Chapter 16

Arterial Blood Gas Case Studies

Case 1 NARCOTIC OVERDOSE

A 25-year-old man residing at sea level is brought to the emergency department after a narcotic overdose and possible aspiration. He is placed on a pulse oximeter, and arterial blood gases are drawn.

Arterial Blood Gases

FIO2	0.21
pH	7.45
PaCO2	36 mm Hg
[HCO3]	23 mEq/L
PaO2	145 mm Hg

Vital Signs

Pulse	55/min
Blood pressure (BP)	100/60
Temperature	37° C
Respiration rate (RR)	8/min

Pulse Oximetry

SpO2

80%

1A Questions

1. Is the pulse oximeter reading congruent with the PaO₂?

2. Which of the two readings must be wrong?

3. (Air in the sample/Venous sampling) could explain these results.

Repeat blood gases are as follows:

Arterial Blood Gases

FIO2	0.21
pH	7.21
PaCO2	64 mm Hg
[HCO3−]	24 mEq/L
PaO2	48 mm Hg

Vital Signs

Pulse	55/min
BP	100/60
Temperature	37° C
RR	8/min

Pulse Oximetry

SpO2

80%

1B Questions

1. Classify the arterial blood gas.

2. What are the four common causes of hypoxemia in hospitalized patients?

3. Hypoventilation (is/is not) a cause of hypoxemia in this patient.

4. What index can be used to differentiate simple hypoventilation from hypoven- tilation in conjunction with increased physiologic shunting?

5. Write the clinical form of the alveolar air equation while breathing room air.

6. What is this patient’s P(A—a)O₂ on room air?

7. This patient (does/does not) have abnormal increased physiologic shunting.

8. Treatment of this patient’s acid-base status may require (sodium bicarbonate/mechanical ventilation).

Case 2 UNEXPLAINED ACIDEMIA

A 46-year-old woman who is comatose and has an unknown history is admitted to the emergency department. Arterial blood gases and laboratory data are as follows:

Arterial Blood Gases

FIO2	0.21
pH	7.22
PaCO2	25 mm Hg
[HCO3]	10 mEq/L
PaO2	96 mm Hg
SaO2	96%

Vital Signs

Pulse	118/min
BP	170/110
Temperature	37° C
RR	18/min

Plasma Electrolytes

Na+	137 mEq/L
CO2	12 mEq/L
Cl−	104 mEq/L
K+	5.5 mEq/L

Bloodwork

Glucose	110 mg/dL
Creatinine	11 mg/dL
BUN	130 mg/dL
Lactate	12 mg/dL

2 Questions

1. Classify the arterial blood gas.

2. The anion gap is (high/low/normal).

3. The patient (appears/does not appear) to be hypoxic.

4. The lactate is (normal/increased).

5. The glucose is (normal/increased).

6. The creatinine is (normal/increased).

7. The BUN is (normal/increased).

8. The [K⁺] is (normal/increased).

9. What is the cause of the metabolic acidosis?

10. The hypocapnia appears to be (compensatory/a primary acid-base problem).

Case 3 GASTROINTESTINAL DISTURBANCE

A woman is admitted to the hospital with salmonella enteritis and a history of severe diarrhea for about 10 days before admission. Vital signs, blood gases, and electrolytes taken at admission are shown below:

Arterial Blood Gases

FIO2	0.21
pH	7.15
PaCO2	15 mm Hg
[HCO3]	5 mEq/L
PaO2	96 mm Hg
SaO2	93%

Vital Signs

Pulse	112/min
BP	100/70
Temperature	37° C
RR	24/min

Plasma Electrolytes

Na+	134 mEq/L
CO2	7 mEq/L
Cl−	113 mEq/L
K+	3.2 mEq/L

3 Questions

1. Classify the arterial blood gas.

2. The anion gap is (high/low/normal).

3. The plasma chloride is (high/low/normal).

4. What is the cause of the metabolic acidosis?

5. The plasma [K⁺] is (high/low/normal).

6. What is the likely cause of the potassium disturbance?

7. The diuretic (Lasix/acetazolamide) could cause an acid-base disturbance similar to this, but the acidemia is usually less severe.

