Arterial/venous blood gases are not as commonly done in some hospital units in Sri Lanka especially those in the periphery due to a lack of equipment. However, in critical care it is an essential investigation that is done numerous times a day. Arterial blood gases (ABGs) are the gold standard, but it is more difficult to obtain and causes immense pain for the patient. The obvious difference between arterial and venous blood samples is the oxygen content – arterial blood has a higher concentration of oxygen. Otherwise, research has found that the results of venous blood gases (VBGs) do correlate to those of ABGs, but VBGs are not clinically recommended when the patient is in shock.

When it comes to my experience, I worked in a neonatal intensive care unit where VBGs were easier to obtain (via an umbilical venous catheter) and less traumatic for the baby. Occasionally I would take an arterial blood gas straight from a pulse point (usually the femoral pulse), but this was very rarely done and only when the venous catheter was blocked. Many find reading a blood gas report to be quite confusing, but luckily for me I loved deciphering ABG questions in medical school (I’m an odd ball, I know) and became a bit of a pro at it. So i’m going to try to explain my method as best I can, it may not be what eveyrone uses, but it has definitely helped me and I have always been able to figure out every blood gas report.

The pH of our blood is guided by 3 main systems – carbon dioxide and bicarbonate. The carbon dioxide level is controlled by the respiratory system – CO2 is got rid of during expiration so if there is lack of ventilation, the CO2 levels build up. The bicarbonate level is controlled by its’ excretion by the kidneys and also by its’ production. Chemical buffers in the blood also guard against sudden shifts in acidity.

First you need to memorize some normal values!

pH 7.35 – 7.45

pCO2 35 – 45 mmHg

pO2 (varies for arterial and venous bloods don’t really need to memorize this)

HCO3- 22 – 28 mmol/L

BE -2 to +2 (chemical buffer system)

Table 1: Normal range of blood gas values

pH

First check the pH value of the blood gas report to decide if the blood is acidotic, alkalotic or within the normal range. If it is in the normal range remember that this can be because of compensation (will explain this in further detail below).

<7.35 is acidotic

7.35 – 7.45 is normal

>7.45 is alkalotic

PCO2 (carbon dioxide)

The carbon dioxide level depends on the respiration and an increase in respiration means more carbon dioxide is expired such as in hyperventilation (reducing the levels). With a decrease in respiration there would be more carbon dioxide retention such as in COPD and guillain-barre syndrome.

Now let the following statements sink in so you remember it!

An increase in carbon dioxide (pCO2) results in respiratory ACIDOSIS.

A decrease in carbon dioxide (pCO2) results in respiratory ALKALOSIS.

<35 mmHg is alkalotic

35 – 45 mmHg is normal

>45 mmHg is acidotic

So now check if the pH corresponds to the pCO2 – if the pH is alkali, is the pCO2 indicating alkalosis? This would mean that the pCO2 (respiratory component) is responsible for the alkalosis. If the two components do not correspond, then move onto the bicarbonate to check for another cause. (Vice versa for an acidotic pH).

HCO3- (bicarbonate ions)

The bicarbonate level depends on the excretion and conservation of the ions by the kidneys. An increase in bicarbonate loss (such as in diarrhoea) means a more acidotic environment. With a decrease in excretion or more production of bicarbonate ions there would be bicarbonate retention (such as in poorly controlled type 1 diabetes mellitus or lactic acidosis or renal failure).

Now let the following statements sink in so you remember it!

An increase in bicabronate ions (HCO3-) results in metabolic ALKALOSIS.

A decrease in bicarbonate ions (HCO3′) results in metabolic ACIDOSIS.

<22 mmHg is acidotic

22 – 28 is normal

>28 mmHg is alkalotic

Check if the pH corresponds to the bicarbonate (HCO3-) levels. If the both correspond and show alkalosis (or acidosis) then the cause is metabolic.

Base excess/deficit

This refers to the amount of base in the blood, of which the predominant base is HCO3-. So if there is an excess of base there is an increase in HCO3- (metabolic alkalosis). If there is a deficit in the base, there is a decrease in HCO3- (metabolic acidosis). It doesn’t matter too much when deciphering the overall result of the blood gas so I wouldn’t worry too much about understanding this if you just want to learn the basics!

So in short:

The respiratory and metabolic buffers in the body are continuously changing to balance one another. If one component is off, the other tried to compensate to maintain a normal overall pH in the blood.

Respiratory acidosis

The pH would be decreased and as the pCO2 reflects respiration it is increased. The bicarbonate can be normal or will be increased (producing alkalosis) if it tries to compensate for the overall acidosis.

Respiratory alkalosis

The pH would be increased, the pCO2 reflecting respiration will be decreased. The bicarbonate can be normal or will be decreased (producing acidosis) if it tries to compensate for the overall alkalosis.

Metabolic acidosis

The pH would be decreased and the HCO3- (reflecting the metabolic component) will also be decreased. The pCO2 can be normal or will be decreased (producing alkalosis) if it tries to compensate for the overall acidosis.

Metabolic alkalosis

The pH would be increased and the HCO3- (reflecting the metabolic component) will also be increased. The pCO2 can be normal or will be increased (producing acidosis) if it tries to compensate for the overall alkalosis.

Compensation

As explained above, the body always tries to maintain a normal pH. If one component of the acid-base buffer system is off, the other part tries to compensate. For instance if there is CO2 trapping like in COPD patients, an acidosis will occur. The kidneys will try to counter this increase by conserving more HCO3- ions there by trying to increase the alkalosis component. The metabolic alkalosis will try to compensate for the underlying respiratory acidosis.

Compensation makes it a little difficult to find out if it is a respiratory or metabolic compensation. Usually the compensation is partial, meaning the overall pH will still be abnormal and for example if it is acidotic, look at the pCO2 and the HCO3- to find out which of these are acidotic to find the underlying cause. Remember that BOTH COMPONENTS CAN BE EITHER ACIDOTIC OR ALKALOTIC. This is referred to as either respiratory and metabolic acidosis or alkalosis depending on the blood gas report.

Conclusion

Blood gases are a little difficult to explain and I do a much better job explaining in person than trying to type out the explaination so if this still doesn’t make sense, I will soon upload a video explaination on blood gases on my instagram by next week. As always, I welcome feedback and contructive criticism so do get in touch!

Additional tips/tricks:

• The procedure to obtain ABGs is a little different and is not easy, i.e. you find the pulse point and go in at a 90 degree angle to the skin. The geeky medics website however has beautifully concised the procedure so I do recommend you take a look (click on the hyperlink) – especially as it can be an OSCE station or a procedure you need to get signed off in your log book! The site also has a marking scheme and some questions to test your knowledge on blood gases.
• The more blood gas questions you do the more of an expert you’ll become. As a medical student, I endlessly searched online for databases with free blood gas questions.