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Dr. Christopher Schneller

Attending Physician, Pediatric Critical Care Dell Children’s Medical Center of Central Texas

Assistant Professor of Pediatrics, Dell Medical School

Associate Program Director, Pediatric Critical Care Fellowship

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Educational Objectives

  • Interpret arterial blood gases from pediatric cases via the 5 step approach
  • Understand the benefits of both arterial and venous blood gases
  • Learn the proper places and technique to obtain arterial blood gases
  • Recognize all components reported on PICU bedside blood gases
  • Calculate the anion gap for metabolic acidosis
  • Review how blood gases can help you evaluate oxygen delivery and consumption
  • Discuss the delta-delta equation
  • Understand the role of albumin in acid-base evaluations

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Obtaining Arterial Blood Gas

  • Best locations for an arterial stick:
    • Radial- easiest, closest to skin
    • Ulnar*
    • Brachial*
    • Dorsalis Pedis
    • Femoral*

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  • Before you graduate residency, you must feel comfortable sticking someone for a blood gas- practice when you can!!

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The typical PICU Blood Gas…

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Arterial vs. Venous Blood Gas

  • Arterial blood gas is almost always preferred- why?
    • More accurate pH
    • Lactate value more accurate
    • paO2 better representation for lung disease
  • What can a central venous saturation help you determine?

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DO2 (Oxygen Delivery) = Cardiac Output x Ca02

VO2 (Oxygen Consumption)= DO2 x (SaO2-SvO2*)

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Strategies to improve DO2/VO2?

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1. Examine the pH

2.

3.

4.

5.

5-step approach to ABG Interpretation

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1. Examine the pH

2. Determine the primary disorder

3.

4.

5.

5-step approach to ABG Interpretation

Acidemia

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Low HCO3-

Metabolic Acidosis

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High pCO2

Respiratory Acidosis

Alkalemia

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High HCO3-

Metabolic Alkalosis

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Low pCO2

Respiratory Alkalosis

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1. Examine the pH

2. Determine the primary disorder

3. Calculate the anion gap

4.

5.

5-step approach to ABG Interpretation

Anion Gap= [Na+] - ([Cl−] + [HCO−3])

Normal AG = 8-16 mEq/L

​

*If the AG is significantly elevated, metabolic acidosis is present- REGARDLESS of pH or HCO−3

*Remember to adjust the anion gap for critical patients with hypoalbuminemia:

​

Anion Gap + 2.5[Normal Albumin (4) – Observed Albumin]

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*When an increased AG metabolic acidosis is present, use the AG - HCO3

-Change > 6, Additional normal anion gap acidosis is present

-Change < -6, Additional metabolic alkalosis is present

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1. Examine the pH

2. Determine the primary disorder

3. Calculate the anion gap

4. Assess for compensation

5.

5-step approach to ABG Interpretation

Quick and dirty rule to assess compensation:

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*When compensation is present, the pCO2 and HCO3 change in the same direction!

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*Failure of compensation often represents an additional disorder

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i.e. Think DKA!

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1. Examine the pH

2. Determine the primary disorder

3. Calculate the anion gap

4. Assess for compensation

5. Define the disorder and construct your differential diagnosis

5-step approach to ABG Interpretation

Ingestion

Lung disease

Medication

Exposure

Metabolic Disease

Hypo-

ventilation

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Case 1

A 16 year old girl with a history of poorly controlled asthma presents with worsening SOB. She is in moderate respiratory distress. ABG indicates a pH of 7.54, pO2 60, pCO2 29. A chemistry panel demonstrates a sodium of 138, chloride 103, and bicarbonate 25.

Which of the following acid-base abnormalities is present in this patient?�

  1. Respiratory acidosis
  2. Respiratory alkalosis
  3. Metabolic acidosis
  4. Metabolic alkalosis

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Case 1

A 16 year old girl with a history of poorly controlled asthma presents with worsening SOB. She is in moderate respiratory distress. ABG indicates a pH of 7.54, pO2 60, pCO2 29. A chemistry panel demonstrates a sodium of 138, chloride 103, and bicarbonate 25.

