Blood Gases Flashcards
Overarching causes of RAGMA:
ADDITIONAL ACIDIC SUBSTANCE:
Lactic acid
- Hypoperfusion
- liver failure, sepsis, seizure
- drugs (MCATPIES)
- inborn errors
Ketones
- diabetes
- starvation
- alcoholic
- profound dehydration
Uraemia
- Renal failure
- GI bleed
MCAT PIES
Causes of metabolic alkalosis:
ACID LOSS
- Diuretics
- GI loss: vomiting, suction
- Low albumin
- CF
BASE EXCESS
- Citrate (transfused blood)
- Antacids
- Sodi bic
ENDOCRINE/ METABOLIC
- HypoK (intracell shift)
- Hypercalcaemia (milk-alkali: calcium supplements, antacids)
- Hyperaldosteronism (ie. hypovolaemia)
- Cushings/ steroids
- Licorice
RENAL
- Gittelman
- Bartter
Overarching causes of lactic acidosis:
A- Hypoperfusion
B1- liver failure, sepsis, seizure
B2- drugs (MCATPIES)
B3- Inborn errors
Product of anaerobic metabolism. Ie. Hypoxia, Hypoperfusion, excessive O2 demand, impaired tissue extraction/utilisation of O2
Normal anion gap:
4-12
(+- 4)
Raised = excess + is cause
Not raised = deficit of - (usually HCO3, Cl) is cause
Na - (HCO3 + Cl)
Delta ratio:
Anion gap -12 / 24 - HCO3
For seeing if a MA is pure, or mixed.
<0.4 NAGMA
Up to 0.8 MIXED
1 or more PURE RAGMA
(>2 equals baseline chronic met alk)
Correction in respiratory acidosis:
For every 10 of CO2 above 40, HCO3 should rise by:
1 (acute)
4 (chronic)
Correction in respiratory alkalosis:
For every 10 CO2 below 40, HCO3 should drop by:
2 (acute)
5 (chronic)
Correction in metabolic acidosis:
Expected CO2 = (1.5 x HCO3) + 8
Correction in metabolic alkalosis:
Expected CO2 = (0.7 x HCO3) + 21
Aa Gradient:
(FiO2 x 713) - (1.25 x PaCO2) - PaO2
713 = baro - water vapour.
If room air 21%, this number is 150.
Applies only sea level and norm temp.
Normal Aa gradient = age/4 + 4
Causes of a hypoxia with raised Aa gradient:
VQ MISMATCH **
R TO L SHUNT
DIFFUSION (rare)
VQ mismatch:
- Poor circ: PE
- Poor vent: APO, pneumonia, asthma, COPD, ARDS
R to L shunt
- Pulmonary: ARDS, APO, pneumonia, pulmonary HTN
- Cardiac: VSD, ASD, RV dysfunction
Diffusion
- Interstitial lung disease
Causes of hypoxia with non-raised AA gradient:
HYPOVENTILATION
- Central
- Peripheral
LOW FiO2
Expected K:
For pH
Where K will end up after correction of the pH:
For every change in pH from 7.4, K should change by 0.5 from 5
Low pH:
5 + (0.5 x points below 7.4)
High pH:
5 - (0.5 x points above 7.4)
Corrected Na:
Na + (glucose/3)
Where Na will be after BSL normalises:
Osmolarity and osmolar gap:
2 x Na + urea + glucose
Normal <300
Osmolar gap = mOsm - cOsm
Raised osmolar gap= >10
Ie. there is an abnormal (foreign) solute present in high amounts:
Eg. Mannitol, methanol, ethylene glycol, ispropyl alcohol, the glycols in IV Diaz-Loraz/phenytoin
*non-toxic causes: severe ketones/ lipids/ lactate.
Drug causes of lactic acidosis:
MCATPIES
Metformin, methanol
Carbon monoxide, cyanide
Aminoglycosides
Theophylline, Toulene (glue)
Propofol, paracetamol, paraldehyde
Isoniazid, iron
Ethylene glycol, ethanol
Salycilates/ aspirin/ ASA
Causes of NAGMA:
BICARB LOSS or FAILED ACID EXCRETION
Renal
- RTA
- Tubuloonterstitial
GI
- Diarrhoea
- Colostomy/ Ileostomy
- Fistulas
Drugs
- Acetazolamide
Endocrine
- Aldosterone deficiency (Addisons)
Excess chloride (eg. saline)
If K high, more likely endocrine/Cl related. If K normal or low, think HCO3 loss
Urea: creatinine ratio
Urea / (Cr/1000)
Around 100 = prerenal
50-100 = normal or post renal
<50 = intrinsic renal
Strong Ion difference
Strong Cations - Strong Anions
Na - Cl
Usually more anions.
Normal = 40-45
Increased difference = Acidosis
- From more Na, or less Cl
Decreased difference = Alkalosis
- From LESS Na, or MORE Cl
How to calculate FiO2 based on L/min:
FiO2 = 20% + (4xL)
ie. 4L
20% + 16 = 36% FiO2
