Oxygen

Question 1

Why is more oxygen not dissolved in the blood instead of mostly being bound to haemoglobin?

Answer 1

The partial pressure of oxygen is not high enough

Question 2

In which two ways can oxygen be carried in the blood?

Answer 2

1. Directly dissolved
2. Bound to haemoglobin

Question 3

Each molecule of haemoglobin can bind how many molecules of oxygen?

Answer 3

4

Question 4

What is the difference between SpO₂ and SaO₂?

Answer 4

• 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

Question 5

How soes a pulse oximeter work?

Answer 5

• 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

Question 6

What is PaO₂?

Answer 6

A measure of the actual oxygen concentration dissolved in arterial blood plasma - has units mmHg or KPa

Question 7

How is PaO₂ related to both SaO₂ and SpO₂?

Answer 7

A higher partial pressure of oxygen will result in a higher saturation of haemoglobin with oxygen

Question 8

What is FiO₂?

Answer 8

This is the amount of oxygen prescribed to a patient

Question 9

Why can a fall in PaO₂ be extremely dangerous when prescribing oxygen to a patient?

Answer 9

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

Question 10

What is type 2 respiratory failure?

Answer 10

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))

Question 11

Which conditions lead to type 2 respiratory failure?

Answer 11

• Cystic fibrosis
• Ptosis
• Obesity
• COPD

Question 12

What is the normal range for blood pH?

Answer 12

7.35-7.45

Question 13

How is normal blood pH levels mainatined?

Answer 13

Through action of buffer systems

Question 14

What is the most important extracellular buffer system for maintaining blood pH?

Answer 14

CO₂ + H₂O ⇌ HCO₃ + H⁺

Question 15

How is accumulation of CO₂ compensated for in metabolic acidosis?

Answer 15

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

Question 16

What is respiratory acidosis?

Answer 16

A respiratory problem causes CO₂ to build up in the blood

Question 17

How is respiratory acidosis counteracted?

Answer 17

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

Question 18

What will differ in the fndings for acute respiratory acidosis vs chronic?

Answer 18

Both will have reduced pH and increased CO₂

Only chronic wil have increased bicarbonate as this takes a while to accumulate

Question 19

What is metabolic alkalosis?

Answer 19

An overall net loss of acid from the body causing a surplus of alkali (bicarbonate)

This could be due to vomiting for example

Question 20

How is metabolic alkalosis counteracted?

Answer 20

Hypoventilation - CO₂ is retained

Question 21

What will metabolic alkalosis present with?

Answer 21

• Elevated pH
• Marginal CO₂ increase - hypoventilation is an inefficient mechanism

Question 22

What will the findings for metabolic acidosis be?

Answer 22

• Reduced pH
• Reduced [H⁺]
• Reduced bicarbonate
• Tachypnoea

Question 23

What is respiratory alkalosis?

Answer 23

Caused due to excessive CO₂ loss from hyperventilation

This is due to pain, stress, anxiety and early sepsis

Question 24

What would be the compensatory mechanism of chronic respiratory alkalosis be if it did occur, which it usually doesn’t?

Answer 24

Loss of bicarbonate

This is a slow process and takes a while, whilst respiratory alkalosis is usually a short lived condition

Question 25

What are the findings in respiratory alkalosis?

Answer 25

• Elevated pH
• Hyperventilation
• Low CO₂
• Little, if any, HCO₃ change

Question 26

In relation to acidosis, what do elevated bicarbonate levels indicate?

Answer 26

Chronic respiratory acidosis

Bicarbonate levels take a while to rise as a compensatory mechanism

Question 27

Why can respiratory acidosis ccur when prescribing oxygen to patients with type II respiratory failure?

Answer 27

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

Question 28

What is hypoxaemia?

Answer 28

Abnormally low amount of oxygen in the blood

This causes cyanosis, dyspnonea, tachypnoea and arrhythmias

Question 29

Why does hyperventilation lead to loss of conciousness?

Answer 29

• 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

Question 30

What is hypoxia?

Answer 30

An abnormally low amount of oxygen reaching tissues

Question 31

Why is oxygen given regardless of the consequences in life threatening conditions such as cardiac arrest, sepsis etc?

Answer 31

The effects are of acidosis are minimal compared with certain death

Question 32

What is type I respiratory failure?

Answer 32

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

Question 33

What may cause type I respiratory failure?

Answer 33

• 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)

Question 34

what causes respiratory acidosis

Answer 34

Respiratory acidosis occurs due to a decrease in the removal of carbon dioxide from the body,thus increase in the PaCO2 in the blood

Question 35

when does low PCO2 is caused by hyperventilation instead of respiratory type 2 failure

Answer 35

when the level of O2 is very low the body trying to compensate and the value of PCO2 is slightly low

Question 36

what would ABG show for patients with COPD

Answer 36

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.