Prescription Drug Information: Sodium Nitroprusside (Page 3 of 4)

OVERDOSAGE

Overdosage of nitroprusside can be manifested as excessive hypotension or cyanide toxicity (see WARNINGS) or as thiocyanate toxicity (see ADVERSE REACTIONS).

The acute intravenous mean lethal doses (LD 50 ) of nitroprusside in rabbits, dogs, mice, and rats are 2.8, 5.0, 8.4, and 11.2 mg/kg, respectively.

Treatment of Cyanide Toxicity: Cyanide levels can be measured by many laboratories, and blood-gas studies that can detect venous hyperoxemia or acidosis are widely available. Acidosis may not appear until more than an hour after the appearance of dangerous cyanide levels, and laboratory tests should not be awaited. Reasonable suspicion of cyanide toxicity is adequate grounds for initiation of treatment.

Treatment of cyanide toxicity consists of

• discontinuing the administration of sodium nitroprusside;

• providing a buffer for cyanide by using sodium nitrite to convert as much hemoglobin into methemoglobin as the patient can safely tolerate; and then

• infusing sodium thiosulfate in sufficient quantity to convert the cyanide into thiocyanate.

The necessary medications for this treatment are contained in commercially available Cyanide Antidote Kits. Alternatively, discrete stocks of medications can be used.

Hemodialysis is ineffective in removal of cyanide, but it will eliminate most thiocyanate.

Cyanide Antidote Kits contain both amyl nitrite and sodium nitrite for induction of methemoglobinemia. The amyl nitrite is supplied in the form of inhalant ampoules, for administration in environments where intravenous administration of sodium nitrite may be delayed. In a patient who already has a patent intravenous line, use of amyl nitrite confers no benefit that is not provided by infusion of sodium nitrite.

Sodium nitrite is available in a 3% solution, and 4 to 6 mg/kg (about 0.2 mL/kg) should be injected over 2 to 4 minutes. This dose can be expected to convert about 10% of the patient’s hemoglobin into methemoglobin; this level of methemoglobinemia is not associated with any important hazard of its own. The nitrite infusion may cause transient vasodilatation and hypotension, and this hypotension must, if it occurs, be routinely managed.

Immediately after infusion of the sodium nitrite, sodium thiosulfate should be infused. This agent is available in 10% and 25% solutions, and the recommended dose is 150 to 200 mg/kg; a typical adult dose is 50 mL of the 25% solution. Thiosulfate treatment of an acutely cyanide-toxic patient will raise thiocyanate levels, but not to a dangerous degree.

The nitrite/thiosulfate regimen may be repeated, at half the original doses, after two hours.

DOSAGE AND ADMINISTRATION

Dilution to Proper Strength for Infusion: Depending on the desired concentration, the solution containing 50 mg of Sodium Nitroprusside Injection must be further diluted in 250 to 1000 mL of sterile 5% dextrose injection. The diluted solution should be protected from light, using the supplied opaque sleeve, aluminum foil, or other opaque material. It is not necessary to cover the infusion drip chamber or the tubing.

Verification of the Chemical Integrity of the Product: Sodium nitroprusside solution can be inactivated by reactions with trace contaminants. The products of these reactions are often blue, green, or red, much brighter than the faint brownish color of unreacted Sodium Nitroprusside Injection. Discolored solutions, or solutions in which particulate matter is visible, should not be used. If properly protected from light, the freshly diluted solution is stable for 24 hours.

No other drugs should be administered in the same solution with sodium nitroprusside.

Avoidance of Excessive Hypotension: While the average effective rate in adult and pediatric patients is about 3 mcg/kg/min, some patients will become dangerously hypotensive when they receive Sodium Nitroprusside Injection at this rate. Infusion of sodium nitroprusside should therefore be started at a very low rate (0.3 mcg/kg/min), with upward titration every few minutes until the desired effect is achieved or the maximum recommended infusion rate (10 mcg/kg/min) has been reached.

Because sodium nitroprusside’s hypotensive effect is very rapid in onset and in dissipation, small variations in infusion rate can lead to wide, undesirable variations in blood pressure. Since there is inherent variation in blood pressure measurement, confirm the drug effect at any infusion rate after an additional 5 minutes before titrating to a higher dose to achieve the desired blood pressure. Sodium nitroprusside should not be infused through ordinary I.V. apparatus, regulated only by gravity and mechanical clamps. Only an infusion pump, preferably a volumetric pump, should be used.

Because sodium nitroprusside can induce essentially unlimited blood-pressure reduction, the blood pressure of a patient receiving this drug must be continuously monitored , using either a continually reinflated sphygmomanometer or (preferably) an intra-arterial pressure sensor. Special caution should be used in elderly patients, since they may be more sensitive to the hypotensive effects of the drug.

When sodium nitroprusside is used in the treatment of acute congestive heart failure, titration of the infusion rate must be guided by the results of invasive hemodynamic monitoring with simultaneous monitoring of urine output. Sodium nitroprusside can be titrated by increasing the infusion rate until:

• measured cardiac output is no longer increasing,

• systemic blood pressure cannot be further reduced without compromising the perfusion of vital organs, or

• the maximum recommended infusion rate has been reached, whichever comes earliest. Specific hemodynamic goals must be tailored to the clinical situation, but improvements in cardiac output and left ventricular filling pressure must not be purchased at the price of undue hypotension and consequent hypoperfusion.

Table 2 below shows the infusion rates corresponding to the recommended initial and maximal doses (0.3 mcg/kg/min and 10 mcg/kg/min, respectively) for both adult and pediatric patients of various weights. This infusion rate may be lower than indicated in the table for patients less than 10 kg. Note that when the concentration used in a given patient is changed, the tubing is still filled with a solution at the previous concentration.

Table 2: Infusion Rates (mL/hour) to Achieve Initial (0.3 mcg/kg/min) and Maximal (10 mcg/kg/min) Dosing of Sodium Nitroprusside Injection

Volume

Sodium Nitroprusside Injection

concentration

250 mL

50 mg

200 mcg/mL

500 mL

50 mg

100 mcg/mL

1000 mL

50 mg

50 mcg/mL

pt

weight

kg

lbs

init

max

init

max

init

max

10

22

1

30

2

60

4

120

20

44

2

60

4

120

7

240

30

66

3

90

5

180

11

360

40

88

4

120

7

240

14

480

50

110

5

150

9

300

18

600

60

132

5

180

11

360

22

720

70

154

6

210

13

420

25

840

80

176

7

240

14

480

29

960

90

198

8

270

16

540

32

1080

100

220

9

300

18

600

36

1200

Avoidance of Cyanide Toxicity: As described in CLINICAL PHARMACOLOGY above, when more than 500 mcg/kg of sodium nitroprusside is administered faster than 2 mcg/kg/min, cyanide is generated faster than the unaided patient can eliminate it. Administration of sodium thiosulfate has been shown to increase the rate of cyanide processing, reducing the hazard of cyanide toxicity. