1. How do you analyze arterial blood gas values?

3 basic points:

a. pH tells you whether the primary event is acidosis (less than 7.4) or alkalosis (greater than 7.4).  (secondary event is compensation for primary event.)

b. CO2 is acidic.  Bicarb is basic.

c. Look at CO2 and bicarb, then pH:

i. high CO2 and bicarb:

-acidosis:  respiratory acidosis

-alkalosis: metabolic alkalosis

ii. low CO2 and bicarb:

-acidosis: metabolic acidosis

-alkalosis: respiratory alkalosis

2. T or F:  The body does not compensate beyond normal pH.

True.  There is no "overcorrection".  pH won't pass 7.4 in the other direction when compensating.

3. List the common causes of acidosis.

Respiratory acidosis: decreased exhale

-COPD, asthma

-respiratory depressant meds: opioids, benzodiazepines, barbiturates, alcohol

Metabolic acidosis: acid (MUDPILES)

-methanol

-uremia

-DKA, diarrhea

-paraldehyde (instead carbonic anhydrase inhibitors)

-iron, INH (isoniazid)

-lactic acid  (sepsis, shock, bowel ischemia)

-ethanol, ethylene glycol

-salicylates, aspirin  

4.  List the common causes of alkalosis.

Respiratory alkalosis: increased breathing

-anxiety/hyperventilation

-aspirin/salicylate overdose

Metabolic alkalosis: loss of bicarb

-diuretics (except carbonic anhydrase inhibitors)

-vomiting

-volume contraction

-hyperaldosteronism

-antacid abuse/ milk-alkali syndrome

5. Aspirin overdose: what type of acid-base disturbance?

Acid-base: metabolic acidosis, respiratory alkalosis

(2 different primary disturbances!)

5b. What sx with aspirin overdose?

sx: tinnitus, hypoglycemia, vomiting, hx of swallowing several pills

5c. What tx for aspirin overdose?

tx: Bicarbonate alkanilizes urine (speeds excretion)

6. What happens to the blood gas of patients with chronic lung conditions?

-pH may be alkaline during the day b/c breathing better when awake (esp. w/ sleep apnea)

-just after bronchitis or other respiratory disorder (not retaining air as much), metabolic alkalosis that compensates for respiratory acidosis becomes the primary disorder

-elevated pH and bicarbonate

-sleep apnea and other chronic lung diseases can cause right-sided heart failure (cor pulmonale)

7. Should you give bicarbonate to a patient with acidosis?

Almost never.  

Try IV fluids and correct the underlying disorder.

Note:  If all other measures fail and pH<7.0, can try bicarb.  

8.  The blood gas of a patient with asthma has changed from basic to normal, and the patient is sleeping.  Can they go home?

No.  Patient is probably crashing.  Will progress to acidosis --> intubate!  Prepare for elective intubation, and continue agressive medical treatment of asthma (beta2 agonists, steroids, O2). 

Explanation: Usually, pH is basic in patients with asthma b/c they are eliminating CO2.  If pt. is tired and doesn't blow off CO2 as well, CO2 levels increase and pH decreases towards normal.  

9.  Signs and sx of hyponatremia.

Lethargy, Mental status changes/confusion, Coma

Anorexia

Seizures, Cramps

Hyponatremia:  "Decreased natural state" = Love: Can't think, can't eat, can't stop shaking.

10. Determine cause of hyponatremia?

1st: look at volume status.

Hypovolemic: peeing excessively

-Dehydration, diuretics, diabetes, addison's disease/hypoaldosteronism (high potassium)

Euvolemic:  too much water

-SIADH, psychogenic polydipsia, oxytocin use

Hypervolemic: Organ failure

-heart failure, nephrotic syndrome, cirrhosis, toxemia, renal failure

11.  How is hyponatremia treated?

tx:

hypovolemic hyponatremia: normal saline

euvolemic and hypervolemic hyponatremia: water/fluid restriction

Note: hypervolemic hyponatremia may require diuretics.

12.  What medication is used to treat SIADH if water restriction fails?

Demeclocycline

-induces nephrogenic diabetes insipidus

13.  What happens if hyponatremia is corrected too quickly?

You may cause brainstem damage via central pontine myelinolysis due to demyelination from fluid loss into the hypernatremic serum.

Hypertonic saline is used only when a patient has seizures from severe hyponatremia, and only cautiously.

Normal saline is a better choice 99% of the time on the boards!

14.  What causes spurious (false) hyponatremia?

-Hyperglycemia

(once glucose is >200mg/dl, sodium decreases by 1.6mEq/L for each rise of 100mg/dL in glucose)

-Hyperprotenemia

-Hyperlipidemia

"Low salt diet can't compensate for high sugar, high protein, and high fat diet."

