Chapter Five - Interpretation of Arterial Blood Gases

Question 1

What are the three components that we are assessing in the interpretation of blood gases? What can indicate the status of these components (how can we mesure them)?

Answer 1

Assessment of:

 Acid-base balance

 pH, PaCO2, HCO3, base excess (BE)

 Oxygenation status

 PaO2, SaO2, CaO2, PvO2

 Adequacy of ventilation

 PaCO2

PaO2: partial pressure of O2, Sa = saturation, Ca = content of oxygen (volume), Pv = partial pressure of veinous oxygen

Question 2

In the metabolism, a lot of acid is produced every day. How is it removed? What is that acidity reflected by? What are the normal, high and low ranges of this mesure called, are what are their limits? What does -emia indicate?

Answer 2

 Metabolism → about 12,000 mEq of acid/day (don’t have to know this number)
 removal by lungs (CO2) – Respiratory buffer
 removal by kidneys (HCO3-) – Renal/Metabolic buffer

 Reflected by pH (H+)
 Normal value 7.35 – 7.45

 Acidemia – pH < 7.35
 Alkalemia – pH > 7.45

We have to know these numbers

Reminder: -emia indicates blood

Question 3

What does the paCO2 reflect? Where does an excess of CO2 come from and what does it do to the blood?

Answer 3

Assessment of Acid-Base Balance

Respiratory Component

CO2 +H2O↔H2CO3 ↔HCO3- +H+

 ↑CO2 - ↑ acidity

 PaCO2

 Reflects effectiveness of ventilation or hypoventilation

Question 4

What are the two components that alter the pH and O2 and CO2 levels? What is the normal PaCO2 value, what is a high value and a low one, and what are they called? What indicates a respiratory acidosis? Alkalosis? Why does an abnormality have to be from a respiratory cause?

Answer 4

We have a respiratory component (related to lung function), and a metabolic component (related to renal function (kidneys))

Respiratory Component

 PaCO2
normal value: 35 – 45 mmHg
Hypercapnia/hypercarbia: PaCO2 > 45 mmHg if

pH < 7.35 = Respiratory acidosis
Hypocapnia/hypocarbia: PaCO2 < 35 mmHg

pH > 7.45 = Respiratory alkalosis

Must be because of respiratory causes because the CO2 was high, or low in the next case

Know these values

Question 5

What can affect the metabolic component of acid-base balance (4)?

Answer 5

Metabolic Component

 Deficit of base or an excess of acid (other than CO2)
 Kidneys excrete or retain HCO3- (base)
 Loss of stomach acid (vomiting)
 Production of metabolic acids (diabetic ketoacidosis)

Question 6

What is the normal HCO3- value, what is a high value and a low one, and what are they called? What indicates a respiratory acidosis? Alkalosis?

Answer 6

Metabolic Component

HCO3-
 normal value: 22 – 26 mEq/L

 alkalosis: HCO3- > 26 mEq/L

pH > 7.45 = Metabolic alkalosis

 acidosis: HCO3- < 22 mEq/L

pH < 7.35 = Metabolic acidosis

Know these numbers

If your bicarb was high and the pH is high, that would be a metabolic alkalosis

If your bicarb was low and the pH is low, that would be a metabolic acidosis

Question 7

What is the method to interpret simple acid-base disorders? What can cause acidosis? Alkalosis?

Answer 7

Interpreting Simple Acid-Base Disorders

Must consider the clinical situation!

The Method:

Assess pH

First we look at the pH, then we look at the mechanism

Second, we have to determine which one of our two mechanisms is making the pH alkalic or acidic

 Acidosis? = pH < 7.35

↑ pCO2 or ↓HCO3

(Respiratory) (Metabolic)

 Alkalosis? = pH > 7.45

↓ pCO2 or ↑HCO3

(Respiratory) (Metabolic)

Question 8

Describe the Acid-Base Disorder:

pH=7.22, pCO2=55, HCO3=25

pH=7.50, pCO2=42, HCO3=33

Answer 8

Top: pH low, acidosis, CO2 high, bicarb normal, so we have a respiratory acidosis

Low: pH is high, CO2 is normal, bicarb is high, so we have a metabolic alkalosis

Question 9

What is compensation? Where are the primary compensation mechanisms? What is a full compensation vs a partial one?

Answer 9

Compensation

When there is an acid-base disturbance the body attempts to bring the pH back to balance. This is called compensation.

Lungs & kidneys are the primary mechanisms

Full compensation:
pH is within normal limits, other values are not

Partial compensation:
pH is trending to normal limits, other values are not

Question 10

Which compensation would be happening in a respiratory acidosis? A metabolic alkalosis? A respiratory alkalosis? Before answering this, what is the pH status and what is responsible for it?

Answer 10

The Method:

Assess Compensation

An attempt to return the pH to within normal limits

 Respiratory Acidosis = pH < 7.35 d/t ↑ pCO2

Compensation = ↑ HCO3
 MetabolicAlkalosis=pH > 7.45 d/t ↑ HCO3

Compensation = ↑ pCO2
 RespiratoryAlkalosis=pH > 7.45 d/t ↓ pCO2

Compensation = ↓ HCO3

Question 11

Interpret these ABGs:

 pH = 7.32
 pCO2 = 32
 HCO3= 18

Describe the acid-base status.

