Respiratory Physiology: VA/Q and Gas Exchange

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Questions and Answers

What is the primary cause of low PCO2 in ventilation-perfusion mismatching?

Ventilatory response (correct)

Alveolar hypoventilation

Chronic hypoventilation

Ventilation-perfusion ratios (VA/Q)

What is the characteristic of regions with low VA/Q in ventilation-perfusion mismatching?

They have normal ventilation and perfusion

They have high oxygen content

They qualitatively behave like right to left shunt (correct)

They qualitatively behave like alveolar dead space

What is the effect of high VA/Q areas on oxygen content?

They increase oxygen content (correct)

They decrease oxygen content

They have no effect on oxygen content

They normalize oxygen content

What is the pH level in acute respiratory acidosis?

7.30 Signup and view all the answers

What is the effect of chronic hypoventilation on pH levels?

It normalizes pH levels Signup and view all the answers

What is the characteristic of ventilation-perfusion mismatching?

Regions with varying ventilation-perfusion ratios Signup and view all the answers

What is the effect of low VA/Q areas on oxygen content?

They decrease oxygen content Signup and view all the answers

What is the primary cause of high PCO2 in ventilation-perfusion mismatching?

Alveolar hypoventilation Signup and view all the answers

What is the overall VA/Q ratio?

0.8 Signup and view all the answers

What is the PAO2 in the normal lung?

100mmHg Signup and view all the answers

What is the percentage of venous blood that passes through the lungs in normal right to left shunts?

98% Signup and view all the answers

What is the effect of a shunt on gas exchange?

No gas exchange occurs Signup and view all the answers

What is the PACO2 in the normal lung?

40mmHg Signup and view all the answers

What is the PaO2 in the dead space?

159mmHg Signup and view all the answers

What is the VA/Q ratio in the dead space?

Infinity Signup and view all the answers

What is the effect of a dead space on gas exchange?

No gas exchange occurs Signup and view all the answers

What is the most likely reason for the low PaO2 and PaCO2 values in the blood gases provided?

Right to left shunt due to Ventricular Septal Defect Signup and view all the answers

What is the effect of a 20% shunt on arterial O2 content?

Decreased O2 content Signup and view all the answers

Why does a moderate rise in CO2 content cause a very small rise in PCO2?

Due to the shape of the CO2 dissociation curve Signup and view all the answers

What is the effect of increased ventilation on blood gases in a right to left shunt?

Low PaO2 and normal or low PaCO2 Signup and view all the answers

What is the effect of breathing 100% oxygen on arterial PO2 in a right to left shunt?

Small increase in PaO2 Signup and view all the answers

What is the consequence of high pressure in the pulmonary circulation in Ventricular Septal Defect?

Pulmonary vascular remodelling and high resistance Signup and view all the answers

What is the initial effect of Atrial Septal Defect and Ventricular Septal Defect on blood flow?

Left to right shunt Signup and view all the answers

Why can't you average the pressures in the calculation of the effect of a shunt on arterial O2 and CO2 contents?

Because the pressures are not directly proportional to contents Signup and view all the answers

What is the effect of hypoxic pulmonary vasoconstriction on blood flow?

Diverts blood flow from poorly ventilated areas to well ventilated areas Signup and view all the answers

What is the normal upper limit of the 'A-a' PO2 gradient in healthy young people?

15 mmHg (2 kPa) Signup and view all the answers

Which of the following is NOT a mechanism leading to arterial hypoxia (low PaO2) that inevitably leads to an increase in PaCO2?

Hypoventilation Signup and view all the answers

What is the effect of systemic vasodilators on VA/Q matching?

Worsens VA/Q matching by increasing blood flow to poorly ventilated areas Signup and view all the answers

What is the effect of increased pulmonary vascular resistance on the right heart?

Increases the load on the right heart Signup and view all the answers

What is the purpose of isotope ventilation and perfusion scans?

To assess VA/Q matching Signup and view all the answers

What is the effect of regional VA/Q differences on disease localization?

They localize diseases to the top or bottom of the lung Signup and view all the answers

What is the effect of hypoxic pulmonary vasoconstriction in the presence of global hypoxia?

