Acid base regulation Flashcards by Emily Southall

sO2

amount of Hb saturated with O2

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FCOHb

CO bound to HB

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FiO2

Fraction of inspired O2

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pAtm

biometric pressure

normally lefty at sea level

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Blood gases values in systemic arteries

PaO2 >10
SaO2 >95%
PaCO2 4.7-6.4Kpa

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blood gas values in systemic veins

PaO2 5.3Kpa
SaO2 75%
PaCO2 6.1Kpa

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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

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what occurs to compensate for change in pH

acidosis to comp for alkalosis

alkalosis to comp for acidosis

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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

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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

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what can you tell from an arterial blood gas (ABG)

low or high pH
PaCO2
BE excess - assess metabolic component
is the patient hypoxemic

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guidelines for PaO2

> 10pKa normal
8-10 mild hypoxaemia
6-8 moderate hypoxaemia
<6 severe hypoxaemia

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what causes acidosis

diahorrea

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what causes alkalosis

vomiting

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ABG interpretation procedure

type of disturbance: acidosis/alkalosis/normal
aetiology of imbalance: resp/metabolic/normal
haemostatic comp: uncomp/partially/fully
oxygenation: hypoxaemia/normoxaemia/hyperoxaemia

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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