Ventilation and Perfusion Flashcards
What is ventilation?
The process by which air moves in and out of lungs
What is perfusion?
Process by which deoxygenated blood passes through the lung and becomes oxygenated
What does the ventilation/perfusion relationship (V/Q ratio) determine?
Normal gas exchange and thus the level of PO2 and PCO2 in the blood
What happens is V/Q is mismatched?
Indicates a respiratory disorder
Why is ventilation not uniformly distributed throughout the lungs?
Gravity, compliance and resistance
Describe the alveoli in apex when in an upright position
The alveoli in apex are more expanded in an upright position (gravity)
What effect does gravity have on the lungs and chest wall?
Pulls the lungs downwards and away from the chest wall
What is pleural pressure?
Pressure in the pleural space - always negative because lung is always pulling inwards and chest wall pulling outwards
Pleural pressure is more negative at which part of the lung?
The apex of the lung
Transpulmonary pressure is greater at which part of the lung?
The apex of the lung
What is compliance?
Compliance refers to how much effort is required to stretch the lungs and chest wall
What does high compliance mean in terms of the lungs and chest wall?
High compliance means that the lungs and chest wall expand easily
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)
What is resistance?
Any narrowing or obstruction of the airway that might reduce airflow
Describe the resistance in large diameter airways
Decreased resistance
When might increased resistance occur?
Pulmonary conditions:
1) Asthma
2) COPD (emphysema, chronic bronchitis) due to obstruction or collapse or airways
What is COPD?
Chronic Obstructive Pulmonary Disease
Includes emphysema and chronic bronchitis
What are the 2 types of dead space?
1) Anatomical dead space
2) Physiological dead space
What is anatomical dead space?
Volume of gas during each breath that fills the conducting airways
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
Why is surfactant important?
- Reduces surface tension/friction between pleural layers
- Important in maintaining elasticity of lungs
The lung has 2 separate blood supplies. What are they?
1) Pulmonary circulation
2) Bronchial circulation
Describe the pulmonary circulation
Brings deoxygenated blood from the heart to lung and oxygenated blood from the lung to the heart
Describe the bronchial circulation
Brings oxygenated blood from heart to lung parenchyma
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
