Ventilation and Perfusion Flashcards by Michaela Netto

What is ventilation?

The process by which air moves in and out of lungs

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What is perfusion?

Process by which deoxygenated blood passes through the lung and becomes oxygenated

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What does the ventilation/perfusion relationship (V/Q ratio) determine?

Normal gas exchange and thus the level of PO2 and PCO2 in the blood

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What happens is V/Q is mismatched?

Indicates a respiratory disorder

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Why is ventilation not uniformly distributed throughout the lungs?

Gravity, compliance and resistance

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Describe the alveoli in apex when in an upright position

The alveoli in apex are more expanded in an upright position (gravity)

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What effect does gravity have on the lungs and chest wall?

Pulls the lungs downwards and away from the chest wall

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What is pleural pressure?

Pressure in the pleural space - always negative because lung is always pulling inwards and chest wall pulling outwards

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Pleural pressure is more negative at which part of the lung?

The apex of the lung

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Transpulmonary pressure is greater at which part of the lung?

The apex of the lung

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What is compliance?

Compliance refers to how much effort is required to stretch the lungs and chest wall

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What does high compliance mean in terms of the lungs and chest wall?

High compliance means that the lungs and chest wall expand easily

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When might decreased compliance occur?

Pulmonary conditions:

1) Scarring in lung tissue (tuberculosis)
2) Lung filled with fluid (oedema)
3) Deficiency in surfactant produced
4) Destruction of elastic fibres (emphysema)

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What is resistance?

Any narrowing or obstruction of the airway that might reduce airflow

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Describe the resistance in large diameter airways

Decreased resistance

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When might increased resistance occur?

Pulmonary conditions:

1) Asthma
2) COPD (emphysema, chronic bronchitis) due to obstruction or collapse or airways

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What is COPD?

Chronic Obstructive Pulmonary Disease

Includes emphysema and chronic bronchitis

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What are the 2 types of dead space?

1) Anatomical dead space

2) Physiological dead space

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What is anatomical dead space?

Volume of gas during each breath that fills the conducting airways

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What is physiological dead space?

Total volume of gas in each breath that does not participate in gas exchange e.g. alveoli that are ventilated but NOT perfused

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Why is surfactant important?

Reduces surface tension/friction between pleural layers

- Important in maintaining elasticity of lungs

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The lung has 2 separate blood supplies. What are they?

1) Pulmonary circulation

2) Bronchial circulation

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Describe the pulmonary circulation

Brings deoxygenated blood from the heart to lung and oxygenated blood from the lung to the heart

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Describe the bronchial circulation

Brings oxygenated blood from heart to lung parenchyma

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Describe the differences between the pulmonary circulation and systemic circulation

1) Systemic circulation = high pressure (120/80 mm Hg). Pulmonary circulation = low pressure (24/9 mmHg) 2) Systemic circulation = high resistance. Pulmonary circulation = low resistance. 3) Systemic circulation = smooth muscle in vessels. Pulmonary circulation = wider vessels with less smooth muscle

Pulmonary circulation is a ___ pressure and ___ resistance system

Low pressure | Low resistance

Which of the following 2 circulations is more influenced by gravity: pulmonary circulation or systemic circulation

Pulmonary circulation

When upright, which part of the lungs receive greater blood flow?

The base (gravity)

What is the V/Q ratio?

The ratio of ventilation to blood flow | Ratio can be defined for a single alveolus, a group of alveoli or entire lung

Describe the V/Q ratio for a single alveolus

Ratio defined as alveolar ventilation divided by capillary flow

Describe the V/Q ratio for a lung

Ratio defined as total alveolar ventilation divided by cardiac output

What is the V/Q ratio for a healthy individual?

0.8-1.2

What are the typical values for alveolar ventilation and perfusion in a healthy individual?

Alveolar ventilation = ~4-6L/min | Pulmonary blood flow = ~5L/min

What is the V/Q ratio when ventilation exceeds perfusion?

V/Q > 1

What is the V/Q ratio when perfusion exceeds ventilation

V/Q < 1

What happens when ventilation and perfusion are mismatched?

