Arterial Blood Gases (ABGs)

Arterial blood gases

Acid-base balance

It's all about homeostasis...

• Constant acid-base balance
• Chemical reactions occur to maintain homeostasis.
• Acids: substances that release H+
• Carbonic acide (H2CO3)
• Bases: substances that trap H+
• Bicarbonate (HCO3-)

pH Levels

• Range from 1-14
• 7 is neutral
• < 7 is acidic
• > 7 is basic
• Normal blood pH is 7.35-7.45
• Acidosis: Excess H+ or loss of bicarb = pH ↓7.35
• Alkalosis: lack of H+ or excess bicarb = pH ↑ 7.45

Two Main Body Systems Involved

• Renal
• Reacts the slowest—hours to days
• Excrete or retain H+

OR

• Form or excrete bicarb (HCO3-)

• Respiratory
• Reacts the fastest to changes—minutes
• Exhales or retain CO2

What is an ABG?

• Arterial Blood Gas
• Sample of arterial blood (usually radial)
• Assesses effectiveness of breathing and acid-base balance
• We’re concerned with 3 parts here:
• Blood pH, PaCO2, and HCO3-
• Can also measure PaO2

Lab Values You MUST Know

• Normal serum pH: 7.35-7.45
• Normal PaCO2: 35-45 mmHg
• Normal HCO3-: 22-26 mEq/L
• Normal PaO2: > 80
• Normal SaO2 > 95

It's All in the Name

1. Everyone in ABGs has a first name, a middle name, and a last name.
2. You have to look at the names one at a time.
1. Acidosis or Alkalosis?
2. Respiratory or Metabolic?
3. Compensated or Noncompensated?

Acidosis or Alkalosis

Compensated or Noncompensated

Respiratory or Metablic

First, We Need a Last Name

1. Look at the pH (normal is 7.35 – 7.45)
2. If the pH is < 7.35 the last name is Acidosis
3. If the pH is > 7.45 the last name is Alkalosis

Let's try out a few on the next slide...

Now We Need a Middle Name

• This is determined by which system is the origin of the problem.
• Two options:
• Respiratory
• Metabolic

Respiratory

• Carbon Dioxide
• PaCO2: gas removed from the body by the lungs
• This is ACID component
• What’s normal?
• 35-45mmHg

Respiratory

• ↑ 45 = too much CO2
• Respiratory acidosis
• pH ↓ 7.35
• ↓ 35 = not enough CO2
• Respiratory alkalosis
• pH ↑ 7.45

Metabolic

• Bicarbonate
• HCO3-: ion removed from or retained by the kidneys
• This is BASE or ALKALINE component
• What’s normal?
• 22-26 mEq/L

Metabolic

• ↑ 26 = too much HCO3-
• Metabolic alkalosis
• pH ↑ 7.45
• ↓ 22 = not enough HCO3-
• Metabolic acidosis
• pH ↓ 7.35

Are the Values Below High, Low or Normal?

Are the Values Below High, Low or Normal?

Are the Values Below High, Low or Normal?

Therefore...

• Decreased pH with decreased HCO3-
• Metabolic Acidosis
• Increased pH with increased HCO3-
• Metabolic Alkalosis
• Decreased pH with increased CO2
• Respiratory Acidosis
• Increased pH with decreased CO2
• Respiratory Alkalosis

What do we do if BOTH CO2 and HCO3- are abnormal?

We Still Need a First Name...

• What has the body done so far to take care of this problem?
• Options:
• Non-compensated
• Partially compensated
• Fully compensated (pH will be normal)

Compensation: Acidosis

• The kidneys attempt to correct by excreting H+ and retaining HCO3-

• The respiratory system attempts to compensate by ↑ ventilation to “blow off” CO2

Compensation: Alkalosis

• The kidneys attempt to correct by retaining H+ and excreting HCO3-

• The respiratory system attempts to compensate by ↓ ventilation to retain CO2

Putting It All Together

1. Is the pH normal?
2. If not, is it alkalosis or acidosis?
3. Determine whether it is metabolic or respiratory (which value correlates with the pH?)
4. Then, determine compensation.
1. Uncompensated: pH and ONE other value are abnormal.
2. Partially compensated: ALL values are abnormal.
3. Compensated: pH is normal, both CO2 and HCO3- are abnormal.....

Let's Practice: pH = 7.29, CO2 = 17 HCO3- = 19

What's Your Interpretation?

Let's Practice: pH = 7.29, CO2 = 17 HCO3- = 19

• pH = acidic
• CO2 = alkalotic
• HCO3- = acidotic

Therefore, this is partially compensated metabolic acidosis.

