Gas Transfer: Objectives, Hypoxia Case Studies - PDF

Gas transfer objectives 1. Clinical scenarios 2. The pulmonary and bronchial circulations 3. Partial pressure gradients 4. Three components to gas transfer 4.1 The diffusion of gases across the alveolo-capillary membrane and factors which affect diffusion rate 4.2 The reaction...

Gas transfer objectives 1. Clinical scenarios 2. The pulmonary and bronchial circulations 3. Partial pressure gradients 4. Three components to gas transfer 4.1 The diffusion of gases across the alveolo-capillary membrane and factors which affect diffusion rate 4.2 The reaction of gases with blood 4.3 Ventilation/perfusion matching 5. The effects of a shunt 6. The effects of increased dead space 7. How to recognise whether hypoxic hypoxia is due to impaired ventilation or impaired gas transfer 8. Gas exchange in the tissues 9. Measurement of transfer factor (Diffusing capacity) ©J P Jamison Physiology for medical students  Hypoxic hypoxia with and without hypercapnia Normal Patient 1 Patient 2 PaO2 12.7 kPa PaO2 8 kPa PaO2 8 kPa PaCO2 5.4 kPa PaCO2 8 kPa PaCO2 4 kPa ↓Ventilation ↓Transfer O2 therapy O2 therapy unsafe safe ©J P Jamison Physiology for medical students  Hypoxic hypoxia with and without hypercapnia causes Ventilation ↑PaCO2 Transfer ↓PaCO2 ↓PaO2 ©J P Jamison Physiology for medical students  Gas transfer objectives2 1. Clinical scenarios 2. The pulmonary and bronchial circulations 3. Partial pressure gradients 4. Three components to gas transfer 4.1 The diffusion of gases across the alveolo-capillary membrane and factors which affect diffusion rate 4.2 The reaction of gases with blood 4.3 Ventilation/perfusion matching 5. The effects of a shunt 6. The effects of increased dead space 7. How to recognise whether hypoxic hypoxia is due to impaired ventilation or impaired gas transfer 8. Gas exchange in the tissues 9. Measurement of transfer factor (Diffusing capacity) ©J P Jamison Physiology for medical students  Pulmonary circulation Hypoxia Pulmonary capillary constricts (wedge) pressure 8 mmHg Pulmonary Resistance is affected by tension in the pulmonary arterioles arterioles Cardiac output Pulmonary Pulmonary venous pressure artery pressure 7 mmHg 25/10, mean 15 mmHg Perfusion pressure 15-7 Pulmonary vascular resistance = = Cardiac output 5 ©J P Jamison (mmHg∙min∙L Physiology for medical students -1)  Bronchial circulation Supplies nutritive needs of bronchial walls – muscle and glands Systemic arterial supply –oxygenated blood Small rate of blood flow Shunt= deoxygenated blood moves from the right to left without mixing with the oxygenated blood High vascular resistance – sympathetic tone, metabolite dilatation Capillaries – oxygen extracted Venules – deoxygenated admixture with pulmonary capillaries Physiological shunt Slightly lower PO2 in systemic arteries than equilibrated with alveoli ©J P Jamison Physiology for medical students  Gas transfer objectives3 1. Clinical scenarios 2. The pulmonary and bronchial circulations 3. Partial pressure gradients 4. Three components to gas transfer 4.1 The diffusion of gases across the alveolo-capillary membrane and factors which affect diffusion rate 4.2 The reaction of gases with blood 4.3 Ventilation/perfusion matching 5. The effects of a shunt 6. The effects of increased dead space 7. How to recognise whether hypoxic hypoxia is due to impaired ventilation or impaired gas transfer 8. Gas exchange in the tissues 9. Measurement of transfer factor (Diffusing capacity) ©J P Jamison Physiology for medical students  Journey of oxygen and carbon dioxide PatmosO2 = 150 mmHg, 20 kPa PatmosCO2 ≈ 0 Gas Transfer PAO2 = 100 mmHg, 13.3 kPa PACO2= 40 mmHg,5.3kPa PvO2 = 40 mmHg, 5.3 kPa PaO2 = 95 mmHg, 12.7 kPa PvCO2 = 46 mmHg, 6.1 kPa PaCO2 = 40 mmHg, 5.3 kPa ©J P Jamison Physiology for medical students  Gas transfer objectives4 1. Clinical scenarios 2. The pulmonary and bronchial circulations 3. Partial pressure gradients 4. Three components to gas transfer 4.1 The diffusion of gases across the alveolo-capillary membrane and factors which affect diffusion rate 4.2 The reaction of gases with blood 4.3 Ventilation/perfusion matching 5. The effects of a shunt 6. The effects of increased dead space 7. How to recognise whether hypoxic hypoxia is due to impaired ventilation or impaired gas transfer 8. Gas exchange in the tissues 9. Measurement of transfer factor (Diffusing capacity) ©J P Jamison Physiology for medical students  Gas transfer Diffusion