Respiratory Physiology - Pulmonary Gas Exchange Flashcards
Hypothetical oxygen cascade in a hypothetical perfect lung
What causes the drop in PO2 in alveoli from atmosphere
Both ventilation (addition of O2 to alveoli) and pulmonary blood flow (removal of O2 from alveoli) can be thought of as continuous
(there is an element of pulsatility but overall negligible)
Taking both into account, overall result is a lower PO2
Effect of hypoventilation on oxygen cascade
Reduced ventilation so less O2 added
Therefore lower PO2
Alveolar gas equation
PAO2 = alveolar PO2
PiO2 = inspired PO2
PACO2 = alveolar PCO2
R = respiratory exchange ratio
F is generally neglected as represents very minimal effect when breathing air
Respiratory exchange ratio
Ratio of CO2 production to O2 uptake
Also known as respiratory quotient when referring to tissue level
Normal respiratory exchange ratio at rest
~0.8
Equation for PiO2 (inspired PO2)
PiO2 = FiO2 x 713 mmHg
Causes of hypoxaemia
Hypoventilation
Diffusion limitation (eg pulmonary fibrosis)
Shunt
Ventilation perfusion mismatch - most common
Causes of hypoventilation
1, 2 = central respiratory centres
3 = interference with nerve tracts from respiratory centres
4 = anterior horn disease
5 = nerve disease
6 = neuromuscular junction
7 = muscular disease
8 = cage wall abnormality
9 = upper airway obstruction
Examples of central respiratory centre abnormalities causing hypoventilation
Encephalitis
Drugs eg opioids
Examples of interference with nerve tracts causing hypoventilation
C spine dislocation causing spinal cord compression
Example of anterior horn disease causing hypoventilation
Polio myelitis
Example of nerve disease causing hypoventilation
Neuritis eg Guillan-Barre Syndrome
Example of alterations at neuromuscular junction causing hypoventilation
Neuromuscular blockade drugs in anaesthesia
Example of muscular disease causing hypoventilation
Muscular dystrophy
Example of thoracic cage wall abnormality causing hypoventilation
Rib fractures
Flail chest
Examples of upper airway obstruction causing hypoventilation
Cancer
Enlarged lymph nodes
Oxygen cascade in physiological lung - not a perfect lung
Drop in PO2 with diffusion and shunt
Shunt definition
Blood reaching the arterial system without passing through ventilated areas of lung
Exists in small amount even in healthy lung - bronchial artery circulation and small amount from cardiac thebesian vein into left ventricle
Drainage of bronchial arterial system
Most enters right atrium via azygous vein and bronchial venous system
Small amount drains downstream of pulmonary capillaries contributing to shunt
Shunt equation for calculating degree of shunt
Method to measure mixed venous concentration of oxygen
CvO2
Pulmonary artery catheter required
Clinically sometimes a CVC near the right atrium is used as an estimate
Why does increase FiO2 to 1.0 not increase PaO2 in shunt (to the expected level)
Increasing FiO2 only impacts blood passing by ventilated lung, but this is not the cause of low PaO2 with shunt
Unoxygenated blood still mixes with end capillary blood
What is normal V/Q ratio
1.0 (in all regions of healthy lung it clusters around this value despite regional lung differences)
Ventilation and perfusion should match in healthy situations
Therefore can assume alveolar PO2 is the same as end pulmonary capillary PO2
Effects of changing ventilation perfusion ratio
A = normal
B = reducing ventilation (shunt)
C = reducing perfusion (dead space)
Can think about it as the scale at the bottom (Ventilation Perfusion line)
Effect of changing V/Q ratio on alveolar PO2 and PCO2
Changes in ventilation, blood flow and V/Q ratio in the upright lung from bottom to top
Why is pulmonary tuberculosis typically at the apices of the lung
Alveolar PO2 at top of lung is higher than at bottom (as V/Q ratio higher) so more favourable for pulmonary TB
Cause for difference between alveolar and arterial PO2
Difference in blood flow between top and bottom of lung is greater than ventilation difference
Therefore relative more blood flow gets O2 from base of lung (with lower PO2) than from the top, but end expired gas is more mixed from top and bottom (as difference in ventilation is lower)
Cause for arterial PCO2 being higher than alveolar PCO2
Similar concept to O2 differences with relative differences in ventilation and blood flow, but now base of lung has higher PCO2 and top has lower PCO2 due to V/Q ratio differences
Therefore mixed arterial blood has higher PCO2 than mixed expired alveolar gas
Why does reduced V/Q ratio cause hypoxaemia
Shunted blood has lower PO2 therefore reducing overall mixed arterial PO2
Effect of emphysema on V/Q ratio
Emphysema results in alveolar wall breakdown including loss of capillaries
Therefore some areas of lung have maintained V/Q around 1.0, but other areas of damage have higher V/Q (still ventilated but loss of blood flow)
Tends to have higher effect on PCO2 as no areas of shunt
Effect of chronic bronchitis on V/Q ratio
Some areas of lung have maintained V/Q around 1.0, but other areas have lower V/Q (maintained blood flow but lower ventilation due to airway inflammation / mucus to lung units)
Stages of impairment of gas exchange when ventilation perfusion mismatch enforced
Moves down stages as physiology aims to maintain PO2 supply for tissue demand and to maintain normal PCO2 by eventually increasing ventilation
How to calculate the alveolar-arterial PO2 difference
Use alveolar equation to calculate alveolar PO2 minus the arterial PO2 from this
For alveolar PCO2, use arterial PCO2 as this is almost equivalent to the “ideal alveolar PCO2”
Normal alveolar-arterial PO2 difference
< 10 mmHg
Significance of raised Alveolar-arterial PO2 difference
Suggests ventilation perfusion mismatch
