ABGs Indepth

I. The Background Stuff

The body tries to maintain homeostasis with the Acid Base balance using acids and bases contained within the body. Each acid and base counter balances with each other (the alkaline part of your ABG). The body enzymes cannot work outside of the balance. The ABG is an arterial Blood measurement of this acid base status.

  • The Respiratory System (Acid); CO2 is a volatile acid
  • The Renal System (Base); the kidneys rid the body of the nonvolatile acids H+ (hydrogen ions) and maintain a constant bicarb (HCO3). Bicarbonate is the body’s base
  1. If you increase your respiratory rate (hyperventilation) you “blow off” CO2 (acid) therefore decreasing your CO2 acid—giving you ALKLAOSIS
  2. If you decrease your respiratory rate (hypoventilation) you retain CO2 (acid) therefore increasing your CO2 (acid)—giving you ACIDOSIS
  1. You have Acidosis when you have excess H+ and decreased HCO3- causing a decrease in pH.

The Kidneys try to adjust for this by excreting H+ and retaining HCO3- base.

The Respiratory System will try to compensate by increasing ventilation to blow off CO2 (acid) and therefore decrease the Acidosis.

    1. You have Alkalosis when H+ decreases and you have excess (or increased) HCO3- base.

The kidneys excrete HCO3- (base) and retain H+ to compensate.

The respiratory system tries to compensate with hypoventilation to retain

CO2 (acid)

To decrease the alkalosis

  • Compensation
  1. The respiratory system can effect a change in 15-30 minutes
  2. The renal system takes several hours to days to have an effect.

II. The Big Four

RESPIRATORY ACIDOSIS: pH < 7.35 (Normal: 7.35 – 7.45) CO2 > 45 (Normal: 35 – 45)

1. Causes: Hypoventilation

a. Depression of the Respiratory Center (sedatives, narcotics, drug

overdose, CVA, cardiac arrest, MI)

b. Respiratory muscle paralysis (spinal cord injury, Guillian-Barre,


c. Chest wall disorders (flail chest, pneumothorax)

d. Disorders of the lung parenchyma (CHF, COPD, pneumonia, aspiration,


e. Alteration in the function of the abdominal system (distension)

2. Signs and Symptoms

a. CNS depression (decreased LOC)

b. Muscle twitching which can progress to convulsions

c. Dysrhythmias, tachycardia, diaphoresis (related to hypoxia secondary to


d. Palpitations

e. Flushed skin

f. Serum electrolyte abnormalities including elevated K+ (potassium leaves

the cell to replace the H+ buffers leaving the cell)

3. Treatment

a. Physically stimulate the patient to improve ventilation

b. Vigorous pulmonary toilet (chest PT, coughing and deep breathing,

inspirometer, respiratory treatments with bronchodilators)

c. Mechanical ventilation (to increase the respiratory rate and tidal volume)

d. Reversal of sedatives and narcotics

e. Antibiotics for infections

f. Diuretics for fluid overload

(NOTE: beware of NaHCO3- sodium bicarbonate—can compensate and cause metabolic alkalosis. Also, if patient has been hypoxic and this is a lactic acidosis; NaHCO3- can be dangerous)

Respiratory Alkalosis: pH > 7.45 (Normal: 7.35 – 7.45) CO2 < 35 (Normal: 35 – 45)

Causes: Alveolar Hyperventilation

  1. Psychogenic (fear, pain, anxiety)
  2. CNS stimulation (brain injury, ETOH, early salicylate poisoning, brain tumor)
  3. Hypermetabolic states (fever, thyrotoxicosis)
  4. Hypoxia (high altitude, pneumonia, heart failure, pulmonary embolism)
  5. Mechanical overventilation (ventilator rate too fast)

Signs and Symptoms

  1. Heachache
  2. Vertigo
  3. Paresthesias (numb fingers /toes, circumoral, carpal pedal spasms and tetany)
  4. Tinnitus (ringing in the ears)
  5. Electrolyte abnormalities (decreased Ca+, K+)

