Symptom to Diagnosis - Acid-Base Abnormalities Flashcards
Step 2 in evaluation of acid-base disorders?
Determine whether the primary disorder is due to a metabolic or respiratory process. Check HCO3 and PaCO2.
Step 3 in evaluation of acid-base abnormalities?
- Calculate whether compensation is appropriate. 2. Formulas predict the expected degree of compensation. 3. Greater or less than predicted compensation suggests that an additional process is affecting the compensating system.
Step 4 in evaluation of acid-base abnormalities?
Calculate the anion gap –> Na -(HCO3+Cl). If ELEVATED –> Anion gap metabolic acidosis!
When does anion gap acidosis occur?
When an acid is produced and the associated unmeasured anion accumulates - ie ketones, lactate, sulfates, phosphates, or organic anions –> Increasing the anion gap.
Anion gap is affected by?
Serum albumin level –> Albumin is negatively charged so that lower serum albumin levels are associated with a lower anion gap.
What is the expected drop in the normal value for the anion gap?
2.5mEq/L for every 1g/dL drop in the serum albumin (below 4.4g/dL).
Step 1 in evaluation of acid-base abnormalities?
Check pH: PRIMARY disorder is acidosis. >7.4 –> PRIMARY disorder is alkalosis.
Compensation in respiratory acidosis - Acute?
HCO3 UP 1mEq/L per 10mmHg UP PaCO2.
Compensation in metabolic alkalosis (either acute or chronic)?
PaCO2 UP 0.7mmHg per 1 mEq/L HCO3 increase.
Compensation in acid-base disorders - Metabolic acidosis (either acute or chronic)?
Expected compensation: PaCO2 DOWN 1.2mmHg per 1mEq/L HCO3 decrease. To a MINIMUM of 10-15mmHg PaCO2.
Compensation of respiratory acidosis - Chronic?
HCO3 UP 3.5mEq/L per 10mmHg UP PaCO2.
Metabolic or respiratory compensation is slower and becomes more complete with time?
Metabolic.
Compensation in respiratory alkalosis - Chronic?
HCO3 DOWN 4mEq/L per 10mmHg DOWN PaCO2.
Normal baseline of PaCO2 and HCO3?
40mmHg and 24mEq/L.
Non anion gap metabolic acidosis occurs when?
HCO3 is lost in the urine or stool.
The normal anion gap is due to?
Negatively charged proteins such as albumin, phosphates, and sulfates.
Etiology of metabolic acidosis - Anion gap acidoses?
- Ketoacidosis - DKA, starvation, alcoholic. 2. Lactic acidosis - 2o to any impairment of aerobic metabolism. 3. Uremia - associated with sulfate and phosphate accumulation. 4. Toxin, drugs, and miscellaneous - Aspirin, methanol, ethylene glycol, rhabdomyolysis, D-lactic acidosis.
The DDX of lactic acidosis includes?
Any disease that interrupts O2 transport from the environment to the cell’s mitochondria –> Common causes include hypoxia and hypotension (shock).
Non anion gap metabolic acidosis - Etiology?
- Diarrhea. 2. Renal tubular acidosis (RTA) (type IV most common in adults). 3. Carbonic anhydrase inhibitor. 4. Dilutional - large volume normal saline administration. 5. Early renal failure.
Metabolic alkalosis - Etiology?
- Vomiting or nasogastric tube. 2. Volume depletion - Diuretics, vomiting. 3. Hypokalemia. 4. Incr. mineralocorticoid activity –> Primary hyperaldosteronism/Hypercortisolism/Excessive licorice ingestion (!).
Respiratory acidosis - Etiology?
Any process that participates in normal ventilation - brain, brainstem, spinal cord, nerve, neuromuscular junction, muscle, chest wall, or lung.
Respiratory acidosis - Specific etiology?
- Brain –> Stroke, drugs, hemorrhage, trauma, sleep apnea. 2. Brainstem herniation. 3. Spinal cord –> Trauma, ALS, polio. 4. Nerve –> Gullain-Barre. 5. Neuromuscular junction –> Myasthenia gravis. 6. Chest wall or muscle –> Flail chest, muscular dystrophy. 7. Pleural disease –> Effusions, pneumothorax.
Respiratory acidosis - MCC?
Lung diseases: 1. COPD. 2. Asthma. 3. Pulm. edema. 4. Pneumonia.
Respiratory alkalosis - Etiology?
- Hypoxemia. 2. Pulm. disorders - via both hypoxic and vagal mechanisms. 3. Extrapulmonary disorders.
Respiratory alkalosis - Pulmonary disorders?
- Pneumonia. 2. Asthma. 3. Pulm. embolism. 4. Pulm. edema. 5. Interstitial lung disease. 6. Mechanical ventilation.
Respiratory alkalosis - Extrapulmonary disorders?
- Anxiety. 2. Pain. 3. Fever. 4. Pregnancy. 5. CNS insult. 6. Drugs (salicylates, nicotine, catecholamines). 7. Cirrhosis.
