A man has suffered a witnessed out-of-hospital arrest. Unfortunately it was 67 minutes before the ambulance arrived during which time no resuscitation was attempted. The paramedics subsequently restored a spontaneous circulation. He is brought into hospital intubated with a pulse of 120 bpm and a good blood pressure. His Glasgow coma score is three.

Arterial blood gas analysis reveals:

Inspired oxygen   50% (FiO2 0.5)                  normal values

 

PaO2                  8.4 kPa                                   >10 kPa on air

pH                      7.18                                        7.35 – 7.45

PaCO2               6.5 kPa                                   4.7 – 6.0 kPa

Bicarbonate     15 mmol l-1                             22 – 26 mmol l-1

 

  1. Assess the patient’s condition.

During his arrest the patient was not ventilated nor was he circulation intact. As a consequence his cells would have had to metabolise and anaerobically. This is metabolism without oxygen which causes accumulation of lactic acid. This lactic acid will cause an acidaemia. In the acute phase much of this acid will combine with the bicarbonate causing the bicarbonate levels to also fall. His PaCO2  may have initially been raised but now he is ventilated this may have improved.

 

  1. Assess the oxygenation.

As a rule of thumb the patients PaO2 should be no less than 10 points lower than the inspired oxygen that is being delivered. So in this case if there was no problem with the oxygenation you would expect the PaO2 to be around 30kPa. In this example the PaO2 is significantly lower than you would expect so there is severe hypoxia.

 

  1. Determine the pH concentration.

We next need to ask ourselves whether there is an acidaemia or an alkaleamia. In this case the pH is less than 7.35 so there is an acidaemia.

 

  1. Determine the respiratory component.

We then look at the PaCO2, remembering that values above 6 would indicate a respiratory acidosis, and values below 4.7 would indicate a respiratory alkalosis. In this example the PaCO2 is above 6 so the patient has a respiratory acidosis although this is very slight in this example.

 

  1. Determine the metabolic component.

The final part of the process is to look at the bicarbonate to determine whether there is a metabolic disorder. The bicarbonate has dropped quite markedly which would indicate that there is a metabolic acidosis, which I said before is probably because the bicarbonate is being used to mop up the excess hydrogen ions.

So to summarise this blood gas we have an acidaemia caused by a metabolic acidosis and little respiratory compensation.

Jevon

Jevon

Possibly the book I most highly recommend for nurses to use as their pocket guide. Phil Jevon is a practitioner in Walsall and has produced an easily read, pocket sized tool. You can click on the picture above  to purchase this excellent book.
Talley and O'Conner
Slightly less 'weighty' than Macleods but still with lots of useful detail and information. The latest copies also have a CD with good, well narrated examples of clinical examination.
Macleods
This title is 'Highly Commended' in the 2006 British Medical Association Awards! 'an incredibly thorough book which is very well illustrated - a must in a book explaining how to perform examinations' - ("Medical Student Review"). This book will show you how to: talk with a patient; take the history from the patient; examine a patient; formulate your findings into differential diagnoses and rank these in order of probability; and, use investigations to support or refute your differential diagnosis.