradient

of oncotic pressures

H 20

arteriolevenule capillary

Oncotic pressure gradient

Hydraulic pressure gradient

filtrate movementLymphatic drainage

Lymphatic drainage

proteins

proteins

Interstitialliquid

Movement of H2O across the lung capillary

arteriolevenule capillary

Oncotic pressure gradient

Hydraulic pressure gradientFiltrate movement

Lymphatic drainage

Lymphatic drainage

proteins

proteins

Interstitialfluid

ARDS

Causes of ARDS

• Shock and inflammation– Circulatory shock and

severe hypotension – Pulmonary embolism– DIC – Extensive burns– Extensive pneumonia – Sepsis and septic shock– Post-transfusion TRALI

• Damage from alveolar side– Inhalation of toxic

gases and fumes – Long-time O2 toxicity– Aspiration of gastric

content– Water aspiration in

drowning• Other

– Lung contusion and chest trauma

– Head trauma– Pancreatitis– Heroin overdose

Summary