Physiology - Arterial Hypoxaemia Continued Flashcards
What does arterial hypoxaemia mean?
Oxygen cannot be delivered to cells
What mechanisms cause arterial hypoxaemia?
Reduced PB or FIO2 Hypoventilation Impaired diffusion Shunt V/Q mismatch
What are the three gas exchange problems?
Impaired diffusion
Shunt
V/Q mismatch
What causes reduced PB or FIO2
Oxygen is not there (e.g. top of mount Everest)
What happens in hypoventilation?
Raise carbon dioxide levels
Decrease oxygen levels
What causes hypoventilation?
High work of breathing - compliance/airways resistance
Damage to chest wall or fatigue/paralysis of resp muscles
Respiratory depressants - morphine/barbiturates
Sleep (relative)
What are key things to remember about hypoventilation?
Always increases the PaCO2
Decreases PO2 unless additional oxygen is inspired
How is hypoxaemia reversible in hypoventilation and why?
Add oxygen
- less air entering the alveoli but if that air is richer in oxygen then the amount of oxygen delivered to the alveoli and available for gas transfer to blood is increased
- BUT IT DOES NOT FIX THE CARBON DIOXIDE EXCESS
Name the two types of respiratory failure.
Type I - PaO2 low, PaCO2 normal
Type II - PaO2 low, PaCO2 high
Explain Type I Resp failure.
PaO2 low, PaCO2 normal
Gas exchange problems - V/Q, shunt e.g. pneumonia, pulmonary oedema
Does not involve ventilation
Describe Type II Resp failure.
Both PaO2 and PaCO2 low.
Ventilatory failure e.g. chronic bronchitis, emphysema
Can both Type I and Type II resp failure occur?
Yes
What does hypoventilation lead to?
Hypercapnia
What does hyperventilation lead to?
Hypocapnia
What problems does hypercapnia give rise to?
pH problems due to altering the PCO2
List the important equation showing how CO2 can alter the pH.
CO2 + H20 H2CO3 H+ + HCO3-
Describe what happens if less ventilation (hypoventilation occurs).
Less ventilation
CO2 not removed –> accumulates
Equation shifts to the right –> excess H+ (decrease in pH)
Respiratory acidosis
What does hypoventilation lead to?
Respiratory acidosis
What happens if there is too much ventilation (hyperventilation)?
Too much ventilation
Extra CO2 removed
Equation shifts to the left –> decreased H+ (increase in pH)
Respiratory alkalosis
Describe the compensatory actions for respiratory control of CO2.
Renal control of HCO3 levels
Integrated response - lungs/kidneys work together to compensate for pH abnormalities
What compensates for respiratory acidosis/alkalosis?
Renal compensation
What compensates for metabolic acidosis/alkalosis?
Respiratory compensation
Explain how the kidneys compensate for respiratory acidosis. (hypoventilation)
Excrete more H+
Synthesize more HCO3
Describe how the lungs compensate for metabolic acidosis (e.g. diabetic ketoacidosis, low insulin, metabolize fats, make ketoacids = extra H+)
Lungs compensate by exhaling more CO2 –> reduces H+ level
Kidneys can also enhance compensation by excreting more H+
- decrease GFR to reduce filtered load of HCO3-
- decrease HCO3- filtered
- increase H+ secretion
Describe what hypoxaemia vs hypo/hypercapnia leads to.
Impairment of metabolism/function - hypoxaemia
pH abnormalities - hypo/hypercapnia
How is oxygen transported?
Dissolved in the blood
Combined with Hb
Describe why dissolved oxygen is not very efficient for transport.
Amount dissolved depends on PO2.
For each mmHg PO2, only 0.03ml dissolved O2 per liter of blood.
Very ineffective
Oxyhaemoglobin dissociation curve go over
xxx
What is oxygen saturation?
The oxygen saturation of Hb is the percentage of available binding sites that have oxygen attached.
What is the O2 saturation of arterial blood with a PO2 of 100mmHg?
98%
What is the oxygen saturation of venous blood (SvO2) with a PO2 of 40mmHg?
70%
Explain why the shape of the dissociation curve has advantages.
Upper flat part of the curve - moderate changes in PO2 around the normal value in the lung have only small effects on the saturation and the amount of oxygen carried by arterial blood.
Steep part of the curve at the lower PO2 - helps with unloading of oxygen to the tissues. Small changes in PO2 result in the unloading of large amounts of O2 to the tissues.
What is the oxygen capacity?
The maximal amount of oxygen that can be combined with Hb.
How much oxygen the blood could carry.
How do we calculate the oxygen capacity?
Normal blood has about 150g Hb/litre
One gram of Hb can combine with 1.34ml O2
Oxygen capacity = 1.34 x 150 = 200ml/litre of blood
What is the oxygen content?
How much oxygen the blood is actually carrying
How do we calculate the oxygen content?
O2 capacity x saturation
Describe what a leftward shift of the dissociation curve means.
More loading of PO2 in the lungs
Describe what a rightward shift of the dissociation curve means.
More unloading of oxygen at a given PO2 in tissues
What explains the changeable conditions of the oxygen dissociation curve.
Bohr effect
What is the Bohr Effect?
The oxygen dissociation curve is shifted to the right by an increase in:
- H+ concentration
- PCO2
- Temperature
- 2,3 DPG in red blood cells
Remember: an exercising muscle is acidic, hypercapnic and hot and it benefits from increased unloading of oxygen.
This is why the Bohr effect is needed.
xx
What is 2,3 DPG?
By-product of glycolysis (red blood cells rely on glycolysis because they contain no mitochondria)
When does 2,3 DPG increase?
Intense exercise training
Altitude
Severe lung diseases
Anaemia
What does 2,3 DPG do?
Helps deliver oxygen to tissues (due to rightward shift of ODC which allows more oxygen to be released from Hb at a particular PO2)
What causes cyanosis?
Low saturation/content
What is the appearance of cyanosis?
Blue-purple colour, most obvious in the skin, nail beds and mucosal membranes (mouth) caused by lower SaO2 and is indicative of blood with low CaO2.
When is cyanosis detectable?
When there is at least 50g/L of deoxy-Hb
What causes central cyanosis?
(blue mouth and tongue) - due to poor oxygenation
What causes peripheral cyanosis?
Lungs are healthy, but there is poor circulation
What happens to the saturation curve in anaemia?
Stays the same, but oxygen content is reduced
What effect does carbon monoxide have on the oxygen content?
CO interferes with oxygen transport by combing with Hb to form carboxyhaemoglobin (COHb) - blocks oxygen binding site.
Unavailable for oxygen carriage = reduced O2 content
Shifts curve to the left - more difficult to unload oxygen to the tissues.
What implications does anaemia have on an individual?
At rest they are fine
But with exercise they develop SOB and hypoxia
Decrease in Hb leads to decrease in content
Name the three ways carbon dioxide is transported.
Dissolved in plasma (10%)
As bicarbonate (70%)
Combined with proteins as carbonamino compounds (20%)
What forms bicarbonate in cells?
Carbonic anhydrase
What does the deoxygenation of blood cause?
Increases oxygen carriage - Haldane effect