through 1 alveolo-capillary Matching 3 ventilation to membrane 2 perfusion Reaction with blood Alveolus Blood Alveolo-capillary membrane ©J P Jamison Physiology for medical students  Gas transfer objectives4.1 1. Clinical scenarios 2. The pulmonary and bronchial circulations 3. Partial pressure gradients 4. Three components to gas transfer 4.1 The diffusion of gases across the alveolo-capillary membrane and factors which affect diffusion rate 4.2 The reaction of gases with blood 4.3 Ventilation/perfusion matching 5. The effects of a shunt 6. The effects of increased dead space 7. How to recognise whether hypoxic hypoxia is due to impaired ventilation or impaired gas transfer 8. Gas exchange in the tissues 9. Measurement of transfer factor (Diffusing capacity) ©J P Jamison Physiology for medical students  Gas transfer1-3 Diffusion through 1 alveolo-capillary 3 Matching ventilation to membrane 2 perfusion Reaction with blood Alveolus Blood Alveolo-capillary membrane ©J P Jamison Physiology for medical students  Fick’s law of diffusion Partial pressure surface solubility gradient × area × in water Rate of diffusion ∞ membrane × √ molecular weight thickness Gas must dissolve in the membrane to create the partial pressure gradient. Diffusion is slow through the alveolar wall. CO2 is more diffusable than O2 as it is much more soluble Partial pressure Step down – solubility in membrane Membrane ©J P Jamison Physiology for medical students  Equilibration of alveoli with blood ¾ second Process occurs very quickly ¼ sec 13 6 PcapO2 6 13 PcapCO2 kPa kPa 5.4 5.4 Distance along pulmonary capillary ©J P Jamison Physiology for medical students  Gas transfer objectives4.2 1. Clinical scenarios 2. The pulmonary and bronchial circulations 3. Partial pressure gradients 4. Three components to gas transfer 4.1 The diffusion of gases across the alveolo-capillary membrane and factors which affect diffusion rate 4.2 The reaction of gases with blood 4.3 Ventilation/perfusion matching 5. The effects of a shunt 6. The effects of increased dead space 7. How to recognise whether hypoxic hypoxia is due to impaired ventilation or impaired gas transfer 8. Gas exchange in the tissues 9. Measurement of transfer factor (Diffusing capacity) ©J P Jamison Physiology for medical students  Gas transfer2 Diffusion through 1 alveolo-capillary Matching 3 ventilation to membrane 2 perfusion Reaction with blood Alveolus Blood Alveolo-capillary membrane ©J P Jamison Physiology for medical students  Reactions with blood Oxygen 4 O2 + Hb4 Hb4O8 Remember Carbon Dioxide never plateaus as it will always react with water Carbon dioxide carbonic anhydrase CO2 + H2O H2CO3 H+ + HCO3─ [CO2 + Hb(NH2) Hb(NHCOOH) ] ©J P Jamison Physiology for medical students  Gas transfer objectives4.3 1. Clinical scenarios 2. The pulmonary and bronchial circulations 3. Partial pressure gradients 4. Three components to gas transfer 4.1 The diffusion of gases across the alveolo-capillary membrane and factors which affect diffusion rate 4.2 The reaction of gases with blood 4.3 Ventilation/perfusion matching 5. The effects of a shunt 6. The effects of increased dead space 7. How to recognise whether hypoxic hypoxia is due to impaired ventilation or impaired gas transfer 8. Gas exchange in the tissues 9. Measurement of transfer factor (Diffusing capacity) ©J P Jamison Physiology for medical students  Gas transfer3 Diffusion through 1 alveolo-capillary Matching 3 ventilation to membrane 2 perfusion Reaction with blood Alveolus Blood Alveolo-capillary membrane ©J P Jamison Physiology for medical students  Ventilation/perfusion mismatch PAO2↓ PACO2↑ PAO2↑ Greater ventilation PACO2↓ Ventilation and perfusion mismatch Greater amount of load on the left They will not compensate as no extra oxygen can be produced due to the plateau in the oxygen disassociation CcapO2↓ curve The total ventilation of the lung CcapCO2↑ responds to he hypoxic hypoxia-CO2 is CcapO2─ blown off C CcapCO2↓ CaO2↓ PaO2↓ CaCO2 ↓ PaCO2 ↓ PP ©J P Jamison Physiology for medical students  Compensation for V/Q mismatch PAO2↓ PACO2↑ PAO2↑ PACO2↓ Hypoxic pulmonary vasoconstriction. Will increase the O2 and prevent hypercapnia. Raise the VQ ration Pulmonary hypertension can be caused duet= to this Hypoxic pulmonary vasoconstriction ©J P Jamison Physiology for medical students  Gas transfer objectives7,8 1. Clinical scenarios 2. The pulmonary and bronchial circulations 3. Partial pressure gradients 4. The diffusion of gases across