1. It aiN’T All that Simple Dr alex Hieatt Consultant ED
ACID BASE DISORDERS
It aiN’T All that Simple
Dr alex Hieatt
Consultant ED

2. What is an ABG? The Components Desired Ranges
pH / PaCO2 / PaO2 / HCO3 / O2sat / BE
Desired Ranges pH
PaCO2 – 4.5 – 6 kPa
PaO2 – 10.5 – 13.5 kPa
HCO
O2sat %
Base Excess - +/-2 mEq/L

3. Acid Base Balance The body produces acids daily 15,000 mmol CO2
mEq Nonvolatile acids
The lungs and kidneys attempt to maintain balance
Buffering also occurs in the liver through ammonia metabolism to urea / glutamate

4. Acid Base Balance
Assessment of status via bicarbonate-carbon dioxide buffer system
Henderson-Hasselbalch
pH= pK + log ([HCO3-] / [H2CO3 ])
CO2 + H2O <--> H2CO3 <--> HCO3- + H+
ph = log ([HCO3] / [0.03 x PCO2])

5. The Terms ACIDS BASES Acidemia Acidosis Alkalemia Alkalosis
Respiratory
CO2
Metabolic
HCO3
BASES
Alkalemia
Alkalosis
Respiratory
CO2
Metabolic
HCO3

6. Respiratory Acidosis ph, CO2, Ventilation Causes CNS depression
Pleural disease
COPD/ARDS
Musculoskeletal disorders
Compensation for metabolic alkalosis

7. Respiratory Acidosis Acute vs Chronic
Acute - little kidney involvement. Buffering via titration via Hb for example
pH by 0.1 for 1.25 kPa  in CO2
Chronic - Renal compensation via synthesis and retention of HCO3 (Cl to balance charges  hypochloremia)
pH by approx 0.05 for 1 kPa in CO2

8. Respiratory Alkalosis
pH, CO2, Ventilation
 CO2   HCO3 (Cl to balance charges  hyperchloremia)
Causes CHAMPS
C – CNS Disease e.g. Intracerebral hemorrhage/ Cirrhosis
H – Hypoxia
A – Anxiety
M – Over ventilation
P – Progesterone
S – Salicylate/Sepsis

9. Respiratory Alkalosis
Acute vs. Chronic
Acute - HCO3 by 1.5 mEq/L for every 1 kPa  in PCO2
Chronic - Ratio increases to 3 mEq/L of HCO3 for every 1 kPa  in PCO2
Decreased renal bicarb reabsorption and decreased ammonium excretion to normalize pH

10. Metabolic Acidosis pH, HCO3
12-24 hours for complete activation of respiratory compensation
PCO2 by 0.15 kPa for every 1 mEq/L HCO3
The degree of compensation is assessed via the Winter’s Formula
 PCO2 = {1.5(HCO3) +8  2 } x [converts to kPa]

11. The Causes Metabolic Gap Acidosis Non Gap Metabolic Acidosis
M - Methanol
U - Uremia
D – DKA - AKA
P - Paraldehyde
I – Isoniazid / Iron
L - Lactic Acidosis
E - Ethylene Glycol
R- Rhabdomyolysis
S - Salicylate
Non Gap Metabolic Acidosis
H - Hyperalimentation
A - Acetazolamide
R - RTA
D - Diarrhoea
U - Uretero-pelvic shunt
P - Pancreatic Fistula
S – Spironolactone

12. Osmolar Gap
OG = Measured osmolality – calculated osmolality OG = 2 x [ Na mmol/L] + [glucose mmol/L] + [urea mmol/L] + (1.25 x [Ethanol mmol/L]) Should be <10 Causes: Methanol Glycine (TRUP) Ethylene Glycol Propylene Glycol Sorbitol Polyethylene Glycol Mannitol Maltose (IV IG)

13. OG For raised AG Metabolic Acidocis Common Causes: Ketones Lactate
Renal Failure
NO –
Ingestion possible
YES – Measure OG
Raised – Then likely Ethylene Glycol / Methanol
Normal – Salicylate, Paraldehyde, Iron + Isoniazid

14. Metabolic Alkalosis pH, HCO3 PCO2 by 0.1 for every 1mEq/L  in HCO3
Causes – CLEVER PD
C- Contraction
L - Liquorice
E - Endocrine: Conn’s / Cushing’s / Bartter’s
V - Vomiting / NG Suction
E - Excess Alkali
R - Refeeding Alkalosis
P - Post Hyper-capnoea
D - Diuretics and Chronic diarrhoea

15. Mixed Acid-Base Disorders
Patients may have two or more acid-base disorders at one time Corrected Bicarbonate = AG – 12 + Serum HCO3- If > 30 then there is also underlying metabolic alkalosis If < 23 then there is an underlying non-AG metabolic acidocis

16. The Steps
Start with the pH – acidaemia or alkalaemia Note the PCO2 Look for disorders revealed by failure of compensation Calculate anion gap Calculate Corrected Bicarbonate

17. Sample Problem #1
An ill-appearing alcoholic male presents with nausea and vomiting.
ABG / 5.4 / 11.3 / 22
Na- 137 / K- 3.8 / Cl- 90 / HCO3- 22

18. Sample Problem #1 Winter’s Formula = {1.5(22) + 8  2} x 0.133
= {39  2} x = 5.3 kPa
 compensated
Anion Gap = ( ) = 25
 anion gap metabolic acidosis
Corrected Bicarbonate = = 13
= 35
 metabolic alkalosis

19. Sample Problem #2
22 year old female presents for attempted overdose. She has taken an unknown amount of Midol containing aspirin, cinnamedrine, and caffeine. On exam she is experiencing respiratory distress.

20. Sample Problem #2
ABG / 2.5 / 15.7 / 14 Na- 145 / K- 3.6 / Cl- 109 / HCO3- 17 ASA level mg/dL

21. Sample Problem #2 Winters Formula = {1.5 (17) + 8  2} x 0.133
= 4.65 kPa
 uncompensated
Anion Gap = ( ) = 19
 anion gap metabolic acidosis
Corrected HCO3- = = 7
= 24
 no metabolic alkalosis

22. Sample Problem #3
47 year old male experienced crush injury at building site. ABG / 4.2 / 12.8 / 15 Na- 135 / K-5 / Cl- 98 / HCO3- 15

23. Sample Problem #3 Winters Formula = {1.5 (15) + 8  2} x 0.133 = 4 kPa
 compensated
Anion Gap = ( ) = 22
 anion gap metabolic acidosis
Corrected Bicarb = = 10
= 25
expected no additional deficit

24. Sample Problem #4
1 month old male presents with projectile vomiting for x 2 days. ABG / 5.33 / 13 / 30 Na- 140 / K- 2.9 / Cl- 92 / HCO3- 32

25. Sample Problem #4
Metabolic Alkalosis, hypochloremic Winters Formula = {1.5 (30) + 8  2} x = 53  2 = 7.3 kPa  uncompensated

26. Questions
Practice makes perfect MD Calc App has Winters and ABG analysis with SI units. Josh Steinberg MD App – ABG eval (but US units.)