Ventilation perfusion relationships Flashcards
What is the formula for calculating minute ventilation?
Minute ventilation = Tidal volume × Respiratory rate.
Example: 500 mL × 15/min = 7500 mL/min.
How is alveolar ventilation calculated?
Alveolar ventilation = (Tidal volume - Anatomical dead space) × Respiratory rate.
Example: (500 mL - 150 mL) × 15 = 5250 mL/min.
What is the approximate pulmonary blood flow at rest?
Pulmonary blood flow is approximately 5000 mL/min.
What is the ventilation-perfusion (V/Q) ratio for the whole lung?
The V/Q ratio is approximately 1, indicating balanced ventilation and perfusion.
What is the normal ventilation-perfusion (V/Q) ratio in the lungs?
The normal V/Q ratio is approximately 0.8, indicating balanced ventilation and perfusion.
What happens when ventilation (𝑉𝐴 ) is zero but perfusion(𝑄) is normal?
There is no gas exchange, resulting in a V/Q ratio of 0 (e.g., shunt)
What happens when perfusion (𝑄) is zero but ventilation (𝑉𝐴) is normal?
There is no gas exchange, and the V/Q ratio becomes infinity (e.g., dead space).
Why can the overall V/Q ratio of 0.8 be misleading?
Ventilation and perfusion must be matched at the alveolar capillary level, because that is where gas exchange occurs. Overall VA/Q may be misleading
What happens to alveolar PO₂ and PCO₂ in a shunt?
PAO2 =40mmHg
PACO2 =46mmHg
What is the V/Q ratio in a shunt, and why?
The V/Q ratio is 0 because there is no ventilation despite normal perfusion.
What are the normal alveolar gas values during effective gas exchange?
PAO2 =100mmHg
PACO2 =40mmHg
What happens to alveolar PO₂ and PCO₂ in dead space?
PAO2 =150mmHg
PACO2 =0mmHg
What is the V/Q ratio in dead space, and why?
The V/Q ratio is infinity because there is ventilation but no perfusion.
What percentage of venous blood normally passes through the lungs for gas exchange?
Over 98% of venous blood.
What veins contribute to the normal right-to-left shunt?
Bronchial veins (draining the lungs).
Thebesian veins (small veins draining the walls of the left ventricle).
What is an abnormal right-to-left shunt?
It is when blood bypasses the lungs without gas exchange, often due to specific pathological conditions.
What 3 conditions can cause an abnormal right-to-left shunt?
Describe abnormal right to left shunts.
- No ventilation in part of a lung but perfusion is still occurring.
- That would be a right to left shunt.
- The blood would be O2 poor and CO2 rich and just gets added to the oxygenated blood.
What are the blood gas values for the patient with a consolidated lung?
What is an Atrial Septal Defect (ASD)?
A defect in the atrial septum allowing blood to flow from the left atrium (LA) to the right atrium (RA), typically causing a left-to-right shunt.
What is a Ventricular Septal Defect (VSD)
A defect in the ventricular septum allowing blood to flow from the left ventricle (LV) to the right ventricle (RV), also causing a left-to-right shunt initially.
How do ASDs and VSDs affect arterial oxygen content initially?
They usually do not lower arterial oxygen content or 𝑃𝑂2 because they are left-to-right shunts, meaning oxygenated blood mixes with deoxygenated blood in the right heart and lungs.
What can happen with prolonged left-to-right shunting in VSDs?
High pressure in the pulmonary circulation can cause pulmonary vascular remodeling, leading to increased resistance and a reversal to a right-to-left shunt.
What are the four components of Fallot’s tetralogy?
What is the effect of a 20% shunt on arterial oxygen and carbon dioxide contents?
The arterial oxygen and carbon dioxide contents are calculated as weighted averages of shunted and unshunted blood.
How is the oxygen content in the blood affected by a 20% shunt?
How is the carbon dioxide content in the blood affected by a 20% shunt?
Can pressures (PO2 and PCO2) be directly averaged in a shunt calculation?
No, pressures cannot be averaged; contents must be calculated using dissociation curves.
How does a moderate rise in CO2 content affect PCO2?
A moderate rise in CO2 content causes a very small rise in PCO2
How does a moderate fall in O2 content affect PO2?
A moderate fall in O2 content causes a large fall in PO2
What is the effect of low arterial 𝑃𝑂2 and high arterial 𝑃𝐶𝑂2 in a right-to-left shunt?
What are the final blood gas levels in a right-to-left shunt with increased ventilation?
Low 𝑃𝑂2, normal or low 𝑃𝐶𝑂2
Why does breathing 100% oxygen have only a modest effect on arterial 𝑃𝑂2 in a right-to-left shunt?
