Blood Gas Analysis Flashcards
what does blood gas analysis do?
measures the partial pressures of gases in the blood
what is an acid?
a proton donor
what is a base?
a proton acceptor
what is normal body pH?
7.35-7.45
what happens to [H+] when pH falls?
[H+] increases in bigger and bigger steps as pH falls
why is knowledge of a patients blood pH important?
effects rate of enzymatic reactions - has big impact on physiology
pH is often deranged in disease
what are the limits of pH change the body can cope with?
death usually occurs if pH is less than 6.8 (acidaemia) or greater than 7.6 (alkalaemia)
what does electroneutral mean?
all negative and positive charges in the body add up
how does the body try and mitigate any charge differences?
H+ is generated from water
what is classed as acidaemia?
blood pH less than 7.35
what is classed as alkalaemia?
blood pH more than 7.45
which systems does the body use to maintain a normal pH?
chemical buffers, respiratory and renal systems
what are the most key parts in the maintenance of normal pH?
phosphates, proteins and the bicarbonate/carbonic acid system
what is the bicarbonate/carbonic acid system?
H2O + CO2 <–> H2CO3 <–> H+ + HCO3-
what is a buffer?
any particle capable of accepting or donating H+
how do buffers work?
act to soak up any excess H+ and release it when necessary
how quickly do buffers work?
fast - seconds
give some examples of buffers in the body
bicarbonate (HCO3-)
haemoglobin
blood proteins
phosphate
lactate
what can cause pH derangements?
respiratory acidosis or alkalosis
metabolic acidosis or alkalosis
what is respiratory acidosis/alkalosis?
a change in PCO2 leading to a change in pH
what is metabolic acidosis/alkalosis?
a change in anything else that can affect pH (not pCO2)
which ion is usually involved in metabolic acidosis/alkalosis?
usually see a change in HCO3-, either due to loss/gain or its role as a buffer to other acids
what is the relationship between arterial CO2 (PaCO2) and ventilation?
inversely proportional
what type of breathing will cause PaCO2 to fall?
hyperventilation
what type of breathing will cause PaCO2 to rise?
hypoventilation
how quickly can the respiratory system change PaCO2?
rapidly - within seconds/minutes of sensing a pH change
which parts of the brain/body sense changes in PaCO2?
medulla and carotid body
what is the advantage of the quick response of the respiratory system to changes in pH?
allows the body to respond to sudden changes in pH e.g. exercise
what is the main way of getting rid of excess acid from the body?
renal system
how quickly does the renal system act to regulate ion levels in the body?
slowly - hours to days
what is acute respiratory acidosis?
increased PCO2 due to hypoventilation
what is the body’s response to acute respiratory acidosis?
compensatory response is to increase HCO3- (buffer to mop up excess H+)
how do the kidneys react to chronic respiratory acidosis?
more acid is excreted and HCO3- retained
what causes respiratory alkalosis?
a fall in PCO2 caused by increased ventilation (pain, stress, hypoxaemia)
what signifies metabolic acidosis? why does this occur?
signified by decreased HCO3-
due to direct loss of bicarbonate or its consumption due to excessive levels of acids
how does the body respond to metabolic acidosis?
respiratory compensation - a fall in PCO2 due to increased ventilation
what signifies metabolic alkalosis? why does this occur?
signified by increased HCO3-
compensatory increase, usually due to loss of chloride ions or albumin
how does the body respond to metabolic alkalosis?
respiratory compensation - an increase in PCO2 due to decreased ventilation
what is base excess?
amount of acid required to titrate 1L of blood to a pH of 7.4 at 37C and a PaCO2 of 40mmHg
is base excess positive or negative?
can be either
what is the relevance of base excess in blood gas interpretation?
fixes the PaCO2 at 40mmHg - means any remaining pH change is due to metabolic processes
what is normal PCO2 (mmHg)?
arterial 36-46
venous 38-48
what is normal HCO3- (mmol/l)?
22-29 (arterial and venous)
what is normal base excess (mmol/l)
2-5 (arterial and venous)
what are normal arterial values for blood when breathing room air?
90-100mmHg
what is the normal venous value for blood?
40-50mmHg
what is hypoxaemia defined as?
less than 80mmHg oxygen in the blood
what is the normal PaO2 under anaesthesia?
400-500mmHg (due to 100% oxygen being given)
which samples can be used for acid-base balance?
arterial or venous
which samples can be used for gas exchange values in the body?
ideally arterial sample, blood taken over a few breaths
samples should be collected anaerobically
can glass syringes be used for sample collection?
yes - impermeable to gas
must keep sample on ice
which type of syringe is best to use if blood sample analysis will be delayed?
glass - impermeable to gas
how long can purpose-designed plastic syringes hold a sample?
5-10 mins at room temperature
what do purpose-designed plastic collection syringes for blood gas analysis contain?
heparin
why do we discard the first drop of blood from the sample?
often clotted
what common artifacts are seen with blood gas analysis?
air contamination - CO2 unexpectedly low
saline contamination - high chloride
clotting - low PCV/HCT
when is saline contamination of blood gas samples often seen?
when blood is sampled from an IV catheter
