Blood gas interpretation

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Introduction

Blood gas interpretation is an important skill that Physicians, Nurses, Respiratory Therapists and other healthcare providers use to determine the optimal course of treatment for critically ill patients. In order to help healthcare providers, I have developed an easy 4 step process to interpreting abnormal blood gases.

Normal Values

Before interpreting blood gases it is important to know the acceptable range of normal values.

pH PaO2 PaCO2 HCO3 B.E.
Arterial 7.35 to 7.45 80 to 100mmHg 35 to 45mmHg 22 to 26mEq/L -2 to +2mEq/L
Venous 7.32 to 7.42 28 to 48mmHg 38 to 52mmHg 19 to 25mEq/L -2 to +2mEq/L

Normal Blood Gas Values

Step 1 - Is the pH acidic or alkalotic?

pH is a measure of the amount of hydrogen ions in a liquid solution on a scale of 0-14. The closer a pH is to 0 the more acidic it is considered, while a pH closer to 14 is considered alkalotic. Based on the normal pH of a healthy adult we can assume that in an arterial blood gas:


A pH less than 7.35 indicates acidosis

A pH greater than 7.45 indicates alkalosis


If the pH is within normal limits but the other blood gas values are abnormal you would continue with your analysis.

Step 2 - Is the the cause of the abnormality respiratory or metabolic?

The two primary compounds that affect the acid-base balance in our body are carbon dioxide \(CO_2\) and bicarbonate \(HCO_3\).


An abnormal \(CO_2\) indicates a respiratory cause and an abnormal \(HCO_3\) indicates a metabolic cause.


An ↑\(CO_2\) in the blood will cause an acidosis and an ↑\(HCO_3\) in the blood will cause an alkalosis.


Conversely, a \(CO_2\) will cause alkalosis and a ↓\(HCO_3\) will cause acidosis.


It is possible that you have a combined acidosis that would manifest as an ↑\(CO_2\) and ↓\(HCO_3\) and a combined alkalosis ↓\(CO_2\) and ↑\(HCO_3\).


Heres a handy dandy chart to make it easier:

pH \(CO_2\) \(HCO3\) Interpretation
<7.35 Respiratory Acidosis
<7.35 Metabolic Acidosis
>7.45 Respiratory Alkalosis
>7.45 Metabolic Alkalosis
<7.35 Combined Acidosis
>7.45 Combined Alkalosis

Step 3 - is the pH compensated, partially compensated, or uncompensated?

Since \(CO_2\) and \(HCO_3\) are opposing forces in the acid-base balance, their respective systems will often attempt to correct a prolonged abnormality. Combined acidosis or alkalosis can not be compensated.


A compensated pH will be within normal limits with both an abnormal \(CO_2\) and/or \(HCO_3\). The causative factor will be determined by the mean normal value for pH. A pH of 7.35-7.40 will have an acidic cause and a pH of 7.4-7.45 will have an alkalotic cause.


A partially compensated pH will be abnormal but shows an attempt to correct the imbalance. For example an acidic pH of 7.30 with a \(CO_2\) of 65 (acidic) and a \(HCO_3\) of 30 (alkalotic).


An uncompensated pH will be abnormal with no attempt at compensation. Either \(CO_2\) or \(HCO_3\) will be normal while the other value is abnormal.

Step 4 - Is the patient hypoxic?

Examine the \(PaO_2\)


\(PaO_2\) > 100 is High

\(PaO_2\) 80-100 Normal

\(PaO_2\) 60-80 Mild

\(PaO_2\) 40-60 Moderate

\(PaO_2\) <40 Severe

Conclusion

Though this 4 step process makes it easy for healthcare providers to interpret blood gases, the acid-base balance is complex and many different factors contribute to its homeostasis. This simplified process is not intended to replace a comprehensive education in blood gas interpretation, but should be used as a reference to reinforce existing knowledge.