Answer 11

No answer

Question 12

Interpret these ABGs:

 pH = 7.35
 pCO2 = 48
 HCO3= 28

Describe the acid-base status.

Answer 12

pH: Within normal limits, trending towards acidosis

pCO2: Higher

HCO3: A bit higher, so a bit alkalitic

Respiratory acidosis, and the bicarb is trying to balance it out, so just barely fully compensated respiratory acidosis

Question 13

Interpret these ABGs:

 pH = 7.33
 pCO2 = 62
 HCO3= 35

Describe the acid-base status.

Answer 13

No answer.

Question 14

Interpret these ABGs:

 pH = 7.43
 pCO2 = 48
 HCO3= 36

Describe the acid-base status.

Answer 14

No answer.

Question 15

What is respiratory acidosis? What are its respiratory causes? Non-respiratory causes?

Answer 15

Respiratory Acidosis (pH \< 7.35) (low ventilation)
 ↓ alveolar ventilation relative to CO2 production

 Respiratory Causes

 Acute or severe airways obstruction

 Overwhelming pulmonary edema (alveoli get filled up with fluid)

 Nonrespiratory Causes

Drug overdose
Spinal cord injury

Neuromuscular disease

Brain injury
 Trauma to thoracic cage

Question 16

What are respiratory acidosis’ signs and symptoms?

Answer 16

Respiratory acidosis

Signs & Symptoms

• Dyspnea (shortness of breath), respiratory distress, ± shallow breathing

• Headache, restlessness, confusion
• Tachycardia, dysrhythmias

We have to know those signs and symptoms

Question 17

What is respiratory alkalosis? What is its respiratory cause? Non-respiratory causes?

Answer 17

Respiratory Alkalosis

 ↑ alveolar ventilation relative to CO2 production

 Respiratory Causes

 hypoxemia

 Nonrespiratory Causes

 pain

 anxiety

Question 18

What are respiratory alkalosis’ signs and symptoms?

Answer 18

Respiratory alkalosis (pH >7.45)

Signs & Symptoms

• Light-headedness, numbness & tingling, confusion, poor concentration, blurred vision
• Dysrhythmias

Question 19

What is metabolic acidosis? What are its causes? What is an example of compensation in this case?

Answer 19

Metabolic Acidosis (pH < 7.35)

Plasma HCO3 is below normal

 Insufficient production or xs loss of buffer

 Diarrhea (diarrhea is a base, so you are losing some)

 Renal disease
Increased production of metabolic acid

 Lacticacidosis
 Ketoacidosis (most commonly seen with diabetes)

 Ingestion of certain toxins (methanol) (Certain infections (anaerobic bacteria, that produce lactic acid))

Compensation
 Rapid, deep respiration – Kussmaul Respiration

Question 20

What is metabolic acidosis’ signs and symptoms?

Answer 20

Metabolic acidosis

Signs & Symptoms

• Kussmaul respiration
• Headache, restlessness, confusion, lethargy, stupor, coma
• Dysrhythmias

Question 21

What is metabolic alkalosis? What are its causes?

Answer 21

Metabolic Alkalosis (pH > 7.45)

Plasma HCO3 is above normal

 HCO3 accumulates in the plasma or xs H+ lost

 Persistent vomiting
 Diuretics (medications that make us pee)
 N-Gsuction

 Steroid therapy
 xs sodium bicarb
 hypokalemia/hypochloremia

Question 22

What is metabolic alkalosis’ signs and symptoms?

Answer 22

Metabolic alkalosis

Signs & Symptoms

• dizziness, lethargy, disorientation, seizures, coma

• Muscle weakness, twitching, cramps, tetany
• Nausea, vomiting, respiratory depression

Question 23

Mr. Frank is a 60 year-old with pneumonia. He is admitted with dyspnea, fever, and chills. His blood gas is below:

pH = 7.28 pCO2 = 56 pO2 = 70 HCO3 = 25 SaO2 =89%

What is your interpretation?

Answer 23

Acid-base assessment: Uncompensated respiratory acidosis

Oxygenation:
Hypoxemia
(This is probably d/t V/Q mismatch as alveoli will be filled with pus and therefore gasexchangeisimpaired.Iwouldn’texpect you to know this part as you haven’t covered pneumonia yet.)

Question 24

Ms. Strauss is a 24 year-old college student. She has a history of Crohn’s disease and is complaining a of a four day history of bloody- watery diarrhea. A blood gas is obtained to assess her acid/base balance:

pH = 7.28 pCO2 = 43 pO2 = 88 HCO3 = 20 SaO2 = 96%

What is your interpretation?

Answer 24

Acid-base assessment: Uncompensated metabolic acidosis

Oxygenation:
pO2 is lower than expected. SaO2 is within normal limits.
No obvious cause for this as the person is young with no history of lung disease. We would have to know more about her S&S to investigate this further.

Question 25

Mrs. Lauder is a thin, elderly-looking 61 year-old COPD patient. She has an ABG done as part of her routine care in the pulmonary clinic. The results are as follows:

pH = 7.37 pCO2 = 63 pO2 = 58 HCO3 = 35 SaO2 = 89%

What is your interpretation?

Answer 25

Acid-base assessment:
Compensated respiratory acidosis
CO2 is high. Have started to retain HCO3 pH is within normal limits.

Oxygenation: Hypoxemia!