acid base, oximetry, and blood gas Flashcards
What type of sample is needed to assess oxygenation?
arterial only
How does ventilation effect the blood pH?
Hyperventilation –> blows off more CO2 –> increases the blood pH
Hypoventilation = decreases the pH
H2+ + HCO3- <–> H2CO3 <–> H2O + CO2
How is bicarbonate considered as an open buffer system?
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
What parameter is evaluated for respiratory acid/base disturbance?
respiratory acidosis, PCO2 = increased
respiratory alkalosis, PCO2 = decreased
What parameter is evaluated for metabolic acid/base disturbance?
metabolic acidosis, increased H+ –> buffering by HCO3- –> leading to a decrease in HCO3-
metabolic alkalosis, increased HCO3-
What does base excess mean?
It’s the of base that above or below the normal buffer base (HCO3-)
- normal is within +/- 5mEq/L
What are the normal values for pH, PCO2, HCO3- and base excess?
pH = 7.35-7.45
pCO2 = 40-45mm Hg
HCO3- = 19-24mm Hg
Base excess = -5 to +5 mEq/L
What are the causes of respiratory acidosis?
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
What are the clinical signs of hypercapnia?
hypoventilation
How is respiratory acidosis treated?
correct the underlying reason for the hypercapnia
Is bicarbonate therapy indicated in respiratory acidosis?
No, H2+ + HCO3- <–> H2CO3 <–> H2O + CO2
it will make more CO2
Is O2 indicated in respiratory acidosis?
No, respiratory rate is driven by hypoxemia –> increasing the O2 saturation may actually worsen the hypoventilation/ hypercapnia
What are some causes of respiratory alkalosis?
hyperventilation/ hyocapnia
- hypoxemia due to pulmonary, circulatory, nervous or iatrogenic conditions that lead to hyperventilation
What’s the treatment of respiratory alkalosis?
target the underlying condition that is causing the hyperventilation/ tachypnea
Is O2 indicated in respiratory alkalosis?
Yes, it can be helpful
What are some causes of metabolic acidosis?
- ketones/ DKA
- uremic acids
- lactic acidosis (hypoperfusion, infection)
- exogenous (ex. ethylene glycol)
What’s the equation for anion gap?
AG = [Na+ + K+] - [Cl- + HCO3-]
What’s the cause if there is metabolic acidosis with an increased anion gap?
This will be due to a gain of acid
- ex. exogenous (ethylene glycol, salicylate intoxication)
- DKA
- lactic acidosis
What’s the cause if there is metabolic acidosis with a normal anion gap?
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
What’s the normal value for anion gap?
16 +/- 4 mEq/L
What are some abnormalities associated with of metabolic acidosis?
- lethargy, decreased cardiac output, systemic hypotension, decreased hepatic/ renal flow
What are the clinical signs of respiratory acidosis/ alkalosis?
few clinicals signs suggesting specific acid/base derangement
How is metabolic acidosis treated?
- IV fluid to promote better perfusion
- elimination of ingested toxins
- correct the underlying metabolic/ GI/ renal disease
Should bicarbonate be infused for severe metabolic acidosis?
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
What’s the pathophysiology of metabolic alkalosis?
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
What are some signs of metabolic alkalosis?
depends on the underlying cause
- can see seizures/ twitching
- signs of hypokalemia: weakness, cardiac arrhythmia, renal dysfunction, and decreased GI motility
How is metabolic alkalosis treated?
NaCl IV fluids (NOT lactated Ringers)
- address the underlying cause
What’s the goal of O2 saturation?
SpO2 = 90%, PaO2 = 60mm Hg
