8 - ABGs and acid base

Question 1

normal ABG values

Answer 1

PaO2 > 60 mmHg. SpO2/SaO2 > 90%. PaCO2 40 mmHg. HCO3 24 mmol/L. pH 7.4

Question 2

Pgas = (formula)

Answer 2

partial pressure o that gas = (barometric pressure - water vapor pressure) X Fgas

Question 3

PA vs Pa

Answer 3

A = alveolar. a = arterial

Question 4

how does partial pressure of O2 change with altitude?

Answer 4

increased altitude = decreased PAO2 and thus decreased PaO2

Question 5

7 causes of low PaO2

Answer 5

things that cause low PAO2: high PCO2 (displaces O2). low FIO2. low barometric alt. (with increased altitude). low exchange of CO2/O2 (decreased respiration). things that cause a mismatch of ventilation and perfusion: shunt, decreased V/Q, increased dead space

Question 6

what controls PaCO2

Answer 6

= VCO2/VA X 0.863. so depends on CO2 production, and ventilation

Question 7

how does PaCO2 change with exercise?

Answer 7

healthy person: it shouldn’t. increased production, but also increased ventilation

Question 8

A-aDO2? what is normal?

Answer 8

PaO2 minus PaO2. normally should be 0.4 X age

Question 9

effects of alveolar ventilation on PCO2 and PO2

Answer 9

increased ventilation = higher PA O2 and higher Pao2 (PAO2 goes up more = higher Aa-DO2). and lower PaCO2

Question 10

interpreting ABG: three steps?

Answer 10

PaCO2 to assess ventilation. PaO2 to determine if it is normal for FIO2 and ventilation rate (can also calculate A-aDO2. pH and assess it relative to PaCO2.

Question 11

fundamental relationship in acid/base physl

Answer 11

CO2 + H2O = HCO3 - + H+

Question 12

what is base excess? what does +/- BE mean?

Answer 12

concentration of bicarb caused by metabolic changes. + means excess of bicarb over what is produced by existing PCO2 so metabolic alkalosis. - means bicarb is lower than can be explained by existing PCO2 so metabolic acidosis.

Question 13

what is PCO2, pH and BE in pure respiratory acidosis? partially compensated resp acid? completely compensated?

Answer 13

pure: high PCO2, low pH, BE 0. part compensated: high PCO2, low pH (but slightly more normal), + BE. complete comp: high PCO2, normal pH, ++ BE

Question 14

what is PCO2, pH and BE in pure respiratory alkalosis? partially compensated resp acid? completely compensated?

Answer 14

pure: low PCO2, high pH, BE 0. partial: low PCO2, high pH, -BE. complete: low PCO2, normal pH, –BE

Question 15

what is PCO2, pH and BE in metabolic acidosis? metabolic alkalosis?

Answer 15

met acid: normal/low PCO2 with low pH, –BE. met alk: normal/high PCO2, high pH, ++BE