15.  What causes hyponatremia in postoperative patients?

Pain and narcotics (causing SIADH) with overaggresive IV fluids.  

Rare cause on USMLE:  adrenal insufficiency

-high K+, low bp

Think post-surgery (narcotics and IV fluids) vs. adrenal insufficiency.

16.  What is the classic cause of hyopnatremia in pregnant patients about to deliver?

Oxytocin (antidiuretic hormone effect)

17.  Signs and sx of hypernatremia.

Same as hyponatremia:

mental status changes/confusion, coma, seizures

hyperreflexia 

Falling "out of love": Can't think, can't stop shaking, kicking the person.

18.  What causes hypernatremia?

Dehydration (free water loss) due to inadequate fluid intake relative to bodily needs.

Also diuretics, diabetes insipidus, diarrhea, renal disease and too much hypertonic IV fluid.

Impaired kidney concentration:

-sickle cell disease:  may lead to renal damage and isosthenuria (inability to concentrate urine).

-hypokalemia

-hypercalcemia

19.  How is hypernatremia treated?

Water replacement with normal saline.

If stable, can switch to 1/2 normal saline.

Do NOT use dextrose in water (D5W) for hypernatremia.

20.  Signs and sx of hypokalemia?

think opp. of hyperkalemia signs and sx:

hypokalemia: EKG  (or cardiac)

-loss of T wave or T-wave flattening, U waves, premature complexes, tachyarrhythmias

muscular weakness --> paralysis, ventilation failure

smooth muscle weakness --> ileus, hypotension

21.  What is the effect of pH on serum potassium?

alkalosis: hypokalemia

= pH increases (bicarb increases), K+ decreases

acidosis: hyperkalemia

=pH decreases (bicarb decreases), K+ increases

--> give bicarb

"Bicarb will decrease K+."

Also correct pH, and potassium will normalize w/o any other effort.

22.  Describe interaction b/t digitalis and potassium.

Heart is sensitive to hypokalemia in patients on digitalis.  Monitor potassium levels carefully in pts. on digitalis, esp. if also taking diuretics (commonly pts. take both).

23. How should potassium be replaced?

Correct potassium slowly.

Oral replacement preferred.  

IV potassium can be given for severe hypokalemia, but :

-don't give more than 20mEq/hr.

-EKG monitor for potential fatal arrhythmia with IV potassium

24.  When hypokalemia persists even after lots of potassium given, what should you do?

Check the magnesium level.  If low, body cannot retain potassium effectively, so correct low mag level and potassium level can return to nl.

25.  Signs and sx of hyperkalemia?

hyperkalemia = EKG or cardiac

Hyperkalemia is the SCARIER of the two:

-vfib, vtach --> asystole

-don't sit on a peaked T wave (don't sit and don't wait!)

-think: peaked T + widened QRS looks sinusoidal

also prolonged PR --> loss of P waves 

Weakness and paralysis.

26.  What causes hyperkalemia?

hyperkalemia: k = kidneys, a = aldosterone

Failure of kidneys (acute or chronic renal failure)

Failure of kidneys to respond to aldosterone

(diabetes--> kidney damage--> kidneys not responding properly to aldosterone)

Failure of adrenals to produce enough aldosterone for kidneys to respond to

-Adrenal insufficiency (w/ low sodium and low bp)

-Hypoaldosteronism

Severe tissue destruction (release of intracellular K)

Medications (stop potassium-sparin diuretics, ACE-Inhibitors, beta blockers, NSAIDs)

27.  What should you suspect if an asymptomatic patient has hyperkalemia?

Think hemolyzed lab specimen (high intracellular K), and repeat test.

28.  The specimen is not hemolyzed. What is the first treatment?

1st: EKG to assess cardiotoxicity (peaked T waves, etc.)

Best therapy for hyperkalemia: decrease K+ intake, give oral sodium polystyrene resin.

K+>6.5 and/or cardiac toxicity apparent: IV therapy

-1st: calcium gluconate (cardioprotective)

-2nd: sodium bicarb and insulin w/ glucose

(alkalosis and insulin shift K+ into cells)

-3rd: beta2 agonists (also drive K+ into cells)

If renal failure (high Cr) or IV therapy not working: dialysis!!

29.  Signs and sx of hypocalcemia?

Neuro: tetany

-tap facial nerve, contraction (Chveostek's sign)

-bp cuff causes hand mm spasm (Trousseau's sign)

EKG:  QT prolonged (calcemia)

Other:

-depression

-encephalopathy, dementia

-convulsions/seizures

-laryngospasm

30.  What to do if calcium level is low?

Hypoproteinemia (low albumin) can cause hypocalcemia b/c protein-bound fraction of calcium is decreased.

