Causes of Hypoxaemia and Hypercapnia

Question 1

What can cause hypoxaemia and tissue hypoxia?

Answer 1

Pathology can arise from dysfunction occurring at any part of 'respiratory journey'
	1. Insufficient ventilation:
	Obstruction of airways
	Failure to breath adequately
	2. Insufficient gas exchange
	Exchange surface dysfunction
	VQ mismatch
	3. Insufficient oxygen carrying capacity
	4. Insufficient oxygen in atmosphere

Question 2

How can we determine the cause of hypoxaemia?

Answer 2

The alveolar gas equation and alveolar-arterial oxygen gradient are used to investigate hypoxaemia
A-a O2 gradient is the difference between alveolar and arterial pressure:
=PAO2- PaO2
Normally= around <2kPa
Interpreting ABG readings using AGE and A-a gradient to investigate cause of hypoxaemia:
1. Is hypoventilation contributing to hypoxaemia?
Is PaCO2 > 6kPa
2. Is the oxygen that reaches the alveoli diffusing into the blood?
Use AGE and ABG reading to calculate A-a gradient (A-a should be less than= 2kPa)

Question 3

What is the difference between type 1 and type 2 respiratory failure?

Answer 3

‘Respiratory failure’= PaO2 < 8kPa when breathing air at sea level
Type 1- gas exchange problem; from alveoli to blood
Caused by decreased oxygenation (altitude, VQ mismatch, pneumonia)
PaO2 decreased (so increased A-a gradient)
Normal PaCO2 and pH
Type 2- ventilation problem; from atmosphere to lungs
Caused by decreased ventilation (asthma, COPD, NMD, drug overdose, apnoea)
Decreased PAO2 and PaO2 (so no change in gradient)
Increased PaCO2 (hypercapnia)
Decreased pH

Question 4

What are the clinical signs and symptoms of respiratory distress?

Answer 4

Shortness of breath, tachypnoea, dyspnoea
Laboured breathing, audible lung sounds
Tiredness, drowsiness, loss of consciousness
Fatigue (generally or upon exercise)
Cyanosis

Question 5

How can airway obstruction cause hypoventilation?

Answer 5

Asphyxia, choking etc.
Obstructive sleep apnoea
Bronchial obstruction (asthma, chronic bronchitis, cystic fibrosis)

Question 6

How can problems with initiation or mechanics of breathing cause hypoventilation?

Answer 6

Drug overdose (typically opioids)
Neuromuscular disease (muscular dystrophy, motor neuron disease)
Stroke
Issues with lung mechanics (COPD, pulmonary fibrosis, NRDS, pneumothorax)

Question 7

How does reduced perfusion of lung regions cause a V/Q mismatch?

Answer 7

Oxygenation problems (V/Q mismatch) more detail in Gas Exchange lecture
Reduced perfusion of lung regions, causes an increase in V/Q ratio (‘dead-space effect’):
Heart failure (cardiac arrest)
Blocked vessels (pulmonary embolism)
Loss/damage to capillaries (emphysema)
Reduced ventilation or limited diffusion causes a decrease in V/Q ratio (shunt effect):
Pneumonia, atelectasis, respiratory distress syndrome

Question 8

What are some oxygen transport disorders?

Answer 8

Oxygen transport disorders- insufficient oxygen carrying capacity
Anaemia (insufficient RBCs or haemoglobin):
Iron deficiency (↓production)
Haemorrhage (↑loss)
Carbon monoxide poisoning (CO prevents O2-Hb binding)

Question 9

What are the effects of acute hypoxaemia and hypercapnia?

Answer 9

Effects of insufficient O2 supply:
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
Effects of insufficient carbon dioxide removal:
Clinical signs= dyspnoea, confusion, seizure, unconsciousness
Hypercapnia -> acidosis -> organ failure + cardiac arrhythmia -> death