8. (Azotemic renal failure/Renal tubular acidosis) may cause a normal anion gap metabolic acidosis.

Case 4 STATUS ASTHMATICUS

A 17-year-old boy with a history of asthma has been continuously short of breath for approximately 2 days. He enters the hospital wheezing and with air hunger. Arterial blood gases and vital signs are as follows:

Arterial Blood Gases

FIO2	0.21
pH	7.35
PaCO2	22 mm Hg
[HCO3]	12 mEq/L
PaO2	41 mm Hg
SaO2	77%

Vital Signs

Pulse	132/min
BP	150/90
Temperature	37° C
RR	28/min

4A Questions

1. Classify the blood gas according to the basic rules for blood gas classification discussed in Chapter 2.

2. Are there any signs to suggest that this is a mixed acid-base disturbance?

3. Do these values fall under the band on the acid-base map for simple metabolic acidosis? (See acid-base map in Chapter 14.)

4. Reclassify the acid-base status.

5. What is the cause of the respiratory alkalosis?

6. What is the probable cause of the metabolic acidosis?

7. What therapy is indicated?

8. Is it important to administer a low concentration of oxygen to this patient?

9. What could explain the elevated blood pressure, pulse, and RR in this patient?

Oxygen therapy and aerosol therapy with bronchodilators are administered, and the following blood gases are obtained about 3 hours later:

Arterial Blood Gases

FIO2	0.5
pH	7.47
PaCO2	24 mm Hg
[HCO3]	16 mEq/L
PaO2	55 mm Hg
SaO2	92%

Vital Signs

Pulse	120/min
BP	140/90
Temperature	37° C
RR	22/min

4B Questions

1. Classify the arterial blood gas.

2. Lactate (can/cannot) be quickly metabo- lized in the presence of adequate oxygen.

3. The low bicarbonate concentration at this point is most likely due to (compensation/lactic acidosis).

4. The current values (do/do not) fall within the band for simple respiratory alkalosis on the acid-base map.

The patient’s wheezing continued to be severe for the next 48 hours. He looked very tired at this point, and arterial blood gases were as follows:

Arterial Blood Gases

FIO2	0.5
pH	7.32
PaCO2	35 mm Hg
[HCO3]	17 mEq/L
PaO2	52 mm Hg
SaO2	83%

Vital Signs

Pulse	135/min
BP	150/100
Temperature	37° C
RR	18/min (wheezing is less audible)

4C Questions

1. Classify the arterial blood gas.

2. It (can/cannot) be assumed that the patient has almost completely recovered.

3. The decreased wheezing (is/is not) clearly a positive sign.

The next day, the patient appears to be more comfortable and under much less stress. Arterial blood gases are as follows:

Arterial Blood Gases

FIO2	0.28
pH	7.32
PaCO2	40 mm Hg
[HCO3]	20 mEq/L
PaO2	90 mm Hg
SaO2	96%

Vital Signs

Pulse	90/min
BP	120/80
Temperature	37° C
RR	14/min

4D Questions

1. Classify the arterial blood gas.

2. What is the most likely cause of the metabolic acidemia at this time?

Case 5 ACUTE RESPIRATORY ACIDEMIA

A 34-year-old man involved in an automobile accident arrives in the emergency department with severe head trauma. Arterial blood gases, vital signs, and pulse oximetry readings are as follows:

Arterial Blood Gases

FIO2	0.21
pH	7.10
PaCO2	95 mm Hg
[BE]	− 5 mEq/L
[HCO3]	29 mEq/L
PaO2	60 mm Hg

Vital Signs

Pulse	60/min
BP	100/50
Temperature	37° C
RR	12/min

Pulse Oximetry

SpO2

78%

5 Questions

1. What is the normal SaO₂ at a PaO₂ of 60 mm Hg?

2. Why is the SpO₂ only 78% in this patient despite a PaO₂ of 60 mm Hg?

3. Do these blood gas values fall in the band for acute respiratory acidosis on the acid-base map?

4. Does the plasma bicarbonate concen- tration of 29 mEq/L represent renal compensation?

5. Is it possible for a blood gas to be correct when the base excess of the blood is decreased and the actual bicarbonate is increased?