Which of the following acid-base abnormalities is present in this patient?�

  1. Respiratory acidosis
  2. Respiratory alkalosis
  3. Metabolic acidosis
  4. Metabolic alkalosis?

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Case 1 Explained

  • H+ + HCO3- H2CO3 CO2 + H2O

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  • Any process that increases pCO2 or decreases bicarbonate will shift the equation to the left and produce more protons which lowers pH
  • Any process that increases bicarbonate or decreases pCO2 will shift the equation to the right to generate an alkalosis�
  • This patient has yet to have significant respiratory failure from her asthma exacerbation and has yet to demonstrate metabolic compensation. She likely has forced and a prolonged respiratory phase eliminating CO2

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Case 2

A three-month-old infant develops increased work of breathing. His medical history is significant for prematurity and CLD. His home meds include spironolactone and hydrochlorothiazide. A capillary gas shows a pH of 7.37, pCO2 70, Na 136, Cl 88, and HCO3 37.

Which of the following describes the acid-base abnormality in this patient?

  1. Metabolic alkalosis with respiratory compensation
  2. Metabolic acidosis with respiratory compensation
  3. Respiratory acidosis with metabolic compensation
  4. Respiratory alkalosis with metabolic compensation

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Case 2

A three-month-old infant develops increased work of breathing. His medical history is significant for prematurity and CLD. His home meds include spironolactone and hydrochlorothiazide. A capillary gas shows a pH of 7.37, pCO2 70, Na 136, Cl 88, and HCO3 37.

Which of the following describes the acid-base abnormality in this patient?

  1. Metabolic alkalosis with respiratory compensation
  2. Metabolic acidosis with respiratory compensation
  3. Respiratory acidosis with metabolic compensation
  4. Respiratory alkalosis with metabolic compensation

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Case 2 Explained

There is both a metabolic alkalosis and a respiratory acidosis, right?

Diuretics OR Chronic Lung Disease could be the primary player….so…

​

A few things to note:

  1. The pH is below 7.40- primary process is more likely to be an acidosis as the body doesn’t overcompensate!
  2. The body’s response to a primary process will halt once you come close to a normal physiologic pH and compensation will remain incomplete
  3. Respiratory compensation for metabolic processes is rapid but renal compensation for a respiratory process takes hours to days to complete- an infant with hypercapnia and a normal bicarbonate level would be much more concerning!!

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Case 3

After being left unsupervised, a two-year-old boy is found by his parents in the family garage with open bottles. He is minimally responsive, drooling, and has the following labs values: pH 6.83, pCO2 40, sodium 142, chloride 106,and bicarbonate 6.

Which of the following defines the acid-base disorder in this patient?

  1. Metabolic acidosis
  2. Respiratory acidosis
  3. Mixed metabolic and respiratory acidosis

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Case 3

After being left unsupervised, a two-year-old boy is found by his parents in the family garage with open bottles. He is minimally responsive, drooling, and has the following labs values: pH 6.83, pCO2 40, sodium 142, chloride 106,and bicarbonate 6.

Which of the following defines the acid-base disorder in this patient?

  1. Metabolic acidosis
  2. Respiratory acidosis
  3. Mixed metabolic and respiratory acidosis

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Case 3 Explained

  • Failed compensation→ Additional disorder!
  • The primary disorder, metabolic acidosis, should drive an increase in minute ventilation in order to increase the pH
  • In patients with a metabolic acidosis, the completeness of compensation can be determined by calculating the expected paCO2 with….

​

Winter’s Formula: pCO2 = (1.5 x HCO3-) +8

i.e. this patient’s pCO2= (1.5 x 6) + 8= 17…..not 40!

