Physiology
Respiratory
Regarding V/Q mismatch, which of the following statements is CORRECT:
Answer:
Both ventilation and perfusion increase towards the lung base, because of the effects of gravity, but the gravitational effects are greater on perfusion than ventilation and therefore there is a regional variation in V/Q ratio from lung apex (high V/Q) to lung base (low V/Q). In a true shunt, there is normal perfusion but absent ventilation and the V/Q ratio = 0. In a true shunt increasing oxygen fraction has no effect because the oxygen-enriched air fails to reach the shunted blood. An increased A-a gradient is seen in V/Q mismatch.Ventilation-Perfusion Mismatch
Physiology / Respiratory / Ventilation-Perfusion Relationship
Last Updated: 8th August 2021
At rest, total alveolar ventilation and total pulmonary blood flow are similar, each being around 5 L/min. To achieve efficient gas exchange, it is essential that the flow of gas (ventilation, V) and the flow of blood (perfusion, Q) are closely matched throughout all regions of the lung. Ideally, local ventilation-perfusion (V/Q) ratios should be as close to 1 as possible.
V/Q Mismatch
When there are significant regional variations in ventilation or perfusion, this is referred to as ventilation-perfusion (V/Q) mismatch.
There are two extremes of V/Q mismatch:
- Dead space
- Lung region with normal alveolar ventilation but absent perfusion
- Caused by large pulmonary embolus for example
- Q = 0, therefore V/Q = ∞
- The Po2 and Pco2 of alveolar gas will approach their values in inspired air
- True shunt
- Lung region with normal perfusion but absent alveolar ventilation
- Caused by complete collapse or consolidation of a lung region for example
- V = 0, therefore V/Q = 0
- The Po2 and Pco2 of pulmonary capillary blood (and, therefore, of systemic arterial blood) will approach their values in mixed venous blood
Effect of V/Q Mismatch on Arterial Gases
Regions of the lung with V/Q > 1 have excessive ventilation relative to perfusion with a dead space effect, and blood derived from them will have raised PaO2 and low PaCO2. This may be seen in emphysematous areas where capillaries are destroyed or where pulmonary emboli are partially blocking blood flow.
Regions of the lung with V/Q < 1 have reduced ventilation relative to perfusion with a shunt effect, and blood derived from them will have low PaO2 and raised PaCO2. This may be seen when airways are partly blocked by bronchoconstriction, inflammation or secretions.
Regions of high V/Q cannot compensate for regions of low V/Q and the net effect of mixing blood from areas with V/Q mismatch is a low PaO2 and a normal/low PaCO2. Hypoxic vasoconstriction helps to reduce the severity of V/Q mismatching by diverting blood from regions with low V/Q ratios to regions that are better ventilated.
For an alveolus with a V/Q between 0-1 (V/Q mismatch but not true shunt), there is perfusion but relatively less ventilation, therefore blood passing through this alveolus will be partially oxygenated and increasing oxygen fraction can significantly improve arterial oxygen content (assuming no diffusion limitation). However in a true shunt (V/Q = 0) increasing oxygen fraction has no effect because the oxygen-enriched air fails to reach the shunted blood.
Gravitational Effects on V/Q Mismatch
Both ventilation and perfusion increase towards the lung base, because of the effects of gravity, but the gravitational effects are greater on perfusion than ventilation and therefore there is a regional variation in V/Q ratio from lung apex (high V/Q) to lung base (low V/Q). In young people, this gravitational effect is modest and has little effect on blood gases, but the V/Q mismatch increases with age and contributes to the reduction in PaO2 seen in the elderly.
Regional ventilation and perfusion can be visualised by inhalation and infusion of appropriate radioisotopes on a V/Q scan.
A-a gradient
The cause of a hypoxia can be classified by the alveolar-arterial PO2 gradient (A-a gradient). The alveolar gradient is calculated as PAO2 – PaO2.
A normal A-a gradient is seen in alveolar hypoventilation or low inspired PO2 (e.g. at high altitude). An increased A-a gradient reflects a diffusion defect (rare), V/Q mismatch or a right-to-left shunt.
In healthy young people, there is a small A-a gradient (< 2 kPa) arising from the normal anatomical right-to-left shunts. The normal value for the A-a gradient increases with age.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
