Anesthesia Keywords Defined - 2005
Close Window.

DS083       INTRAOCULAR PRESSURE: GAS SOLUBILITY: SF6, N2O

The goal of using intravitreal gas during reattachment of retina is to have a sustained bubble of stable size holding the retina in place. Sulfur hexafluoride (SF6) is commonly used, and is inert and poorly diffusible.

N2O is 117 times more diffusible than SF6 and rapidly enters the gas bubble, if administered after the injection of SF6 into the vitreal cavity. The injected gas bubble will rapidly increase to three times its original size and IOP increase from 14 to 30 mmHg. When N2O is discontinued, both bubble size and IOP will decrease within 18 minutes. These rapid and wide fluctuations in bubble size may adversely affect the outcome of surgery. Therefore, administration of N2O should be discontinued at least 20 minutes before the injection of gas (washout of N2O 90% complete within 10 minutes).

SF6 gas bubble remains in place for at least 10 days (other intravitreal gases remain 21-28 days). Therefore, N2O should be avoided for general anesthesia within 3-4 weeks. Second exposure might cause reexpansion of the bubble and elevated IOP with resultant occlusion of retinal artery and loss of vision. This is more likely if hypotension also occurs during anesthesia.

1. Donlon JV, Doyle DJ, Feldman MA. Anesthesia for Eye, Ear, Nose, and Throat Surgery. In Miller RD (ed.): Anesthesia. 6th edition. Churchill Livingstone, Philadelphia, PA. 2005:2533.

PR015       DOXORUBICIN: ANESTHETIC IMPLICATIONS

The side effects of doxorubicin include (myelosuppression) thrombocytopenia, leukopenia, anemia, cardiac toxicity, hepatic toxicity, stomatitis and red urine.

Cardiac toxicity manifesting as life threatening cardiomyopathy occurs in 1% to 5% of patients. The rapidly progressive congestive heart failure (CHF) is often refractory to cardiac inotropic drugs or mechanical cardiac assistance. Cardiomegaly and/or pleural effusion may be evident on chest radiographs. QRS voltages on the ECG may be decreased. Patients who have undergone radiation therapy, particularly to the mediastinum, or patients who are on concurrent cyclophosphamide therapy seem to be more susceptible to the development of cardiomyopathy. Impairment of left ventricular function for as long as 3 years after discontinuing the doxorubicin has been observed.

In contrast to life-threatening cardiomyopathy, about 10% of treated patients show nonspecific, usually benign changes on ECG (nonspecific ST-T changes, low QRS voltages, atrial or ventricular premature beats) that do not necessarily reflect an underlying cardiomyopathy.

Mild degrees of cardiomyopathy can be detected preoperatively with echocardiography, or exercise radionuclide angiography.

1. Cancer. In Stoelting RK, Dierdorf SF. (ed.): Anesthesia and Coexisting Disease. 4th edition. Churchill Livingstone, Philadelphia, PA. 2000:588-92.

AS011       DRUGS TO REDUCE THE DOSE OF SODIUM NITROPRUSSIDE

Sodium nitroprusside (SNP) is an extremely potent vasodilator that is available only for IV administration. It acts directly on smooth muscle causing both arterial and venous dilation. This results in decreased pre-load and afterload.

SNP is used in spinal, intracranial (however, remember SNP causes increased ICP by dilating cerebral capacitance and also directly impairs cerebral autoregulation) and other surgeries for induced hypotension to reduce blood loss and facilitate surgery.

The benefits of deliberate hypotension must out weigh the risks as it is associated with many complications. The dose should be titrated to effect, but risk of toxicity is increased if a dose of 10 mg/kg/min is exceeded. Toxicity is recognized by the increasing tolerance to the drug.

Administration of SNP causes a reflex increase in sympathetic tone and renin release. Drugs that blunt these responses markedly enhance its effects (Beta blockers or ACE inhibitors), allowing for lower doses and decreased potential for toxicity due to buildup of cyanide.

Various medications that can be use to reduce the dose of SNP are as follows:
1. Calcium channel blockers (e.g. nicardipine, diltiazem)
2. Beta blockers (e.g. labetalol, esmolol and metoprolol)
3. Alpha 2 agonist such as clonidine
4. Isoflurane, opioids, and propofol
5. Captopril
6. Using nitroprusside in a 1:10 mixture with trimethaphan

1. SJ Lustik. J clin. Anesthesia. 2004;16(1):25-33.
2. Hackman T. Anesth Analg. 2003;96(4):976-81.
3. Sum DC. Acta Anesth Sin. 1996;34(4):203-07.
4. Bendo AA, Kass AS, Hartung J, Cottrell JE. Anesthesia for Neurosurgery. In Barash PG, Cullen BF, Stoelting RK (eds): Clinical Anesthesia. 4th edition. Lippincott Williams & Wilkins, Philadelphia, PA. 2001:773-76.

Close Window.