Nephro - Acid base balance

What is a normal pH?

7.35-7.45 (7.40 avg)

What range pH is compatible with life?

6.8-8.0

What are the body’s 3 buffer systems?

Bicarbonate-carbonic acid buffer - ECF & ICF (most important)

Phosphate buffer - ICF

Protein buffer - albumin & globulin (ECF) & hgb (ICF)

What are the body’s 2 major buffers?

Acid → Carbonic acid - H2CO3 1.2 mEq/L

Base → Bicarbonate HCO3 24 mEq/L

What is the carbonic anhydrase equation that regulates acid bace balance?

[lungs] CO2 + H2O ⇋ H2CO3 ⇋ H⁺ + HCO3 [kidneys]

As CO2, carbonic acid, & H+ increases ______

pH drops < 7.35 → more acidic

As HCO3 increases & H+ decreases, _______

pH rises > 7.45 → more basic

How is pH calculated?

Bicarbonate (kidneys) / carbon dioxide (lungs)

_____ blows off / get rid of CO2 ; ______ retains CO2 for rapid regulation

Hyperventilation ; hypoventilation

What is a normal PaCO2?

35-45 mmHg

How do the kidneys regulate acid base?

Excretion or retention of H+ or HCO3; slow regulation (hours to days)

What is a normal serum HCO3?

22-26 mEq/L

What causes acidosis (pH < 7.35)?

Inc in blood carbonic acid or dec in bicarb

What causes alkalosis (pH > 7.45)?

Inc in bicarb or dec in carbonic acid

What is the normal ratio of HCO3 to H2CO3?

20:1

What is compensation?

Corrective response of kidneys/lung to restore pH & 20:1 ratio; pH approaches near normal

What is uncompensated acid base imbalance?

Inability to adjust pH or 20:1 ratio; pH highly abnormal

What causes respiratory acidosis?

Hypoventilation

What kind of imbalance?

• pH < 7.35

• PaCO2 > 45

• excess carbonic acid, CO2 rises

Respiratory acidosis

What is the compensation for respiratory acidosis?

Kidneys retain HCO3 (slow response)

What causes respiratory alkalosis?

Hyperventilation

What kind of imbalance?

• pH > 7.45

• PaCO2 < 35

• carbonic acid deficit, CO2 decreases

Respiratory alkalosis

What is the compensation for respiratory alkalosis?

Kidney excretes excess bicarb

What causes metabolic acidosis?

Gain in anions → lactic acidosis, ketoacidosis (HAGMA)

Actual loss of HCO3 → renal failure, diarrhea (NAGMA)

What kind of imbalance?

• pH < 7.35

• HCO3 < 22 mEq/L

• Base-bicarb deficit, low plasma bicarb

Metabolic acidosis

What is the compensation for metabolic acidosis?

Lungs hyperventilate to get rid of CO2 → dec PaCO2 & raise HCO3

What causes metabolic alkalosis?

MCC → vomiting, gastric suction (NG tube)

Other → antacid, diuretics (contraction alk)

What kind of imbalance?

• pH > 7.45

• HCO3 > 26 mEq/L

• bicarbonate excess, loss of H+ ions or gain HCO3

Metabolic alkalosis

What is the compensation for metabolic alkalosis?

Lungs hypoventilate → retain CO2, inc PaCO2, inc acid level in blood

What is a normal PaO2?

80-100 mmHg (95)

What is a normal O2 sat?

95-100%

How can you tell if an acid base imbalance is uncompensated?

Abnormally high or low pH

Only 1 component is abnormal (high/low CO2 or HCO3)

The component not causing the imbalance is normal

How can you tell if an acid base imbalance is compensated?

pH near normal

Other values are abnormal in opposite directions (one acidotic, one alkaline)

What formula is used to determine of if a mixed acid base disorder is present in the assessment of elevated anion gap metabolic acidosis?

Delta - delta ratio

What is the formula for the delta delta ratio?

(measured AG - normal AG) / normal HCO3 - measured HCO3 OR

(AG - 12) / 24 - [HCO3]

What delta ratio indicates hyperchloraemic non anion gap metabolic acidosis?

< 0.4

What delta ratio indicates HAGMA + NAGMA (may be isolated renal failure)?

0.4-0.8

What delta ratio indicates uncomplicated HAGMA?

1-2

What delta ratio indicates metabolic acidosis w/ pre-existing elevated HCO3 (metabolic alkalosis or respiratory acidosis)?

> 2

What is the normal PaO2 for adults < 70 y/o on room air?

*pO2 drops ~10 mmHg for each decade above 70

90-100 mmHg

What is the normal PaO2 for ages 70-79 on room air?

80-100 mmHg

What are the ranges for mild hypoxemia?

60-80 mmHg

What are the ranges for moderate hypoxemia?

40-60 mmHg

What are the ranges for severe hypoxemia?

< 40 mmHg

At what pulse ox level would confusion begin?

< 90%

What pulse ox level is life threatening?

< 70%

What formula calculates the lungs compensation in metabolic acidosis?

Winter’s formula

What is winters formula?

pCO2 = 1.5(HCO3) + 8 (±2)

What is a normal anion gap?

10

How do you calculate anion gap?

Na - (Cl + HCO3)

What is another way to estimate the PCO2 compensation in metabolic acidosis?

Last 2 digits of pH (almost always equals pCO2)

What causes non anion gap metabolic acidosis (NAGMA)?

GI bicarb loss (diarrhea), kidney bicarb loss (type 2 proximal RTA) or inability of kidney to adequately excrete acid (type 1 & 4 distal RTA, CKD)

Other: HARDUP

What is used to evaluate the etiology of NAGMA to differentiate between GI loss of HCO3 or renal tubular acidosis?

Urine anion gap (UAG)

What is the formula for urine anion gap (UAG)?

Na⁺ + K⁺ - Cl^-

What does a negative UAG indicate?

GI loss

What does a positive UAG indicate?

Renal loss

What is the mechanism of type 2 renal tubular acidosis (RTA)?

Proximal convoluted tubule defect in bicarbonate reabsorption

What lab findings are associated with type 2 RTA?

Urine pH: initially > 5.5, later < 5.5

Hypokalemia

Positive UAG

What conditions are associated with type 2 RTA?

MM, acetazolamide, fanconi syndrome

What is the only RTA to have hyperkalemia?

Type 4

What are the only RTAs to have urine pH > 5.5?

Type 1 & type 2 (initially)

What is the mechanism of type 1 RTA?

Distal hydrogen ion secretion defect

What is the mechanism of type 4 RTA?

Aldosterone def or resistance to aldosterone effects

What values are expected in metabolic acidosis?

pH: < 7.35

Primary: dec HCO3 < 22

Comp: dec PCO2

What values are expected in metabolic alkalosis?

pH: > 7.45

Primary: inc HCO3 > 26

Comp: inc PCO2

What values are expected in respiratory acidosis?

pH: < 7.35

Primary: inc CO2 > 45

Comp: inc HCO3

What values are expected in respiratory alkalosis?

pH: > 7.45

Primary: dec CO2 < 35

Comp: dec HCO3