The Ultimate ABGs

Blood gas guide you need to calm your nerves

William Kelly, MSN, FNP-C

Author | Nurse Practitioner

This ultimate ABGs Blood gas guide is exactly what you’ve been looking for to understand Arterial Blood Gases! ABGs are used frequently in the ER and ICU settings, and many critical patients will need their blood gases monitored frequently.

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What Are ABGs or a Blood Gas?

ABGs, or an Arterial Blood Gas, is a blood sample that is taken from an artery in the wrist. This is different than normal blood work, which is taken from the veins of the arms. The arterial blood sample is obtained by a respiratory therapist or a critical care nurse.

Arterial samples provide better indicators of oxygen and carbon dioxide levels, but ABGs also look at acidity and bicarbonate levels within the blood.

A blood gas is used to look at acid-base disturbances and/or to evaluate the adequacy of oxygenation/ventilation. When an ABG blood gas is ordered, 4 contents of the arterial blood are tested:

  • Oxygen Levels: This measures PaO2 as well as SaO2
  • Carbon Dioxide levels: PaCO2 levels
  • pH: the acidity of the blood
  • Bicarbonate: The amount of bicarb, which is a buffer

Oxygen (O2) and carbon dioxide (CO2) are the main gases within the blood, and these are measured in blood gas. However, ABGs also provide levels of blood pH and Bicarb.

Of all of the measurements, the most important levels to look at are the CO2, the Bicarb, and the pH in determining acid-base balance.

When are ABGs ordered?

ABGs are very useful in evaluating acid-base disturbances, as well as ventilation/oxygenation disturbances. The patients who are ordered ABGs are often sick – usually ICU bound. The most common patients who might have a blood draw include:

Things to keep in Mind with ABGs

There are some important factors to keep in mind when thinking about ABGs and interpreting them.

Things Aren't Always Simple

Patients can have mixed acid-base disturbances, which can make it confusing. That’s why the interpretation is ultimately best left up to the critical care physicians and other Providers within their care.

Compensation

Remember the body is always trying to maintain homeostasis. The respiratory system will attempt to compensate for the metabolic system and vice versa.

Underlying Cause

Always focus on treating the underlying cause.

THE ABGs: Blood Gas Measurements

Okay, so lets dive a little deeper into what each measurement is on the ABG results, and what their levels mean.

The pH

pH is the “potential of Hydrogen”, which measures how acidic a solution is. The more hydrogen ions present in a solution, the more acidic it is. 

  • Normal pH of blood: 7.35-7.45
  • Low pH (<7.35): Indicates acidosis, and is termed acidemia
  • High pH (>7.45): Indicates alkalosis (metabolic or respiratory), and is termed alkalemia

pH may be normal or near-normal in chronic acid-base disturbances from compensation, or the patient can have multiple different acid-base disturbances going on at once.

The PaCO2

The PaCO2 is the partial pressure of Carbon Dioxide within the arterial blood. Essentially this is just a measure of the amount of carbon dioxide gas within the blood.

Remember that the lungs breathe in oxygen, deliver the oxygen to the cells, and the cells use that oxygen to create energy. To create energy (ATP), the cells utilize the Kreb’s Cycle, and a byproduct of that cycle is carbon dioxide. That CO2 is then breathed out when you exhale.

CO2 isn’t acidic by itself, but in the blood forms something called carbonic acid, which is acidic. Breathing out less CO2 will cause acidosis, and breathing out too much CO2 can cause alkalosis.

IF THE PaCO2 AND the pH are both high, think RESPIRATORY ACIDOSIS.

  • Normal: 35-45 mmHg
  • Elevated: >45 mmHg, termed hypercapnia
  • Decreased: <35 mmHg, termed hypocapnia

My Blood is BOILING

When you hold your breath, eventually you need to breathe again because it feels like your blood is boiling. This always helped me remember that when you aren’t breathing enough, the CO2 makes it boil – aka acidosis!

The HCO3 (Bicarb)

HCO3 on an ABG blood gas is the serum bicarb levels within the arterial blood. Bicarb acts as a buffer to make acidity less acidic. Think of it as the opposite of hydrogen ions. The less bicarb there is, the more acidic the blood is. To get technical, Bicarb reacts with H+ to form carbonic acid, which the body breaks down into CO2 and water – which it breaths out.

  • Normal: Between 22-26 mEq/L
  • Elevated: >26 mEq/L, associated with metabolic alkalosis
  • Decreased: <22 mEq/L (associated with metabolic acidosis)

The PaO2

PaO2 is the partial pressure of oxygen within arterial blood. This basically measures the actual oxygen blood gas content.

