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MP - Lecture 30

Ventilation and Perfusion Matching

Medical Physiology – Lecture 30 Ventilation and Perfusion Matching
Inspired (150 mmHg) to alveolar (100 mmHg) gradient is determined by: Alveolar ventilation and O2 ventilation
Alveolar (100 mmHg) to arterial (95 mmHg) gradient is determined by: Air-blood matching
Arterial (95 mmHg) to venous (40 mmHg) gradient is determined by: tissue blood flow, VO2, and Hb dissociation
At the beginning of an asthma attack, arterial PO2: Decreases
At the beginning of an asthma attack, alveolar and arterial PCO2: Decreases
How can both PO2 and PCO2 decrease if alveolar O2 and CO2 move in opposite directions? Arterial PO2 goes down by under ventilated areas, alveolar and arterial PCO2 goes down by over ventilated areas
Likely cause of decreased arterial PO2 and low to normal arterial PCO2 is: Air-blood mismatch
A-a gradient of O2 and CO2 in healthy people: PAO2 ~ PaO2, PACO2 ~ PaCO2
A-a gradient of O2 and CO2 in many respiratory diseases: PAO2 >> PaO2, PACO2 ~ PaCO2
In a normal steady state, homogenous lung alveolar ventilation and blood flow are: Uniform throughout lung
If VA in a local region increases while Q remains constant: PAO2 increases, PACO2 decreases
If VA in a local region decreases while Q remains constant: PAO2 decreases, PACO2 increases
If both VA and Q change by the same factor: PAO2 and PACO2 are unchanged
Local alveolar gas composition is determined by: VA/Q ratio
Arterial PO2 in a right to left shunt: Always decreases
In a right to left shunt arterial PO2 depends on: How much cardiac output flows through shunt
If 50% of Q goes through shunt, then effect of increased ventilation: Corrects CO2 (small A-a) but not O2 (large A-a)
VA/Q in absolute shunt = 0
VA/Q in physiological shunt = Low but not 0
Physiological shunt results in: Low PaO2, increased A-a O2 difference
Absolute shunt and physiological shunt effect on PaCO2 is minor because: Increased ventilation to good part of lung makes up for under ventilated areas since small A-a difference
To determine absolute shunt from physiological shunt: Breathing 100% O2 will increase PaO2 in physiological shunt
Compared to base, ventilation and perfusion in apex is: Lower
Perfusion in the apex compared to ventilation in the apex is: Much lower
VA/Q is highest in the: Apex (low ventilation / lower perfusion)
For each value of VA/Q: There are specific PAO2 and PACO2 values
At VA/Q = 0 VA = 0, PAO2 = 40, PACO2 = 46 (equal to mixed venous blood)
At VA/Q = 0.8 (Normal) PAO2 = 100, PACO2 = 40
At VA/Q = infinity Q = 0, PAO2 = 150, PACO2 = 0 (equal to atmosphere)
Breathing 100% O2 with an absolute shunt results in: Increased A-a PO2 difference
Very high VA/Q results in: Wasted ventilation by physiological dead space
Physiological Dead Space Volume of expired air in each breath that does not receive CO2 from blood
Physiological dead space is calculated by: VD = VT x [(PACO2 – PECO2)/PACO2]
Normal value for physiological dead space ~30% of tidal volume
Pulmonary embolism results in: Local Q~0 (large VA/Q), rest of lung Q is high (small VA/Q) and large A-a PO2 difference with low PaO2
Physiological shunt and physiological dead space in pulmonary embolism are: Both increased
A massive embolism can cause: Large absolute shunt
Created by: emyang



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