Blood Gas Interpretation Review
for Pandemic
Blood Gases
• Important diagnostic tool
• Reveals:
1. acid-base balance
2. oxygenation status
**arterial gases only**
3. abnormalities of ventilation
2
Acid- base balance
• The body is designed for optimum performance at a
specific pH level
• Cell division
• Metabolism
3
Components of Acid- Base Balance
• pH- measures the bloods acidity
– Normal range 7.35- 7.45
– Overall H+ from both respiratory and metabolic factors
• pCO2- partial pressure of carbon dioxide in the blood
– Normal range 35-45 mmHg
– Snapshot of adequacy of alveolar ventilation
• HCO3- the amount of bicarbonate in the blood
– Normal range 22- 26 mEq/L
4
Acid – Base Balance
Bicarbonate – carbonic acid buffer equation
(H+)(HCO3)
(H2CO3)
(CO2)(H2O)
It’s not that complicated!
pH 1
Acidic
7
Neutral
14
Alkaline
5
Acid – Base Balance
• Lungs
• Respiratory
• CO2 (acid)
• Kidneys
• Metabolic
• HCO3 ( base/alkaline)
6
Making sense of it…
pH
7.35 – 7.45
Respiratory
Metabolic
CO2=Acidosis
HCO3=Acidosis
CO2=Alkalosis
HCO3=Alkalosis
7
Interpretation: 4 steps
• Normal Values
– pH
– pCO2
– HCO3
7.35 – 7.45
35 – 45 mmHg
22 - 26 mEq/L
• Evaluate each component as Acid or Base
8
Step 1…
• Evaluate pH and determine acidosis or
alkalosis
7.35
7.40
7.45
Acid
Acidosis
Normal
Base
Alkalosis
9
Step 2…
• Evaluate pCO2 (respiratory)
35
Base
40
Normal
45
Acid
10
Step 3…
• Evaluate HCO3 (metabolic)
22
24
Acid
Normal
26
Base
11
Step 4…
• Determine which regulatory system is responsible for
the imbalance by checking to see which component
matches the pH.
– If pH and pCO2 match = respiratory
– If pH and HCO3 match = metabolic
12
ABG Analysis
pH
pCO2
HCO3
Resp. Acidosis
A
(<7.35)
A
(>45)
N
(22-26)
Resp.
Alkalosis
B
(>7.45)
B
(<35)
N
(22-26)
Metabolic
Acidosis
A
(<7.35)
N
(35-45)
A
(<22)
Metabolic
Alkalosis
B
(>7.45)
N
(35-45)
B
(>26)
13
Let’s practice…
pH
pCO2
HCO3
7.26
55
23
A
B
A
N
N
B
7.54
N
43
39
7.39
B
41
25
7.51
A
29
7.29
A
40
7.28
61
N
B
N
A
N
N
Respiratory Acidosis
Metabolic Alkalosis
Normal
Respiratory Alkalosis
24
A
Metabolic Acidosis
17
A
Mixed Acidosis
18
14
Compensation
• When an acid – base imbalance exists, over time the
body attempts to compensate.
15
Understanding Compensation
• Uncompensated – the alternate system has not attempted to
adjust (remains within normal range), and the pH remains
abnormal
• Example
– pH
7.30
A
– pCO2
60
A
– HCO3
25
N
Uncompensated Respiratory Acidosis
16
Understanding Compensation
• Partial Compensation – the alternate system is trying to create
a balanced environment and bring the pH back within normal
limits, but hasn’t yet succeeded.
• Example
– pH
7.34A
– pCO2
59 A
– HCO3
28 B
Partially Compensated Respiratory Acidosis
17
Understanding Compensation
• Fully Compensated – the alternate system has adjusted enough
to restore balance and normalize the pH
• Example
– pH
7.36
N (but slightly A)
– pCO2
58
A
– HCO3
31
B
Compensated Respiratory Acidosis
18
Let’s Practice Compensation…
pH
pCO2
B
A
7.51
49
A
7.29
N
A
53
B
7.37
N
25
7.35
65
B
7.46
A
7.34
B
22
A
52
A
HCO3
B
Metabolic Alkalosis partially
compensated
40
N
Respiratory Acidosis uncompensated
22
A
18
28
A
20
B
B
Metabolic Acidosis fully compensated
Respiratory Acidosis fully compensated
Respiratory Alkalosis partially
compensated
Respiratory Acidosis partially
compensated
27
19
A Final Step…
• Determine level of oxygenation
(arterial samples only)
• Normal = 80 – 100 mmHg
• Mild hypoxemia = 60 – 80 mmHg
• Moderate hypoxemia = 40 – 60 mmHg
• Severe hypoxemia = less than 40 mmHg
20
Respiratory Acidosis
• Excessive CO2 retention
• Causes
– Airway obstruction
– Depression of respiratory drive
• Sedatives, analgesics
• Head trauma
– Respiratory muscle weakness resulting from muscle disease
or chest wall abnormalities
– Decreased lung surface area participating in gas exchange
21
Respiratory Acidosis
• Clues
– Confusion, restlessness
– Headache, dizziness
– Lethargy
– Dyspnea
– Tachycardia
– Dysrhythmias
– Coma leading to death
22
Respiratory Acidosis
• Solutions
– Improve ventilation
• Ensure adequate airway; positioning,
suctioning
• Encourage deep breathing and coughing
• Frequent repositioning
• Chest physio/ postural drainage
• Bronchodilators
• Decrease sedation/analgesia
• Oxygen therapy
23
Respiratory Alkalosis
• Excessive CO2 loss due to hyperventilation
• Causes
– CNS injury: brainstem lesions, salicylate overdose, Reye’s
Syndrome, hepatic encephalopathy
– Aggressive mechanical ventilation
– Anxiety, fear or pain
– Hypoxia
– Fever
– Congestive heart failure
24
Respiratory Alkalosis
• Clues
– Light headedness
– Confusion
– Decreased concentration
– Tingling fingers and toes
– Syncope
– Tetany
25
Respiratory Alkalosis
• Solutions
– Decrease respiratory rate and depth
• Sedation/analgesia as appropriate
• Rebreather mask
• Paper bag
• Emotional support/encourage patient to slow
breathing
• Calm, soothing environment
26
Metabolic Acidosis
• Excessive HCO3 loss, or acid gain
• Causes
– Diabetic ketoacidosis
– Sepsis/shock
– Diarrhea (fluid losses below gastric sphincter)
– Renal Failure
– Poison ingestion
– Starvation
– Dehydration
27
Metabolic Acidosis
• Clues
– Stupor
– Restlessness
– Kussmaul’s respirations (air hunger)
– Seizures
– Coma leading to death
28
Metabolic Acidosis
• Solutions
– Replace HCO3 while treating underlying cause
– Monitor intake and output
– Monitor electrolytes, especially K+
– Seizure precautions
29
Metabolic Alkalosis
• HCO3 retention, or loss of extracellular acid,
• Causes
– GI losses above gastric sphincter
• Vomiting
• Nasogastric suction
– Antacids
– Diuretic therapy causing electrolyte loss
30
Metabolic Alkalosis
• Clues
– Weakness, dizziness
– Disorientation
– Hypoventilation
– Muscle twitching
– Tetany
31
Metabolic Alkalosis
• Solutions
– Control vomiting
– Replace GI losses
– Eliminate overuse of antacids
– Monitor intake and output
– Monitor electrolytes
32