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Oral case 17

Created: 6/9/2004

A 6-year old 20 kg boy is admitted to the hospital for dental extractions of carious teeth. He has Tetralogy of Fallot with a left-sided systemic to pulmonary artery shunt performed at two years of age. Lately, the cyanotic spells have increased in severity. The blood pressure is 95/70 mm Hg, heart rate is 106 bpm, respirations are 34, and the hematocrit is 68.

 Preoperative evaluation

I. Evaluation of cardiac status

1. What are possible reasons for the extractions?

2. What is the significance of the cyanotic spells?

3. Describe the pathophysiology of Tetralogy of Fallot. Why is this important to you?

4. What do you expect to find on the chest radiograph of this patient?

 Preoperative therapy requirements

1. Would you give propranolol?

2. Would you give digitalis?

3. What is the mechanism of action of digitalis?

Digitalis increases intracellular sodium and calcium. It does this by binding to sodium-potassium adenosine triphosphatase in the sarcolemma. More calcium is available for interaction with troponin C, increasing excitation-contraction coupling and thus the strength of contraction.

4. Why is digitalis good to use in a patient with a failing myocardium?

Positive inotropy results in decreased end-diastolic pressure and volume, decreasing heart size and therefore decreased myocardial wall tension, oxygen demand, and angina. It also decreases systemic vascular resistance, and improves congestive heart failure in patients on doses not causing significant inotropism.

5. What are some risk factors for developing digitalis toxicity?

Hypokalemia, hypercalcemia, hypomagnesemia, hypoxia, advanced age, hypothyroidism, and drugs like propranolol, verapamil, quinidine, or amiodarone.

6. What does intravenous calcium chloride cause?

With severe hypocalcemia, exogenous calcium may cause exaggerated inotropy, and with normocalcemia or hypercalcemia, exogenous calcium transiently increases systemic vascular resistance.

7. Would you phlebotomize the child?

8. What fluids would you give?

9. Would you give a premedication? Why or why not?

 Intraoperative course

I. Equipment requirements

1. Would you use a central venous catheter? Explain.

2. Which anesthesia system would you choose? Why?

3. Describe the mechanics of the system you chose.

II. Management of anesthesia

1. Which drugs would you use for induction? Explain.

2. What can cause reversal of the shunt?

3. Compare ketamine alone versus endotracheal technique.

4. Which muscle relaxant is preferable for intubation? Explain.

5. Which maintenance agents would you utilize? Why?

6. Compare the central, myocardial, arrhythmic, and peripheral effects of the agent of your choice.

 Intraoperative hypoxia and acidosis: Evaluation and management

1. During the operative procedure, paO2 drops from 45 to 25. Why?

2. What is your treatment?

3. Would positive end-expiratory pressure be helpful?

4. What is the role of the kidneys in maintaining acid/base balance?

The kidneys filter bicarbonate and then reabsorb it; they excrete acid into the urine. Renal tubular acidosis occurs either when the proximal tube does not reabsorb bicarbonate, which is renal tubular acidosis type II, or when distal tubule excretion of ammonium is impaired, which is renal tubular acidosis type I.

4. The pulse drops to 40 bpm, with a blood pressure now of 30/20 mm Hg. Why?

5. What is your treatment?

6. The blood gas now reveals a pH of 6.9, paCO2 of 65, and a paO2 of 50 mm Hg. Should sodium bicarbonate be given? Why or why not?

Bicarbonate therapy is best used on patients with adequate ventilation; so more CO2 does not form and then cause increased intracellular acidosis. Patients should also have a pH less than 7.20, when generalized enzymatic and metabolic dysfunction begins.

Lactic acidosis due to hypovolemia and subsequent tissue hypoperfusion may exist in this patient, and volume resuscitation should be performed first, before bicarbonate therapy is instituted.

Bicarbonate is useful in renal failure, hyperkalemia, bicarbonate-wasting states, and to alkalinize the urine for such things as tricyclic antidepressant overdosage.

7. How much bicarbonate would you give?

The weight in kilograms multiplied by the 0.3 times the base excess gives the total base deficit, of which half is replaced and then another arterial blood gas is taken to further assess deficit.

 Management of fluid therapy

1. What fluids are needed? Why?

The goal in resuscitation for hypovolemic patients is to restore perfusion and reverse the hypovolemia. The extracellular fluid volume needs to be replaced, with a balanced salt solution being the first choice, for it distributes into both the blood volume and the interstitial space. Because one third of such a solution remains in the blood volume, actual blood loss is replaced three to one with crystalloid.

The patient may be in a state of compensated shock, where a decreased cardiac output exists, and there is still tachycardia and decreased urine output. A persistent metabolic acidosis may ensue, with increased lactate and possible multiple organ failure and death. Blood transfusion remains the treatment of choice for ongoing intraoperative blood loss in a patient with hypovolemia.

2. What is your limit of infusion rate? 3. Damage to the tongue occurs, with major associated blood loss. Would you change your fluid management?

4. What are your limits to allowable blood loss?

5. Does the child need a high hematocrit?

 Postoperative care

I. Management of hypothermia

1. The patient arrives in the recovery room with a temperature of 33 degrees centigrade. Is this a problem? Explain.

II. Interpretation and management of stridor

1. Upon arousal after extubation, the patient is noted to have moderate stridor. What is your differential diagnosis?

2. What is your treatment?

 Management of anesthesia for reoperation

1. The patient continues to bleed excessively in the recovery room, and requires reoperation. How do you proceed with anesthesia?

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