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
What are the findings in respiratory alkalosis?
- Elevated pH
- Hyperventilation
- Low CO2
- Little, if any, HCO3 change
In relation to acidosis, what do elevated bicarbonate levels indicate?
Chronic respiratory acidosis
Bicarbonate levels take a while to rise as a compensatory mechanism
Why can respiratory acidosis ccur when prescribing oxygen to patients with type II respiratory failure?
By increasing oxygen the perfusion problem is solved and enough oxygen can enter the blood
Oxyegn displaces carbon dioxide from haemoglobin due to the Haldane effect
However, due to the poor ventilation, carbon dioxide cannot escape and will dissolve in the blood causing acidosis
Effectiviely, administering oxygen in these situations promotes acidosis
What is hypoxaemia?
Abnormally low amount of oxygen in the blood
This causes cyanosis, dyspnonea, tachypnoea and arrhythmias
Why does hyperventilation lead to loss of conciousness?
- Carbon dioxide is removed at a greater rate then it is produced
- This raises blood pH - respiratory alkalosis
- This has symptoms of dizziness and collapse
What is hypoxia?
An abnormally low amount of oxygen reaching tissues
Why is oxygen given regardless of the consequences in life threatening conditions such as cardiac arrest, sepsis etc?
The effects are of acidosis are minimal compared with certain death
What is type I respiratory failure?
Hypoxaemia without an increase in carbon dioxide levels in the blood (PaCO2)
Typically, caused by a ventilation/perfusion (V/Q) mismatch - the volume of air flowing in and out of the lungs is not matched with the flow of blood to the lungs
What may cause type I respiratory failure?
- Low PaO2 oxygen (high altitude)
- V/Q mismatch (parts of the lung receive oxygen but not enough blood to absorb it, e.g. pulmonary embolism)
- Alveolar hypoventilation
- Diffusion problem (oxygen cannot enter the capillaries due to parenchymal disease, e.g. in pneumonia or ARDS)
- Shunt (oxygenated blood mixes with non-oxygenated blood from the venous system, e.g. left to right shunt)
what causes respiratory acidosis
Respiratory acidosis occurs due to a decrease in the removal of carbon dioxide from the body,thus increase in the PaCO2 in the blood
when does low PCO2 is caused by hyperventilation instead of respiratory type 2 failure
when the level of O2 is very low the body trying to compensate and the value of PCO2 is slightly low
what would ABG show for patients with COPD
Patients with longstanding COPD rely on the hypoxic drive in order to drive respiration because the respiratory centre in their brain is insensitive to CO₂. Patients are therefore hypercapnic and exhibit type 2 respiratory failure. The longstanding hypercapnia eventually causes a respiratory acidosis which over time becomes compensated by HCO₃. These patients classically have a low % O2 and high CO2.
