Mixed Acid Base Disorders: It’s Complicated - Schedschd.ws/hosted_files/psnannual2015/64/4.00...
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Mixed Acid Base Disorders:
It’s Complicated
Roberto C. Tanchanco, MD, MBA Associate Professor, Ateneo School of Medicine & Public Health
Head, Section of Nephrology, The Medical City
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ABG Interpretation:
A Teaching Tool to Detect
Mixed Acid Base Disorders:
It’s Complicated…not?
Roberto C. Tanchanco, MD, MBA
Associate Professor,
Ateneo School of Medicine & Public Health
PSN 35th Annual Convention Apr 17, 2015 Tanchanco RC
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Objectives
• To present an ABG worksheet that can be used to teach ABG interpretation
• To emphasize that the first step in interpretation is to anticipate the findings based on the clinical scenario
• To emphasize the importance of a step-wise approach to ABG interpretation – Identifying the primary disorder
– Differentiating simple from mixed disorders
• To emphasize that the utility of ABG interpretation is ultimately to understand what is going on in a patient to guide further management
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References
• Rose BD, Post TW, Clinical Physiology of
Acid Based and Electrolyte Disorders, 5th Ed
2001
• Brenner & Rector’s The Kidney, 9th Ed 2012
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Steps in ABG Analysis
1. From the clinical scenario, what acid base disorder do you anticipate?
2. Does the patient have acidemia or alkalemia?
3. What is the primary acid base disorder?
4. Is it a simple or mixed acid base disorder? 1. Based on the limits of compensation
2. Based on the Anion Gap and Delta Ratio
5. If hypercapnic, what is the A-a gradient (alveolar-arterial gradient)?
6. State your complete interpretation of the ABG results, including possible etiologies of all disorders.
7. State your plan of care.
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62 year old male, known diabetic and hypertensive, with CKD
Brought to the E.R. unconscious after a generalized tonic-clonic seizure episode
Hypertensive, tachypneic
No vomiting, no diarrhea
From the clinical scenario, what acid
base disorder do you anticipate?
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Respiratory Alkalosis
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Metabolic Acidosis
High [H+]
Normal Gap
Can’t excrete [H+]
Losing too much [HCO3]
High Gap High [H+] Load MUDPILES
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Respiratory Acidosis
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Metabolic Alkalosis
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62 year old male, known diabetic and hypertensive, with CKD
Brought to the E.R. unconscious after a generalized tonic-clonic seizure episode
Hypertensive, tachypneic
EXPECTED ACID BASE DISORDERS:
• Respiratory alkalosis
• Metabolic acidosis, high gap
• Metabolic acidosis, normal gap
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Does the patient have acidemia or alkalemia?
What is the primary acid base disorder?
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Is it a simple or mixed acid base disorder? Based on the limits of compensation
ePaCO2 = 14.7 + 15 = 29.7 +/- 2 mmHg
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Based on the Limits of
Compensation:
• ePaCO2 = 29.7 +/- 2 mmHg
• Actual PaCO2 = 34.5 mmHg
• Hence, the patient has a Mixed Acid Base
Disorder:
– Metabolic Acidosis
– Respiratory Acidosis
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[H+] nmol/L = 24 x PaCO2 mmHg
[HCO3] mmol/L
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pH [H+]
• pH = -log [H+] in mol/L
• 10-pH = [H+] in mol/L
• Normal pH = 7.4
• 10-7.4 = 3.98 x 10-8 mol/L
• 10-7.4 = 40 x 10-9 mol/L
• 10-7.4 = 40 nmol/L
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pH [H+]
• pH = -log [H+] in mol/L
• 10-pH = [H+] in mol/L
• Normal pH = 7.4
• 10-7.4 = 3.98 x 10-8 mol/L
• 10-7.4 = 40 x 10-9 mol/L
• 10-7.4 = 40 nmol/L
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pH [H+]
• pH = -log [H+] in mol/L
• 10-pH = [H+] in mol/L
• 10(9 – pH) = [H+] in nmol/L
• Normal pH = 7.4
• 10-7.4 mol/L = 3.98 x 10-8 mol/L
• 10(9 – 7.4) nmol/L= 101.6 nmol/L
• 101.6 = 39.8 nmol/L = 40 nmol/L
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pH [H+]
• pH = -log [H+] in mol/L
• 10-pH = [H+] in mol/L
• 10(9 – pH) = [H+] in nmol/L
• Normal pH = 7.4
• 10-7.4 mol/L = 3.98 x 10-8 mol/L
• 10(9 – 7.4) nmol/L= 101.6 nmol/L
• 101.6 = 39.8 nmol/L = 40 nmol/L
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Convert pH to [H+] nmol/L
10(9 – pH) = [H+] nmol/L
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Convert pH to [H+] nmol/L
10(9 – 7.23) = [H+] nmol/L
101.77 = [H+] nmol/L
101.77 = 58.9 nmol/L
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[H+] nmol/L = 24 x PaCO2 mmHg
[HCO3] mmol/L
[HCO3] mmol/L = 24 x PaCO2 mmHg 10(9 – pH) nmol/L
*The ABG machine directly measures pH and PaCO2 but only derives HCO3.
