Acid-Base Disorders

MICU Resident Lecture Series

Ian J Barbash, MD

Overview

• Framework for Interpretation

• Case-Based Application of Principles

General Definitions

• Alkalemia/Acidemia: the state of the patient

– i.e. The patient (or patient’s blood) is alkalemic/acidemic

• Alkalosis/Acidosis: the underlying processesthat contribute to the state of alkalemia or acidemia

– i.e. The patient is acidemic due to an acute respiratory acidosis

• 45yo man arrives in ED, obtunded

• Anything amiss?– Anion Gap?

– Osm Gap?

– Mixed Disorder?

– Important Therapeutic Implications

Why should I care about this Acid-Base stuff?

4.7

138

17

103

1.5

2596ABG: 7.21 / 45 / 60

Systematic Approach to Acid-Base Analysis

• Step 1:

– Assess data for internal consistency using Henderson-Hasselbach Equation

– i.e. did the machine or computer make an error

[H+] = 24(PaCO2)[HCO3-]

http://www.thoracic.org/clinical/critical-care/clinical-education/abgs.php

• Step 2: Is the patient alkalemic or acidemic?

– Alkalemia = pH >7.45

– Acidemia = pH <7.35

– Remember: In a mixed disorder, acidosis and alkalosis can be present together and result in normal pH—have to look at HCO3 and PaCO2

– However a patient cannot be alkalemic and acidemic at the same time!

• Step 3: Is the primary disturbance respiratory or metabolic?...Look at the pH and PaCO2

pH PaCO2

Acidosis Metabolic

Respiratory

Alkalosis Metabolic

Respiratory

• Step 4: Is there appropriate compensation for the primary disturbance?

– Uncommon for compensation to correct pH to normal…if so, likely a mixed disorder

Acidosis Metabolic PaCO2 = (1.5 x [HCO3]) + 8 ± 2

Acute Respiratory [HCO3] = PaCO2/10

Chronic Respiratory [HCO3] = 3.5 ( PaCO2/10)

Alkalosis Metabolic PaCO2 = 40 + 0.6 ( [HCO3])

Acute Respiratory [HCO3] = 2 ( PaCO2/10)

Chronic Respiratory [HCO3] = 5 ( PaCO2/10)

You’re likely to need to look this up until you’ve done lots of acid-base problems

• Step 5: If there is a metabolic acidosis, calculate the anion gap– AG = [Na] – ([Cl] + [HCO3])

• Normal AG = 12 ± 2

• In hypoalbuminemia, AG falls by ~2.5 for every 1gm/dLdecrease in plasma albumin

– If AG elevated without obvious cause, or if ingestion suspected, calculate Osm Gap• Osm Gap = Osmmeasured – (2[Na+] + [gluc]/18 + [BUN]/2.8 +

[EtOH]/4.6)

• Osm Gap >10 is abnormal

• Step 6: If there is an anion-gap metabolic acidosis, calculate the “delta-delta”

– ΔΔ < 1 indicates concurrent non-AG metabolic acidosis (drop in bicarb greater than expected)

– ΔΔ 1-2 indicates simple AG acidosis

– ΔΔ > 2 indicates concurrent metabolic alkalosis (drop in bicarb smaller than expected)

ΔΔ= AGobserved – AGexpected

[HCO3]normal – [HCO3]observed

Adjust for albumin

Assess Internal Validity

Assess Acid-Base Status

Acidemia Alkalemia

Assess Primary Disorder Assess Primary Disorder

Metabolic Acidosis

RespiratoryAcidosis

Metabolic Alkalosis

RespiratoryAlkalosis

Compensation as Expected? If not, assess for Secondary Disorder

AG Present?

Osm Gap?ΔΔ?

Yes No

Oh Yeah…Examine the Patient and Think About the Clinical Scenario

Cases

Heroin OD

• 21yo F found unresponsive in a bathroom

4.3

139

26

105

0.6

1579ABG: 7.21 / 65 / 45

Diarrhea from Food Poisoning

• 95yo F with dizziness following a buffet meal at bingo

2.9

124

18

94

1.9

3589ABG: 7.33 / 35 / 85

Lactic Acidosis and CKD

• 85yo M presenting with syncope and massive BRBPR in the ED

5.1

134

15

101

2.3

75147ABG: 7.32 / 30 / 95

ΔΔ= AGobserved – AGexpected

[HCO3]normal– [HCO3]observed

Lactate 5.6

ΔΔ= 18 – 1224 – 15

= 0.66 < 1

Overdiuresis…and nervous about the ABG!

• 55yo with CHF in the hospital

3.2

149

34

102

1.0

29134ABG: 7.51 / 45 / PaO2

Ethylene Glycol Ingestion

• 13yo F presenting with altered mental status

Osm Gap = Osmmeasured – (2[Na+] + [gluc]/18 + [BUN]/2.8)

=321 – 292 = 29!!

4.4

140

14

98

1.5

2275ABG: 7.34 / 27 / 85 Osm = 321

ASA Ingestion

• 19yo M presenting to ED with agitation

3.4

138

15

95

0.8

1382ABG: 7.45 / 22 / 86

DKA with Vomiting

• 28yo F presenting with N/V

4.1

141

19

95

1.7

35452ABG: 7.33 / 38 / 92 3+ Urine Ketones

ΔΔ= AGobserved – AGexpected

[HCO3]normal– [HCO3]observed

ΔΔ= 27 – 1224 – 19

= 3 > 2

Sepsis in patient with COPD

• 67yo M in ICU with pneumonia and shock

5.9

133

25

93

2.8

45105ABG: 7.22 / 65 / 58 Lactate = 3.4

Albumin = 2.0

ΔΔ= AGobserved – AGexpected

[HCO3]normal– [HCO3]observed

ΔΔ= 15 – 1224 – 25

= -3??

Adjust for albumin

Could there be a chronic respiratory acidosis?

Recent Outpatient HCO3 = 32ΔΔ= 15 – 732 – 25

= 1.14

Acknowledgements/References

• Matt Gingo, MD MS

• ATS Education Serieshttp://www.thoracic.org/clinical/critical-care/clinical-education/abgs.php