2. WHAT IS AN ARTERIAL BLOOD GAS?
The Components
• pH / PaCO2 / PaO2 / HCO3 / O2sat / BE
Normal Ranges
• pH - 7.35 - 7.45
• PaCO2 – 4.5 – 6 kPa
• PaO2 – 10.5 – 13.5 kPa
• HCO3 - 21-27
• O2sat - 95-100%
• Base Excess - +/-2 mEq/L
3. ACID BASE BALANCE
The body produces acids daily
• 15,000 mmol CO2
• 50-100 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 = 6.10 + log ([HCO3] / [0.03 x PCO2])
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 bicarbonate 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 0.133 [converts to kPa]
11. THE CAUSES
Metabolic Gap
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 Acidosis
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 - 7.4 / 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 0.133 = 5.3 kPa
compensated
Anion Gap = 137 - (90 + 22) = 25
anion gap metabolic acidosis
Corrected Bicarbonate = 25 - 12 = 13
13 + 22 = 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.
