Mednotes.

Respiratory Failure

Definitions

  • Respiratory failure- ‘Blood PaO2 < 8.0kpaon air’
    • This is ultimately due to ‘Inadequate gas exchange by the respiratory system’
    • This hypoxemia can then cause hypoxia

Pathophysiology

  • Respiratory failure is a complication/presentation of conditions

  • Gas exchange determines the type of failure that different pathologies cause

  • Oxygengas exchange is dependent on
    1. Surface area
    2. PO2
    3. Ventilation
    4. Diffusion pathway length

  • All of these can occur through the various mechanisms of RF

  • Carbon dioxideexchanges 20x faster
    • Mainly dependent just on overall ventilation
      • Poor inspiration and expiration causes T2RF
    • Therefore less affected in in T1RF

  • 2 main types although there are others

  • Type 1- Normocapnic
    • Localised V/Q mismatch:
      1. Pathological shunts
      2. Fluid in the alveoli

  • Type 2- Hypercapnia (PCO2 >6kPa)
    • Problems with generalised alveolar hypoventilationcausing a global V/Q mismatch in all alveoli

  • Both can be acute or chronic

  • Summary of causes and features

    • Type I

      Type II

      Acute

      Chronic

      Acute

      Chronic

      H+

      		 → or ↑

      →

      ↑

      		 → or ↑

      Bicarbonate

      →

      →

      →

      ↑

      Causes

      Acute asthma

      Emphysema

      Acute severe asthma

      COPD

      Pulmonary oedema

      Lung fibrosis

      Acute exacerbation COPD

      Sleep apnoea and obesity

      Pneumonia

      Lymphangitis carcinomatosa

      Upper airway obstruction

      Spinal problems

      • Kyphoscoliosis
      • Ankylosing spondylitis

      Lobar collapse

      Right-to-left shunts

      Paralysis

      • Central
      • Neuropathies
      • Narcotic drugs

      Myopathies/muscular dystrophy

      Pneumothorax

      Brain-stem lesion

      Flail chest injury

      Pulmonary embolus

      Primary alveolar hypoventilation

      ARDS

Clinical features- are similar in both types :

  • Hypoxemia
    • Acute
      • Tachypnoeaanddyspnoea with accessory muscle usage
      • Cyanosis
      • Tachycardia
      • Cognitive disturbance – e.g. agitation and confusion- CNS is very vulnerable to hypoxia
      • May even develop cardiorespiratory arrest
    • Chronic
      • Polycythaemia
      • Pulmonary HTN
        • Cor pulmonale

  • Hypercapnia
    • Vasodilation- Flushing
    • Bounding pulse
    • Increased cerebral bloodflowhas various effects
      • Headache
      • CO2 retention flap
      • Confusion => coma
      • Papilloedema and ICP
    • Respiratory acidosis

Investigate

  • History
  • Respiratory exam

  1. Immediate ABGto diagnose the type of respiratory failure
    • Repeat after an hourof supportive care to see if there is improvement

  1. Investigat the cause
    • Bloods
    • Orifices – sputum, throat swabs
    • CXR
    • ECG
    • Further imaging and other special tests

Management

  1. Supportive care – treat physiological derangement
    1. Oxygen –
      1. Hudson- Aim for 94-98% SaO2 in most patients
      2. Non-rebreatheif acute
      1. Venturi- NB in COPD patients hypoxia drives ventilation rather than hypercapnia
        • Therefore give oxygen carefully targeting SaO2 88-92%
      • May require intubationif mask oxygen is insufficient

  1. Hypercapnia treatment
    • BIPAP is used if it is severe and not improved with medical management
    • Treat cause

  1. Then can manage and investigate the cause
    • Use additional measures if required
      • E.g. nebulised salbutamol, antibiotics, diuretics for CCF