Acid base regulation Flashcards
sO2
amount of Hb saturated with O2
FCOHb
CO bound to HB
FiO2
Fraction of inspired O2
pAtm
biometric pressure
normally lefty at sea level
Blood gases values in systemic arteries
PaO2 >10
SaO2 >95%
PaCO2 4.7-6.4Kpa
blood gas values in systemic veins
PaO2 5.3Kpa
SaO2 75%
PaCO2 6.1Kpa
Compensation
ventilation - rapid compensatory response to change in CO2 elimination and therefore pH
vent can comp for metabolic problem
changes in HCO3- and H+ retention/secretion in kidneys - slow comp response to change in pH
kidney can correct lung problem
what occurs to compensate for change in pH
acidosis to comp for alkalosis
alkalosis to comp for acidosis
type 1 resp failure
failure of pul gas exchange V/Q inequality hypoxic O2 into blood impaired CO2 out - fine because it is more diffusible PaO2<8KPa PaCO2 - low/normal hypoventilation diffusion abnormality pul oedema pneumonia atelectasis - collapse or closure of the lung - worse GE
Type 2 resp failure
hypercapnic resp failure getting gas there problem V/Q problem PaO2<8KPa - low but not really the problem - O2 down greater conc grad so fine PaCO2 >6.7KPa increased CO2 production decreased VCO2 elimination decreased CNS drive increased work of breathing pul fibrosis neuromuscular disease increased physiological dead space obesity
what can you tell from an arterial blood gas (ABG)
low or high pH
PaCO2
BE excess - assess metabolic component
is the patient hypoxemic
guidelines for PaO2
> 10pKa normal
8-10 mild hypoxaemia
6-8 moderate hypoxaemia
<6 severe hypoxaemia
what causes acidosis
diahorrea
what causes alkalosis
vomiting
ABG interpretation procedure
type of disturbance: acidosis/alkalosis/normal
aetiology of imbalance: resp/metabolic/normal
haemostatic comp: uncomp/partially/fully
oxygenation: hypoxaemia/normoxaemia/hyperoxaemia
order of reporting an ABG
compensation
aetiology
disturbance
oxygenation
normal values for ABG
[Hb] 130-170g/l pH 7.35 - 7.45 pCO2 4.7-6.4Kpa 35-48mmHg PO2 >10Kpa/>10mmHg HCO3- 22-26mEq/L BE -2-+2mmol/L
Base excess
bicarbonate measured compared to excreted because of CO2
bicarb and co2 in dynamic eq - should be able to calculate one form the other - how ever much the true value is out from the calculated value is the BE
HCO3- range
if out by enough something is wrong
standard BE
normal BE for 5g/dL [Hb]
corrective BE assuming specific HB conc
Hb conc and CO2 bound to Hb affect ratio of expected bicarb to actual
only useful in critical care
hyperoxaemia
on oxygen
ifd hyperventilate for long time increase conc grad of O2 and CO2 between air in lung and blood
get rid of CO2 = high pH
base actively mops it up so this doesn’t happen
interpreting AGM
if co2 match ph - resp problem - if BE normal uncomp - if BE opp to pH - comp
if co2 opp to ph - met problem being comp for by resp
if pH normal - fully comp
if both CO2 and BE match pH it is mixed
Alkalaemia
higher than normal pH in blood
proton conc in blood at a point in time
acidaemia
lower than normal pH in the blood
proton conc in the blood at a point in time
alkalosis
circumstanbces that will decrease [H+] and increase pH
cause CO2 to be cleared and O2 to be used
acidosis
circumstances that will increase [H+] and decrease pH
what is an acid
any molecule with loosely bound H+ ions (protons)
dissociation depends on strength of acid
higher conc H+ = lower pH
what is a base
anionic
capable of reversibly binding to protons
acid and bases in the body
acidity of blood has to be closely monitored - otherwise changes 3D structure of channels, enzymes and hormones
bases and conjugate acids are in equilibrium
le chatelier’s principle - change 1 thing and it causes a change in the other direction to keep the ratio - amount of unbinding and binding changes
equilibrium between CO2 and bicarb
H2O + CO2 – H2CO3 – HCO3-
Pitts and Swann experiment
identified the huge buffering capacity of blood that reacts almost immediately to imbalances
dog anethesised - baseline bloods drawn
dog injected with 14M acid
thought dog would die
actually pH only decreased a really small amount
proton acceptor mop up extra protons - prevent change in pH
converting between [H+] and pH
small value of [H+] inconvenient to use
Sorensen scaled data using log10 conversion, add - in front to make value +ve
pH = -log10[H+]
[H+] = 10(to power -pH)
where do acids come from in body
respiratory acids - CO2
metabolic acid - pyruvate, lactic, HCL, septic
ratio - resp:met = 100:1
Sorensen eqn
pH = -log10[H+]
Henderson eqn
to calculate the dissociation constant Ka
Ka = {H+][HCO3-]/[CO2][H2O]
Henderson-hasselbach eqn
combination of Henderson and Sorenson eqn
pH = pKa + log[HCO3-]/[CO2]
units for [H+] used in eqns
mol/L
