Acid-Base Interactions - Aviram

Question 1

Normal serum values for pH, [H+], pCO2 and [HCO3-]

Answer 1

Normal serum pH :7.36-7.44 (+2 SD)
Normal serum [H+]: 40 nEq/L
Normal serum [HCO3-]: 24 mEq/L
Normal serum pCO2: 40 mmHg

Question 2

Normal plasma cation concentrations

Answer 2

ion nmoles/L mEq/L

 H+ 40 40x 10-5
 Na+ 140,000,000 140
 K+ 4,000,000 4
 Ca++ 2,500,000 5
 Mg++ 1,000,000 2

Question 3

Definition of pH in acidemia and alkalemia

Answer 3

Acidemia: Serum pH < 7.35
Alkalemia: Serum pH > 7.45

Question 4

How does the kidney compensate for pH changes?

Answer 4

Changing rates of both tubular acid secretion and HCO3- absorption. Takes about 1-3 days.

Question 5

Alveolar ventilation equation (VA)

Answer 5

0.863 x VCO2 / PaCO2

VCO2 is the rate of CO2 production

Question 6

Common causes of primary respiratory acidosis

Answer 6

 Depression of respiratory control centers:
anesthetics, sedatives, brain injury,
severe hypercapnia and hypoxemia.
 Neuromuscular disorders: spinal cord or
phrenic nerve injury, myopathies,
neuropathies.
 Chest wall restriction: severe
kyphoscoliosis, extreme obesity.

 Airway obstruction: COPD, upper airway
obstruction, asthma.
 Pulmonary parenchymal disease:
pulmonary edema, pneumonia, ARDS,
Cystic fibrosis.
 Lung restriction: Pulmonary fibrosis,
pneumothorax, hemothorax, pleural
effusion.

Question 7

Common causes of respiratory alkalosis

Answer 7

 CNS: Anxiety, hyperventilation syndrome,
fever, pain encephalitis, meningitis, CVA,
brain tumor.
 Drugs or hormones: Salicylates (aspirin),
progesterone, nicotine.
 Sepsis.
 Pulmonary disease: Acute asthma, pulmonary
embolism, interstitial fibrosis.
 Over-ventilation with mechanical ventilation.
 Hypoxia: High altitude.

Question 8

Anion Gap

Answer 8

An approximation of the total concentration of anions other than Cl- and HCO3-

 Na+ and K+ account for 95% of all cations , whereas Cl- and HCO3- account for 85% of all
anions.
 Unmeasured anion include: sulfate, phosphate,
urate, lactate, oxalate and pyruvate.
 Anion Gap = [Na+] – {[HCO3- + Cl-]}
Anion Gap = 12 +/- 3

Question 9

Common causes of primary metabolic acidosis

Answer 9

2 major causes:
A. Loss of HCO3 via GIT – Diarrhea.
B. Loss of HCO3 in the urine - RTA (renal tubular acidosis)
Urinary Anion Gap: [Na+ + K+] - Cl-
Urinary AG << 0 (- 20 or more ) – GIT loss.
Urinary AG = or >0 . RTA

Diruetics

Question 10

How does metabolic compensation change between acute and chronic respiratory acidosis?

Answer 10

Acute respiratory acidosis:
 For each 10 mmHg rise in PaCO2, pH will fall by
0.08, and bicarbonate will increase by 1 mmol/L.
 Chronic respiratory acidosis:
For each 10 mmHg rise in PaCO2, pH will fall by
0.03, and bicarbonate will increase by 4
mmol/L

Question 11

How does metabolic compensation change between acute and chronic respiratory alkalosis?

Answer 11

Acute respiratory alkalosis:
 For each 10 mmHg decrease in PaCO2, pH will rise by 0.08, and bicarbonate will decrease by 1-3 mmol/L.
 Chronic respiratory alkalosis:
For each 10 mmHg decrease in PaCO2, pH will rise by 0.03, and bicarbonate will decrease by 3-5 mmol/L.

Question 12

How does respiratory compensation change between metabolic acidosis and alkalosis?

Answer 12

Metabolic acidosis:
 For each 1 mmol/L fall in bicarbonate,
PaCO2 will decrease by 1-1.5 mmHg.
(PaCO2 = 1.5 [HCO3-] + 8.
 Metabolic alkalosis:
For each 1 mmol/L rise in bicarbonate,
PaCO2 will increase by 0.25-1 mmHg.
(PaCO2 = 0.9 [HCO3-] + 16.

Question 13

Calculation for excess anion gap

Answer 13

Total anion gap minus the normal anion
gap (12) and add this value to the
measured [HCO3-]
The sum > 30 mmol/L : Metabolic alkalosis
The sum < 23 mmol/L : non-anion gap
metabolic acidosis.