Respiration - Ventilation (V) & Perfusion (Q) Relationship Flashcards
What is ventilation?
Process by which air moves in and out of lung.
What is perfusion?
Process by which deoxygenated blood passes through the lung and becomes oxygenated
What is the V/Q ratio?
ratio of ventilation to blood flow.
Why is ventilation not uniformly distributed in the lungs?
gravity
How does gravity affect the lungs?
pulls lungs down and away from chest wall
What is the distribution of ventilation in the lungs (in terms of pressure and volume) ?
In upright position, alveoli in apex more expanded than at base (increased volume). due to compression and weight at base
pleural pressure (Ppl) more negative at apex than at base.
transpulmonary pressure (PL) is greater at apex than at base.
What affects distribution of ventilation apart from gravity?
compliance (C) & resistance (R) affect distribution of ventilation.
What is compliance?
how much effort is required to stretch the lungs and chest wall.
What does high compliance mean?
the lungs and chest wall expand easily.
What pulmonary conditions is decreased compliance a factor in?
Scarring in lung tissue (tuberculosis)
Lung filled with fluid (edema)
Deficiency in surfactant production
Destruction of elastic fibres (emphysema)
What is resistance?
any narrowing or obstruction of the airway that may reduce airflow.
What do larger diameter airways mean?
decreased resistance
What pulmonary conditions is increased resistance a factor in?
Asthma
COPD- chronic obstructive pulmonary disease (emphysema, chronic
bronchitis) due to obstruction or collapse of airways.
What are the two types of dead space?
anatomical
physiological
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., alveolar + anatomical (ventilated but not perfused)
In healthy individuals, the physiological dead space is typically less than 30% of the tidal volume
What are the characteristics of pulmonary circulation?
a low pressure and low resistance system – influenced by gravity more than systemic circulation is.
upright – greater blood flow to base than apex.
wide variations in arterial and venous pressure from apex to base.
What do the wide variations in arterial and venous pressure from apex to base in pulmonary circulation affect?
both flow and ventilation-perfusion relationships.
What is the V/Q ratio?
ratio of ventilation to blood flow.
For a single alveolus, what is the V/Q ratio defined as?
ratio defined as alveolar ventilation divided by capillary flow.
For the entire lung, what is the V/Q ration defined as?
ratio defined as total alveolar ventilation divided by cardiac output.
What is the alveolar ventilation per min for a normal individual?
4-6 L/min
What is the pulmonary blood flow per min?
5 L/min
What is the V/Q ratio a major determinant in?
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 the V/Q ratio when ventilation exceeds perfusion?
ventilation exceeds perfusion: V/Q > 1
What is the V/Q ratio when perfusion exceeds ventilation?
perfusion exceeds ventilation: V/Q < 1.
When does V/Q mismatching occur?
when either the ventilation (airflow) or perfusion (blood flow) in the lungs is impaired, preventing the lungs from optimally delivering oxygen to the blood.
What is the V/Q in a single alveolus (assuming perfection)? And what is a perfect model?
inspired gas and cardiac output shared equally between alveoli: V/Q = 1
(this is ‘perfect’ model so alveolar PO2 and arterial PO2 are same.)
What oxygen value is arterial hypoxemia diagnosed at?
arterial PO2 < 80 mmHg (normal ~100 mmHg)
What is hypoxia (and value)?
when insufficient O2 to carry out metabolic functions – when arterial PO2 < 60 mmHg
What is hypercapnia (and value)?
increase in arterial PCO2 > 40 mmHg.
What is hypocapnia (and value)?
decrease in arterial PCO2 < 35 mmHg.
What is an anatomical shunt
Mixed venous blood ‘shunted’ directly into arterial blood. bypassing lungs
Right to left shunt’: blood being shunted
is deoxygenated.
What changes during anatomical shunting?
Alveolar ventilation same, distribution of blood flow changed.
Where do most anatomical shunts occur?
Most anatomic shunts occur within heart - blood from right atrium or ventricle crosses septum to left atrium or ventricle: right to left shunt.
What do anatomical shunts result in?
varying degrees of hypoxemia
What happens to CO2 in anatomical shunts?
PCO2 not increased because central
chemoreceptors very sensitive to
CO2 changes – leads to
increase ventilation – leads to
reduction in PCO2.
What is physiological shunt?
blood flows through the lungs without participating in gas exchange.
blood leaves lungs mixed as it doesn’t exchange - (perfused not ventilated)
this happens because some areas of the lungs receive less ventilation than others, due to factors such as collapsed airways, fluid in the lungs, or other lung diseases.
What are the causes of physiological shunts?
Atelectasis (condition in which one or more areas of the lung collapse or become partially deflated causing obstruction of ventilation)
due to mucous plugs, airway
oedema, foreign bodies,
tumours in airways.
Occurs normally (2% of blood)
What happens in V/Q mismatching?
Some alveoli V/Q > 1, some V/Q < 1.
Results in varying alveolar and capillary
gas contents.
Why we do not all have hypoxemia despite having some degree of V/Q mismatch due to gravity?
normal V/Q mismatch is typically mild and does not result in significant hypoxemia (low oxygen levels in the blood).
body has mechanisms that may be disrupted in respiratory disorders such as increased ventilation, increased cardiac out and vasoconstriction to direct blood
Where is V/Q mismatching most frequent?
V-Q mismatching most frequent cause of arterial hypoxemia in patients with respiratory disorders.