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[H+] nmol/L = 24 x PaCO2 mmHg
[HCO3] mmol/L
[HCO3] mmol/L = 24 x 34.5 mmHg
58.9 nmol/L
= 14.1 mmol/L
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• Anion Gap = Na – (Cl + HCO3)
• Anion Gap = 137 – 106 – 14.7
• Anion Gap = 16.3 mmol/L
Is it a simple or mixed acid base disorder? Based on the Anion Gap and Delta Ratio
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The Anion Gap
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The Normal Anion Gap = 10 – (2 x (4-[Alb g/dl]) – (1.6 x (1.2 – [Phos mmol/L])
• Normal Anion Gap
oMostly Albumin and Phosphorus
o Normal Gap = (2 x [Alb] g/dL) + (1.6 x [Phos] mmol/L)
oNormal Albumin = 4 g/dL
oNormal Phosphorus = 1.2 mmol/L
oNormal Gap = (2 x 4) + (1.6 x 1.2) = 9.92, or ~10 mmol/L
o Normal Gap
= 10 – (2 x (4-[Alb g/dl]) – (1.6 x (1.2 – [Phos mmol/L])
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High Gap Metabolic Acidosis
Patient’s Anion Gap = 16.3 mmol/L
Normal Anion Gap = 10 mmol/L
High Anion Gap Metabolic Acidosis
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Normal Gap Metabolic Acidosis
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Normal Gap Metabolic Acidosis
Normal [Na] = 140 mmol/L
Normal [Cl] = 100 mmol/L
Normal [Na]:[Cl] = 1.4
[Na]:[Cl] < 1.4 Hyperchloremia
[Na] = 137
[Cl] = 106
[Na]:[Cl] = 1.29
Hyperchloremic Acidosis
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High and Normal Gap Acidosis
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High Gap Acidosis
with Metabolic Alkalosis
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High Gap Acidosis with Metabolic Alkalosis
High Gap Acidosis with Normal Gap Acidosis
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Delta Ratio
Delta Ratio = Patient’s Anion Gap – Patient’s Normal Anion Gap
( 24 – [HCO3])
Delta Ratio = Δ AG .
Δ [HCO3]
• >2 = HAG metabolic acidosis with metabolic alkalosis
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Delta Ratio
Delta Ratio = Patient’s Anion Gap – Patient’s Normal Anion Gap
( 24 – [HCO3])
Delta Ratio = Δ AG .
Δ [HCO3]
• <1 = HAG with NAG metabolic acidosis
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Delta Ratio = ΔAG / Δ[HCO3]
Delta Ratio > 2
Delta Ratio < 1
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Delta Ratio
Delta Ratio = Δ AG .
