Anesthesia and Pain Control

Question 1

82. A 79 year old white male presents to your office for removal of carious teeth. Medical history review reveals chronic obstructive pulmonary disease (COPD), hypertension, peptic ulcer disease, athlerosclerosis with occasional angina, and osteoarthritis. Daily medications include isosorbide dinitrate, furosemide, and acetaminophen. After conscious sedation with midazolam and local anesthesia with prilocaine, you note that in recovery he has slowly become ashen looking and the pulse oximetry reading has fallen to 85%. Which of the following measures is most appropriate?

A. Intubation and hyperventilation with 100% oxygen
B. Titrated administration of 0.4 mg flumazenil IV
C. Methylene blue administration 1 mg/kg IV
D. Assisted ventilation by face mask with room air.

Answer 1

ANSWER: C

RATIONALE:
This situation may appear to be pulmonary in origin, but in fact represents acquired methemoglobinemia. This condition can be precipitated by nitrates, (such as isosorbide dinitrate) acetaminophen, prilocaine, articaine, and a number of other medications, especially in genetically susceptible individuals. The oxidized (ferric) state of the methemoglobin molecule cannot be reversed by increasing the FIO2, which also may decrease the respiratory drive in COPD. Sedation reversal by flumazenil will have no effect on the condition. Cautious administration of methylene blue will reduce methemoglobin back to a ferrous state, normalizing the oxygen binding/delivering capacity of hemoglobin.

REFERENCE:
Benumof JL Anesthesia & Uncommon Diseases, 4th ed. WB Saunders, 1998 pp288-9

Question 2

83. Which of the following medication is least indicated for acute control of asthma?

A. Cromolyn sodium
B. Metaproterenol
C. Dexamethasone
D. Diphenhydramine

Answer 2

ANSWER: A

RATIONALE:
Chromolyn sodium is a mast cell stabilizer preventing the release of histamine in the mucosa of the tracheobronchial tree when used chronically, but acts too slowly to be useful on an emergent basis. Metaproterenol is a beta agonist used to dilate the airway and reverse bronchoconstriction. Dexamethasone may be used intravenously to control the mucosal inflammatory component of acute asthma. Diphenhydramine, a histamine antagonist, may be administered concomitantly with a beta agonist and a steroid to decrease histamine-mediated bronchoconstriction and mucosal edema.

REFERENCE:
Harrison’s Principles of Internal Medicine 13th ed., McGraw-Hill, 1994 pp 1170-2

Question 3

84. Which of the following medications is the most appropriate agent when considering intubation general anesthesia for the patient with chronic bronchitis and emphysema?

A. Desflurane
B. Methohexital
C. Vecuronium
D. Nitrous oxide

Answer 3

ANSWER: C

RATIONALE:
Vecuronium, a non-depolarizing muscle relaxant, is a steroidal medication and therefore lacks histamine-releasing tendencies that occur with the benzoisoquinolone non- depolarizers. Histamine release can increase tracheobronchial mucosal edema and cause bronchoconstriction. Desflurane, although a potent bronchodilator, is also an airway irritant, causing coughing and increasing sympathetic tone; and may not be desirable in the patient with chronic bronchitis. Methohexital when used as an induction agent can precipitate laryngospasm and bronchospasm in airways already irritated by chronic disease. Nitrous oxide’s extremely low blood solubility will cause it to come out of solution to expand areas of hypoventilation in the lung, which may cause pneumothorax in patients with emphysematous changes and pulmonary blebs.

REFERENCE:
Weinberg, G (ed.) Basic Science Review of Anesthesiology McGraw-Hill, 1997

Question 4

85. Which statement regarding acute respiratory failure is correct?

A. It represents inadequate ventilation, caused by partial obstruction of the airway at the level of the trachea
B. It involves the inability of the lungs to provide adequate exchange of O2 and CO2
C. Generally, diagnosis is made by clinical signs and chest radiography.
D. A PaO2 of <80mm Hg or PaCO2 >40 mmHg is consistent with a diagnosis of acute
respiratory failure.

Answer 4

ANSWER: B

RATIONALE:
Acute respiratory failure is the inability of the lungs to provide adequate arterial oxygenation with or without acceptable elimination of carbon dioxide. It culminates in hypoventilation, hypercarbia, and hypoxemia.
Partial obstruction of the airway at the level of the trachea represents stridor. Measurement of arterial blood gases and pH are mandatory in the diagnosis and management of acute respiratory failure. It is distinguished from chronic respiratory failure on the basis of the relationship of the PaCO2 to pH (acute respiratory failure shows no partial compensation of hypercarbia.) Respiratory failure is diagnosed with arterial hypoxemia (PaO2 <60mm Hg despite supplemental O2), hypercarbia (PaCO2 >50mm Hg), and SaO2 <92%. On room air,
normal PaO2 is 90-100 mmHg, normal PaCO2 is 40 mm Hg, and oxygen saturation is 98- 100%. Functional residual capacity and lung compliance are also reduced in respiratory failure

REFERENCE:
Stoelting, RK and Dierdorf, SF. Handbook For Anesthesia and Co-Existing Disease. New York. 1993. Churchill-Livingstone, Inc. pp. 129-133.
Hurford WE, et.al. Clinical Anesthesia Procedures of the Massachusetts General Hospital. 5th Edition. 1998. Lippincott-Raven. pp. 618-621.