Session 5 - Control of breathing, Hypoxia and Respiratory Failure Flashcards
What is hypoxia?
• A fall in alveolar, thus arterial pO2
What is hypercapnia?
• A rise in alveolar, thus arterial CO2
What is hypocapnia?
• A fall in alveolar, thus arterial CO2
What is hyperventilation?
- Ventilation increases with no change in metabolism
* (breathing more than you actually have to)
What is hypoventilation?
- Ventilation decreases with no change in metabolism
* (breathing less than you have to)
How does hyperventilation affect plasma pH?
- pCO2 down
* pH increases
How does hypoventilation affect CO2 and plasma pH?
- pCO2 up
* pH down
What is the normal metabolic pH?
• 7.4
What is the body’s normal pH range?
• 7.38 - 7.42
What happens if plasma pH falls below 7.0?
- Plasma k+ rises to dangerous levels and enzymes are lethally denatured
- Function of heart affected
What happens if plasma pH rises above 7.6?
- Free calcium concentration falls enough to produce fatal tetany
- Calcium salts soluble in acid conditions - In alkalosis, calcium forms complexes. Nerves become excitable, causing tetany
Give two events which will occur when hypoventilation occurs?
- Hypercapnia
- Respiratory acidosis
- pH falls below 7.0
- Enzymes become lethally denatured
Give two events which will occur when hyperventilation occurs?
• Hypo capnia and respiratory alkalosis
• pH rises above 7.6
• Free calcium concentration falls enough to produce fatal tetany
○ Ca2+ is only soluble in acid, so pH rises Ca2+ cannot stay in blood. Nerves become hyperexcitable.
What is respiratory acidosis?
- CO2 produced more rapidly than it is removed by the lungs (hypoventilation).
- pCO2 rises, so (dissolved CO2) rises more than HCO3-, producing a fall in plasma pH
What is compensated respiratory acidosis?
• Respiratory acidosis persists, and the kidneys responsd to low pH by reducing excretion of HCO3-, thus restoring ratio of (dissolved CO2) to (HCO2-), producing a rise in pH
How long does compensation take?
• 2-3 days
What is respiratory alkalosis?
- CO2 is removed from alveoli more rapidly than it is produced (hyperventilation)
- Alveolar pCO2 decreases, changing the ratio of (dissolved CO2) to (HCO3-) producing an increase in plasma pH
What is 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.
What is metabolic acidosis?
- HCO3- displaced by metabolically produced acids
* Blood pH form
How can metabolic acidosis be compensated for?
• Ratio of (dissolved CO2) to (HCO3-) may be restored to near normal by increasing ventilation rate to decrease pCO2
What does acidosis mean?
- Reduction in HCO3-
* NOT PH
What is metabolic alkalosis?
- Plasma HCO3- rises, causing the pH of blood to rise (after vomiting?)
- Stomach produces HCO3- when acid generated
- If acid removed from stomach, gastrin released which produces more acid
- HCO3- produced in excess as a result of this increased production
How can metabolic alkalosis be compensated for?
- Ratio of dissolved CO2 to HCO3- may be restored to near normal by raising pCO2
- Lungs decrease ventilation to correct pH
- Dangerous
What is the control of our breathing moderated by?
- pH
* Oxygen requirements are secondary
Give three variables which affect breathing?
- pH
- Decrease O2
- Increased CO2
What occurs when there is falling inspired pO2?
- Detected by peripheral chemoreceptors located in carotid and aortic bodies
- Increase the tidal volume and rate of respiration
- Changes in circulation direction more blood to the brain and kidney
- Increased pumping of blood by the heart
What occurs when there is an increase in inspired pCO2
- Central chemoreceptors in medulla more sensitive than peripheral
- Small rise in pCO2 -> Increase ventilation
- Small decrease in pCO2 -> Decrease ventilation
- Basis of negative feedback control of breathing
What is low O2 detected by?
• Peripheral chemoreceptors
Where are peripheral chemoreceptors found?
- Carotid bodies
* Aortic bodies
Why do peripheral chemoreceptors only respond to large changes in O2?
- High blood flow in carotid and aorta
* Lots of O2, usually
What are peripheral chemoreceptors stimulated by?
- Low O2
* High CO2 (minor function)
Give three ways in which chemoreceptors response to a decrease in O2
- Increase tidal volume and rate of respiration
- changes in circulation directing blood to the brain and kidney
- Increased pumping of blood by the heart
What is a central chemoreceptor and where is it found?
- Found in medulla of the brain
* Much more sensitive, altering breathing on a second to second basis
What do the central chemoreceptors do in response to arterial pCO2?
- Small rise in pCO2 -> Increase ventilation
* Small fall in pCO2 -> Decrease ventilation
hat do central chemoreceptors respond to?
- Changes in the pH of cerebro-spinal fluid (CSF)
* CSF separated from blood by the blood-brain barrier
What is CSH (HCO3-) controlled by?
