arterial blood gases and acid base regulation Flashcards
how do blood gasses differ in arterial and venous circulation?
Pa O2: >10 kPa
Sa O2: >95%
Pa CO2: 4.7-6.0 kPa
Pv O2: 4.0-5.3 kPa
Sv O2: ~75%
Pv CO2: 5.3-6.7 kPa
what is pulmonary transit time?
The amount of time that the erythrocytes and plasma have for gas exchange (in the lungs)
Takes about a third of this time to equilibrate
CO2 moves quicker
Even during exercise when the blood moves quicker theres still enough time
Pulmonary transit time: 0.75 s
Gas exchange time: 0.25 s
partial pressure of oxygen rises from 5.3 to about 13.5 kPA
PP of CO2 drops from 6.1 to 5.3
what is the pH calculation?
𝑝𝐻= −𝑙𝑜𝑔_10 [𝐻+]
reverse:
[H] = 10^ -pH
what are acids and bases?
An acid is any molecule that has a loosely bound H+ ion that it can donate
H+ ions are also called protons (because an H atom with a +1 valency has no electrons or neutrons)
PARADOX: A greater concentration of H+ ions refers to a lower pH
The acidity of the blood must be tightly regulated, marked changes will alter the 3D structure of proteins (enzymes, hormones, protein channels)
A base is an anionic (negatively charged ion) molecule capable of reversibly binding protons (to reduce the amount that are ‘free’)
H+A- H+ and A-
This relationship is in an equilibrium. Increasing something on one side will push the equation in the opposite direction
H2O + CO2 H2CO3 H+ + HCO3-
the body makes a lot of acid, what are the proportions of the two main categories of this?
The body produces significant amounts of acid, but how much of this is respiratory acid, and how much is metabolic acid?
Respiratory: 99% (main is carbonic acid)
metabolic: 1% (eg. lactic acid, HCL, keto acids etc)
what is CO2 flux?
PaCO2 = 5.3 kPa (40 mmHg) CO2 as HCO3-= 43 mL·dL-1 HbCO2 = 2.5 mL·dL-1 Cd CO2 = 2.5 mL·dL-1 Ca CO2 = 48.0 mL·dL-1 pH = 7.40
Δ = +4 mL·dL-1
Δ = +200 mL CO2·min-1
this is CO2 flux
PaCO2 = 6.1 kPa (46 mmHg) CO2 as HCO3-= 45.2 mL·dL-1 HbCO2 = 3.0 mL·dL-1 Cd CO2 = 3.8 mL·dL-1 Ca CO2 = 52 mL·dL-1 pH = 7.36
look at slide for good diagram
some CO2 is dissolved in the blood
in erythrocytes, conversion tocarbonic acid is enzymatic
what is the buffering capacity of blood like?
The blood has an ENORMOUS buffering capacity that can react almost IMMEDIATELY to imbalances
Dog is anaesthetised.
Baseline bloods drawn
pH 7.44 ->
Dog was injected with
14 Molar acid
pH 7.44 ->
hypothesis: dead dog pH 2.5
actual: survival pH 7.14
what is corrective compensation?
Changes in ventilation can stimulate a RAPID compensatory response to change CO2 elimination and therefore alter pH
Changes in HCO3- and H+ retention/secretion in the kidneys can stimulate a SLOW compensatory response to increase/decrease pH
An acidosis will need an alkalosis to correct
An alkalosis will need an acidosis to correct
(Hyperventilation – you clear out more CO2, -> alkalosis)
-osis is the process that will cause an -aemia
what is the procedure for interpreting an ABG?
Type of imbalance?
Acidosis (or acidaemia) / Alkalosis (or alkalaemia) / Normal
Aetiology of imbalance?
Respiratory (acidosis or alkalaemia) / Metabolic (acidosis or alkalosis) / Mixed (respiratory and metabolic) / Normal
Any homeostatic compensation?
Uncompensated / Partially compensated / Fully compensated
Oxygenation?
Hypoxaemia / Normoxaemia / Hyperoxaemia
(If CO2 is high its prob respiratory)
(metabolic/respiratory: If both have changed there is prob compensation
If only one has changed it is uncompensated)
(Partially compensated is pH not back to normal yet)
how do you interpret ABGs?
look at the screenshot table
uses: pH (normal: 7.35-7.45) Pa CO2 (normal: 4.7-6.4) Base excess (normal: -2 - 2) Pa O2 ( normal: 10-13.5)
what is base excess?
BE= base excess
Comparison of actual bicarbonate to the expected bicarbonate (due to the fact that CO2 should be in equilibrium with bicarb)
