Oxygen Flashcards
Why is more oxygen not dissolved in the blood instead of mostly being bound to haemoglobin?
The partial pressure of oxygen is not high enough
In which two ways can oxygen be carried in the blood?
- Directly dissolved
- Bound to haemoglobin
Each molecule of haemoglobin can bind how many molecules of oxygen?
4
What is the difference between SpO2 and SaO2?
- SpO2 - saturation of blood with oxygen as measured by a pulse oximeter - non invasive
- SaO2 - Oxygen saturation of arterial blood is measured by a blood test which analyses blood gases
How soes a pulse oximeter work?
- The pulse oximeter emits both red light and IR radiation from an LED
- Arterial blood is bright red so will absorb more of the IR wavelengths
- Venous blood is dark red will absorb more red light and let more IR pass through
- A photodiode will measure the received wavelengths and determine the level of oxygenation based on the ratios of light absorption
What is PaO2?
A measure of the actual oxygen concentration dissolved in arterial blood plasma - has units mmHg or KPa
How is PaO2 related to both SaO2 and SpO2?
A higher partial pressure of oxygen will result in a higher saturation of haemoglobin with oxygen
What is FiO2?
This is the amount of oxygen prescribed to a patient
Why can a fall in PaO2 be extremely dangerous when prescribing oxygen to a patient?
It depends on what the patient’s starting oxygen saturation is
Since the percentage saturation/oxygen partial pressure curve is sigmoidal, a fall in partial pressure when the starting saturation is near 100% may not affect the patient
However if the patient has a saturation of around 90% and then PaO2 falls, this can lead to a massive drop in saturation due to the PaO2 drop coinciding with the steepest part of the sigmoidal curve
What is type 2 respiratory failure?
An abnormally high amount of carbon dioxide is retained in the blood meaning blood pH stays low
This is due to poor alveolar ventilation
(Hypoxemia (PaO2 <8kPa) with hypercapnia (PaCO2 >6.0kPa))
Which conditions lead to type 2 respiratory failure?
- Cystic fibrosis
- Ptosis
- Obesity
- COPD
What is the normal range for blood pH?
7.35-7.45
How is normal blood pH levels mainatined?
Through action of buffer systems
What is the most important extracellular buffer system for maintaining blood pH?
CO2 + H2O ⇌ HCO3 + H+
How is accumulation of CO2 compensated for in metabolic acidosis?
Metabolic acidosis - an excess of H+ is produced due to disease e.g. renal failure, sepsis etc
CO2 + H2O ⇌ HCO3 + H+
The equilibrium shifts to the left because there is an increase in H+
This means tachypnoea will occur to blow off excess CO2
What is respiratory acidosis?
A respiratory problem causes CO2 to build up in the blood
How is respiratory acidosis counteracted?
Kidneys retain HCO3 - this is a slow process as bicarbonate levels take a while to rise
This happens because bicarnonate is a base and can aid the stabilisation of pH
What will differ in the fndings for acute respiratory acidosis vs chronic?
Both will have reduced pH and increased CO2
Only chronic wil have increased bicarbonate as this takes a while to accumulate
What is metabolic alkalosis?
An overall net loss of acid from the body causing a surplus of alkali (bicarbonate)
This could be due to vomiting for example
How is metabolic alkalosis counteracted?
Hypoventilation - CO2 is retained
What will metabolic alkalosis present with?
- Elevated pH
- Marginal CO2 increase - hypoventilation is an inefficient mechanism
What will the findings for metabolic acidosis be?
- Reduced pH
- Reduced [H+]
- Reduced bicarbonate
- Tachypnoea
What is respiratory alkalosis?
Caused due to excessive CO2 loss from hyperventilation
This is due to pain, stress, anxiety and early sepsis
What would be the compensatory mechanism of chronic respiratory alkalosis be if it did occur, which it usually doesn’t?
Loss of bicarbonate
This is a slow process and takes a while, whilst respiratory alkalosis is usually a short lived condition