Let's Practice: pH = 7.52, CO2 = 42, HCO3- = 33

What's Your Interpretation?

Let's Practice: pH = 7.52, CO2 = 42 HCO3- = 33

• pH = alkalotic
• CO2 = normal
• HCO3- = alkalotic

Therefore, this is uncompensated metabolic alkalosis.

Metabolic

Respiratory

Causes and compensation

Respiratory Acidosis

• ↓ respiratory rate or depth (hypoventilation)
• ↑ CO2 retention  ↑ arterial Pa CO2 = pH < 7.35
• = Respiratory Acidosis

Respiratory Acidosis: Causes

• Chest wall disorders
• Flail chest
• Pneumothorax
• Disorders of the lung parenchyma
• CHF
• COPD
• Pneumonia
• Aspiration
• ARDS

• Depression of the respiratory center
• Sedatives
• Narcotics
• Drug OD
• CVA and head injuries
• Respiratory muscle paralysis
• Scleroderma
• Guillian – Barre

Respiratory Acidosis: Signs and Symptoms

• CNS depression
• ↓ LOC and mental cloudiness
• Muscle twitching
• Dizziness
• Weakness
• Headache

Respiratory Acidosis: Treatment

• Physically stimulate patient to ↑ ventilation
• Vigorous pulmonary toilet (CPT, PD w/ P, TCDB, IS, respiratory tx w/ bronchodilators)
• Mechanical ventilation (to ↑ RR/tidal volume)
• Reversal of sedatives/narcotics
• Adequate hydration

Respiratory Alkalosis

• ↑ respiratory rate (hyperventilation)
• “blow off” CO2
• ↓ arterial pCO2 = ↑ pH > 7.45
• Respiratory Alkalosis

Respiratory Alkalosis: Causes

• Hypermetabolic states
• Fever
• Thyrotoxicosis
• Pregnancy
• Early sepsis
• Hypoxemia
• High altitude
• Pneumonia
• Heart failure
• Pulmonary embolism

• Psychogenic
• Fear
• Pain
• Anxiety (most common cause)
• CNS stimulation
• Brain injury involving the respiratory center
• Mechanical overventilation (ventilator rate too fast)

Respiratory Alkalosis: Signs and Symptoms

• Increased CNS excitability
• Lightheadedness, decreased concentration

• Tinnitus
• Paresthesias
• Palpitations
• Sweating
• Dry mouth
• Convulsions
• Loss of consciousness

Respiratory Alkalosis: Treatment

• Treat the underlying cause
• Sedatives or analgesics
• Correction of hypoxia (diuretics, mechanical ventilation to ↓ RR/tidal volume)
• Antipyretics (fever)
• Treat hyperthyroidism
• Breathe into paper bag (inhalation of exhaled CO2)

Metabolic Acidosis: Causes

• Impaired renal excretion of acid
• Renal Failure
• Abnormal loss of HCO3-
• Diarrhea
• Intestinal fistulas
• Ingestion of acid
• Salicylate overdose
• Oral anti-freeze

• pH is < 7.35 (Normal is 7.35 – 7.45)
• HCO3- <22 (Normal is 22 -26)

• Over production of organic acids
• Starvation
• Ketoacidosis

Metabolic Acidosis: Signs and Symptoms

• CNS Depression
• Confusion to Coma
• Cardiac Dysrhythmias
• Electrolyte Abnormalities
• K+, Cl-, Ca++
• Flushed Skin
• Nausea
• Kussmaul’s respirations

Metabolic Acidosis: Treatment

• Treat underlying cause
• NaHCO3- (Sodium Bicarbonate) based on ABGs only and with caution
• IV Fluids and insulin for DKA (Diabetic Ketoacidosis)
• Dialysis for renal failure

• Rehydrate and monitor I & O

Metabolic Alkalosis

• pH is > 7.45 (Normal is 7.35 – 7.45)
• HCO3- > 26 (Normal is 22 -26)
• Loss of gastric juices
• Vomiting
• Prolonged NG suction
• Antacid overuse
• K+ wasting diuretics

Metabolic Alkalosis: Signs and Symptoms

• Dizziness
• Shallow breathing
• Hypertonic muscles
• Tingling of fingers and toes

• Confusion progressing to lethargy

Metabolic Alkalosis: Treatment

• Treat underlying cause
• Replace K+
• Monitor I & O
• Administer IV fluids with saline
• Monitor neuro status
• Seizure precautions

Questions?