  • Normal: > 80 mmHg
  • Elevated: > 100 mmHg, usually due to over-oxygenation
  • Decreased: <80 mmHg, associated with respiratory failure, although could be from severe anemia as well

Oxygen Toxicity

Don’t forget that too much oxygen can be bad too. Oxygen toxicity can produce reactive oxygen species and cause cellular injury, inflammation, and cell death. It can also worsen hypercapnia like in patients with COPD.

The SaO2

The SaO2 is the peripheral oxygenation, which is equivalent to the Pulse Ox reading.

  • Normal: Above 94-96%
  • Decreased: <90-92%

ACID-BASE DISTURBANCES

When interpreting ABGs and blood gases, there are 4 general categories we use:

  • Respiratory Acidosis
  • Respiratory Alkalosis
  • Metabolic Acidosis
  • Metabolic Alkalosis

Using these categories, we can better understand what the possible underlying cause of the acid-base disturbance is!

Don’t forget someone can have multiple acid-base disturbances going on at one time, and this makes clinical interpretation difficult – everything is not black and white in medicine, but this should give you a pretty good idea of what may be causing your patient’s acid-base disturbance.

RESPIRATORY ACIDOSIS

Respiratory acidosis is due to alveolar hypoventilation. The lungs are NOT able to remove enough carbon dioxide quickly enough, so CO2 and Hydrogen build up in the blood.

High CO2 tends to occur late in the lung disease or when respiratory muscles are fatigued – this is usually seen in severe respiratory failure. The acidosis can be acute or chronic.

This classically can happen to patients with COPD because they are less responsive to hypoxia and hypocapnia. There is also increased dead-space ventilation and decreased diaphragmatic function due to fatigue and hyperinflation.

Acute Respiratory Acidosis

Acute respiratory acidosis could be from multiple different reasons including:

  • Respiratory Failure & Airway Obstruction: Severe asthma, COPD, CHF
  • CNS disease: Stroke, Traumatic brain injury
  • Drug-induced: Opioid or benzodiazepine overdose
  • Neuromuscular Disease: Myasthenia Gravis, ALS, Guillan Barre

Chronic Respiratory Acidosis

Chronic respiratory acidosis may occur when the PaCO2 is elevated, but the pH remains normal or near-normal because the body adjusts (metabolic compensation). Causes of chronic respiratory acidosis include: be Obesity-Hypoventilation syndrome (Pickwickian syndrome), ALS, interstitial fibrosis, and thoracic skeletal deformities.

  • COPD with Chronic CO2 retention
  • Obesity-hypoventilation Syndrome: Also called Pickwickian syndrome
  • Others: ALS, Interstitial fibrosis, thoracic skeletal deformities

COPD & Oxygen

When your patient with COPD is on a lot of oxygen, there is always a risk of hypoventilation and CO2 retention. This is the classic patient you should be thinking about with respiratory acidosis.

The treatment for respiratory acidosis is treating the underlying cause (i.e. giving Narcan to someone who overdosed on opioids), but more often than not the treatment is BIPAP or Intubation.

RESPIRATORY ALKALOSIS

This acid-base disturbance is due to alveolar hyperventilation. The lungs remove too much carbon dioxide too quickly, so hypocapnia (low PaCO2) and alkalosis occur.

It is commonly found in those who are critically ill, but can be found in various other conditions such as:

Ventilated Patients

The settings on the ventilator could be incorrect, and the patient may have a rate that is too fast

Hyperventilation

Patients experiencing panic attacks, severe anxiety, or psychosis can experience respiratory alkalosis. However, the patient’s with panic attacks almost never have ABGs ordered (it’s unnecessary)

Early-Intermediate Lung Disease

Pneumonia, pneumothorax, pulmonary embolism, asthma, bronchitis. This is more the increased respiratory rate compensating for the disease, but eventually, these issues can cause respiratory acidosis instead

Numbness & Tingling with Hyperventilation

Acute low CO2 levels lead to potassium and phosphorus shifting into the cells and cause calcium to increase its binding to albumin. This can cause temporary symptoms such as numbness/tingling in extremities that many patients may experience with acute panic attacks!

The treatment for respiratory alkalosis is treating the underlying cause, such as adjusting ventilator settings, administering anxiolytics, etc. 

Metabolic Acidosis

A bicarb level <22 mEq/L in addition to a pH <7.35 is metabolic acidosis. This acid-base disturbance is due to increased plasma acidity. Metabolic Acidosis is further broken down into whether or not the Anion Gap is normal or elevated.

Severe HCO3 levels <12 are almost always caused by some degree of metabolic acidosis, instead of just compensation for respiratory alkalosis.

