Basic sciences

Respiratory failure

Carbon Monoxide Poisoning

The O2 sats are normal in CO poisoning - if you suspect it you must check ABG


  • Carbon monoxide is an odourless colourless gas which binds avidly to Hb x 240 affinity than Oxygen
  • Saturation probes treat CoHb as HbO2 giving a FALSE NORMAL SaO2


  • Carbon monoxide from poorly fitted or ventilated gas heating is inspired and the result is tissue hypoxia and metabolic acidosis
  • Carbon monoxide also inhibits cytochrome oxidase a3. The result is leftward shift in oxygen haemoglobin dissociation curve.


  • Drowsiness, headaches, fatigue, breathless proceeding to coma and death despite a pink rosy colouration
  • If another house member affected (?dead) unless obvious other explanation treat as CO until proven otherwise
  • Arterial blood gas and Measure CoHb is crucial > 10%
  • ECG - may show ischaemia and arrhythmias
  • CT head - exclude other causes. May show cerebral oedema


  • Opiates, Subdural, Intracranial lesion


  • Severe cases cause fitting and cardiorespiratory arrest.
  • Give 100% Oxygen at least 12 hours and some may need hyperbaric oxygen
  • They are also prone to Cerebral oedema so do neuro observations and they may need mannitol.
  • There may be long term neuropsychiatric damage and parkinsonism.

Indications for Hyperbaric oxygen

  • CoHb > 40%
  • Coma
  • Neurological or psychiatric problems
  • ECG changes
  • Pregnancy NB fetal CoHB is to the left of mothers

Restrictive vs Obstructive lung disease, Smoking, COPD and Asthma


Understanding the patterns will make everything much easier. Not all patients and their diseases fit nicely into discrete categories but the differentiation into restrictive and obstructive is useful and is applicable to many respiratory illnesses

Pulmonary physiology

  • Tidal volume : volume of air shifted per normal quiet breath
  • FEV1.0 : amount of air expelled in the first second from a forced expiration at maximal inspiration
  • FVC : amount of air that can be expelled in a single breath
  • Residual volume : the volume of air remains in the lungs after full expiration. Can only be measured indirectly using total body plethysmography or helium dilution.
  • PEFR : the maximum rate at which air can be expelled from the lungs and used as a simple guide often in asthma to airways obstruction

These measurements are dependent on the patient giving their best effort and the less the patient tries the worse the values. This can lead to misdiagnosis and and an underestimation of lung function in the less compliant patients. [picture of spirometry showing FEV and FVC]

Restrictive vs Obstructive

Restrictive lung disease

  • Think of it as the Lungs getting smaller and stiffer usually due to a diffuse parenchymal disease, Fibrosing lung disease
  • Reduced Vital capacity, Reduced FEV1, FEV/FVC normal, Reduced Transfer factor
  • CXR shows lung fibrosis and ABG show Type 1 Respiratory failure
  • Cyanosed, inspiratory crackles, breathless, small poorly expanding lungs