Critical Care Flashcards

1
Q

How does an acid differ from a base?

A

Acids donate protons

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2
Q

The equilibrium between what two compounds is most important to the human body’s ability to buffer?

A

Carbon Dioxide and Water

H2O + CO2 ↔H2CO3 ↔ H+ + HCO3-

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3
Q

In the human body, what ratio determines the extracellular pH?

A

ratio of the bicarbonate concentration to the PaCO2

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4
Q

Metabolic acids are largely produced through what kind of metabolism?

A

anaerobic metabolism

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5
Q

What comprises the most abundant and hence most important buffering substrate in the human body.

A

Proteins, in particular - Hemoglobin

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6
Q

How does the respiratory system eliminate acid?

A

medullary chemoreceptors –> affects the pH of the ECF through regulation of the PaCO2

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7
Q

How does the renal system eliminate acid?

A

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.

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8
Q

What five (5) values are present on an arterial blood gas report?

A

◾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)

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9
Q

What is a normal PaO2 at sea level?

A

~100mmHg

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10
Q

What is the alveolar gas equation?

A

PAO2 = FIO2(PB – PH2O) – (1/RQ)PaCO2

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11
Q

What is a normal arterial to alveolar oxygen tension: PaO2/ PAO2?

A

~0.85

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12
Q

Acutely, what is the expected rise in HCO3- for a patient with a pCO2 of 70 mmHg?

A

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.
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13
Q

How do you calculate base excess?

A

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
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14
Q

Calculate the amount of NaHCO3- required to correct the metabolic acidosis of an adult 100kg man with a base deficit of 10.

A

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.
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