The respiratory system and acid–base disorders

Question 1

How is CO2 carried in the blood

Answer 1

• Dissolved in the plasma
• As bicarbonate (Majority)
• Bound to amino acids on Hb as carbamino compounds

Question 2

Ventilatory response to changes in CO2

Answer 2

• As CO2 increases there is a linear rise in minute ventilation until hypercapnic sedation and muscle fatigue kicks in flattening the response curve
• As CO2 falls there is also a linear drop in minute ventilation until the patient is apnoeic, this does not occur in conscious patients as the pre-frontal cortex can modulate this but does occur under general anaesthetic.

Question 3

Factors affecting the ventilatory response to hypercapnia

Answer 3

Decreased response
- Obesity
- Circadian rhythm
- Opioids
- Anaesthetic drugs

Increased response
- Doxapram
- Pregnancy (progesterone related)

Question 4

Causes of respiratory acidosis: Increased inspired CO2

Answer 4

• Iatrogenic CO2 in the breathing circuit e.g. wrong cylinder providing gas
• Rebreathing due to anaesthetic circuits and fresh gas flow or soda lime exhaustion

Question 5

Causes of respiratory acidosis: Increased production of CO2

Answer 5

• Sepsis
• Thyrotoxicosis
• Malignant hyperthermia
• Laparoscopic surgery

Question 6

Causes of respiratory acidosis: Impaired removal of CO2 due to alveolar hypoventilation (shunt)

Answer 6

• Opioids, sedatives, obesity, raised ICP
• Restrictive lung disease: polio, MN disease, muscular dystrophies, scoliosis, myopathies etc
• Small airway obstruction: asthma, COPD
• Upper airway obstruction: infection, inhaled FB, tumour, OSA

Question 7

Causes of respiratory acidosis: Impaired removal of CO2 due to increased dead space

Answer 7

• Low CO state
• PE,
• general anaesthetic
• emphysema

Question 8

Effects of hypercapnia on the immune system

Answer 8

• Reduced expression of pro-inflammatory cytokines (TNF and IL-6). Attenuating phagocytosis from B-cells and macrophages
• Impaired proliferation of alveolar epithelial cells

Question 9

Effects of hypercapnia on the respiratory system

Answer 9

• Increased MV
• Shift of oxyhaemoglobin dissociation curve to the right
• Pulmonary vasoconstriction

Question 10

Effects of hypercapnia on the cardiovascular system

Answer 10

• Direct myocardial depressant
• Systemic vasodilation
• This is partially offset by sympathetic nervous system stimulation causing increased ionotropy and afterload however venodilatation tends to continue reducing preload
• Increased CO coupled with pulmonary vasoconstriction can cause acute cor pulmonale

Question 11

Effects of hypercapnia on the CNS

Answer 11

• Sedation
• Seizures
• Headaches (due to vasodilation)
• Raised ICP due to vasodilation