Physiology
Respiratory
Regarding right to left shunts, which of the following statements is CORRECT:
Answer:
Part of the venous effluent of the bronchial and coronary circulations bypasses the lungs and enters the pulmonary vein and left ventricle respectively resulting in anatomical right to left shunts. In healthy people, anatomical right to left shunts are equivalent to 2% or less of the cardiac output but they explain why arterial PO2 is less than alveolar PO2 even though pulmonary capillary blood equilibrates with alveolar. In a right to left shunt, the shunted blood will not have been oxygenated or been able to offload CO2 and thus its levels of PO2 and PCO2 are that of mixed venous blood (i.e. low PO2 and high PCO2). Treatment with high-flow oxygen in patients with pathological right to left shunts is of little benefit, as since blood is being shunted, the oxygen-enriched air fails to reach the shunted blood and the arterial PO2 will remain low.Right to Left Shunt
Physiology / Respiratory / Ventilation-Perfusion Relationship
Last Updated: 11th April 2019
Anatomical Right to Left Shunts
Part of the venous effluent of the bronchial and coronary circulations bypasses the lungs and enters the pulmonary vein and left ventricle respectively. Oxygenated blood from the lungs is therefore diluted by venous blood. In healthy people, these anatomical shunts are equivalent to 2% or less of the cardiac output but they explain why arterial PO2 is less than alveolar PO2 even though pulmonary capillary blood equilibrates with alveolar air.
Right to Left Shunts in Disease
Shunting may become significant in some disease states when regions of the lung are not ventilated (e.g. atelectasis, pulmonary oedema, pneumonia) or due to cyanotic congenital heart disease where blood bypasses the pulmonary circulation completely (e.g. tetralogy of Fallot).
The shunted blood will not have been oxygenated or been able to offload CO2 and thus its levels of PO2 and PCO2 are that of mixed venous blood. Changes in PCO2 and PO2 stimulate the chemoreceptors and increase ventilation, so that arterial PCO2 returns to normal. However, increased ventilation cannot increase blood O2 content, as the haemoglobin of the blood passing through the lungs is already close to saturation. Thus right to left shunts commonly result in a low arterial PO2 but a normal or low PCO2.
Treatment with high-flow oxygen in these patients is of little benefit, as since blood is being shunted, the oxygen-enriched air fails to reach the shunted blood and the arterial PO2 will remain low.
Report A Problem
Is there something wrong with this question? Let us know and we’ll fix it as soon as possible.
Loading Form...
- 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 |