Normal Gap Metabolic Acidosis

This type of metabolic acidosis usually has high chloride. This is when Bicarbonate is lost within the GI tract or kidneys (is peed or pooped out). This can be caused by:

Diarrhea

Diarrhea can cause loss of Bicarb within the stool but tends to save chloride, which does not increase the anion gap.

Chronic Renal Failure

Typically when GFR is between 20-50ml/min

Renal Tubular Acidosis

In RTA, the kidneys do not remove acid from the blood like they should

Large-Volume Fluid Replacement

Replacing large volumes of Normal Saline can cause a modest metabolic acidosis that is termed dilutional acidosis. This can worsen kidney injury. Using Lactated Ringers is a possible benefit to this, as the lactate is used as a buffer.

Elevated Gap Metabolic Acidosis

The anion gap is the difference between the positive ions in the blood (sodium), and the negative ions in the blood (chloride, bicarb, lactic acid, ketones, etc). Common causes of elevated gap metabolic acidosis include:

    Diabetic Ketoacidosis

    DKA causes a massive increase of ketone bodies which are acidic, in addition to severe dehydration

    Lactic Acidosis

    Lactic acidosis, especially in setting of sepsis, can cause metabolic acidosis

    Acute Kidney Injury

    Injury to the kidneys can cause a decreased ability to excrete hydrogen ions as well as the ability to increase bicarb levels to help buffer the acidosis

    Ingestion of certain Poisons

    Certain substances are toxic and can cause metabolic acidosis including alcohols, salicylates, cyanide, and carbon monoxide

    The treatment of metabolic acidosis is to correct the underlying issue causing the acidosis in the first place. Bicarb drips can be used in severe cases of acidosis (pH < 7.1 or 7.2).  

    METABOLIC ALKALOSIS

    This acid-base disturbance is caused by increased serum bicarb and decreased acidity. Bicarb levels >35 mEq/L are almost always caused by some degree of metabolic alkalosis as opposed to just compensation.

    For metabolic alkalosis, the acidity or hydrogen ions (H+) are usually lost in some manner, either through the GI tract or the kidneys:

    Excessive Vomiting

    Gastric secretion has a high content of hydrogen ions, so excessive vomiting can reduce overall acidity within the body

    NG Tube

    Over time, NG tubes remove a lot of gastric fluid, similar to excessive vomiting, this can cause a decrease in hydrogen ions

    Alkalotic overdose

    Rare, but if you consume massive amounts of milk products or antacids this can cause metabolic alkalosis

    Renal Losses

    The use of certain diuretics or mineralocorticoid excess, and some other rare disorders can cause the kidneys to pee out too many hydrogen ions.

    STEPS TO INTERPRET ABGs

    It always helps to have a systematic approach when interpreting ABGs, as blood gasses can be somewhat confusing if you miss a step!

    Determine the pH

    First, see whether or not the patient is acidic (pH <7.35), or alkalotic (pH >7.45). This will tell you if there is an acute acid-base disturbance going on.

    PCO2 + HCO3 Abnormalities

    See which levels are abnormal. Are they leaning acidic or alkalotic?  

    Correlate with pH

    See which one (CO2 or HCO3) correlates with pH. For example, if the pH is 7.2 (acidic), which abnormality is also leaning towards acidity? If CO2 was 56 and HCO3 was 30, the CO2 correlates with the pH because both are acidic.

    Determine Compensation

    Now check the level that doesn’t correlate with the pH. Is this also abnormal but in the opposite direction? If so this is termed compensation. If the pH is abnormal, it is only partial compensation. 

    Check the Anion Gap

    This step is optional and done if there is metabolic acidosis. This will help give you a better idea of which type of acidosis it is. If it is high, think of kidney failure, sepsis, or DKA. If it is low, think severe diarrhea. 

    Hopefully, this gave a good idea of how to interpret ABGs, as well as the treatment involved with abnormal results.

    REFERENCES

    Emmett, M., & Szerlip, H. (2022). Approach to the adult with metabolic acidosis. In T. W. Post (Ed.), Uptodate. https://www.uptodate.com/contents/approach-to-the-adult-with-metabolic-acidosis

    Emmett, M., & Szerlip, H. (2022). Causes of metabolic alkalosis. In T. W. Post (Ed.), Uptodate. https://www.uptodate.com/contents/causes-of-metabolic-alkalosis

    Hopkins, E., Sanvictores, T., & Sharma, S. (2020, September 14). Physiology, Acid Base Balance. National Library of Medicine. https://www.ncbi.nlm.nih.gov/books/NBK507807/

    Sood, P., Paul, G., & Puri, S. (2010). Interpretation of arterial blood gas. Indian J Crit Care Med, 14(2), 57-64. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2936733/

    Theodore, A. C. (2022). Arterial blood gases. In Uptodate. https://www.uptodate.com/contents/arterial-blood-gases

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