15. Blood gas analysis and pulse oximetry Flashcards
6 questions to ask when interpreting a blood gas
1) how is the pt
2) hypoxaemic?
3) pH?
4) PaCO2?
5) bicarb?
6) any other imp values to consider?
what is the partial pressure of a gas?
the contribution it makes to the total pressure of the mixture
what is atmospheric pressure?
what about partial pressure of oxygen in the atmosphere?
this gives an arterial PaO2 of ___kPa
close to 100kPa
21kPa
10-13 (in normally functioning lungs)
when you give a pt 40% oxygen, what is the partial pressure of oxygen you are giving them?
40kPa (almost twice that which they were breathing in air)
normal arterial PaO2 on room air?
10-13kPa
why is the PaO2 always lower than the atmospheric partial pressure?
because the air is humidified as it’s breathed in, then it mixes with expired CO2 in the alveoli, both of which dilute the oxygen a little
rough rule of thumb, PaO2 should be about ___ less than the inspired concentration in a healthy pt
10
i.e. in a pt breathing 40% oxygen, would expect the PaO2 to rise to 30kPa
the acidity/ alkalinity of the blood is determined by the concentration of _____ ions
normal levels in the body?
hydrogen (the greater the conc, the more acid)
normally very low, around 40nmol/L
normal pH of arterial blood?
7.35-7.45
(36-44 H+)
T/F: small changes in pH represent big changes in H+ concentration
true
why does too much CO2 make pts acidaemic?
because CO2 combines with water to generate hydrogen ions and bicarbonate
normal PaCO2?
4.7-6.0 kPa
T/F: the respiratory centre in the brain stem is usually very sensitive to blood H+ concentraiton
true - within a few minutes of sending a rise in hydrogen concentration it stimulates the person to breathe more increasing CO2 excretion
T/F: The lungs are the primary mechanism by which [H+] is adjusted by regulating PaCO2
true
normal base excess and bicarbonate?
BE -2 to +2
Bicarbonate 22-26
_______ is the most important buffer for hydrogen ions in the body and is generated by the ______
bicarbonate
kidneys
what is an excess of bicarbonate normally caused by?
loss of acid, e.g. from persistent vomiting
will cause excessive buffering of hydrogen ions and produce a metabolic alkalosis
what is base excess a measure of?
the amount of acid or base that would need added to neutralise the pH of their blood
BE of +8 means 8mmol strong acid would be needed to return pH to normal > indicates metabolic alkalosis
BE of -8 means 8mmol strong base would be needed to return pH to normal > indicates metabolic acidosis
T/F: metabolic compensation takes several days
true
T/F: during cardiac arrest, ABG values correlate closely with the severity of hypoxaemia, hypercapnia and acidosis in the tissues
false - poor correlation so interpret with caution
VBG values may reflect more accurately the acid-base state of the tissues
inspired oxygen concentration on 6L face mask?
about 40%
where is the best site for arterial cannulation?
the radial artery
why is ABG more accurate than pulse oximetry
the relationship between oxygen saturation and PaO2 is a sinusoid shape on the oxyhaemoglobin dissociation curve
an initial decrease from a normal PaO2 is not accompanied by a drop of similar magnitude in the oxygen saturation of the blood (>early hypoxaemia may be masked)
T/F: pulse oximeters may cause a falsely low reading in anaemia or jaundice
false - not affected by these factors
may, however, get false readings in presence of other Hbs (CO poisoning, sickling red cells), imaging dyes (methylene blue), nail varnish, high ambient light levels, motion artefact and reduced pulse volume (hypotension, low CO, vasoconstriction, hypothermia)
T/F: pulse oximetry does not provide a reliable signal during CPR
true - relies on the presence of a pulse for reliable measurement
