Arterial Blood Gases (ABG) Calculator
The arterial blood gases calculator calculates whether an individual is in metabolic acidosis, metabolic alkalosis, respiratory acidosis, respiratory alkalosis, or is normal. The calculator also determines whether the state is compensated or uncompensated.
Arterial blood gases are blood taken from an artery, normally the radial artery, which determines how well oxygenated a person's blood is.
Arterial blood gases determine the pH of the the person's blood, the bicarbonate level of a person's blood, and the amount of carbon dioxide in a person's blood. It can also tell the amount of oxygen in a person's blood, but we don't use the level of oxygenation to determine the state the person is in.
So the first thing an arterial blood gas can tell is the pH of the blood. This is huge and alone determines whether the person is in an acidotic state or an alkalotic state. Normal blood pH ranges from 7.35 to 7.45. If the person's pH is below 7.35, the person is in an acidotic state. If the person's pH is above 7.45, the person is in an alkalotic state. So the pH is the first thing we look at to determine what state- acidotic or alkalotic- a person is in.
Another component of arterial blood gas is the bicarbonate level. Bicarbonate is an alkaline substance produced by the kidneys in order to buffer against acidity in the body. It is the metabolic component of the arterial blood gases. If a person is in a state of respiratory acidosis, to compensate for this acidotic state, the person's kidneys will produce more bicarbonate. This helps to neutralize the acidity. If a person is in a state of respiratory alkalosis, to compensate for this alkaline state, the person's kidneys will produce less bicarbonate. The normal range of bicarbonate in the blood is 22-28. During states of metabolic acidosis, the bicarbonate will be below 22. In states of metabolic alkalosis, the bicarbonate level will be above 28.
The other component of arterial blood gases is the partial pressure of carbon dioxide, referred to as PaCO2. The partial pressure of carbon dioxide is the amount of carbon dioxide in the blood. This represents the respiratory component of the arterial blood gases. If a person is in a state of metabolic acidosis, the person will hyperventilate, as compensation. When a person hyperventilates, he blows off CO2. This helps to reduce the overall acidotic state of the body. If a person is in a state of metabolic alkalosis, the person will hypoventilate, as compensation. This will help retain carbon dioxide, so that the pH will be lowered back to a normal state. The normal level of PaCO2 is 35-45mmHg.
Compensation is the body compensating from one state to going back to a normal state, a pH between 7.35 and 7.45.
So if the body is in a metabolic state, it will compensate to go back to a normal state. If the body is in metabolic acidosis, it will compensate to go back to
So, now that you know the individual components that makes up the arterial blood gases, how do we interpret them to know what state a person is in and if it is compensated or uncompensated?
So, the first thing we do is look at the person's pH.
If the pH is below 7.35, the person is acidotic.
If the pH is above 7.45, the person is alkalotic.
If the pH is 7.35 to 7.45, the person is normal, neither acidotic nor alkalotic.
So once we look at the pH and determined the state, the next thing we do is look at the bicarbonate and paCO2. Based on the person's state, match that to either the bicarbonate or paCO2. See whether the bicarbonate or the paCO2 value shows that state. Only one will.
As an example, let's say a person is in a state of acidosis. The pH is 7.24. The bicarbonate value is 36 and the PaCO2 value is 25. Is the person is respiratory acidosis or respiratory acidosis?
Okay, so based on the pH, we know for sure that the person is acidotic, since the pH is less than 7.35. Is it metabolic or respiratory in nature? Based on the 2 values, it's respiratory in nature, since the PaCO2 value is below 35. This is what is meant by matching the states together. If the pH is below 7.35, the person is acidotic. Now you just have to figure out whether the bicarbonate (metabolic value) is acidotic or the PaCO2 is acidotic. Based on which one matches the state of the pH determines whether it's respiratory or metabolic in nature. This is why all you have to do is match the pH state to either the bicarbonate or PaCO2 value.
The last part is to tell whether there is compensation or not. Once you've figured out the state of the pH and whether it's respiratory or metabolic in nature, you then look at the last value. If that value is out of range in favor to neutralize the pH state the person is in, there is compensation. If it lies within its normal range, then the state is uncompensated.
We'll go back to the same example, of the person with a PH of 7.24, a bicarbonate of 36, and a PaCO2 value of 25. Based on the pH, the person is acidotic. The paCO2 is below 7.35, so it is respiratory acidosis. So we now detail down on the bicarbonate value, which is 36. Remember the normal range for bicarbonate in the blood is 22-28. The bicarbonate here is 36, so it is well out of range in order for the body to compensate for the acidotic state. Therefore, this person has respiratory acidosis with compensation. If the bicarbonate was within its normal range of 22-28, then it would be uncompensated.
So this is the nature of arterial blood gases. It allows us to determine the pH of a person's blood which allows us to determine whether the person is in an acidotic or alkalotic state. We then can tell whether it is metabolic or respiratory in nature and with or without compensation by looking at the bicarbonate and PaCO2.
There is one more uncertain situation that may happen with arterial blood gases.
The pH may be normal, that is between 7.35-7.45, but the other 1 of the other 2 values may be out of proportion or both of them may be out of proportion. So even though the pH is normal, the other 2 values may not be, which is abnormal.
How we approach is this situation is we take the midline of the pH, which is 7.40. If a value is below 7.40, the person is considered to be recovering from an acidotic state. If the pH is above 7.40, the person is considered to be recovering from an alkalotic state. We then look at the other 2 values to see whether it is metabolic or respiratory and. Usually in a case like this, there must always be compensation. If there is no compensation, it wouldn't be possible for the pH to be within a normal range.
So, for example, let's say a person's pH is 7.43, the bicarbonate is 38, and the PaCO2 is 25. The pH is normal but both of the other values are out of range. Thus, the arterial blood gases are still abnormal. Since the pH is above 7.40, the person is in a alkalotic state. Since the bicarbonate is above 28, the person is recovering from a state of metabolic alkalosis. To compensate for this alkalosis, the person's PaCO2 is below 7.35.
So this is all really to know about arterial blood gases.
This calculator simply asks for the 3 values of pH, bicarbonate, and PaCO2.
It can then determine the state the person is in and whether there is compensation or not.
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