1. Jeff Kaufhold, MD FACP 2013 Source: The ICU Book Chapter 36-38
Approach to Acid Base
Jeff Kaufhold, MD FACP
2013
Source: The ICU Book
Chapter 36-38

2. Acid Base Disorders Basic Physiology Approach to A-B disorders
Equations
Cases
Electrolytes

3. Acid Base Physiology Balance based on Henderson-Hasselbach equation
Hydrogen Ion measured in NanoEquivalents , i.e. very tightly regulated.
Buffers for acid:
Bicarbonate, Hemoglobin, Albumin, Bone
Acid excretion: Lungs and Kidneys

4. Acid - Base Physiology Sources of Acid:
70 mEq daily as inorganic acids
H2SO4, H3PO4
Excreted by kidney
16,000 mMole as CO2
Excreted by lungs
32,000 mEq organic acid which gets metabolised

5. Acid – Base Physiology Lung blows off acid as CO2 Kidney
H2CO3  H2O + CO2
Kidney
Proximal tubule has lumenal carbonic anhydrase to reclaim bicarb from urine
Distal tubule has basolateral CA to pull H+ out of blood, complexed to NH3 to form NH4Cl (ammonium chloride)
Can measure activity by urinary Anion Gap (Na+K – Cl)

6. ABG reporting Normals: Reported as pH / pCO2 / pO2 / HCO3
pCO2 36 – 44 mmHg
pO2 over 60
HCO3 (bicarb) 22 – 26
Reported as pH / pCO2 / pO2 / HCO3
Example / 40 / 80 / 23

7. Compensatory Mechanisms
Disorder
Resp Acidosis
Resp Alkalosis
Met Acidosis
Met Alkalosis
End point is a constant ratio of:
Primary / Compens
Pco2 up HCO3 up
Pco2 down, HCO down
HCO down, Pco2 down
HCO up Pco2 up
PCO2 / HCO3

8. Rules for interpretation
1. Primary Metabolic disorders:
PCO2 and pH move in same direction
Can be quickly identified if last 2 digits of pH = pCO2
i.e / 20 / 70
7.51 / 50 / 120
If / 40 / 70, not simple disorder
Respiratory compensation not adequate

9. Rules for interpretation
3. Primary Respiratory disorders:
PCO2 and pH move in opposite direction
Resp acidosis: 7.20 / 58 / 68
Resp alkalosis: / 25/ 80
7.25 / 48 / 52 Not a simple disorder
Neither is 7.40 / 25 / 68

10. Rules for interpretation
Rule 5 Mixed Disorders:
Compensatory responses do not completely correct primary problem
Can have BOTH metabolic acidosis and metabolic alkalosis simultaneously (sepsis plus vomiting is common)
But can only hyperventilate (resp alk) OR hypoventilate (resp acidosis).
Usually due to failure to adequately lower pCO2 (resp fatigue)

11. Expected Changes Rules of Thumb
Met Acidosis
Pco2 = 1.5 * bicarb + 8 +/- 2
7.20 / 20 / 68 / HCO3 = 8
Met Alkalosis
Pco2 = 0.7 * HCO /- 2
7.50 / 50 / 120 / HCO3 = 40

12. Expected Changes Acute Resp Acidosis Acute Resp Alkalosis
Change in pH = * change pCO2
7.30 / 50 / 68 / HCO3 = 22
Acute Resp Alkalosis
7.50 / 20 / 120 / HCO3 = 20
Shortcut: D pH = 0.08 for each 10 mmHg change in pCO2

13. Expected Changes Chronic Resp Acidosis Chronic Resp Alkalosis
Change in pH = * change pCO2
7.33 / 50 / 68 / HCO3 = 32
Chronic Resp Alkalosis
Change in pH = * change pCO2
7.48 / 25 / 120 / HCO3 = 20

14. Expected Changes
Note that the chronic respiratory conditions induce chronic changes in bicarbonate handling by the kidney, which takes time.

