T2 L7: Respiratory failure & V-Q mismatch

• What is respiratory failure?

  When the respiratory system fails in one or both of its gas exchange functions:

  • oxygenation
  • carbon dioxide elimination
• What are the two types of respiratory failure?

  1. Lung failure - gas exchange / causes hypoxaemia (not enough oxygen taken in)
  2. Pump failure - ventilatory / hypercapnia (not enough carbon dioxide taken out)
• What is type II respiratory failure?

  not breathing enough = low ventilation:

  • breathing shallow
  • slow breathing
  • obstructed

  example: narcotic (eg opioid) overdose

  Result: low PaO2, high PaCO2, acidosis
• What is obstructive Type 2 respiratory failure and what are some examples?

  obstruction in airways increases resistance to airflow

  more difficult to exhale so rate of exhalation smaller whilst conserving total lung capacity (TLC)

  Examples:

  • increased airway resistance (COPD, asthma, suffocation)
  • Partial closure of airways (chronic bronchitis)
• What is restrictive Type 2 respiratory failure and what are some examples?

  insufficient negative pressure / space in lungs

  lung/chest stiffness limit ability of lung to pull airflow in

  more difficult to inhale; TLC decreased so FVC smaller

  examples:

  • reduced breathing effort (eg narcotic drugs, opiates)
  • neuromuscular problems (eg myasthenia gravis)
  • chest defects that interfere with breathing
  • loss of lung (eg lung cancer surgery)
• What are some obstructive airways examples?

  COPD

  asthma

  cystic fibrosis
• What is COPD?

  chronic obstructive pulmonary disease

  often mixture of emphysema and chronic bronchitis

  • emphysema: alveolar walls destroyed, bulla; SOB & fatigue
  • chronic bronchitis: airways narrow; mucus, cough
• What are 'blue bloaters' in COPD?

  mostly bronchitis

  air trapping, cyanotic
• What are 'pink puffers' in COPD?

  mostly emphysema

  not cyanotic; increase internal airway pressure during exhalation to keep alveoli open
• Why should you not give a COPD patient too much oxygen?

  CO2 retention

  lose O2 respiratory drive (hypoventilation and respiratory failure)
• What are the possible causes of pump failure (type 2)?

  • brain - insufficient central respiratory drive
  • musculoskeletal - insufficient peripheral anatomical movements
  • lungs - excessive work of breathing
• What are the 4 pathological mechanisms of Hypoxaemic (Type I) Respiratory failure?

  1. Diffusion impairment
  2. Increased shunt
  3. Ventilation / perfusion inequality
  4. Alveolar hypoventilation
• What is Type 1 respiratory failure?

  Low O2 in blood even though breathing is not low

  • poor gas diffusion
  • lung vasculature & perfusion altered
  • Mixing of oxygenated & deoxygenated blood
  • atmospheric gases fail to oxygenate inside alveoli
• How does oedema effect gas exchange?

  increases diffusion path distance

  so decreases oxygenation
• What is the lung parenchyma?

  tissue in lungs involved in gas exchange, esp in alveoli and adjacent capillaries
• What is the effect of fibrosis on lung parenchyma?

  thickens tissue

  longer diffusion distance

  
• How does pneumonia cause V/Q mismatch?

  infection creates mucus:

  • take up space lowering alveolar ventilation
  • thicken diffusion path

  makes local ventilation less than perfusion (V<Q)

  when blood arrives in left heart, will dilute saturation like a partial shunt (50%)
• What is the most common cause of hypoxaemia?

  V/Q mismatch

  A-a gradient enlarged; regions of over-perfusion (low O2) dilute total blood O2
• What is the V/Q mismatch in the following conditions?

  Pulmonary embolism: V>Q

  Lobar pneumonia: V<Q
• What is V<Q?

  Normal perfusion with impaired ventilation

  like shunt but responds to supplemental O2

  caused by conditions that cause collapse in subset of alveoli and lower local alveolar ventilation, eg:

  • lobar pneumonia (!)
  • asthma
  • pulmonary oedema
  • bronchitis
• What are the differences and similarities between a shunt and low ventilation V/Q mismatch?

  shunt is anatomical, V/Q mismatch physiological

  for V<Q, supplementary O2 helps, does not help in shunt

  however both can have similar effect (around 50% hypoxaemia)
• How does alveolar collapse lead to low O2 saturation in pneumonia?

  eg one lobe is filled with fluid (consolidation)

  pulmonary circulation rule: where CO2 is high, vasoconstriction prevents blood from perfusing

  So when alveolus collapses, CO2 does not build up

  
• What is V>Q mismatch?

  low perfusion V/Q mismatch

  full ventilation, insufficient perfusion

  physiological dead space

  eg pulmonary embolus or emphysema

  can be due to: decreased CO, shock

  mixes poorly oxygenated Hb with normal HbO2
• What can hypoxaemia due to V/Q mismatch be treated with?

  100% O2
• What is the effect of low perfusion V/Q (V>Q) on healthy regions?

  overtake healthy regions

  as blood rapidly detours

  Pathological V > Q in one region causes V < Q in another healthy region
• What is the effect of gravity on V & Q?

  increases both in lower part (dependent region)
• What is the physiological V/Q inequality?

  base (dependent region): V<Q

  Apex (non-dependent region): V>Q
• What is the mechanism of breathing rate compensation in Type 1 Respiratory failure?

  CO2 in brain goes up → breathe faster → respiratory rate increases → minute ventilation increases → until fatigued → enter type 2 respiratory failure → hypercapnia