L86: Ventilation (V), Perfusion (Q) & (V/Q) Relationship Flashcards
Define ventilation (V):
Process by which air moves in and out of the lungs
Define perfusion (Q):
Process by which deoxygenated blood passes through the lungs and becomes oxygenated
What is the (V/Q) ratio?
The relationship between ventilation and perfusion (blood flow)
What does the (V/Q) ratio determine?
Normal gas exchange and thus, the level of PO2 and PCO2 in the blood.
Which forces affect ventilation?
- Gravity;
- Compliance;
- Resistance.
How does gravity affect ventilation?
Gravity leads to a non-uniform distribution of ventilation within the lungs
Explain how gravity affects ventilation within the lungs:
In an upright position:
- Alveoli at the apex are more expanded than those at the base;
- Therefore greater alveolar volume at the apex;
- Pleural pressure is more negative at the apex than at the base;
- Transpulmonary pressure is greater at the apex than the base.
What is compliance, with regards to the lungs?
Effort required to stretch the lungs
High compliance is:
Easy expansion of the lungs
In pulmonary conditions, compliance is:
Decreased
What is resistance, with regards to the lungs?
Narrowing or obstruction of the airway that may reduce air flow
Large diameter airways have:
Decreased resistance
In pulmonary conditions, resistance is:
Increased
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
What are the blood supplies for the lungs?
- Pulmonary Circulation (heart and lungs);
- Bronchial Circulation (O2 to parenchyma)
What are the main features of pulmonary circulation?
- Low pressure;
- Low resistance;
- Affected by gravity more than systemic circulation.
How is the V/Q ratio calculated for a single alveolus?
Alveolar ventilation: Capillary flow
How is the V/Q ratio calculated for entire lung?
Total alveolar ventilation: Cardiac output
In healthy individuals, the typical V/Q range for lung is:
0.8-1.2
When ventilation exceeds perfusion:
V/Q > 1
When perfusion exceeds ventilation:
V/Q < 1
In a perfect model…
inspired gas and cardiac output are shared equally between alveoli, V/Q = 1. Alveolar and arterial PO2 are the same.
What is arterial hypoxemia?
Arterial PO2 < 80 mmHg
What is hypoxia?
When insufficient O2 to carry out metabolic functions, PO2 < 60 mmHg
What is hypercapnia?
Increase in arterial PCO2 > 40 mmHg
What is hypocapnia?
Decrease in arterial PCO2 < 35 mmHg
What is anatomical shunt?
When a proportion of the deoxygenated blood entering the lungs, is shunted away from the alveoli and is not oxygenated before leaving the lungs in the pulmonary veins. This leads to a lower PO2, hypoxemia, in blood leaving the lungs.
Anatomical shunt can also happen in the heart e.g. R to L shunt.
How can hypoxemia caused by anatomical shunt be confirmed, clinically?
Treatment using 100% oxygen will not increase the overall PO2 (by much).
What is physiological shunt?
Blood perfusing non-ventilated alveoli in the lungs due to obstructions etc.
What is V-Q mismatching?
Variation of V/Q ratios in alveoli. Some V/Q <1, some V/Q >1.
What is COPD?
What are the symptoms of COPD?
A condition where airflow is obstructed. Includes emphysema and bronchitis;
Chronic cough, chest tightness, shortness of breath, mucous.
What are the features of emphysema?
What are the symptoms of emphysema?
Structures in alveoli are over inflated;
Patients can inhale but exhalation is difficult due to decreased elasticity.
What are the features of chronic bronchitis?
What are the symptoms of chronic bronchitis?
Inflammation of bronchi causing excessive mucous production and swelling;
Shortness of breath and mild exertion.
Give an example and the features of a restrictive lung disease:
Pulmonary fibrosis;
Scarring and thickening of tissue, decreased elasticity and gas exchange.
When measuring lung volumes, what are the 4 major recorded values?
- Forced vital capacity (FVC);
- Forced expiratory volume in 1 second
(FEV1); - Ratio of FEV1 to FVC (FEV1/FVC);
- Average midmaximal expiratory flow (FEF25-75).
In a healthy individual, what is FEV1/FVC?
> 70%, i.e. >70% of lung volume is expired in 1 second
In an individual with obstructive lung disease, what is FEV1/FVC?
<70%, i.e. <70% of lung volume is expired in 1 second
In an individual with restrictive lung disease, what is FEV1/FVC?
> 70%, because lung volume (FVC) is much smaller than that of a healthy individual
What is PO2 at rest?
100 ± 2 mmHg
What is PCO2 at rest?
40 ± 2 mmHg
What is the rate of ventilation at rest?
~6-7 L/min (12-15 breaths/min, 500ml/breath)
What is the rate of gas exchange at rest?
~ 250 mL/ O2 consumed;
~ 200 mL/ CO2 expired.