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Case 3 Explained Further

  • You also should have calculated the anion gap! �
  • Remember: �Anion Gap= [Na+] - ([Cl−] +[HCO−3]) ��142 – (106+6🡪112)= 30�
  • When an another acid is added to the blood and bicarbonate losses are not driving the acidosis, the body cannot compensate by retaining chloride in the kidneys�
  • This patient problem ingested antifreeze (ethylene glycol) leading to his AG acidosis- how could we know for sure?

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Case 3 Explained Further

  • Calculate the Osmolar Gap= Measured laboratory osmolality – Calculated Osmolality

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  • If measured osmolality is >15-20 higher than the calculated osmolality then an osmolar gap is present and can likely confirm the presence of additional osmotically active solutes!

​

Serum osmolality=

2Na + BUN/2.8 + Glucose/18

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Case 4

Which of the following is the most complete description of the acid-base disturbance in this patient?

  1. Normal AG metabolic acidosis with compensatory respiratory alkalosis
  2. Increased AG metabolic acidosis with compensatory respiratory alkalosis
  3. Respiratory alkalosis with compensatory metabolic acidosis

A child with biliary atresia awaiting a liver transplant has the following lab results: Na 130, Cl 100, Bicarbonate 16, Creatinine 0.4, Albumin 1.8. Capillary gas shows a pH of 7.31 and pCO2 of 32

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Case 4

Which of the following is the most complete description of the acid-base disturbance in this patient?

  1. Normal AG metabolic acidosis with compensatory respiratory alkalosis
  2. Increased AG metabolic acidosis with compensatory respiratory alkalosis
  3. Respiratory alkalosis with compensatory metabolic acidosis

A child with biliary atresia awaiting a liver transplant has the following lab results: Na 130, Cl 100, Bicarbonate 16, Creatinine 0.4, Albumin 1.8. Capillary gas shows a pH of 7.31 and pCO2 of 32

​

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Case 4 Explained

  • Correct for hypoalbuminemia!
  • Albumin is the primary component of the anion gap and it carries a net negative charge-> therefore in hypoalbuminemic states, the calculated AG may be much lower than it actually is!

Calculated Anion Gap + 2.5[Normal Albumin (4) – Observed Albumin]

or

(130-100-16=14) + 2.5[4-1.8] = 20

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Case 5

A seven-year-old girl with IDDM develops fever, vomiting, diarrhea, and worsening glycemic control. Lab evaluation shows the following: Na 144, Cl 118, Bicarb 6, Glucose 495. An arterial pH is 7.14, pCO2 15

Which of the following is the best description of this patient’s acid base disorder?

  1. Increased AG metabolic acidosis with respiratory compensation
  2. Normal AG metabolic acidosis with respiratory compensation
  3. Increased AG metabolic acidosis and normal AG metabolic acidosis with respiratory compensation
  4. Increased AG metabolic acidosis with respiratory acidosis

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Case 5

A seven-year-old girl with IDDM develops fever, vomiting, diarrhea, and worsening glycemic control. Lab evaluation shows the following: Na 144, Cl 118, Bicarb 6, Glucose 495. An arterial pH is 7.14, pCO2 15

Which of the following is the best description of this patient’s acid base disorder?

  1. Increased AG metabolic acidosis with respiratory compensation
  2. Normal AG metabolic acidosis with respiratory compensation
  3. Increased AG metabolic acidosis and normal AG metabolic acidosis with respiratory compensation
  4. Increased AG metabolic acidosis with respiratory acidosis

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Case 5 Explained

  • The presence of an anion gap acidosis in a child with DKA for example does not preclude the existence of a simultaneous non-gap acidosis or alkalosis!
  • In this case, the girl likely developed DKA after a protracted course with a gastrointestinal illness! She is acidemic with a low bicarbonate level and has an elevated AG due to the presence of ketones and acetoacetate in her blood
  • Here we can calculate the delta-delta or AG - HCO3-

​

This principle demonstrates that the addition of an acid to the blood should cause an equimolar change in the bicarbonate level. This patient’s AG is 20 so the change is 8. However, the change in HCO3- is 20 indicating another process has lowered the bicarbonate further. This patient has lost bicarbonate in her stool and has additional acid production in her blood.