the alveolo-capillary membrane and factors which affect diffusion rate 5. The reaction of gases with blood 6. The effect of ventilation/perfusion mismatch 7. The effects of a shunt 8. The effects of increased dead space 9. How to recognise whether hypoxic hypoxia is due to impaired ventilation or impaired gas transfer 10. Gas exchange in the tissues 11. Measurement of transfer factor (Diffusing capacity) ©J P Jamison Physiology for medical students  V/Q mismatch, dead space & shunts Dead space is the portion of the lung where there is ventilation but n gas exchange Shut causes hypoxia and hypocapnia. Shunt cannot be corrected by deep breathing V/Q mismatch DEAD SPACE SHUNT ©J P Jamison Physiology for medical students  Gas transfer objectives7 1. Clinical scenarios 2. The pulmonary and bronchial circulations 3. Partial pressure gradients 4. Three components to gas transfer 4.1 The diffusion of gases across the alveolo-capillary membrane and factors which affect diffusion rate 4.2 The reaction of gases with blood 4.3 Ventilation/perfusion matching 5. The effects of a shunt 6. The effects of increased dead space 7. How to recognise whether hypoxic hypoxia is due to impaired ventilation or impaired gas transfer 8. Gas exchange in the tissues 9. Measurement of transfer factor (Diffusing capacity) ©J P Jamison Physiology for medical students  Hypoxic hypoxia with and without hypercapniaRx Hypoxia with hypercapnia Oxygen therapy can impair ventilation further and make it more harmful Hypoxia with hypocapnia-impaired gas transfer CO2 diffuses readily as the plateauedoxygen cannot compensate for the Normal Patient 1 Patient 2 pleural PaO2 12.7 kPa PaO2 8 kPa PaO2 8 kPa PaCO2 5.4 kPa PaCO2 8 kPa PaCO2 4 kPa ↓Ventilation ↓Transfer O2 therapy O2 therapy unsafe safe ©J P Jamison Physiology for medical students  Gas transfer objectives8 1. Clinical scenarios 2. The pulmonary and bronchial circulations 3. Partial pressure gradients 4. Three components to gas transfer 4.1 The diffusion of gases across the alveolo-capillary membrane and factors which affect diffusion rate 4.2 The reaction of gases with blood 4.3 Ventilation/perfusion matching 5. The effects of a shunt 6. The effects of increased dead space 7. How to recognise whether hypoxic hypoxia is due to impaired ventilation or impaired gas transfer 8. Gas exchange in the tissues 9. Measurement of transfer factor (Diffusing capacity) ©J P Jamison Physiology for medical students  Gas exchange in tissues Vasomotor effect in capillaries Variable proportion of capillaries closed PvCO2 PaO2 PvO2 PaCO2 Equilibration with tissues does not occur ©J P Jamison Physiology for medical students  Gas transfer objectives9 1. Clinical scenarios 2. The pulmonary and bronchial circulations 3. Partial pressure gradients 4. Three components to gas transfer 4.1 The diffusion of gases across the alveolo-capillary membrane and factors which affect diffusion rate 4.2 The reaction of gases with blood 4.3 Ventilation/perfusion matching 5. The effects of a shunt 6. The effects of increased dead space 7. How to recognise whether hypoxic hypoxia is due to impaired ventilation or impaired gas transfer 8. Gas exchange in the tissues 9. Measurement of transfer factor (Diffusing capacity) ©J P Jamison Physiology for medical students Measuring the capacity of the lungs to transfer gases The single breath carbon monoxide transfer test This is studied in the practical laboratory ©J P Jamison Physiology for medical students A 20 year old man presents with severe wheezy dyspnoea. He is given a 2 agonist and 100% oxygen. Arterial blood samples on 30 minutes later admission Wheezy dyspnoea breathing air breathing 100% normal suggest obstructed ow and turbulent ow from the O2 ranges bronchus B2 agonist results n pH 7.49 7.34 7.35-7.45 dilation of blood vessels in the bronchioles PaO2 9 13 11.3-12.6 kPa PaCO2 4.2 6 4.7-6.0 kPa HCO3¯ 23 24 20-28 mmol/L Acute res. Alkalosis Inadequate gas transfe What is the cause of the hypoxic hypoxia on admission? and hypoxia with hypocapnia Acute resp. Acidosis What has happened 30 minutes later? Severe asthma and the muscles of respiration Analyse his acid/base status. cannot keep up with ventilation ©J P Jamison Physiology for medical students  Hypoxic hypoxia with and without hypercapnia case Ventilation ↑PaCO2 30 min later Transfer ↓PaCO2 at presentation ↓PaO2 Respiratory muscles are overworked and he cannot be breather for much longer ©J P Jamison Physiology for medical students