ABG and Acid/Base

Question 1

Normal levels for PaO2, SpO2, PaCO2, HCO3, and pH

Answer 1

PaO2 > 60 mmHg

SpO2 or SaO2 > 90% (saturation)

PaCO2 = 40 mmHg (less means hyperventilation, more is hypo)

HCO3- = 24 mmol/L

pH = 7.40

Question 2

Define: partial pressure; Dalton’s law; PiO2; PAO2; PEO2; PaO2; PvO2

Answer 2

Partial pressure: the part of the atmospheric pressure contributed by a specific gas Dalton’s Law is just about partial pressures adding to total.

PIO2: inspired PO2

PAO2: Alveolar PO2.

PEO2: Expired PO2

PaO2: Arterial PO2

PvO2: Venous PO2

Question 3

Describe 3 sites where arterial punctures are commonly performed in order to perform arterial blood gas sampling

Answer 3

Most common is radial a. Brachial ok. Femoral for emergencies.

Question 4

Describe the “P02 cascade” (how does the partial pressure of oxygen vary at different site within the cardiopulmonary systems?)

Answer 4

It allows O2 to flow in the right direction. Highest outside lung, lower in alveoli and a bit lower in arteries.

Question 5

Describe the “PCO2 cascade” (how does the partial pressure of oxygen vary at different site within the cardiopulmonary systems?)

Answer 5

Higher in veins, lower in alveoli, and much much lower outside body.

Question 6

Describe the relationship between the partial pressure of oxygen in the inspired air and altitude

Answer 6

21% O2 everywhere in the atmosphere. BUT, barometric pressure falls = decrease in PIO2. That’s the problem.

Question 7

Describe the pathophysiologic mechanisms of hypoxemia

Answer 7

1. High alveolar PCO2 (low PAO2)
2. Low FIO2 (low PAO2)
3. Low Barometric pressure (↑altitude)
4. Low exchange of CO2 vs O2 (↓R)
5. Matching of ventilation and perfusion: ↑Shunt, ↓V/Q, ↑Dead space

Question 8

Recall and explain the alveolar gas equation

Answer 8

PAO2 = FIO2(PB – 47) – (PaCO2/R*)

R = Respiratory quotient - depends on diet (esp. lipid and carbohydrate). Just assume it’s .8

FIO2 = fraction of inspired O2

Question 9

Define: normal alveolar-arterial oxygen tension difference (A-aDO2 (normal) < 0.4 x patient’s age in years)

Answer 9

Normal A-aDO2 ~ 0.4 x Age Tells you if lung is working properly.

Question 10

Explain, in both graphic and verbal terms, the relationship between ventilation and PaCO2

Answer 10

PaCO2 = (VCO2 / VA)0.863 (both V’s have dots) VA = alveolar ventilation - amount of air flowing into alveoli VCO2 = how much CO2 produced

Question 11

Explain, in both graphic and verbal terms, the relationship between ventilation and PaO2

Answer 11

PAO2 – PaO2 - difference in gradients.

Question 12

Interpreting ABG - steps

Answer 12

1. Look at the PaCO2 value to access ventilation 2. Look at the PaO2 value and determine if it is normal for the FIO2 and the ventilation rate (If unsure calculate A-aDO2) 3. Look at the pH and access it relative to the PaCO2

Question 13

Describe, in graphic terms, the relationship between pH, [HCO3-] and PaCO2

Answer 13

Picture

Question 14

Describe both the primary change as well as the compensatory response that occurs in acute and chronic respiratory acidosis

Answer 14

Resp. drive suppressed from drugs, etc. hypoventilate. Move from PCO2 = 40 to 60. Increases bicarb, increases H.

Kidneys don’t like acidosis , so they generate more bicarb to move the pH back towards normal.

Chronic: Reach normal, base excess is ++

Question 15

Describe both the primary change as well as the compensatory response that occurs in acute and chronic respiratory alkalosis

Answer 15

Hyperventilate and clot interferes with ventilation so CO2 drops.

Kidneys don’t like alkalosis, so they excrete more bicarb.

Chronic: Reach normal, base excess is - -

Question 16

Describe both the primary change as well as the compensatory response that occurs in metabolic acidosis

Answer 16

Addition of acid to or loss of base from body. ex DKA. adds lots of acid.

Hyperventilate to partially compensate, then kidneys do the rest. Should be BE = 0.

Question 17

Describe both the primary change as well as the compensatory response that occurs in metabolic alkalosis

Answer 17

Addition of bass to or loss of acid from body. ex DKA. adds lots of acid.

Hypoventilate to partially compensate, then kidneys do the rest. Should be BE = 0.