Δ [HCO3]
• <1 = HAG with NAG
metabolic acidosis
Delta Ratio = 16.3 – 10 = 0.68
24 – 14.7
• <1 = HAG with NAG metabolic
acidosis
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• Anion Gap = 16.3 mmol/L (Normal Anion Gap = 10 mmol/L)
• [Na]:[Cl] = 1.29, or <1.4
• Delta Ratio = 0.68, or <1
• PaCO2 = 34.5 mmHg (ePaCO2 = 29.7+/-2 mmHg)
• Mixed HAG and NAG Metabolic Acidosis and
Respiratory Acidosis
Is it a simple or mixed acid base disorder? Based on the Anion Gap and Delta Ratio
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Respiratory Acidosis
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The A-a gradient
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If hypercapnic, what is the A-a gradient
(alveolar-arterial gradient)?
PAO2 = .40 x 713 – (34.5/0.8) = 285 – 43 = 242
A-a gradient = 242 – 138 = 104
Expected A-a gradient for age = 62/4 + 4 = 19.5 mmHg
Therefore, the patient’s A-a gradient is elevated.
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So far, we know the patient has:
• Metabolic Acidosis
– High Anion Gap (16.3 mmol/L), AND
– Normal Anion Gap (Na:Cl <1.4, Delta Ratio <1),
AND
• Respiratory Acidosis
– Pulmonary Cause
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State your complete interpretation of the ABG
results, including possible etiologies of all
disorders.
Acid Base Disorder Possible Etiology
Metabolic Acidosis, High Gap Rhabdomyolysis?
Lactic Acidosis?
Uremia?
Ingestion?
Metabolic Acidosis, Normal Gap Uremia?
RTA Type IV?
Respiratory Acidosis, Pulmonary Pneumonia?
Aspiration?
Ingestion?
CNS Lesion?
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State your plan of care.
Acid Base
Disorder
Possible Etiology Diagnostic Plan Therapeutic Plan
Metabolic
Acidosis,
High Gap
Rhabdomyolysis?
Lactic Acidosis?
Ketoacidosis?
Uremia?
Ingestion?
CK Total/MM, iCa, Phos
Serum Lactate
RBS, Serum Ketones
Creatinine
Review History
Hydrate, Alkalinize?
Underlying cause?
Insulin etc?
Renal Support…
Metabolic
Acidosis,
Normal Gap
Uremia?
RTA Type IV?
Creatinine
K, Urine pH
Renal Support…
Review meds, diet.
Address HyperK?
Respiratory
Acidosis,
Pulmonary
Pneumonia?
Aspiration?
Ingestion?
CNS Lesion?
Chest Xray
Review History
Review Neuro Exam, CT
scan head
Antibiotic coverage
Pulmonary support
Neurology support
and management
PSN 35th Annual Convention Apr 17, 2015 Tanchanco RC
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Steps in ABG Analysis
1. From the clinical scenario, what acid base disorder do you anticipate?
2. Does the patient have acidemia or alkalemia?
3. What is the primary acid base disorder?
4. Is it a simple or mixed acid base disorder? 1. Based on the limits of compensation
2. Based on the Anion Gap and Delta Ratio
5. If hypercapnic, what is the A-a gradient?
6. State your complete interpretation of the ABG results, including possible etiologies of all disorders.
7. State your plan of care.
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• To present an ABG worksheet that can be used to teach ABG interpretation
• To emphasize that the first step in interpretation is to anticipate the findings based on the clinical scenario
• To emphasize the importance of a step-wise approach to ABG interpretation – Identifying the primary disorder
– Differentiating simple from mixed disorders
• To emphasize that the utility of ABG interpretation is ultimately to understand what is going on in a patient to guide further management
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3
1
7
2
2
1
3
1
20
PSN 35th Annual Convention Apr 17, 2015 Tanchanco RC
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THANK YOU!
PSN 35th Annual Convention Apr 17, 2015 Tanchanco RC
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For Primary Respiratory Disorders:
• The pH changes by 0.008 units
for every 1 mmHg change in PaCO2
• Given PaCO2:
– expectedpH = 7.4 + 0.008 x (40 – PaCO2)
• If pH = exppH, then the respiratory disorder is
a simple acute respiratory disorder.
• If pH <> epH, then the respiratory disorder is
not acute and possibly not simple.
PSN 35th Annual Convention Apr 17, 2015 Tanchanco RC