Textbook presentation of DKA:
Often begins as an acute illness (ie pneumonia, UTI, MI) in a type I diabetic.
DKA precipitants in type II diabetes:
- Severe stress. 2. Marked hyperglycemia that may transiently impair insulin secretion.
DKA incidence:
4.6-8 cases/1.000 person years in patients with diabetes.
General precipitants of DKA:
- Low insulin. 2. High insulin counter-hormones (cortisol, Epi, glucagon, GH). 3. BOTH.
DKA - MC precipitants:
- New-onset type I DM. 2. Non compliance with insulin. 3. Infection (UTI and pneumonia are MC. Patients may be AFEBRILE).
DKA - Other precipitants:
- Other infections. 2. MI. 3. CVA. 4. PE. 5. Acute pancreatitis. 6. GI hemorrhage. 7. Severe emotional stress. 8. Drugs (eg steroids, thiazides, cocaine).
Most frequent cause of mortality in DKA:
The precipitant.
DKA Pathogenesis:
Marked decrease in insulin levels together with an increase in counterregulatory hormones.
DKA - Following events:
- Hyperglycemia. 2. Ketoacidosis. 3. Volume depletion. 4. Hypokalemia. 5. Hyperkalemia. 6. Hyponatremia.
DKA - Hyperglycemia:
- Reduced glucose uptake by cells leads to hyperglycemia. 2. Increased hepatic glycogenolysis and gluconeogenesis –> augment hyperglycemia. 3. Glucosuria helps prevent extreme hyperglycemia (>500-600mg/dL). 4. More extreme hyperglycemia occurs if urinary output falls.
DKA - Ketoacidosis:
- Marked insulin deficiency increases acetyl CoA production within the liver. 2. Massive production of acetyl CoA overwhelms Krebs cycle –> Ketone production and ketonemia (primarily β hydroxybutyric acid and to a lesser extent acetoacetic acid). 3. Ketonemia leads to anion gap metabolic acidosis.
DKA - Volume depletion:
Ketonemia and hyperglycemia result in OSMOTIC DIURESIS which results in profound dehydration and typical fluid losses of 3-6L.
DKA - Hypokalemia:
- Osmotic diuresis also causes significant K losses. 2. Dehydration-induced hyperaldosteronism –> K loss. 3. Typical K deficit is 3-5mEq/kg body weight.
DKA - HYPERkalemia:
- Despite the total body K, hyperkalemia is frequent. 2. Etiology is multifactorial. a. Insulin normally drives glucose + K INTO the cells. Insulin def. causes HYPERkalemia. b. Plasma hypertonicity drives H2O + K OUT of the cell –> HYPERkalemia. c. Acidosis –> K OUT of the cells –> HYPERkalemia.
DKA - Hyponatremia:
Hyperglycemia leads to an osmotic shift of water from the intracellular space to intravascular space, resulting in hyponatremia.
DKA mortality rate:
5-15%.
Compensation in respiratory alkalosis - Acute?
HCO3 DOWN 2mEq/L per 10mmHg DOWN PaCO2.
DKA - Main symptoms/signs:
- Polyuria and increased thirst are common. 2. Lethargy and obtundation may be seen with markedly increased effective osmolality (>320mOsm/L). 3. Abdominal pain. 4. Nausea and vomiting are common and nonspecific.
DKA - Diagnostic criteria established by the American Diabetes Association (ADA):
- Glucose >250mg/dL.
- pH
- HCO3
- Positive serum ketones.
DKA - Effective osmolality can be calculated:
2x Na + Gluc/18.
DKA - Abdominal pain:
Present in 50-75% of DKA cases.
DKA - Abdominal pain due to:
2o to the DKA or another process precipitating DKA (appendicitis, pancreatitis, cholecystitis, abscess). –> Increasingly common with increasing severity of DKA.
DKA - When to consider an intra-abdominal cause?
Always, especially if the abdominal pain persists, occurs in patients with mild acidosis (HCO3 >10mEq/L), or in patients older than 40yrs.
DKA - Patients with glucose
15% - Particularly in pregnancy or with poor oral intake.
Sensitivity of glucose >250 for DKA?
POOR (11%).
DKA - 3 ketones:
- β-hydroxybutyrate. 2. Acetoacetate. 3. Acetone.
DKA - Use of nitroprusside reaction?
To detect ketones - Acetoacetate but NOT beta-hydroxybutyrate.
Prominent ketone in severe DKA:
Beta hydroxybutyrate.
Which drug causes a false(+) nitroprusside reaction?
Captopril.
DKA diagnosis - Measurement of beta hydroxybutyrate:
Directly and rapidly. 98% sensitive. 85% specific. LR+ 6.5. LR- 0.02.
DKA diagnosis - urine ketones helpful?
Sensitive for DKA but NOT specific (69%). Blood measurement is preferred.
Anion gap - In patients with DKA and glucose >250 in the ED - Sensitivity/specificity, LR+, LR-?
84-90% sensitive. 85-99% specific. LR+ 6-84. LR- 0.11-0.16.