6. How much will the plasma bicarbonate increase acutely for every 10-mm Hg increase in PaCO₂ due to the hydrolysis effect?

7. What supportive treatment is indicated for this patient’s acid-base status?

Case 6 NASOGASTRIC SUCTION

A nasogastric tube was placed in a 32-year-old woman with intestinal obstruction. For several days, large amounts of fluid were suctioned from the nasogastric tube. Arterial blood gases and electrolytes were as follows:

Arterial Blood Gases

FIO2	0.21
pH	7.53
PaCO2	49 mm Hg
[HCO3]	39 mEq/L
PaO2	92 mm Hg
SaO2	98%

Vital Signs

Pulse	105/min
BP	110/70
Temperature	37° C
RR	18/min

Plasma Electrolytes

Na+	142 mEq/L
CO2	42 mEq/L
Cl−	86 mEq/L
K+	3.2 mEq/L

6 Questions

1. Classify the arterial blood gas.

2. Do the values fall within the band on the acid-base map for simple metabolic alkalosis?

3. What is the cause of the metabolic alkalosis?

4. Metabolic alkalosis is usually associated with (hyperchloremia/hypochloremia).

5. Hypokalemia (is/is not) common with a loss of gastric fluid.

6. Loss of body fluids (is/is not) an important aspect of this type of metabolic alkalosis.

7. What is the appropriate treatment for this type of metabolic alkalosis?

Case 7 UNEXPLAINED ALKALEMIA

A 28-year-old woman in her eighth month of pregnancy is admitted to the hospital after having severe vomiting for several days. Arterial blood gases, vital signs, and electrolytes are as follows:

Arterial Blood Gases

FIO2	0.21
pH	7.58
PaCO2	31 mm Hg
[HCO3]	28 mEq/L
PaO2	65 mm Hg
SaO2	96%

Vital Signs

Pulse	110/min
BP	130/80
Temperature	37° C
RR	18/min

Plasma Electrolytes

Na+	130 mEq/L
CO2	32 mEq/L
Cl−	86 mEq/L
K+	3.1 mEq/L

7 Questions

1. Classify the arterial blood gas.

2. What is the likely cause of the metabolic alkalosis?

3. What is the likely cause of the respiratory alkalosis?

4. What mechanisms are responsible for hyperventilation during late pregnancy?

Case 8 OXYGENATION DISTURBANCE

A 4-month-old infant is admitted to the emergency department with cyanosis and mild cardiopulmonary distress. The family was from a rural area, and the infant had been receiving formula prepared with water taken from a well. Arterial blood gases, before the infant was given oxygen, were drawn and the blood specimen was noted to be dark. The blood gas results, pulse oximetry readings, and vital signs were as follows:

Arterial Blood Gases

FIO2	0.21
pH	7.30
PaCO2	28 mm Hg
[BE]	−12 mEq/L
PaO2	105 mm Hg

Vital Signs

Pulse	140/min
BP	140/100
Temperature	37° C
RR	40/min

Pulse Oximetry

SpO2

94%

8A Questions

1. Does the pulse oximeter reading and PaO₂ concur with the clinical picture of cyanosis and the appearance of a dark blood sample?

2. Should another blood gas sample be drawn?

The child is then placed on oxygen and repeat arterial blood gases are drawn. Surprisingly, when a small amount of the sample accidentally escapes from the syringe, the blood appears rusty brown or chocolate in color. Blood gas results and vital signs are as follows:

Arterial Blood Gases

FIO2	0.5
pH	7.28
PaCO2	28 mm Hg
[HCO3]	13 mEq/L
PaO2	240 mm Hg

Vital Signs

Pulse	140/min
BP	130/90
Temperature	37° C
RR	38/min

Pulse Oximetry

SpO2

90%

8B Questions

1. What is the predicted normal PaO₂ on FIO₂ of 0.5?

2. Does the infant appear to have abnormal shunting?

3. Could the cyanosis and metabolic acidosis be due to hypoxia?

4. What type of oxygenation disturbance could be associated with cyanosis despite a normal PaO₂ and rust- colored blood on exposure of the blood to air?

