Control of Breathing: Hypoxia - Respiratory System

Control of breathing hypoxia By Ali baay Terms : SPO2 : O 2 level by blood by oximetry SO2 : oxygen saturation in the blood PAO2 : oxygen level in the alveoli PaO2 : oxygen level in the arterial blood A-a gradient : difference between PAO2 & PaO2 should be < 12 LO 5.1 Define...

Control of breathing hypoxia By Ali baay Terms : SPO2 : O 2 level by blood by oximetry SO2 : oxygen saturation in the blood PAO2 : oxygen level in the alveoli PaO2 : oxygen level in the arterial blood A-a gradient : difference between PAO2 & PaO2 should be < 12 LO 5.1 Define the terms hypoxia, hypercapnia, hypocapnia, hyperventilation, hypoventilation Hypoxia : A fall in alveolar, thus arterial pO2 Hypercapnia – A rise in alveolar, thus arterial, CO2 Hypocapnia – A fall in alveolar, thus arterial CO2 Hyperventilation – Ventilation increases with no change in metabolism Hypoventilation – Ventilation decreases with no change in metabolism  LO 5.2 Describe the effects on plasma pH of hyper- and hypo-ventilation pCO2 affects plasma pH (Henderson- Hasselbach) PCO2= high H=low pH= acidosis o Hyperventilation  pCO ê Hypocapnia and Respiratory 2 Alkalosis  pH é above 7.6  Free calcium concentration falls enough to produce fatal tetany (Ca2+ is only soluble in acid, so when pH rises, Ca2+ cannot stay in the blood. Nerves become hyper-excitable.) o Hypoventilation  pCO2 é Hypercapnia and Respiratory Acidosis  pH ê  Enzymes become lethally denature Respiratory Acidosis CO2 is produced more rapidly than it is removed by the lungs (hypoventilation). Alveolar pCO2 rises, so [Dissolved CO2] rises more than [HCO3-], producing a fall in plasma pH. Compensated Respiratory Acidosis Respiratory Acidosis persists, and the kidneys respond to low pH by reducing the excretion of HCO3- therefore the pH be just below 7.3. Respiratory Alkalosis CO2 is removed from alveoli more rapidly than it is produced (hyperventilation). Alveolar pCO2 falls producing a rise in plasma pH. Compensated Respiratory Alkalosis Respiratory Alkalosis persists, and the kidneys respond to the high pH by excreting HCO3-, thus restoring the ratio of [Dissolved CO2] to [HCO3-], and therefore the pH. Metabolic Acidosis Metabolic production of acid displaces HCO3- from plasma as the acid is buffered; therefore the pH of blood falls. Compensated Metabolic Acidosis The ratio of [Dissolved CO2] to [HCO3-] may be restored to near normal by lowering pCO2. The lungs increase ventilation to correct pH. Metabolic Alkalosis Plasma [HCO3-] rises, causing the pH of blood to rise (e.g. after vomiting). Compensated Metabolic Alkalosis The ratio of [Dissolved CO2] to [HCO3-] may be restored to near normal by raising pCO2. The  Factors affect ventilation Falling Inspired pO2 The falling arterial pO2 is detected by Peripheral Chemoreceptors located in the Carotid and Aortic bodies. The carotid and aortic bodies are stimulated by a decrease in oxygen supply relative to their own oxygen usage, which is small. They only respond to large drops in O2. Stimulation of the receptors: o Increases the tidal volume and rate of respiration o Changes in circulation directing more blood to the brain and kidneys o Increased pumping of blood by the heart Increase in Inspired pCO2 The Peripheral Chemoreceptors in the Carotid and Aortic bodies also detect changes in pCO2, but are insensitive. Central Chemoreceptors in the Medulla of the brain are much more sensitive, altering breathing on a second to second basis. Central chemoreceptors detect changes in Arterial pCO2. o Small rise in pCO2 Increase Ventilation o Small falls in pCO2 Decrease Ventilation The central chemoreceptors actually respond to changes in the pH of cerebro-spinal fluid (CSF). The CSF is separated from the blood by the blood-brain barrier. The pCO2 of the CSF is determined by arterial pCO2, but HCO3- and H+ cannot cross. CSF [HCO3-] is controlled by Choroid Plexus Cells. The pH of CSF is determined by the ratio of [HCO3-] to pCO2. In the short term, [HCO3-] is fixed (cannot cross BBB), so falls in pCO2 raize pH and rises in pCO2 Lower pH. Persisting changes compensated for via the Choroid Plexus Cells altering CSF [HCO3-]. Respiratory failure Failure to achieve adequate O2 for the tissue & metabolic requirements Type 1 Respiratory Failure o Respiratory Rate é o pO2 ê o CO2 - / ê Type 2 Respiratory Failure o Respiratory Rate é o pO2 ê o CO2 é Type 3 : postoperative Type 4 respiratory failure : Due to increase requirements as hyper dynamic state  Mechanism of hypercapnea Only hypoventilation as the CO2 is highly diffused gas with rapid efficient penetration Hypoxia – A fall in alveolar, thus arterial pO2 Respiratory Failure Arterial pO2 falls below 8kPa when breathing air at sea level. o Type 1 Respiratory Failure Arterial hypoxia, accompanied by a normal or low pCO2 causing Breathlessness, Exercise Intolerance, Central Cyanosis o Type 2 Respiratory Failure Arterial hypoxia, accompanied by an elevated pCO2  Mechanisms of hypoxia 1. Low pO2 in inspired air Everything is normal, the air breathed in just has low pO2 as People living at high altitudes 2. Hypoventilation Always associated with increased pCO2 (Type 2 Respiratory Failure) can caused by any part of respiration problems as: 1- Central causes  Respiratory depression due to opiate overdose  Head injury 2- Neuromuscular Problems as myasthenia gravis dis. 3- Muscle weakness (NMJ/Nerve/Muscle 4- Chest wall problems (Mechanical) Scoliosis/kyphosis Morbid obesity Trauma Pneumothorax 5- Hard to Ventilate lungs: Airway obstruction COPD & Asthma when the airway narrowing is severe and widespread Severe fibrosis 3. Diffusion Impairment O2 diffuses much less readily than CO2, so is always affected first pCO2 is therefore low/normal with wide A-a gradient Always Type 1 Respiratory Failure - Structural Changes Lung fibrosis causing thickening of alveolar capillary membrane - Increased Path Length Pulmonary Oedema - Total area for diffusion reduced Emphysema Ventilation Perfusion Mismatch The most common mechanism of hypoxia O2 diffuses much less readily than CO2, so is always affected first pCO2 is therefore low/normal with wide A-a gradient Always Type 1 Respiratory Failure - Reduced Ventilation of some Alveoli Lobar Pneumonia - Reduced Perfusion of Some Alveoli Pulmonary Embolism Shunt With O2 affection more with wide A-a gradient abnormal Right to Left Shunts to bypass the gas exchange areas o E.g. o - Cyanotic Heart Disease such as VSD o - extra-cardiac shunt as tetrology of Fallot o thanks