Causes of Hypoxaemia and Hypercapnia

Question 1

How does insufficient ventilation cause hypoxaemia and tissue hypoxia?

Answer 1

→ Obstruction of airways

→ Failure to breath adequately

Question 2

How does insufficient gas exchange cause hypoxaemia and tissue hypoxia?

Answer 2

→ Exchange surface dysfunction

→ V/Q mismatch

Question 3

How does insufficient oxygen carrying capacity lead to hypoxia?

Answer 3

→ eg. there isn’t enough Hb

Question 4

How does insufficient oxygen in the atmosphere cause hypoxia?

Answer 4

Not enough being exchanged

→ ability to extract oxygen from the blood decreases

Question 5

What are the types of respiratory failure?

Answer 5

→ Type I respiratory failure involves low oxygen, and normal or low carbon dioxide levels.
→ Type II respiratory failure involves low oxygen, with high carbon dioxide.

Question 6

Describe the PAO2, PaO2, PaCO2, and pH in type 1 respiratory failure

Answer 6

PAO2= normal
PaO2= reduced
PaCO2= normal
pH= normal
→ increased A-a gradient

Question 7

Describe the PAO2, PaO2, PaCO2, and pH in type 2 respiratory failure

Answer 7

PAO2= reduced
PaO2= reduced
PaCO2= increased(hypercapnia)
pH= reduced

Question 8

What are the causes of type 1 and type 2 respiratory failure?

Answer 8

TYPE 1:
↓ oxygenation
(altitude, VQ mismatch, pneumonia)

TYPE 2:
↓ ventilation
(asthma, COPD, NMD, drug overdose, apnoea)
→ inability to overcome increased resistance to ventilation

Question 9

What is respiratory failure?

Answer 9

PaO2 < 8kPa when breathing air at sea level

Question 10

What are the causes of hypoventilation?

Answer 10

→ Aphyxia, choking etc.
→ Obstructive sleep apnoea
→ Bonchial obstruction (asthma, chronic bronchitis, cystic fibrosis- excess mucus secretion)r

Question 11

Problems with initiation or mechanics of breathing…

Answer 11

→ Drug overdose- neural impulses are impaired (typically opioids
→ Neuromuscular disease (muscular dystrophy, motor neuron disease)
→ Issues with lung mechanics (COPD, pulmonary fibrosis, NRDS, pneumothorax)
→ Stroke- blood supply is interacted

Question 12

What are the causes of increased distance for diffusion?

Answer 12

↑ basement membrane thickness (fibrosis)
↑ thickness of fluid layer/oedema (pulmonary oedema, pneumonia)
→ Alveolar injury (ARDS)

Question 13

What can cause reduced surface area for diffusion?

Answer 13

emphysema

Question 14

What is the result of reduced perfusion?

Answer 14

increase in 𝑉̇/𝑄̇ ratio -‘dead-space effect’
→ Heart failure (cardiac arrest)
→ Blocked vessels (pulmonary embolism)
→ Loss/damage to capillaries (emphysema)

Question 15

What are the results of reduced ventilation?

Answer 15

→ a decrease in 𝑉̇/𝑄̇ ratio -shunt effect
→ Pneumonia,
→ atelectasis
→respiratory distress syndrome

Question 16

What is atelectasis?

Answer 16

the airways and air sacs in the lung collapse or do not expand properly.

Question 17

What are the causes of insufficient oxygen carrying capacity?

Answer 17

→Iron deficiency (↓ production)
→Haemorrhage (↑ loss)
→Carbon monoxide poisoning (CO prevents O2-Hb binding):

Question 18

What are the clinical signs/symptoms of respiratory distress?

Answer 18

→Shortness of breath, tachypnoea, dyspnoea
→Tachycardia- CV system tries to compensate for lack of O2 by increasing CO
→Laboured breathing, audible lung sounds- e.g. asthma, or pulmonary fibrosis
→Tiredness, drowsiness, loss of consciousness
→Fatigue (generally or upon exercise)- reduced oxygenation
→Cyanosis- due to ratio between oxy- and deoxy- blood

Question 19

Effects of acute hypoxaemia…

Answer 19

→Clinical signs = dyspnoea (shortness of breath), cyanosis, fatigue, coma, seizure

→Acute = Hypoxaemia → tissue hypoxia (e.g. cerebral hypoxia) → organ failure → death

→Chronic = polycythaemia, hypoxic pulmonary vasoconstriction → pulmonary heart failure

→Reliance on anaerobic respiration which produces lactic acid

Question 20

Effects of insufficient carbon dioxide removal:

Answer 20

→Clinical signs = dyspnoea, confusion, seizure, unconsciousness
→Hypercapnia → acidosis → organ failure + cardiac arrhythmia → death

Question 21

What two main factors affect total arterial content?

Answer 21

→oxygen dissolved in plasma

→oxygen bound to the Hb

Question 22

What influences oxygen dissolved in plasma?

Answer 22

composition of inspired air- eg altitude, O2 therapy
→alveolar ventilation
→oxygen diffusion between alveoli and blood
→adequate perfusion of alveoli

Question 23

What influences alveolar ventilation?

Answer 23

rate and depth of breathing- eg drug overdose, stroke, neuromuscular problems
→lung compliance- eg fibrosis
→airway resistance

Question 24

What affects oxygen diffusion between alveoli and blood?

Answer 24

surface area

→diffusion distance

Question 25

What affects diffusion distance?

Answer 25

→membrane thickness- eg fibrosis

→amount of interstitial fluid - eg oedema

Question 26

What affects oxygen bound to Hb?

Answer 26

→%saturation of Hb

→total number of binding sites- affected by CO poisoning

Question 27

What affects %saturation of Hb?

Answer 27

PCO2
→pH
→2,3DPG
→temperature