15. Common Causes of A-B Disorders
Met Acidosis MUD PILES
Met Alkalosis Vomiting, NG suction, IV infusion of bicarb or Acetate (TPN), Diuretics
Resp Acidosis : respiratory failure, COPD exacerbation, Narcotics
Resp Alkalosis: Hyperventilation/anxiety, Pregnancy, Early sepsis, early stage asthma, chronic alcoholism

16. Anion Gap Useful for evaluation of metabolic acidosis.
Calculate AG every time you see electrolytes, esp on exams.
AG = Anions – Cations
AG = Na + K – (CL + HCO3)
AG = – ( )
= 8 to 16
> 18 nephrologist gets excited

17. Anion Gap Low AG suggests extra plasma proteins, such as Myeloma
Non Anion Gap acidosis suggests loss of bicarbonate through either:
Diarrhea
Renal Tubular Acidosis (RTA)
Or Infusion of acid via NS, TPN
High AG means accumulation of acid
MUDD PILES or other nemonic

18. Anion Gap High AG means accumulation of acid
Organic acid can be metabolised back to bicarbonate
So AG over 16 or so can represent “potential Bicarb” or “delta gap”.
Calculating the potential bicarb is useful for identifying mixed disorders, since both met acidosis and met alkalosis can be present at same time.
Delta Gap = AG – 12 (extra anions)
Potential bicarb = HCO3 + delta gap

19. Potential Bicarb
HCO3 + Delta gap=Potential bicarb

20. Gapped Metabolic Acidosis
Methanol (wood alcohol)
Uremia/acute renal failure
DKA
D lactate (Metformin/blind loop in bowel)
Paraldehyde
Isopropyl alcohol
Lactic acidosis (hypoxia/hypoperfusion)
Ethanol
Starvation Ketosis

21. Osmolal Gap
Used to help identify presence of exogenous acids which may be causing gapped metabolic acidosis
Measured Osm – calculated Osm
Osm calc = 2(Na) + Glucose/18 + BUN/30
OG > 20 suggests Ethanol, Methanol

22. Clinical Problems in A-B
1. Calculate AG and potential bicarb
2. Is pH acidic or alkalotic
3. Is pCO2 alkalotic or acidotic
< > 40
4. Is the bicarb measured by the ABG machine same as that on lytes?
If not the samples are not simultaneous
5. Apply the rules of thumb
If values are consistent, Simple disorder
If values are not consistent: Mixed.

23. Case 1
38 y.o. male with chronic GN. Admitted with weakness, fatigue. BUN 100 cr 6
Labs: Na 134, K 5.6, Cl 100, CO2 14.
ABG: 7.26 / 27 / 72 / 14
What is A-B disturbance?
AG =
Delta gap/Potential bicarb =
Acidosis or alkalosis ?
Direction of CO2 ?
MA rule: pCO2 = 1.5*HCO /-2

24. Case 1 Labs: Na 134, K 5.6, Cl 100, CO2 14. ABG: 7.26 / 27 / 72 / 14
What is A-B disturbance?
AG = 20
Delta gap/Potential bicarb = 8 / 22
Acidosis or alkalosis ? acidosis
Direction of CO2 ? alkalosis
MA rule: pCO2 = 1.5*HCO /-2 works
So simple metabolic acidosis with appropriate respiratory compensation

25. Case 2
23 y.o. garage mechanic presents with acute confusion after drinking antifreeze.
Lab: Na 137 K 5.4 Cl 105 HCO 5
ABG: 6.95 / 15 / 80 / 4
What is A-B disturbance?
AG =
Delta gap/Potential bicarb =
Acidosis or alkalosis ?
Direction of CO2 ?
MA rule: pCO2 = 1.5*HCO /-2

26. Case 2 Lab: Na 137 K 5.4 Cl 105 HCO 5 ABG: 6.95 / 15 / 80 / 4
What is A-B disturbance?
AG = 27
Delta gap/Potential bicarb = 15 / 20
Acidosis or alkalosis ? acidosis
Direction of CO2 ? alkalosis
MA rule: pCO2 = 1.5*HCO /-2 works so simple met acidosis with appropriate resp compensation.

