Beginner’s Guide to Blood Gas Analysis

Presenter – Tobias Grave, TA, GP Cert (SAP), DipACVECC, MRCVS, Vets Now Referrals Glasgow

The words acid-base and anion gap have been wandering aimlessly around my head since my college days without any particular purpose, probably because I never really grasped their meaning or relevance to real life cases. Last week’s veterinary webinar organised by ‘The Webinar Vet’ proved a revelation being the first time  I actually fully understood blood gas analysis with Tobias Grave TA,GPCert(SAP)DipACVECC, MRCVS from Vets Now Glasgow Referrals explaining it concepts in a practical and relevant way.     So for those of us who don’t measure blood gases, including myself, why do we really need to bother? Tobias explained the critical patient has no capacity for error with ‘guestimation’ potentially being lethal in some cases. We really do need to determine the oxygenation and acid base status of these cases to offer their best chance for survival.

The last person you’ll want to explain the ‘open buffer system’ and ‘acid-base balance’ is someone who has just grasped it themselves, so I won’t pretend to try. I will leave that job to Tobias’ webinar but one simple piece of advice which stood out was a relatively simple method to establish the most likely primary disturbance causing either an acideamia or alkalaemia, according to the partial pressures of C02 and bicarbonate.

Tobias explained that CO2 is controlled by the respiratory system and bicarbonate levels are controlled by the metabolic component of the open buffer system. Low bicarbonate and high CO2 levels will cause an acidaemia and high bicarbonate and low C02 levels will cause an alkalaemia. Tobias advised the primary disturbance causing either an acideamia or alkalaemia can be determined by the level of either CO2 or bicarbonate  moving towards the pH with the level ‘going opposite’ most likely being the compensatory mechanism.

For example a patient with a pH of 7.2 is suffering from an acideamia. So what is the primary disturbance – is it metabolic or respiratory? This is best determined by measuring CO2 and bicarbonate which are both low. A low CO2 is moving in the opposite direction to the acidosis so is likely to be a compensatory mechanism, whereas a low bicarbonate is moving towards acidosis and consequently is most likely to be the primary disturbance. Hence a patient with these results is most likely to have a metabolic acidosis.

Another example is a patient with an acidaemia which has high bicarbonate and CO2 levels. The high bicarbonate is moving in the oppositedirection to the pH so is likely to be the compensatory mechanism, whereas the high CO2 is moving towards the acidosis and is most likely to be the primary disturbance. Hence a patient with these results is most likely to have a respiratory acidosis.

Tobias’ explanation of blood gas analysis made it seem so simple and I must apologise to anyone who I have confused with my explanation. I can reassure you that Tobias does a much better job and if acid-base balance is one of those concepts which has never really clicked in the past, I can guarantee this veterinary webinar will provide a number of ‘light bulb’ moments.

The Stethoscope (MRCVS)

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