Past Papers · SAQ
Applied Respiratory Physiology — Hypercapnia
2012B Q21
Exam questionDescribe the physiological consequences of a progressive rise in blood carbon dioxide levels.
CICMWrecks answer
Master answer
Normal Ranges
- Normal 35 < PaCO₂< 45mmHg
- Moderate hypercaponea 45 < PaCO₂< 80mmHg
- Severe hypercaponea 80mmHg < PaCO₂
Sensors
| Central Chemoreceptor (CCR) | Peripheral Chemoreceptor (PCR) | |
|---|---|---|
| Location | Below the ventral surface of the medulla Within the BBB | In Glomus cells of: • Carotid bodies: At the bifurcation of the common carotid arteries • Aortic bodies: Above and below the aortic arch. |
| Mechanism | CO2 rapidly diffuses into CSF • Converted to H+ by carbonic anhydrased • Δ ECF [H+] closes pH sensitive K-channels • ↑ ECF [H+] stimulates ↑ ventilation • CSF has: – Low CSF protein and HCO3 (c.f. plasma) – Poor buffering capacity – Thus ΔCSF pH per ΔPaCO2 is GREATER than that of blood | Type 2 cells: • ↓ PaO2, ↑ PaCO2 and/or ↓pH • ↓ IC [ATP] which leads to ↑ NT production and release • ↑ AP firing rate of AB or CB afferent fibres |
| Contribution to ↑MV | 80% | 20% |
| Response Speed | Fast | Relatively fast compared with CCR |
Physiological effects of Hypercaponea
Moderate 45mmHg < PaCO₂<100mmHg
- Resp
- CO₂ strongest determinant of ↑MV (2-3 L/min per mmHg)
- Hypoxaemia has a synergistic effect on hypercapnoeic-ventilatory drive
- CO₂ is a potent pulmonary vasoconstrictor → ↑ pulmonary vascular resistance
- CVS
- Direct depression of cardiac muscle and vascular SM
- Peripheral vasodilation → ↓ SVR and ↑ regional BF
- ↓ heart rate/contractility
- ↑ RV Afterload due to effect on PVR
- ↑ circulating catecholamines
- Direct depression of cardiac muscle and vascular SM
- Neuro
- ↑ CBF (4% per mmHg ↑PaCO2) → ↑ ICP
- Haeme
- Right shift in OHDC
Severe (PaCO₂ >100mmHg)
- SNS stimulation
- hyperkinetic circulation →
- ↑ C.O., ↑ BP and arrhythmias
- ↑ cardiac MRO2 demand
- ↓ cardiac blood supply
- “CO2 nacrosis”
- PaCO2 > 80 mmHg → ↓ CSF pH
- Disorientation, confusion, headache, unconsciousness
- Hypoxia due to displacement of O2 from alveoli (as per Alveolar gas equation) → further ↑MV as above
- Respiratory acidosis →
- ↑ PaCO2 overwhelms buffering capacity of blood
- ↑ serum K+ and Ca2+ (stewart)
- ↑PaCO2 can cause anuria
Gladwin 2016
Past papers
Exam appearances
| Exam | Exact exam wording | Candidate success |
|---|---|---|
| 2012B Q21 | Describe the physiological consequences of a progressive rise in blood carbon dioxide levels. | 22.7% |