acid base, oximetry, and blood gas

Question 1

What type of sample is needed to assess oxygenation?

Answer 1

arterial only

Question 2

How does ventilation effect the blood pH?

Answer 2

Hyperventilation –> blows off more CO2 –> increases the blood pH
Hypoventilation = decreases the pH

H2+ + HCO3- <–> H2CO3 <–> H2O + CO2

Question 3

How is bicarbonate considered as an open buffer system?

Answer 3

As long as there is HCO3- available and the respiratory system is working, it will help to blow off the excess H2+
- if there is loss of HCO3-, like from renal or GI system, it can induce acidemia

Question 4

What parameter is evaluated for respiratory acid/base disturbance?

Answer 4

respiratory acidosis, PCO2 = increased
respiratory alkalosis, PCO2 = decreased

Question 5

What parameter is evaluated for metabolic acid/base disturbance?

Answer 5

metabolic acidosis, increased H+ –> buffering by HCO3- –> leading to a decrease in HCO3-
metabolic alkalosis, increased HCO3-

Question 6

What does base excess mean?

Answer 6

It’s the of base that above or below the normal buffer base (HCO3-)
- normal is within +/- 5mEq/L

Question 7

What are the normal values for pH, PCO2, HCO3- and base excess?

Answer 7

pH = 7.35-7.45
pCO2 = 40-45mm Hg
HCO3- = 19-24mm Hg
Base excess = -5 to +5 mEq/L

Question 8

What are the causes of respiratory acidosis?

Answer 8

inappropriate ventilation leading to hypercapnia
- abnormal gas exchange
- decreased respiratory rate or effort
Examples:
- circulatory failure/ CPA
- nervous system disease
- respiratory muscle failure
- physical impairment of ventilation (ex. pleural space disease, pain)
- primary pulmonary disease
- iatrogenic under GA

Question 9

What are the clinical signs of hypercapnia?

Answer 9

hypoventilation

Question 10

How is respiratory acidosis treated?

Answer 10

correct the underlying reason for the hypercapnia

Question 11

Is bicarbonate therapy indicated in respiratory acidosis?

Answer 11

No, H2+ + HCO3- <–> H2CO3 <–> H2O + CO2
it will make more CO2

Question 12

Is O2 indicated in respiratory acidosis?

Answer 12

No, respiratory rate is driven by hypoxemia –> increasing the O2 saturation may actually worsen the hypoventilation/ hypercapnia

Question 13

What are some causes of respiratory alkalosis?

Answer 13

hyperventilation/ hyocapnia
- hypoxemia due to pulmonary, circulatory, nervous or iatrogenic conditions that lead to hyperventilation

Question 14

What’s the treatment of respiratory alkalosis?

Answer 14

target the underlying condition that is causing the hyperventilation/ tachypnea

Question 15

Is O2 indicated in respiratory alkalosis?

Answer 15

Yes, it can be helpful

Question 16

What are some causes of metabolic acidosis?

Answer 16

• ketones/ DKA
• uremic acids
• lactic acidosis (hypoperfusion, infection)
• exogenous (ex. ethylene glycol)

Question 17

What’s the equation for anion gap?

Answer 17

AG = [Na+ + K+] - [Cl- + HCO3-]

Question 18

What’s the cause if there is metabolic acidosis with an increased anion gap?

Answer 18

This will be due to a gain of acid
- ex. exogenous (ethylene glycol, salicylate intoxication)
- DKA
- lactic acidosis

Question 19

What’s the cause if there is metabolic acidosis with a normal anion gap?

Answer 19

This will be due to decreased excretion of H+ or loss of HCO3- (so there is an increase in Cl- to remain electro-neutrality –> hyperchloremic metabolic acidosis)
- not as common as metabolic acidosis with an increased anion gap
- could be due to renal tubular impairment
- loss of bicarbonate via severe diarrhea

Question 20

What’s the normal value for anion gap?

Answer 20

16 +/- 4 mEq/L

Question 20

What are some abnormalities associated with of metabolic acidosis?

Answer 20

• lethargy, decreased cardiac output, systemic hypotension, decreased hepatic/ renal flow

Question 21

What are the clinical signs of respiratory acidosis/ alkalosis?

Answer 21

few clinicals signs suggesting specific acid/base derangement

Question 22

How is metabolic acidosis treated?

Answer 22

• IV fluid to promote better perfusion
• elimination of ingested toxins
• correct the underlying metabolic/ GI/ renal disease

Question 23

Should bicarbonate be infused for severe metabolic acidosis?

Answer 23

Yes, but cautiously
- can lead to paradoxical central nervous system acidosis (pushes the equation towards CO2 production)
- rapid correction can lead to hyperosmolarity, hypernatremia, hypokalemia, and hypocalcemia tetany

Question 24

What’s the pathophysiology of metabolic alkalosis?

Answer 24

decreased Cl-, usually due to upper GI obstruction/ sequestration
- initially renal compensation prevents acid/base derragement
- with gastric outflow obstruction, once vomiting leads to hypovolemia –> aldosterone release –> Na+ retention
- Na+ is usually retained with bicarbonate or Cl-, or in exchange with K+
- since Cl- and K+ are high in gastric fluids, there is a depletion of those electrolytes
- so Na+ can only be reabsorbed with bicarbonate

Question 25

What are some signs of metabolic alkalosis?

Answer 25

depends on the underlying cause
- can see seizures/ twitching
- signs of hypokalemia: weakness, cardiac arrhythmia, renal dysfunction, and decreased GI motility

Question 26

How is metabolic alkalosis treated?

Answer 26

NaCl IV fluids (NOT lactated Ringers)
- address the underlying cause

Question 27

What’s the goal of O2 saturation?

Answer 27

SpO2 = 90%, PaO2 = 60mm Hg