Treatment (treat the underlying cause)

  1. Sedatives or analgesics
  2. Correction of hypoxia (possible diuretics, mechanical ventilation to also decrease respiratory rate and decrease the tidal volume)

NOTE: patients with brain injury may need hyperventilation

    1. Antipyretics for fever
    2. Treat hyperthyroidism
    3. Breathe into a paper bag for hyperventilation

Metabolic Acidosis pH < 7.35 (Normal: 7.35 – 7.45) HCO3- < 22 (normal: 22 – 26)

Causes: Increased H+, excess loss of HCO3-

  1. Overproduction of organic acids (starvation, ketoacidosis, increased catabolism)
  2. Impaired renal excretion of acid (renal failure)
  3. Abnormal loss of HCO3- (diarrhea, biliary fistula, Diamox)
  4. Ingestion of acid (salicylate overdose, oral anti-freeze)

Signs and Symptoms

  1. CNS depression (confusion to coma)
  2. Cardiac Dysrhythmias (elevated T wave, wide QRS to ventricular standstill)
  3. Electrolyte abnormalities (elevated K+, Cl-, Ca2+)
  4. Flushed skin (arteriolar dilitation)
  5. Nausea

Treatment (Treat the underlying cause)

  1. NaHCO3- (sodium bicarbonate) based on ABGs only and with caution
  2. IV fluids and insulin for DKA
  3. Dialysis for renal failure
  4. Antibiotics, increased nutrition for tissue catabolism
  5. Increased cardiac output and tissue perfusion for low CO states
  6. Rehydrate, monitor I and O
  7. Treat dysrhythmias, support hemodynamic and respiratory status

Metabolic Alkalosis pH > 7.45 (Normal: 7.35 – 7.45) HCO3- > 26

Causes: Loss of H+ or increased HCO3-

  1. Loss of K+ (diarrhea, vomiting)
  2. Ingestion of large amounts of bicarbonate (antacids, resuscitation)
  3. Prolonged use of diuretics (distal tubule lose ability to reabsorb Na+ and Cl- therefore NaCl); Ammonia is in the urine and then binds with H+

Signs and Symptoms: similar to the disease process

  1. Diaphoresis
  2. Nausea and Vomiting
  3. Increase neuromuscular excitability (Ca2+ binds with protein)
  4. Shallow breathing (respiratory compensation)
  5. EKG changes (increased QT, sinus tachycardia)
  6. May also see confusion progressing to lethargy to coma
  7. Electrolyte abnormality (decreased Ca2+), normal or decreased K+, increased base excess on the ABG

Treatment: Treat the underlying cause

  1. Replace potassium (KCl) losses in 0.9% NaCl (rehydrates and increases HCO3- excretion)
  2. Diamox (acetazolamide, increases HCO3-excretion)
  3. Monitor neuro status, re-orient, seizure precaution, monitor I and O

III. The Land of the ABG**

(**based upon a concept by Laura Gasparis Vonfrolio, RN, PhD)

Once upon a time there was a land known as ABG

Everyone there was related with only a limited number of names for the population.

They were also very polite and had their own etiquette for learning each other’s names.

Now I would like to introduce you to your patient. Let’s figure out what her name is.

All of the people in the land of ABG have a first name, a middle name, and a last name.

You just have to look at them one name at a time.

The Last Name

  1. First, look at her pH (normal is 7.35 – 7.45)
  2. If her pH is < 7.35; her name is ACIDOSIS
  3. If her pH is > 7.45; her last name is ALKALOSIS

(NOTE: To have an absolutely perfect last name; her pH needs to be 7.40. So, keep in mind that if her pH is 7.35 – 7.39 she’s thinking about marrying into the ACIDOSIS family. If her pH is 7.41 – 7.45 she’s thinking about marrying into the ALKALOSIS family)

The First Name

Now that you know your patient’s last name, you would like to also learn her first name.