• Choroid plexus cells
What out of HCO3-, CO2 and H+ can cross the BBB
• CO2 only
What is the pH of the CSF governed by?
• The ratio of HCO2- to pCO2
How is regulation of brain pH different to that of the rest of the body?
• Occurs within hours
Apart from pCO2 and pO2, what do the central chemoreceptors respond to?
• Changes in pH of CSF
How is the CSF separated from the blood?
• By the blood brain barrier
What is the CO2 of the CSF determined by?
- Arterial pCO2
* HCO3- and H+ cannot cross
What is CSF (HCO3-) controlled by?
• Choroid plexus cells
What is the pH of CSF determined by?
- The ratio of HCO3- to pCO2
- In the short term HCO3- is fixed, so falls in pCO2 -> increase in pH
- Rise in pCO2 -> lower pH
- Persisting changes compensated for by choroid plexus cells altering CSF
What is the oxygen transport chain?
• Air -> Airways -> Alveolar gas -> alveolar membrane -> Arterial blood -> Regional arteries -> Capillary blood -> Tissues
Define hypoxia
• A fall in alveolar, thus arterial pO2
Give conditions which cause diffusion impairments
• Fibrotic lung disease
○ Thickened alveolar membrane slows gas exchange
• Pulmonary oedema
○ Fluid in interstitial space increases diffusion distance
• Emphysema
○ Destruction of alveoli reduces surface area for gas exchange
What is respiratory failure?
- Not enough oxygen enters the blood
* Not enough CO2 leaves the blood
What is type 1 respiratory failure?
- Arterial hypoxia (pO2 below 8kPa), accompanied by normal or low pCO2
- Breathlessness, Exercise intolerance, central cyanosis
What alveoli can t1 respiratory failure effect?
• Some or all alveoli
Give three symptoms of T1 Respiratory failure
- Breathlessness
- Exercise intolerance
- Central cyanosis
Can ventilation perfusion matching bring lungs to 100% capacity
no
Give four possible causes of T1 respiratory failure can be caused by conditions affecting some alveoli
- Pulmonary embolism
- Pneumonia
- Consolidation
- Early stages of acute asthma
Give two possible causes of T1 respiratory failure which can be caused by conditions affecting most alveoli
• Pulmonary oedema • Fibrosis ○ Pneumoconiosis ○ Asbestosis ○ Extrinsic allergic alveolitis
What is type 2 respiratory failure?
- Arterial hypoxia, accompanied by an elevated pCO2
* (Arterial hypoxia = 8kPa)
How is O2 saturation measured?
- Pule oximeter
* Blood gas analysis
Give three causes of T2 respiratory failure
• Poor respiratory effort ○ Narcotics ○ Muscle weakness (upper and lower motoneurone) • Chest wall problems ○ Scoliosis/Kyphosis ○ Trauma ○ Pneumothorax • Hard to ventilate lungs ○ High airway resistance ○ COPD ○ Asthma
What is emphysema?
- Destruction of lung tissue ( lack of a1-antitrypsin)
- Changes in compliance
- Ventilation perfusion mismatch
- Affects O2 supply
- Initally T1 failure, then T2
What happens in the body to solve chronic hypoxia?
- There is a renal correction of acid base balance
* Increase in ventilation
Outline the acute effects of Type 2 respiratory failure
• pCO2 rises • Central chemoreceptors detects • Breathlessness ○ Some compensation ○ Poor ventialtion prevents full compensation
What happens in chronic type 2 respiratory failure
- CSF acidity corrected by choroid plexus
- Central chemoreceptors rest to high CO2 level
- Persisting hypoxia
- Reduction of respiratory drive, which is now driven by hypoxia
What happens if you give O2 to someone with COPD (T2 respiratory failure)
• They may stop breathing
What effect does T2 respiratory failure have on pulmonary circulation?
• Effects of hypoxia on pulmonary arterioles ○ Pulmonary hypertension ○ Right heart failure ○ Cor pulmonale • Increased O2 transport capacity • Hb increased • 2,3 BPG
Give five factors necessary to maintain arterial pO2, problems with which will cause hypoxia
- Low pO2 in inspired air
- Hypoventilation
- Diffusion impairment
- Ventilation perfusion mismatch
- abnormal right to left cardiac shunts
What causes low pO2 in inspired air?
• Everything is normal, air has low pO2
What is hypoventilation often associated with?
• Increased pCO2 (type 2 respiratory failure)
What are neuromuscular causes of hypoventilation?
- Respiratory depression due to opiate overdose • Head injury
- Muscle weakness (NMJ/Nerve/Muscle diseases)
What are some Chest wall problems (Mechanical) of hypoventilation
- Scoliosis/kyphosis
- Morbid obesity
- Trauma
- Pneumothorax
Give three things which make it hard to ventilate lungs
- Airway obstruction
- COPD & Asthma when the airway narrowing is severe and widespread
- Severe fibrosis