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Case 6

A five-week-old male infant presents after seven days of projectile vomiting. On exam, the fontanelle is sunken and an olive like mass is noted in the abdomen. Lab evaluation reveals: arterial pH 7.57, pCO2 55, Na 130, Cl 77, Bicarbonate 44

Which of the following is the most accurate statement regarding this patient’s acid-base abnormality?

​

  1. Compensation is maximal and complete in this clinical scenario
  2. Compensation is incomplete—the elevated pH signifies the presence of additional respiratory alkalosis
  3. Compensation is incomplete—the AG should increase due to the generation of a compensatory increased AG acidosis
  4. Compensation is incomplete—the patient should excrete bicarbonate in the urine to generate a compensatory metabolic acidosis

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Case 6

A five-week-old male infant presents after seven days of projectile vomiting. On exam, the fontanelle is sunken and an olive like mass is noted in the abdomen. Lab evaluation reveals: arterial pH 7.57, pCO2 55, Na 130, Cl 77, Bicarbonate 44

Which of the following is the most accurate statement regarding this patient’s acid-base abnormality?

​

  1. Compensation is maximal and complete in this clinical scenario
  2. Compensation is incomplete—the elevated pH signifies the presence of additional respiratory alkalosis
  3. Compensation is incomplete—the AG should increase due to the generation of a compensatory increased AG acidosis
  4. Compensation is incomplete—the patient should excrete bicarbonate in the urine to generate a compensatory metabolic acidosis

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Case 6 Explained

  • Tricky question!! There are limits to compensation…�
  • Pyloric stenosis🡪 loss of hydrochloric acid with perpetuation of volume depletion🡪 this makes kidney compensation difficult because doing so would also result in more sodium and fluid loss and kidneys are stubborn and dumb.�
  • The appropriate compensation here would be a respiratory acidosis with CO2 retention- however, the brain will only allow hypoventilation to a certain point before avoidance of a hypoxemic stimulus predominates (Remember A-a gradient!)�
  • In general, pCO2 maxes out in the mid 50s in healthy lungs. Similarly, pCO2 of 10-15 is the physiologic limit of hyperventilation

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Case 7- Last one!

A 16 year-old girl is brought to the ED by her parents, who are concerned because she is extremely depressed and recently threatened suicide. Tonight, they found her vomiting and lethargic. Lab eval reveals: Na 144, Cl 100, Bicarb 24, arterial pH 7.4, pCO2 40

Which of the following best describes the patient’s acid base status?

​

  1. The patient has no acid base abnormality
  2. The patient has both a metabolic acidosis and a metabolic alkalosis
  3. The patient has primary respiratory acidosis with complete respiratory compensation

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Case 7- Last one!

A 16 year-old girl is brought to the ED by her parents, who are concerned because she is extremely depressed and recently threatened suicide. Tonight, they found her vomiting and lethargic. Lab eval reveals: Na 144, Cl 100, Bicarb 24, arterial pH 7.4, pCO2 40

Which of the following best describes the patient’s acid base status?

​

  1. The patient has no acid base abnormality
  2. The patient has both a metabolic acidosis and a metabolic alkalosis
  3. The patient has primary respiratory acidosis with complete respiratory compensation

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Case 7 Explained

  • This case highlights the importance of calculating the AG on every patient even when metabolic acidosis is not immediately apparent�
  • The AG here is 20 and that is never normal- the body can never generate unmeasured anions…�
  • The delta-delta in this case is 8 (AG 20-12, HCO3 24-24) so clearly a mixed disorder is present�
  • Therefore, this girl has both an increased AG metabolic acidosis and a concurrent metabolic alkalosis- why? �
  • We need to get more information…probably a toxic ingestion followed by significant vomiting

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Thanks!

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