Lecture 4: Anesthesia & Renal Physiology: regulation of acid-base balance Flashcards
Normal ECF H+ concentration is:
40nEq/L
Normal physiologic pH is:
7.4
An acid is defined as a proton (H+) _____.
Donor
A base is defined as a proton (H+) _____.
Acceptor
the Henderson–Hasselbalch equation describes:
the derivation of pH as a measure of acidity (using pKa, the negative log of the acid dissociation constant) in biological and chemical systems.
Buffers are most efficient when _______?
pH=pKa (50/50 mixture)
Name the buffers in the body:
- Bicarbonate (#1) H2CO3 or HCO3
- hemoglobin
- intracellular proteins
- phosphate (H2PO4; HPO4)
- Ammonia (NH3; NH4)
Bicarbonate buffer:
Hydration of CO2 is catalyzed by ____ ____ in the ____ _____.
-Carbonic anhydrase in the proximal tubule
Bicarbonate buffer is effective against _____ acidosis but not _____ acidosis.
Effective against METABOLIC acidosis but not respiratory acidosis.
Carbonic Anhydrase inhibitors control BP by:
Increasing excretion of HCO3 (Bicarbonate), sodium, potassium and water. (diuresis)
_____% of Bicarbonate is reabsorbed in the proximal tubule and the remaining ____% is reabsorbed in the distal tubule.
80-90% in the proximal tubule
10-20% in the distal tubule
H+ secreted in the tubule lumen can combine with HPO4 to form H2PO4 which is not reasorbable and becomes trapped in the urine, this is considered a ____ buffer?
Phosphate buffer.
Ammonium is synthesized from _____? What is generated in the synthesis process?
Glutamine; Bicarbonate
Ammonia (NH4) is an important tubular fluid butter in the _____, _____, and _____.
Proximal tubule, Thick loop, and Distal Tubule.
Renal compensation for Alkalosis:
Large amounts of bicarbonate can be excreted if necessary. Metabolic Alkalosis is mainly possible in 2 situations:
- Na Depletion (reabsorption)- more sodium is reabsorbed in the proximal tubule, and Cl moves with it to preserve electroneutrality. As Cl in the tubule decreases, HCO3 must be reabsorbed. This is called “contraction alkalosis” that can occur with long-term diuretic use.
- Increased Aldosterone (mineralocorticoid) activity increases Na reabsorption and H+ secretion in the distal tubule.
Base Excess is:
The amount of acid or base that must be added to return blood pH to 7.4
A positive base excess indicates _____ _____ and a negative value indicates ____ ____.
Positive= Metabolic Alkalosis
Negative = Metabolic Acidosis
Physiologic effects of acidosis:
K increases ____ for each ___ unit increase in pH.
K increases 0.6 mEq/L for each 0.1 unit increase in pH.
The oxyhemoglobin dissociation curve shifts to the ___ during acidosis.
Right (decreased affinity for oxygen); gives O2 to tissues.
What cardiac effects does acidosis have?
- Decreased responsiveness to catecholamines
- Decreased contractility
Anion gap measures what?
The Anion Gap (AG) is a derived variable primarily used for the evaluation of metabolic acidosis to determine the presence of unmeasured anions. The normal anion gap depends on serum phosphate and serum albumin concentrations. An elevated anion gap strongly suggests the presence of a metabolic acidosis.
What is a NORMAL anion gap value?
Anion GAP= [Na+] - ([Cl-] + [HCO3-]
Normal values= 140- (104 + 24) = 12 mEq/L
Normal range: 7-14 mEq/L
Can you have a NORMAL anion gap and have acidosis?
YES:
Usually seen with hyperchloremia (Cl- takes the place of the bicarbonate anion)
Related conditions:
- Diarrhea
- Renal loss of HCO3-
Acidosis with a HIGH anion gap:
-In this case H+ consumes HCO3- and an unmeasured anion accumalates and takes the place of bicarbonate.
Related conditions:
- Uremia
- Diabetic Ketoacidosis
- Lactic acidosis
Treatment for Metabolic Acidosis:
-Treat the underlying cause of the acidosis:
(Hypovolemia, anemia (Hgb is a buffer), cardiogenic shock.)
Do we treat METABOLIC acidosis with Sodium Bicarbonate (NaHCO3)?
NO!
Rare instances only, like MH when you’re really up shit creek.
Sodium Bicarbonate (NaHCO3) has the effect of increasing the CO2 too much. For a person in respiratory failure or other pulmonary disorders, they cannot expell all the CO2 through respirations.
Why do we check ABGs in the OR?
Mainly for H & H changes and secondarily for electrolyte values.
-blood loss is an important indicator of impending acidosis. That’s why EBL is kept track of in OR cases.
Treatments of Alkalosis:
- aldosterone antagonist spironolactone or with other potassium-sparing diuretics (eg, amiloride, triamterene)
- HypoKalemia will also augment H+ secretion. (correct K imbalances)
ABG interpretation:
What is the normal pH range:
7.35-7.45
Panic Value:
less than 7.25 or greater than 7.55
ABG interpretation:
What is the normal pCO2 range:
pCO2 35-45 mmHg
Panic Value: pCO2 >60
ABG interpretation:
What is normal pO2?
pO2 >80 mmHg
Panic Value: pO2 <50 mmHg
Remember: pO2 is a measure of oxygenation, not ventilation.
ABG interpretation:
22-26 mmol/L
Pure respiratory Alkalosis:
pH = HIGH
pCO2 = LOW
HCO3= decreased or normal
Pure respiratory Acidosis:
pH = LOW
pCO2 = HIGH
HCO3 = normal or high (compensation)
pO2 = normal or low
Pure metabolic Alkalosis:
pH = High
pCO2 = Normal
HCO3 = normal to high
Pure Metabolic Acidosis:
pH= LOW
pCO2= NORMAL
HCO3= decreased
Metabolic Alkalosis with respiratory compensation:
pH = High
pCO2= High
Metabolic Acidosis with respiratory compensation:
pH= low
pCO2= low
