Critical Care Flashcards
How does an acid differ from a base?
Acids donate protons
The equilibrium between what two compounds is most important to the human body’s ability to buffer?
Carbon Dioxide and Water
H2O + CO2 ↔H2CO3 ↔ H+ + HCO3-
In the human body, what ratio determines the extracellular pH?
ratio of the bicarbonate concentration to the PaCO2
Metabolic acids are largely produced through what kind of metabolism?
anaerobic metabolism
What comprises the most abundant and hence most important buffering substrate in the human body.
Proteins, in particular - Hemoglobin
How does the respiratory system eliminate acid?
medullary chemoreceptors –> affects the pH of the ECF through regulation of the PaCO2
How does the renal system eliminate acid?
actively excrete H+ into the urine and reclaim Na+ from the filtrate via the Na+/H+ transport pump.
Cytoplasmic carbonic anhydrase converts the CO2 and H2O –> H2CO3.
H2CO3 –> H+ and HCO3.
HCO3- –> out of the cell’s basolateral membrane and conserved.
What five (5) values are present on an arterial blood gas report?
◾pH (Normal range: 7.35 - 7.45)
◾PaO2 - partial pressure of arterial oxygen. (Normal 75 - 100 mmHg)
◾PaCO2 - partial pressure of arterial CO2. (Normal 35 - 45 mmHg)
◾HCO3- - concentration of bicarbonate ion. (Normal range: 22 - 26 mEq/L)*
◾Base Excess - calculated measure of metabolic component (Normal range: -2 - +2)
What is a normal PaO2 at sea level?
~100mmHg
What is the alveolar gas equation?
PAO2 = FIO2(PB – PH2O) – (1/RQ)PaCO2
What is a normal arterial to alveolar oxygen tension: PaO2/ PAO2?
~0.85
Acutely, what is the expected rise in HCO3- for a patient with a pCO2 of 70 mmHg?
3mEq/L
Respiratory acidosis: For every 10mmHg rise in PaCO2 above 40:
- Acute: HCO3- increases by 1mEq/L.
- Chronic (compensated): HCO3- increases by 4mEq/L.
Respiratory alkalosis: For every 10 mmHg fall in PaCO2 below 40:
- Acute: HCO3- decreases by 2 mEq/L.
- Chronic (compensated): HCO3- decreases by 3 mEq/L.
How do you calculate base excess?
pH = 7.1, PaCO2 = 35, HCO3- = 10
pH is 0.3 units from normal (from 7.1 to 7.4)
- bicarbonate will decrease by 3 mEq/L (0.3 times 10) –> new value of 7 (10 minus 3)
- The BE = 7 – 24, or -14.
pH = 7.0, HCO3 = 8 pH 7.0 or 0.4 units from 7.4 - bicarbonate will decrease by 4 (10 x 0.4). - HCO3 of 8 - 4 = 4. - The BE is thus 4 – 24 = -20
Calculate the amount of NaHCO3- required to correct the metabolic acidosis of an adult 100kg man with a base deficit of 10.
First calculate the ECF fraction which
- 0.5 (premature baby)
- 0.4 (term baby)
- 0.3 (2 year old)
- 0.25 (older child)
- 0.2 (adult).
BE x wt (kg) x ECF fraction = bicarbonate deficit
- Bicarb merely converts metabolic acid to respiratory acid that must be removed, as CO2, by increased minute ventilation.
- Hyperosmolar and can cause cerebral hemorrhage, particularly in children.
- correcting the extracellular acidosis with exogenous bicarbonate may worsen intracellular acidosis.