ABG help

Daytonite, BSN, RN · Feb 21, 2007

It's looks like the patient went from Respiratory Acidosis to Respiratory and Metabolic Acidosis. His second set of ABGs are NOT normal. His bicarb levels are below normal which is now making him also in metabolic acidosis. His body is trying to compensate for the high CO2 levels through his buffering system and using up his bicarb, so now he's in metabolic acidosis.

Advanced Practice Columnist / Guide · Feb 21, 2007

Whenever you interpret ABG's, just like with SpO2's, you would need to know what FiO2 the patient is on and whether they are on a ventilator or not. Your first ABG looks like a post-operative blood gas to me. Usually, when patients arrive in the ICU or recovery from AAA repair, they are still under the influence of anesthesia and will need respiratory support with a ventilator. Orders typically call for Assist/Control ventilation with 100% FiO2, a set tidal volume based on ideal body weight, and a set respiratory rate. Sometimes, PEEP is added as well.

The first ABG showed a pO2 of 419.2. This is not unusual for a post-op patient on 100% FiO2 on the vent. However, high pO2 can be detrimental so it is important to wean the FiO2 from the vent in this case. The patient's pCO2 is high and this can be caused by either breathing too slow (because of low respiratory rate setting on the vent and patient being too drowsy to breathe over the set rate) or too low of a set tidal volume causing shallow breathing. The HCO3 is within normal but it appears that the patient is compensating for the respiratory acidosis based on the low base excess.

The physician or practitioner likely changed the vent settings to improve the ABG in this case by weaning the FiO2 and either increasing the set respiratory rate on the vent or increasing the set tidal volume on the vent. Either approach will decrease the patient's pCO2 and cause the pH to normalize.

Your second blood gas may have been a mixed picture of respiratory and metabolic acidosis but I am leaning more towards respiratory acidosis in this case. However, again, you did not indicate what FiO2 setting the patient was in and whether the patient has already been extubated at the time the ABG was drawn. Oxygenation does not affect the pH. The only values that affect it are pCO2 and HCO3 - the acid-base buffers in the body. I suspect this patient may have been extubated at the time this ABG was drawn and needs to take deep breathing/coughing exercises and use incentive spirometry as part of post-op care to improve the oxygenation and ventilation.

Megsd, BSN, RN · Feb 26, 2007

Whenever you interpret ABG's, just like with SpO2's, you would need to know what FiO2 the patient is on and whether they are on a ventilator or not. Your first ABG looks like a post-operative blood gas to me. Usually, when patients arrive in the ICU or recovery from AAA repair, they are still under the influence of anesthesia and will need respiratory support with a ventilator. Orders typically call for Assist/Control ventilation with 100% FiO2, a set tidal volume based on ideal body weight, and a set respiratory rate. Sometimes, PEEP is added as well.
The first ABG showed a pO2 of 419.2. This is not unusual for a post-op patient on 100% FiO2 on the vent. However, high pO2 can be detrimental so it is important to wean the FiO2 from the vent in this case. The patient's pCO2 is high and this can be caused by either breathing too slow (because of low respiratory rate setting on the vent and patient being too drowsy to breathe over the set rate) or too low of a set tidal volume causing shallow breathing. The HCO3 is within normal but it appears that the patient is compensating for the respiratory acidosis based on the low base excess.
The physician or practitioner likely changed the vent settings to improve the ABG in this case by weaning the FiO2 and either increasing the set respiratory rate on the vent or increasing the set tidal volume on the vent. Either approach will decrease the patient's pCO2 and cause the pH to normalize.
Your second blood gas may have been a mixed picture of respiratory and metabolic acidosis but I am leaning more towards respiratory acidosis in this case. However, again, you did not indicate what FiO2 setting the patient was in and whether the patient has already been extubated at the time the ABG was drawn. Oxygenation does not affect the pH. The only values that affect it are pCO2 and HCO3 - the acid-base buffers in the body. I suspect this patient may have been extubated at the time this ABG was drawn and needs to take deep breathing/coughing exercises and use incentive spirometry as part of post-op care to improve the oxygenation and ventilation.

Unfortunately I do not know what was going on at the time these two ABGs were drawn. I am responsible for interpreting all lab values for my pt for a certain number of days. He did have surgery on this day, but I do not know what time the surgery took place, nor was I able to find any notes about his status at any particular time. I agree, it would be very helpful to know his ventilatory status.

Thanks for the info.

loafin' · Feb 28, 2007

Your second blood gas may have been a mixed picture of respiratory and metabolic acidosis but I am leaning more towards respiratory acidosis in this case. However, again, you did not indicate what FiO2 setting the patient was in and whether the patient has already been extubated at the time the ABG was drawn. Oxygenation does not affect the pH. The only values that affect it are pCO2 and HCO3 - the acid-base buffers in the body. I suspect this patient may have been extubated at the time this ABG was drawn and needs to take deep breathing/coughing exercises and use incentive spirometry as part of post-op care to improve the oxygenation and ventilation.

Actually oxygenation does affect pH. The most profound acidosis I've ever seen was on a client who had cyanide poisoning who had a pH of 6.6. He had a normal p02 and sat with abg. I drew venous gas and was vertually identical. He was in anearobic metabolism(lactic acidosis).It is called hystotoxic hypoxia,that is cyanide prevents unloading of o2 from hgb to tissues. Any situation that causes anearobic metabolism for long enough will cause acidosis.

Advanced Practice Columnist / Guide · Feb 28, 2007

You're right, lactic acidosis is an example of metabolic acidosis resulting from anaerobic metabolism causing release of lactic acid from conditions such as sepsis, drug ingestions, etc. But what I was trying to explain is that when we look at ABG's, it is the pCO2 and HCO3 we look at to determine the underlying acid-base etiology, not the pO2.

I would like to add that in lactic acidosis, we do look at the ABG but only in terms of knowing that the pH is low and the base deficit is high. As your example shows, your pO2 on the ABG was normal in your patient while the pO2 on your venous gas is too high (identical to ABG pO2) since the body is not able to extract oxygen in the tissues. Lactic acidosis is not diagnosed with the ABG alone but with checking for mixed venous gas, anion gap, and a serum lactate level.

loafin' · Mar 1, 2007

Lactic acidosis is not diagnosed with the ABG alone but with checking for mixed venous gas, anion gap, and a serum lactate level.

I know this, but thank you for clarifying that oxygen does play a very inportmant role in pH. A lactate level would be drawn of course,but we would know that it was lactic acidosis just by doing a venous gas with the p02 being virtually the same as the abg.