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