27. Case 3
65 y.o. recently discharged after pneumonia. Presents with C dif Diarrhea.
Lab: Na 132 K 2.4 Cl 105 HCO 15
ABG: 7.30 / 30 / 80 / 14
What is A-B disturbance?
AG =
Delta gap/Potential bicarb =
Acidosis or alkalosis ?
Direction of CO2 ?
MA rule: pCO2 = 1.5*HCO /-2

28. Case 3 Lab: Na 132 K 2.4 Cl 105 HCO 15 ABG: 7.30 / 30 / 80 / 14
What is A-B disturbance?
AG = 12
Delta gap/Potential bicarb = 0 / 15
Acidosis
Direction of CO2 ? alkalotic
MA rule: pCO2 = 1.5*HCO /-2
Works so Non Gap Metabolic Acidosis

29. Case 4 40 y.o. with status asthmaticus.
Lab: Na 135 K 3.4 Cl 100 HCO 21
ABG: 7.50 / 30 / 80 / 20
What is A-B disturbance?
AG =
Delta gap/Potential bicarb =
Acidosis or alkalosis ?
Direction of CO2 ?
Acute RA rule: DpH = * DpCO2

30. Case 4 40 y.o. with status asthmaticus.
Lab: Na 135 K 3.4 Cl 100 HCO 21
ABG: 7.50 / 30 / 80 / 20
What is A-B disturbance?
AG = 14
Delta gap/Potential bicarb = 23
alkalosis
Direction of CO2 ? alkalosis
Acute RA rule: DpH = * DpCO2
works

31. Case 5
32 y.o. chronic pyelo, CRF, pleural effusion. Admitted for thoracentesis
Lab: Na 130 K 5.0 Cl 94 HCO 15
ABG: 7.32 / 32 / 55 / 15
Attempted thoracentesis unsuccessful, became dyspneic and coded as blood drawn:
Na 131/k 7.8/cl 92/CO 9/ /50/40

32. Case 5 CRF with pneumothorax
Admission AG = 20
Appropriate resp compensation
Uremic metabolic acidosis
After thoracentesis:
Resp acidosis, plus met acidosis
Met acidosis worse, gap up to 30.
Due to uremia plus lactic acidosis.

33. Case 6
44 y.o. alcoholic admitted with abd pain and N/V/Fever/chills. SOB and CXR shows pneumonia with effusion.
Lab: Na 142 K 3.4 Cl 98 HCO 20
ABG: 7.28 / 41 / 58 / 20
What is A-B disturbance?
AG =
Delta gap/Potential bicarb =
Acidosis or alkalosis ?
Direction of CO2 ?

34. Case 6
44 y.o. alcoholic admitted with abd pain and N/V/Fever/chills. SOB and CXR shows pneumonia with effusion.
Lab: Na 142 K 3.4 Cl 98 HCO 20
ABG: 7.28 / 41 / 58 / 20
What is A-B disturbance?
AG = 24
Delta gap/Potential bicarb = 12 / 32
Acidosis or alkalosis ? Acidosis
Direction of CO2 ? Not appropriate
What is going on?

35. Case 6
44 y.o. alcoholic admitted with abd pain and N/V/Fever/chills. SOB and CXR shows pneumonia with effusion.
Lab: Na 142 K 3.4 Cl 98 HCO 20
ABG: 7.28 / 41 / 58 / 20
Pneumonia is interfering with resp compensation.
Also see this with mechanical problems of ventilation such as flail chest, paralyzed hemidiaphragm, abdominal compartment syndrome.