Impaired O2 and CO2 transfer | Indicates a respiratory disorder

What is arterial hypoxemia defined as?

Arterial PO2 < 80 mmHg (normal ~100 mmHg)

What is hypoxia?

Insufficient oxygen to carry out metabolic functions - when arterial PO2 < 60 mmHg

What is hypercapnia?

Increase in arterial PCO2 > 40 mmHg

What is hypocapnia?

Decrease in arterial PCO2 < 35 mm Hg

What is the most common cause of arterial hypoxemia in patients with respiratory disorders?

V/Q mismatching Some alveoli: V/Q > 1 Other alveoli: V/Q <1 Results in varying alveolar and capillary gas contents

What is an anatomical shunt? (that can underlie V/Q mismatching)

Extra vessel that results in mixing of oxygenated and deoxygenated blood e.g. mixed venous blood shunted directly into arterial blood

Where do most anatomical shunts occur?

In the heart - e.g. blood from right atrium or ventricle crosses septum to left atrium or ventricle

What does an anatomical shunt result in?

Results in varying degrees of hypoxemia

In the case of an anatomical shunt, why is PCO2 not increased?

Central chemoreceptors are very sensitive to CO2 changes - leads to increased ventilation - leads to reduction in PCO2

What is a physiological shunt?

Ventilation to lungs units is absent with continuing perfusion (venous admixture)

What is the V/Q ratio when there is a physiological shunt present?

V/Q = 0

What is atelectasis?

Obstruction of ventilation due to mucous plug, airway oedema, foreign bodies, tumours in airway Results in absence of gas exchange (absence of ventilation)

When might the V/Q relationship be 0?

- When ventilation to a region is 0 (physiological shunt) - e.g. child inhaling a peanut - Blocks airway - Ventilation redistributed to other alveoli -elevated V/Q in other regions

What might be the cause of a low V/Q relationship (<0.8)?

- Asthma - Chronic bronchitis - Low ventilation, high perfusion

What would cause a V/Q relationship of ∞?

- When perfusion goes to 0/absence of perfusion i.e. no blood flow - Physiological dead space - e.g. pulmonary embolism - blood diverted to other capillaries - low V/Q in other regions

What might cause a high V/Q relationship?

- Emphysema: disrupted gas exchange - Pulmonary fibrosis: decreased gas exchange - High ventilation, low perfusion

What is chronic obstructive pulmonary disease (COPD)?

- Condition in which airflow is obstructed | - COPD encompasses emphysema and chronic bronchitis

What is the most frequent cause of COPD?

Long term smoking

What are the symptoms of COPD?

- Chronic cough - Chest tightness - Shortness of breath - Increased mucous production

What is emphysema?

- Lung condition - Structures in alveoli over-inflated - Lungs lose elasticity, cannot expand and contract - Patients can inhale but exhalation is difficult due to decreased elastic recoil

What is chronic bronchitis?

- Inflammation of bronchi causing mucous production and excessive swelling - Shortness of breath with mild exertion - Chest infections more prevalent

What is pulmonary fibrosis?

- A type of interstitial lung disease - Scarring and thickening of tissue - Decreased elasticity - Decreased gas exchange

What is the FEV1/FVC ratio in healthy individuals?

>70% | i.e. >70% of lung volume expired in 1 second

Describe the FEV1/FVC ratio in patients with restrictive lung disease e.g. pulmonary fibrosis, neuromuscular diseases, respiratory distress syndrome

FEV1/FVC ratio >70% | BUT lung volume (FVC) smaller, <80% of a healthy individual

Describe the FEV1/FVC ratio in patients with obstructive lung disease e.g. emphysema, asthma, pulmonary oedema

FEV1/FVC ratio <70% | Less than 70% of lung volume expired in 1 second

What is the term for the total volume of gas in each breath that does not participate in gas exchange

Physiological dead space

Regional differences in ventilation and perfusion are largely due to what?

Gravity

What is the typical V/Q ratio in a healthy lung?

0.8

When does V/Q ratio exceed 1?

When ventilation exceeds perfusion

When is the V/Q ratio <1?

When perfusion exceeds ventilation