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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
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