Ventilation (V), Perfusion (Q) and the V/Q Relationship Flashcards
What is ventilation?
Process by which air moves in and out of the lungs
What is perfusion?
Process by which deoxygenated blood passes through the lung and becomes oxygenated
A major determinant of normal gas exchange and thus the level of PO2 and PCO2 in blood is the relationship between ventilation and perfusion. What is this relationship called?
The V/Q ratio
Ventilation is not uniformly distributed in the lung. What is one of the main reasons for this?
Gravity
- In an upright position, alveoli in the apex are more expanded than at the base
- Gravity pulls the lung down and away from the chest wall so:
- Pleural pressure is more negative at the apex than the base
- Transpulmonary pressure is greater at the apex than at the base
- Increased alveolar volume in apex
In addition to gravity what 2 other factors affect the distribution of ventilation?
- Compliance
- Resistance
What does compliance refer to in relation to the lungs?
- How much effort is required to stretch the lungs and chest wall
- High compliance means that the lungs and chest wall will expand easily
Decreased compliance is a common feature in pulmonary conditions, due to what 4 conditions/changes of the lung?
1, Scarring in lung tissue (TB)
- Lung filled with fluid (oedema)
- Deficiency in surfactant production
- Destruction of elastic fibres (emphysema)
What does resistance refer to in relation to the lungs?
Any narrowing or obstruction of the airway that may reduce airflow
- Large diameter airways have decreased resistance
Increased resistance is a common feature in pulmonary conditions, what are these conditions?
- Asthma
2. COPD (emphysema, chronic bronchitis) due to obstruction or collapse of airways
What is the definition of dead space?
The volume of gas not participating in gas exchange
What are the 2 different types of dead space?
- Anatomical dead space
- 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 perfused but not ventilated
How should anatomical and physiological dead space compare in a healthy individual?
The should be around the same
What is the pulmonary circulation of the lung?
- Brings deoxygenated blood from the heart to the lung and oxygenated blood from the lung to the heart
What is the bronchial circulation of the lung?
- Brings oxygenated blood to the lung parenchyma
What are the characteristics of systemic circulation in terms of pressure and resistance?
- High pressure - 120/80mmHg
- High resistance (used to control distribution of blood)
- Smooth muscles in vessels
What are the characteristic of pulmonary circulation in terms of pressure and resistance?
- Low pressure - 24/9 mmHg
- Low resistance
- Wider vessels with less smooth muscle
How can the V/Q ratio be defined for a single alveolus?
Ratio defined as alveolar ventilation divided by capillary flow
How can the V/Q ratio be defined for the entire lung?
Ratio defined as total alveolar ventilation divided by cardiac output
For a healthy individual what is the V/Q of the lung?
0.8-1.2
What is the V/Q ratio when ventilation exceeds perfusion?
> 1
What is the V/Q ratio when perfusion exceeds ventilation?
> 1
In a ‘perfect model’ where inspired gas and cardiac output is shared equally so alveolar PO2 and arterial PCO2 are the same, what would the V/Q ratio be?
1
What is the arterial PO2 in arterial hypoxemia?
<80mmHg (whereas normal is around 100mmHg)
What is the arterial PO2 in hypoxia?
- When insufficient O2 to carry out metabolic functions, so arterial PO2 <60mmHg (this is the level in which peripheral chemoreceptors start to kick in to increase ventilation)
What is the arterial PCO2 in hypercapnia?
Increase in arterial PCO2 > 40mmHg
What is arterial PCO2 in hypocapnia?
Decrease in arterial PCO2 < 35mmHg
What is the most frequent cause of arterial hypoxemia in patients with respiratory disorders?
V-Q mismatching
- Results in varying alveolar and capillary gas contents
What is an anatomical shunt?
When mixed venous blood is shunted directly into arterial blood which lowers the levels of oxygen in the blood
- The alveolar ventilation is the same, but distribution of blood flow is changed
- A ‘right to left shunt’ means blood that is being shunted is deoxygenated
Where do most anatomical shunts occur?
Mainly within the heart - blood from the right atrium or ventricle crosses septum to left atrium or ventricle: right to left shunt
- Results in varying degrees of hypoxemia
What is a physiological shunt?
Air is no longer getting to an alveoli, but blood perfusion and gas exchange is still occurring (still get gas exchange occurring initially however there is a build up of CO2 and a low level of O2 which causes gas exchange to stop occurring)
- So V/Q=0
What is atelectasis?
An obstruction of ventilation due to mucous plugs, airway oedema, foreign bodies and tumours in the airways which causes the collapse of the lung tissue
What is happening when V/Q=0 for an area of the lung?
- Ventilation to a region is 0
- Airway is blocked
- Ventilation redistributed to other alveoli - elevated V/q in other regions
What conditions are associated with a low V/Q?
- Asthma
- Chronic bronchitis
What is happening when V/Q= infinity for an area in the lung?
- No blood flow; perfusion to a region is 0
- Physiological dead space e.g. pulmonary embolism
- Blood diverted to other capillaries - low V/Q in other regions
What conditions are associated with a high V/Q?
- Emphysema - disrupted gas exchange
- Pulmonary fibrosis - decreased gas exchange
What is chronic obstructive pulmonary disease (COPD)?
- A condition in which airflow is obstructed
- It encompasses emphysema and chronic bronchitis
What is a frequent cause of COPD?
Long term smoking
What are the symptoms and signs of COPD?
- Chronic cough
- Chest tightness
- Shortness of breath
- Increased mucous production
What is emphysema?
- Structures in the alveoli are over inflated
- Lungs loose their elasticity and cannot fully 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 are more prevalent
What is pulmonary fibrosis?
- A type of interstitial lung disease
- Scarring and thickening of tissue
- Decreased elasticity
- Decreased gas exchange
What is forced vital capacity?
The maximal volume of gas that can be exhaled from a full inhalation by exhaling as forcefully and rapidly as possible
What is the forced expiratory volume of the lungs in 1 second?
The maximal volume of gas that can be exhaled in 1 second from a full inhalation
In healthy individuals, what should the FEV1/FVC ratio be?
> 70%
- In other words 70% of the lung volume should be expired in 1 second
How many breaths should a healthy individual take per minute?
around 12-15