5. Is there normally any methemoglobin present in the blood?

6. What is the normal percentage of methemoglobin in the blood?

7. Are infants more likely to have this particular disorder?

8. How can the level of methemoglobin be reduced?

9. Why is the pulse oximeter reading in the normal range?

10. Methemoglobin is (oxygenated/oxidized).

11. The definitive diagnosis would be made via (co-oximetry, electrolytes)?

Case 9 DIABETIC PATIENT

A 32-year-old woman with a history of diabetes mellitus is admitted to the hospital with lethargy and confusion. Current arterial blood gases, laboratory data, and vital signs are shown below:

Arterial Blood Gases

FIO2	0.21
pH	7.04
PaCO2	15 mm Hg
[HCO3]	10 mEq/L
PaO2	125 mm Hg
SaO2	95%

Vital Signs

Pulse	118/min
BP	90/50
Temperature	37° C
RR	32/min

Plasma Electrolytes

Na+	136 mEq/L
CO2	7 mEq/L
Cl−	95 mEq/L
K+	6.3 mEq/L

Bloodwork

Glucose	750 mg/dL
Acetoacetic acid	250 mg/dL
Blood urea nitrogen (BUN)	38 mg/dL
Lactate	30 mg/dL

9A Questions

1. Classify the arterial blood gas.

2. Why is the PaO₂ greater than 100 mm Hg on room air?

3. What is the maximum PaO₂ that can be achieved during hyperventilation while breathing room air?

4. This is a (high/normal) anion gap metabolic acidosis.

5. The primary cause of the metabolic acidosis is (lactic acidosis/ketoacidosis).

6. It is (expected/unexpected) to have some accumulation of lactic acid during ketoacidosis.

7. State the two ketoacids.

8. The concentration of acetoacetic acid is (normal/high).

9. Severe (hyperglycemia/hypoglycemia) is common during diabetic ketoacidosis and causes (polyuria/oliguria).

10. Hyperkalemia is (unexpected/expected) in ketoacidosis.

11. Dehydration is (common/uncommon) in ketoacidosis. Explain this.

12. The deep, rapid, breathing pattern observed in ketoacidosis is called ______ breathing.

13. Due to hypovolemia in ketoacidosis, blood pressure is frequently (high/low), and BUN is frequently (decreased/increased).

14. Ketosis and ketoacidosis are a result of increased (carbohydrate/protein/fat) metabolism.

15. The fruity odor often present on the breath during ketoacidosis is a result of (acetone/urea).

16. Sodium bicarbonate treatment (is/is not) recommended for this patient.

Follow-up

The patient was treated with bicarbonate, insulin, and fluids. Blood gases and electrolytes were drawn 6 hours later and were as follows:

Arterial Blood Gases

FIO2	0.21
pH	7.54
PaCO2	32 mm Hg
[BE]	4 mEq/L
PaO2	97 mm Hg
SaO2	98%

Plasma Electrolytes

Na+	136 mEq/L
CO2	33 mEq/L
Cl−	90 mEq/L
K+	3.1 mEq/L

9B Questions

1. Classify the arterial blood gas.

2. What mechanisms may be responsible for the metabolic alkalosis?

3. What mechanism is most likely responsible for the continued hyperventilation?

Case 10 ACUTE EXACERBATION OF CHRONIC OBSTRUCTIVE PULMONARY DISEASE

A 62-year-old (60 kg) man with a history of chronic bronchitis is examined in the emergency department for shortness of breath and expectoration of large amounts of yellow sputum. The following blood gases, vital signs, bloodwork, and electrolytes were drawn in the emergency department:

Arterial Blood Gases

FIO2	0.21
pH	7.23
PaCO2	80 mm Hg
[HCO3]	34 mEq/L
PaO2	39 mm Hg
SaO2	52%

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Related posts:

1. Accuracy Check and Metabolic Acid-Base Indices
2. Regulation of Acids, Bases, and Electrolytes
3. Hypoxia: Assessment and Intervention
4. Oxygen Transport and Internal Respiration

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