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Oral case 114

Created: 20/9/2004

Obstetrical anesthesia

After three hours of labor with a functioning epidural in place, beat-to-beat variability in the fetal heart rate is lost. The obstetrician asks you if it may be related to your epidural drug, and asks if you can "give the mother something".

1. What is your response?

2. Late decelerations occur, and an urgent cesarean section is planned. What will your preoperative preparation be?

3. Will you use the epidural, or will you induce general anesthesia? Give your rationale for your choice.

Weaning from cardiopulmonary bypass

A 65-year old male who had a mitral valve replacement for mitral regurgitation is having difficulty separating from cardiopulmonary bypass, secondary to hypotension.

1. How will you evaluate causes?

2. How will you manage?

Epinephrine can be used as a first line agent due to its ease of titration and its potency. Dobutamine may be used because it is less potent and therefore less adversely affects myocardial oxygen demand, and has favorable hemodynamic on both ventricles. For persistent low cardiac output or hypotension, other inotropes like epinephrine or a phosphodiesterase inhibitor may be added. Phosphodiesterase inhibitors may be used as a first line agent, as it causes positive inotropy and vasodilation, improved coronary arterial and collateral flow, and decreased myocardial oxygen demand. If cor pulmonale or pulmonary hypertension exist, a phosphodiesterase inhibitor is the drug of choice.

3. Would you consider using ephedrine after bypass?

Ephedrine, a mixed indirect and direct nonselective alpha and beta agonist, can support blood pressure with safety, but may lead to tachycardia and tachyphylaxis.

4. What effect does cardiopulmonary bypass have on thyroid hormones?

Bypass can decrease triiodothyronine or T3, thyroxine or T4, and thyroid-stimulating hormone for days postoperatively, and euthyroid patients may get a "euthyroid sick syndrome" as well. In patients with normal thyroid function, a study by Novitsky et al showed that if given T3, all patients with impaired ventricular function on preoperative exam had improved stroke volume, cardiac output, and systemic and pulmonary vascular resistance. Also, administration of thyroxine immediately increases cardiac activity in patients who are hypothyroid.

5. How does thyroid hormone increase cardiac inotropy?

There is an increase in mitochondrial adenosine triphosphate, or ATP, increased calcium-ATPase in the sarcolemma, and increased sodium entry into myocardial cells. This increased sodium causes increased calcium concentration and activity.

6. Name two cases in which a patient should receive thyroid hormone for heart surgery.

Hypothyroid patients, and those undergoing heart transplant, as they are more likely to develop euthyroid sick syndrome.

7. Discuss the use of glucose-insulin-potassium infusions to improve myocardial function.

Preoperatively, this increases myocardial glycogen, which preserves myocardium during surgery. It also increases glucose availability and ATP production, which helps myocardial function and tolerance of ischemia. After cardiac surgery, there is less need for inotropic support. The mechanism lies in the decreased uptake of myocardial free fatty acids, increased carbohydrate metabolism, and decreased myocardial oxygen demand.

8. What are complications associated with the use of this infusion?

Complications include hypokalemia with resultant arrhythmias, and hyperglycemia with resultant neurologic ischemia.

3. Are there preoperative or intraoperative predictors of those who will need inotropic support after bypass?

Poor ejection fraction is a good indicator. Also predictive are those with impaired ventricular function, regional wall dyskinesis, advanced age, cardiomegaly, females, and high left ventricular end-diastolic pressures prior and after contrast injection.

Intraoperatively, predictors of the need for inotropic support include those with unfavorable response to anesthetics, inadequate myocardial preservation during aortic cross-clamp, suboptimal revascularization or valve repair, and bypass time over 150 minutes with an ejection fraction over 45%. At partial bypass separation, the left ventricular vent is removed, and the arterial waveform is observed simultaneously with the pulmonary artery pressure. If the pulmonary artery pressure goes up and there is no or little left ventricular ejection, inotropic support will probably be necessary. Also, decreased contractility by direct visualization of the anterior surface of the heart just prior to separation from bypass is relatively predictive.

Stridor in the recovery room

A 28-year old, 110 kg female has undergone cholecystectomy. After forty-five minutes in the recovery room, she is extubated and you are called to evaluate increased respiratory rate and "noisy respiration".

1. What are possible causes?

2. What would you do?

3. How will you differentiate etiologies?

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