The oxygen does not reach shunted blood, and blood in ventilated regions is already near full saturation.
What does the low 𝑃𝐶𝑂2 in ventilation-perfusion mismatching depend on?
It depends on the ventilatory response.
What happens if alveolar hypoventilation occurs?
The ventilatory response becomes poorer, and 𝑃𝐶𝑂2 rises, such as in exhaustion due to acute severe asthma.
What are the blood gas levels in acute respiratory acidosis due to severe asthma?
What are the blood gas levels in acute respiratory acidosis due to severe asthma?
It is an ominous clinical finding.
How does chronic hypoventilation affect pH?
Renal compensation normalizes pH, with 𝐻𝐶𝑂3− levels high and 𝑃𝐶𝑂2 remaining high
What is ventilation-perfusion mismatching?
It refers to the presence of regions with varying ventilation-perfusion ratios (𝑉𝐴/ /Q) in diseases, rather than regions with no gas exchange.
How do under-perfused regions behave?
They behave qualitatively like alveolar dead space.
How do under-ventilated regions behave?
They behave qualitatively like a right-to-left shunt.
What happens in areas with high 𝑉𝐴/Q?
What happens in areas with low 𝑉𝐴/Q?
Can the effects of high 𝑉𝐴/Q areas balance the effects of low 𝑉𝐴/Q areas?
Look at black box
What is the overall effect on ventilation due to 𝑉𝐴/Q mismatch?
Peripheral and central chemoreceptors are stimulated, leading to increased ventilation.
What is the effect of oxygen-enriched inspired air on pure right-to-left shunts?
Any improvement in arterial 𝑃O2 is small because 𝑉𝐴/Q=0 (e.g., completely collapsed lung lobe).
How does ventilation-perfusion mismatching respond to oxygen-enriched inspired air?
Improvement is often marked because 𝑃𝑂2 in under-ventilated lung areas is improved.
Where is the 𝑉𝐴 /Q ratio higher in the lungs?
𝑉𝐴 /Q is higher at the top of the lungs compared to the bottom.
How does gravity affect lung perfusion and ventilation?
Gravity increases both perfusion and ventilation, with a greater effect on perfusion, especially at the bottom of the lungs.
Why is V(A)/Q higher at the top of the lung rather than the bottom?
Why are regional V(A)/Q differences important?
Regional VA/Q differences localise some diseases to top or bottom of the lung e.g. Tuberculosis at apex (high P(A)O2)
What is hypoxic pulmonary vasoconstriction (HPV)?
It is the constriction of pulmonary blood vessels in response to hypoxia, diverting blood flow from poorly ventilated areas to well-ventilated areas to improve 𝑉𝐴/Q matching.
How does HPV improve oxygenation?
By redirecting blood flow to well-ventilated areas, enhancing 𝑉𝐴/Q matching and arterial oxygenation.
Why is HPV not beneficial during global hypoxia?
Global hypoxia, such as in respiratory failure or high altitude, increases pulmonary vascular resistance, causing pulmonary hypertension and strain on the right heart.
which is more detrimental, systemic or topical vasodilators?
systemic
What are the three methods used to assess ventilation-perfusion mismatching?
Around 5–10 mmHg (0.6–1.2 kPa), with a normal upper limit of ~15 mmHg (2 kPa).
What does a ventilation-perfusion scan reveal in pulmonary embolism?
Ventilation remains normal (E), but there is no perfusion to the affected area (F), indicating an occlusion in the pulmonary artery.
What finding on an X-ray suggests a pulmonary embolism?
Occlusion in the right pulmonary artery (D).
What are the mechanisms leading to arterial hypoxia (low PaO2)?
- Low inspired PO2 - the problem is not in the lung but with the air or the alevolar gas has a reduced PO2, will make you hypoxaemic.
- Hypoventilation - air is not moving in or out of the lung, resulting in high alveolar PCO2 and low PO2
- Diffusion impairment - problems with surface area or thickness of the gas exchange interface.
- Right to left shunt - blood isn’t in the right place so you get shunting of the blood without it taking part in gas exchange.
- Ventilation-perfusion mismatch - mix of high V(A)/Q areas (dead space) and low V(A)/Q right to left shunts.
- Only hypoventilation results in high PaCO2 (in hypoxia).
- Diffusion impairment, right to left shunt and ventilation-perfusion mismatch increases the A-a PO2 gradient (alveolar and arterial).
Where must ventilation and perfusion be matched in the lungs?
At the alveolar/capillary level, not the whole lung.
What are the two extremes of 𝑉𝐴 /Q mismatching?
What is the inevitable result of hypoventilation?
