Transcript ACID-BASE DISORDERS

ACID-BASE DISORDERS
Slides by Sherri Clewell D.O.
9/1/05
Plasma Activity
• Normal value: [H+]= 40meq/L
– PH = 7.4
• Linear relationship [H+] to pH
• Plasma [H+]= f(production, excretion,
buffer)
• pKa~physiologic pH
Plasma Acid Hemostasis
• H+ influenced by
– Rate of endogenous production
– Rate of excretion
– Buffering capacity of body
• Buffers effective at physiologic pH
– Hemoglobin
– Phosphate
– Protiens
– bicarbonate
Henderson-Hasselbach Equation
• Demonstrates interrelationship between
– Carbonic acid
– Bicarbonate
– pH
pH = pK + log [HCO3-] /[H2CO3]
Kassirer-Bleich equation
• [H+] = 24 x PCO2/ [HCO3-]
• Can be used to calculate any component
of buffer system provided other 2
components are known
• (how bicarb is calcuated on a blood gas)
Acid production and Excretion
• Lung: PCO2 action is immediate
• Liver: uses HCO3- to make urea
– Prevents accumulation of ammonia and traps
H+ in distal tubule
• Kidney: lose or make HCO3– Proximal tubule reclaims 85% filtered HCO3– Distal tubule reclaims 15%, and excretes H+
Fundamental acid base disorders
• Acidemia = pos net H+ in blood
• Alkalemia = neg net H+ in blood
• Normal or high pH does not exclude
acidosis
• Normal or low pH does not exclude
alkalosis
Fundamental acid base disorders
• Respiratory Disorder – first affect pco2
• Metabolic disorder – first affect HCO3-
Anion Gap
• AG= [Na+] – ([HCO3] + [Cl-])
• Normal anion gap is 7 +/- 4
• Is the unmeasured anion concentration
Metabolic Acidosis
• Caused by an decrease in bicarb this is
replaced by unmeasured anion (elevated
anion gap) or by chloride (no anion gap)
• Loss by GI-vomiting, enterocutaneous
fistula
• Loss by kidney- RTA, carbonic anhydrase
inhibitor therapy
Metabolic Acidosis
• Unopposed metabolic acidosis results in
decreased serum bicarb and increased H+
• H+ stimulates respiratory center to
increase minute ventilation to lower H+ by
reduction in PCO2
Metabolic Acidosis
• The compensatory mechanism calculation
• PCO2 = (1.5 x [HCO3-] + 8) +/- 2
• With normal respiratory compensation the
PCO2 fallby by 1 mm Hg for every 1
meq/L fall in HCO3• If calculation PCO2 differs from pts PCO2
then concominant respiratory disorder
Anion gap metabolic acidosis
•
•
•
•
•
•
•
•
M methanol
U uremia
D DKA
P paraldahyde, propylene glycol
I Isoniazide, Iron
L lactic acidosis
E ethylene glycol, ethanol
S salicylates, starvation ketoacidosis
Non anion gap metabolic acidosis
• Bicarb loss in GI, urine
• Hypoaldosteronism, renal tubular acidosis,
urinary tract obstruction
• Sometimes referred to as hyperchloremic
metabolic acidosis
Metabolic Acidosis
• Treatment is aimed at treating the
underlying cause, restoring normal tissue
perfusion
• Must know if underlying respiratory
disorder because must treat respiratory
first
Buffer Therapy
• Must use bicarb judiciously
• Can cause paradoxical CNS acidosis
• Give if
– Bicarb <4
– pH <7.2 with signs of shock or myocardial
irritability
– Severe hyperchloremic acidemia
Metabolic alkalosis
• Chloride sensitive
– Causes; vomiting, diarrhea, diuretic, CHF
– Treatment: normal saline
• Chloride insensitive
– Cause: excessive mineralcorticoid, no
chloride loss
– Treatment: treat underlying cause
Respiratory acidosis
• Inadequate ventilation
• Diagnosed when PCO2 is greater then
expected value
Acute Respiratory Acidosis
• /\ H+ = 0.8 (/\ PCO2)
• If the [H+] is higher or lower than
suggested by change in PCO2 a mixed
disorder is present
Chronic Respiratory Acidosis
• /\[H+] = 0.3 (/\ PCO2)
Respiratory Alkalosis
• Acute
• /\[H+] = 0.4 (/\PCO2)
• Chronic
• /\[H+] = 0.75(/\PCO2)
Questions
• 1.Causes of anion gap acidosis include all
of the following except
– A. salicylate poisoning
– B. isopropyl alcohol ingestion
– C. uremia
– D. seizures
QUESTIONS
• 2. An elevation anion gap and an elevation
of the osmolar gap may be seen in all of
the following except
– A. uremia
– B. ethanol intoxication
– C. methanol poisoning
– Diabetic ketoacidosis
Questions
• 3. The pulmonary excretion of CO2
– A. Raises the serum H+ concentration
– B. Raises the serum pH
– C. Decreases the renal excretion of
bicarbonate
– D. Raises the serum concentration of
bicarbonate
Questions
• 4. Physiologic compensation for metabolic
acidosis occurs through all of the following
mechanisms except
– A. Persistent vomiting
– B. Pulmonary excretion of CO2
– C. Increased renal H+ excretion
– D. Increased renal bicarbonate losses
ANSWERS
•
•
•
•
1. B
2. B
3. B
4. D