Acid-Base Balance and Arterial Blood Gases
The body maintains a balance between acids and bases by the interplay between:
Acid Production
Acid Buffering
Of Carbonic acid (H2CO3) and
Metabolic acids (any acid that
is not carbonic acid)
Acid Excretion
Via buffers - pairs of chemicals that
work together to maintain normal
pH of body fluids; primary regulator
Acids release hydrogen (H+) ions
The more H+ present = more acidic
Bases (alkaline substances ) take up H+ ions
Degree of acidity = pH
pH 1.0 = very acidic; pH 14.0 = very alkaline (base); pH 7.0= neutral
Acidosis
Acid-Base Imbalance
Results from an alteration in the ratio
of acids and bases
Signifies an underlying health problem
Classified as either respiratory or
metabolic
Respiratory - from retention or excess
of CO2 which affect H2CO3 (carbonic
acid) levels
Metabolic - deals with HCO3
(bicarbonate) levels
Via lungs (excrete carbonic acid)
and kidneys (excrete metabolic
acids)
ABG values provide
objective info about a
patient's acid-base
status and underlying
cause.
Respiratory Acidosis -
A condition where
the blood is too
acidic
2 Types
Alkalosis
What: Excess carbonic acid (H2CO3)
Cause: Hypoventilation/Bradypnea (<12 bpm) due to:
Mechanical hypoventilation
COPD
Pulmonary edema
Sedatives
Chest wall abnormality
Severe pneumonia
Respiratory muscle weakness
Atelectasis
Labs:
pH < 7.35
CO2 > 45
Sign/Symptoms:
RR < 12 bpm
Headache
Lethargy
Coma
Confusion
Low BP
Dizziness
Warm, flushed skin
Compensation: Kidneys conserve bicarbonate and secrete
H+ into urine
Oxygen supplementation
Encourage deep breathing
Suction if pneumonia
Monitor K+ levels for hyperkalemia
May need endotracheal intubation
Bronchodilators
Antibiotics
Respiratory Alkalosis 2 Types
↓carbonic acid (H2CO3)
(↓CO2)
↑
Metabolic Alkalosis - bicarbonate (HCO3)
pH =
Respiratory Acidosis
Treatment/Interventions
↓
Metabolic Acidosis - bicarbonate (HCO3)
pH =
A condition where
the blood is too
basic (alkaline)
↑carbonic acid (H2CO3)
(↑CO2)
Lab Values
to Know!
pH
CO2
HCO3
Acidic
Normal
Alkaline
< 7.35
7.35 - 7.45
> 7.45
22 - 26
> 26
> 45
< 22
35 - 45
< 35
Respiratory Alkalosis
What: Deficit of carbonic acid (H2CO3)
Cause: Hyperventilation/Tachypnea (<20 bpm) due to:
Anxiety, fear, pain, exercise, fever
Stimulated respiratory center (e.g. stroke, brain injury)
Liver failure
Mechanical hyperventilation
Labs:
pH > 7.45
Treatment/Interventions
CO2 < 35
Instruct on rebreathing into
Sign/Symptoms:
paper bag and re-breather mask
RR > 20 bpm
Monitor K+ levels for
Lethargy
hypokalemia
Tetany, numbness
If on mech vent monitor for
Dysrhythmias
hyperventilation
Muscle cramps, N/V
+Chvostek's sign
Compensation: Kidneys secrete bicarbonate
Acid-Base Balance and Arterial Blood Gases
Metabolic Acidosis
Metabolic Alkalosis
What: Loss of bicarbonate or acid accumulation due to:
acid production OR
acid excretion OR
Loss of bicarbonate
Cause: To determine cause, look at anion gap:
↑
↓
Anion gap: The difference between anions and cations.
Usually calculated by MD; normal = 10 - 14 mEq/L
High Anion Acidosis (>14 mEq/L): DKA, lactic acidosis,
aspirin toxicity, renal failure, shock
Normal Anion Acidosis (10 - 14 mEq/L): diarrhea,
ostomies or fistula drainage, Diamox (diuretic)
Labs:
Treatment/Interventions
pH < 7.35
HCO3 < 22
Monitor for need to intubate
Sign/Symptoms:
Monitor K+ levels
Kussmaul respirations
Check neuro status
Confused, weak
IV insulin if DKA
BP
Bicarbonate may be needed
N/V
May need dialysis
↓
Compensation: Kidneys will attempt to excrete excess acid
and lungs will CO2 excretion (Kussmaul respirations)
1
4
Steps of ABG Interpretation
Determine if values are normal/acidic/alkaline
pH
2
↑
CO2
HCO3
Acidic
Normal
Alkaline
< 7.35
7.35 - 7.45
> 7.45
22 - 26
> 26
> 45
< 22
35 - 45
< 35
Analyze CO2 to see if problem is respiratory
↑CO2 + ↓pH = respiratory acidosis
↓CO2 + ↑pH = respiratory alkalosis
R
O
M
E
espiratory
pposite
etabolic
qual
What: Excess bicarbonate due to:
Loss of acid
Gain in bicarbonate
Cause:
Hypokalemia
Vomiting
Excess NaHCO3 intake
Nasogastric suctioning
Mineralocorticoid use
Diuretics
Labs:
Treatment/Interventions
pH > 7.45
HCO3 >26
Depends on cause:
Sign/Symptoms:
Vomiting: give
Lethargy
antiemetic
Confusion
Diuretics: discontinue
Headache
Diamox may be ordered
Tachycardia
Monitor:
Dysrhythmias
K+ and Cl- levels
N/V
For signs of respiratory
Tetany
distress
Tremors
Tingling in fingers/toes
Hypoventilation
Compensation: Kidneys excrete bicarbonate and
respiratory rate slows (hypoventilation)
3 Analyze HCO3 to see if problem is metabolic
↑HCO3 + ↑pH = metabolic alkalosis
↓HCO3 + ↓pH = metabolic acidosis
4 Determine if patient is compensating or not
Look at the component (CO2 or HCO3) that is NOT the
cause of the primary disturbance
If that component is moving in the opposite direction,
the body is attempting to compensate; example:
pH 7.33 (acidic)
respiratory acidosis
CO2 55 mmHg (high)
HCO3 36 mEq/L (high/alkalotic) = compensation
ROME Method for ABG Interpretation
If the CO2 and pH levels are opposite = Respiratory
If the HCO3 and pH levels are the same = Metabolic