  1. Look at her pH again.
  2. If it is 7.35 – 7.45 (normal) then her first name is COMPENSATED.
  3. If the pH is < 7.35 or > 7.45 then her first name is UNCOMPENSATED.

The Middle Name

Now that you know your patient’s first and last name, you would like to know her

middle name.

Name Alert: These people are all related and you have many patients with the same

first and last name. A middle name will give you more information.

First you need to look at the CO2 and HCO3-. Remember : normal CO2 35 – 45; and

HCO3- 22 – 26.

1. The middle name will either be Respiratory or Metabolic.

2. If the CO2 is < 35 or > 45 her middle name is RESPIRATORY.

3. If the HCO3- is < 22 or > 26; her middle name is METABOLIC.

The Family Feud

1. pH and HCO3- are “kissin’ cousins” they like to go in the same direction

2. CO2 is the “black sheep” pH runs the opposite direction when it sees him



3. Decreased pH with decreased HCO3-: ACIDOSIS

4. Increased pH with increased HCO3-: ALKALOSIS

5. Decreased pH with increased CO2-: ACIDOSIS

6. Increased pH with decreased CO2-: ALKALOSIS






Did you notice that I haven’t mentioned O2?

The O2 number has nothing to do with your acid-base ABG interpretation!

What does the PaO2 mean?

      • The O2 tells us if the patient has hypoxemia (decreased oxygen in the blood).
      • Normal PaO2 = 80-100 (hypoxemia = PaO2 < 80)
      • PaO2 assesses perfusion (gas exchange)
      • PaCO2 assesses the adequacy of ventilation (breathing pattern)
      • The PaO2 is very important in determining your patient’s oxygen status and needs – but it is not necessary in determining the BIG FOUR.

What is saturation?

      • SaO2 (oxygen saturation) measures the percent of oxygen bound to hemoglobin. This tells whether the patient has hypoxia (decrease O2 in the tissue).
      • Normal SaO2 = greater than 95%
      • Acceptable SaO2 will vary between MD, but PaO2 dramatically drops when it is less than 92%.
      • This is a noninvasive measurement via pulse oximetry and can be less accurate due to hypoxemia, hypotension, hypovolemia, or vasoactives.

Note: In Carbon Monoxide Poisoning, the Hgb is saturated with Carbon Monoxide. Although the patient is hypoxemic, it is because there is no room on the Hgb for O2 to be carried – the Saturation looks good because it can’t distinguish between the two.

What are some causes of low PaO2?

      • Hypoxic-Hypoxia – there is just not enough of a supply of O2 (COPD, pneumonia, ARDS, suffocation)
      • Anemic-Hypoxia – there is plenty of O2, but not enough Hgb to carry it to the tissue.
      • Stagnant-Hypoxia – there may be enough O2 coming in and enough Hgb to carry it, but the circulation is stagnant due to a decreased Cardiac Output (CO). The O2 is not being adequately carried to the tissue.
      • Histotoxic-Hypoxia – poisoning like Carbon Monoxide or Cyanide. Either the blood can’t carry the O2 or the cells can’t receive the O2 from the blood.

Why do we look at Base Excess (BE)?

      • Base excess is the amount of base needed to return the pH to a normal value
      • Normal range = (-)2 – (+)2, with zero (0) being the middle ground
      • Used mostly to distinguish metabolic alkalosis from acidosis
      • Too much base excess (over +2) may indicate loss of gastric fluids as in vomiting, whereas a base deficit of -5 or lower may indicate a lactic acid problem or diabetic ketoacidosis
      • Used mostly to test for consistency in diagnosing metabolic problems

Can we only be PARTIALLY compensated?

      • The body has attempted to compensate with either the lungs or kidneys, but has not fully compensated to return the pH to normal.
      • pH balance will still be abnormal, either acidotic or alkalotic state.