-Ionized (unbound active fraction) calcium is unchanged

so check albumin level and ionized calcium to make sure "true" hypocacelmia

-For every 1gm/dL decrease in albumin (below 4gm/dL), determine actual calcium levels by adding 0.8mg/dL to calcium level.

31.  What causes hypocalcemia?

think: hypocalcemia--> hypoparathyroid

--> Vit. D--> DiGeorge's

-DiGeorge's syndrome (tetany 24-48 hrs. after birth, absent thymic shadow on x-ray)

-Renal failure (kidney makes vit. D)

-Hypoparathyroidism (watch for a postthyroidectomy patient b/c all 4 parathyroids may have been removed)

-Vit. D deficiency

-Pseudohypoparathyroidism (short fingers, short stature, mental retardation, nl parathyroid hormone w/ end-organ unresponsiveness to parathyroid hormone)

-acute pancreatitis

-renal tubular acidosis

32.  Describe relationship between low calcium and low magnesium.

Difficult to correct hypocalcemia until hypomagnesemia is corrected.

33.  How does pH affect calcium levels?

Same as pH to potassium: bicarb shifts Ca into cells

Alkalosis causes sx similar to hypocalcemia through effects on ionized fraction of calcium  (alkalosis causes calcium to shift intracellularly).  

--> correct pH

Hyperventilation/anxiety --> decrease CO2 --> alkalosis --> hypocalcemia --> perioral and extremity tingling

-tx: reduce anxiety

34.  Relationship b/t calcium and phosphorus.  When is this important and how to treat.

Phosphorus and calcium levels go in opp. directions.

Chronic renal failure:  raise calcium levels (vit. D and calcium supplements) and restrict phosphorus.

35.  Signs and sx of hypercalcemia?

Often asymptomatic, usually only found by routine lab tests.

If sx:  

EKG:  QT-interval shortening

Bones (osteopenia, pathologic fractures)

Stones (kidney stones, polyuria)

Groans (abd. pain, anorexia, constipation, ileus, n/v)

Psychiatric overtones (depression, psychosis, delirium/confusion)

Note:  abd. pain may be due to PUD or pancreatitis, both of which may have hypercalcemia.

36. What causes hypercalcemia?

Outpatients: Hyperparathyroidism.

Inpatients:  Malignancy.

Also Vit. A or D excess, sarcoidosis, thiazide diuretics, familial hypocalciuric hypercalcemia (low urinary calcium, which is rare in hypercalcemia)

Hyperoproteinemia (due to decreased bound fraction, no change to unbound/ionized fraction)

37.  Why is asymptomatic hypercalcemia treated?

Prolonged hypercalcemia can cause nephrocalcinosis and renal failure due to calcium salt deposition in kidneys.

May result in bone disease due to loss of calcium.

38.  How is hypercalcemia treated?

Fluids, furosemide, phosphorus

1st: IV fluids.

Then: furosemide to cause calcium diuresis

(no thiazides --> increases calcium)

Other: phosphorus (oral), calcitonin, disphosphonates (etidronate), plicamycin, prednisone (esp. for malignancy-induced hypercalcemia).

tx: Correct underlying cause of hypercalcemia.

Hyperparathyroidism: surgery!

39.  What usually causes hypomagnesemia?

Alcoholism.  Magnesium is wasted through the kidneys.

40.  What are the signs and sx of hypomagnesemia?

Similar to hypocalcemia: prolonged QT, tetany

41.  When will you most often see hypermagnesemia?

Pregnant patients w/ preeclampsia, treated w/ magnesium sulfate.

Renal failure.

Magnesium toxicity:  decreased deep tendon reflexes --> hypotension --> respiratory failure

42.  How is hypermagnesemia treated?

First: stop any mag. infusion!

Remember ABCs, intubate if necessary.

If stable:  start IV fluids, then furosemide to cause mag. diuresis.  

Last resort: dialysis.

43.  When is hypophosphatemia seen?  What are signs and sx?

Image: Loser Fonz is a diabetic alcoholic w/ wasted mm and bleeding out.

Uncontrolled diabetes and alcoholics.

Signs and sx:  

-muscle weakness or loss (rhabdo, esp. in alcoholics)

-encephalopathy

-anemia

-WBC and platelet dysfunction

44. IV fluid of choice in trauma patients?

Ringer's lactate.  

2nd choice: NS.

45.  What is IV choice in nontrauma, hypovolemic pts?

NS or Ringer's lactate.

46.  What is maintenance fluid of choice for patients who are not eating?

1/2 NS w/ 5% dextrose in adults

1/4 or 1/3 NS w/ 5% dextrose in children (due to renal difference)

47.  Should anything be added to IV fluid for patients who are not eating?

Potassium chloride (10 or 20mEq) is added to 1L of IV fluid each day to prevent hypokalemia (assuming baseline K+ is normal).