ICM - Acid Base Flashcards
Acid definition
Proton Donor
Base definition
Proton Acceptor
What is a Strong acid
Fully dissociates in solution
Weak acid
Partially disociates in solution
What is a Buffer
Chemical substance that prevent large changes in H conc when acid or base is added to a solution
Acidaemia
Decrease in pH
What is Acidosis
Increase in H concentration
Acid-base buffer
Solution of two or more compounds that prevent marked changes in H with acid or base is added.
pK of a buffer
the pH at which the ionised and unionised forms of a chemical are at equilibiurm
Henderson Hasselbach eqn
pH = pKA + Log10 (base/acid)
= 6.1 + log 1o (24/ 2.3X5.3)
= 7.4
If CO2 production is constant, what determines CO2 concentration
alveolar vent
What affects alveolar vent
Resp centre in medulla oblangata is sensitive to change in H+ and alter the ventilation
Acts within minutes
Kidney role in acid base balance
controls secretion of H relative to amount of filtered bicarb
Methods of H secretion
1) secondary active transport of H in PCT, thick segment of asc loop and DCT
CO2 + H20 __(Carbonic) —> Carbonic acid —> H plus bicarb and H sereted into tubule
2) Primary active transport in latter DCT to renal pelvis
Acounts for 5% of H secreted
But can concentrate H 900 fold compared to 4 fold for secondary
Why does saline cause acidosis
Relative excess of chloride given
This reduces the Strong Ion Difference
Reducing SID increases water dissociation
Therefore, more H+
What is the SID?
Strong Ion Difference (SID) is the difference between the concentrations of strong cations and strong anions (Na+K+Ca+Mg) - (Cl - Lact)
What happens when the SID increases
Less water dissociation, therefore reduced H, therefore pH rises
When the SID decrease
More water dissociation making more H, so pH falls
Is PaO2 directly or indirectly measured
Direct
PaCO2 direct or indirect
Direct
pH direct or indirect
Direct (as negative of {H}
Standard bicarb, direct or indirect
Indirect from pH and CO2 using Henerson Hasselback
What is standard bicarb
Conc of bicarb in an equilibriated sample to 37C and PaCO2 5.3
Range 22-26
Base excess/defecity
Amount of acid or base that needs to be added to a sample under standard conditions (37C, PaCO2 5.3) to return the pH to 7.4
Rnage -2 to 2
Anion Gap
(Na+K) - (Cl+HCO3)
10-18
Indicates the presence of non volatile acids
Lactic, Ketoacids, exogenous
What happens to blood gas if it cools
CO becomes more solube
PaCO2 falls
pH increases by 0.015 for every degree drop in temp
Causes of met acidosis with normal AG
GI bicarb loss: diarrhoea, ileostomy. uretosigmoidostomy
Renal bicarb losses - acetazaolamide, proximal RTA, hyperparathyroid
Drugs, heavy metals, paraproteins
TPN
Dilutional
Causes of met acidosis with raised AG
Organic acids (KUSMEL) Ketones Uraemia Salicylate Methanol Ethylene Glycol Lactate AKI/CKD
Ketoacidosis (alcohol excess, starvation, DKA)
Types and causes of lactic acidosis
Type A - Anaerobic metab
Hypotension, arrest, low CO
Sepsis
Poisoning - ethylene glycol, methanol
Extreme muscular activity
Type B - Metformin Haem malig leukamia, lymphoma AIDS Enzyme defects - pytuvate dehydrogenous Decrease liver lactate metab
Causes of metabolic alkalosis
Loss of acid
Vomiting, NG
Hydrogen loss from kidney
Diuretics, hypokalaemia, excess steroid
Low chloride states
Addition of alkali - bicarb
Substances converted to bicarb
Lactae, acetate, citrate
Causes of resp acidosis
Resp depression, drugs, cerebral injury
Weakness, GBS, myasthenia, polio
Trauma
ARDS
COPD
Artifical vent - poor MV< high PEEP
Causes or resp alk
High MV Hypoxia PE Early asthma Salicylate over dose - early
early sepsis
Difference between type A and type B lactic acidosis and how can you tell
A - impaired oxygen delivery
B - normal oxygen delivery but increased cellular production, reduced clearence
Lactate increases with increased glycolysis
Lactate to pyruvate ratio distinguises between tissue hypoxia (ratio > 10:1) and normal conditions (<10:1).
Anion Gap corrected for Albumin
(Serum AG) + (2.5 x (45-Albumin)
Steward approach ideas
Acids increase H+ and bases decrease H+
HCO3 is not independent of pH/H but is dependent on it
Acids are strong if they fully dissociate and weak if they dont
Three things contribute to acid base
SID
Conc of non volatile weak acids
PaCO2
What three things determine acid base balance
The Strong Ion Differnce
Concentration of non volatile weak acids (Atot)
PaCO2
Is PaCO2 an indepedent determinant of acid bases
yes (compaed to HCO3 which isnt)
What is the strong ion difference gap
Difference between the Apparent SID (normal eqn) and the effective SID which involves albumin
> 2 is elevated gap, indicating unmeasured strong cations, ketones, salicylates etc
How does albumin alter acid base
It is a weak acid
Increasing albumin (or phosphate) reduces the Effective SID, and therefore the equilibrium shifts to acidosis.
Low albumin shifts to alkalosis
