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MP - Lecture 30
Ventilation and Perfusion Matching
| Question | Answer |
|---|---|
| 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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