Arterial Blood Gases Flashcards
Describe blood gases in circulation
Arterial blood:
PaO2 > 10kPa
SaO2 > 95%
PaCO2 = 4.7-6.0 kPa
Venous blood:
PaO2 > 4.0-5.3kPa
SaO2 > 75ish%
PaCO2 = 5.3-6.7 kPa
Describe pulmonary transit time
Red blood cell usually able to exchange gases over 0.75 seconds but usually does so in 0.25 seconds. This means if gas exchange is slowed, there is a buffer capacity before function is reduced. CO2 has a lower diffusion gradient and less to exchange so occurs much faster than O2.
What is an acid?
An acid is any molecule that has a loosely bound H+ ion that it can donate. The acidity of the blood must be tightly regulated, marked changes will alter the 3D structure of proteins (enzymes, hormones, protein channels).
What is a base?
A base is an anionic (negatively charged ion) molecule capable of reversibly binding protons (to reduce the amount that are ‘free’).
What are corrective compensation examples?
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.
How is acid base status evaluated? When is it uncompensated?
pH examined, partial pressure of CO2 examined and bicarbonate examined. If pH is high or low with a causative CO2 level and no change in bicarbonate, it is an uncompensated respiratory acidosis/alkalosis. If pH is high or low but not explained by CO2, with a similar high/low in BE, it is an uncompensated metabolic acidosis/alkalosis.
When is an acid base disturbance partially compensated?
When CO2 causes pH change and BE is shifted in the direction of correction, it is a partially compensated respiratory acidosis/alkalosis. Otherwise cause is metabolic.
When is an acid base disturbance fully compensated?
pH is normal but both carbon dioxide and bicarbonate shifted in the same direction.