It worsens VA/Q matching and arterial oxygenation Signup and view all the answers

What is the primary reason why high VA/Q areas do not compensate for the low VA/Q areas in their effects on arterial blood gases?

More blood tends to come from the low VA/Q areas Signup and view all the answers

What is the effect of gravity on VA/Q in the lungs?

VA/Q is lower at the bottom than the top Signup and view all the answers

What is the characteristic of pure right to left shunts in terms of VA/Q?

VA/Q = 0 Signup and view all the answers

What is the effect of ventilation-perfusion mismatching on arterial PO2 in response to O2 enriched inspired air?

Marked improvement Signup and view all the answers

What is the relationship between Q and VA in l/min-1 per unit lung volume?

Q is directly proportional to VA Signup and view all the answers

What is the effect of high VA/Q areas on alveolar PO2?

Alveolar PO2 is increased Signup and view all the answers

What is the response to O2 enriched inspired air in terms of arterial PO2 in pure right to left shunts?

Small improvement Signup and view all the answers

What is the effect of ventilation-perfusion mismatching on normal upright lung?

Some mismatching occurs Signup and view all the answers

Study Notes

Shunt, Dead Space, and VA/Q

VA/Q = 0.8, indicating incomplete gas exchange
Shunt: blood that bypasses alveolar gas exchange, resulting in no oxygenation and no CO2 removal
Dead space: ventilated areas with no perfusion, resulting in no gas exchange

Right-to-Left Shunts

Normal right-to-left shunts: >98% of venous blood passes through the lungs, but a small amount from bronchial veins and Thebesian veins bypasses gas exchange
Abnormal right-to-left shunts: occur in conditions like collapsed lung, pneumonia, and congenital heart diseases
Atrial Septal Defect (ASD) and Ventricular Septal Defect (VSD) can cause right-to-left shunts, leading to low arterial oxygen content and PO2

Effect of Shunts on Arterial O2 and CO2 Contents

Shunt effect: a weighted average of contents in the shunted and unshunted blood
Calculating the effect on PO2 and PCO2 using dissociation curves
O2 content: a moderate fall in O2 content causes a large fall in PO2
CO2 content: a moderate rise in CO2 content causes a small rise in PCO2

Effect of Increased Ventilation or FIO2 on Right-to-Left Shunts

Stimulates chemoreceptors, leading to increased ventilation
Ventilated areas lose more CO2, but gain little extra O2 (haemoglobin already saturated)
Shunt blood remains unaffected
Final blood gases: low PaO2, normal or low PaCO2

Ventilation-Perfusion Mismatching

Inequalities in VA/Q lead to both dead space and shunt effects
Regions with high VA/Q (dead space) and low VA/Q (shunt) cannot be balanced
Low O2 content and high CO2 content in arterial blood

Response to O2 Enriched Inspired Air

Pure right-to-left shunts: minimal improvement in arterial PO2
Ventilation-perfusion mismatching: marked improvement in arterial PO2 due to increased PO2 in under-ventilated areas

Regional VA/Q Differences

Gravity affects perfusion and ventilation, resulting in higher VA/Q at the top of the lungs
Accounts for the PO2 A-a difference

Hypoxic Pulmonary Vasoconstriction

Pulmonary blood vessels vasoconstrict in response to hypoxia
Diverts blood flow from poorly ventilated areas to well-ventilated areas, improving VA/Q matching and arterial oxygenation
Not helpful in global hypoxia, as it increases pulmonary vascular resistance and load on the right heart

Assessment of Ventilation-Perfusion Mismatching

Isotope ventilation and perfusion scans
Measuring alveolar dead space and shunt effect using inert gases and modified Bohr and shunt equations
From "A-a" PO2 gradient, which increases with age

Mechanisms Leading to Arterial Hypoxia

Hypoventilation, shunt, dead space, and ventilation-perfusion mismatching
Only hypoventilation inevitably leads to increased PaCO2, while other causes usually lead to normal or reduced PaCO2.

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Description

This quiz assesses understanding of ventilation-perfusion ratio, shunt, dead space, and gas exchange in respiratory physiology. Calculate VA and Q values and interpret PAO2 and PICO2 levels.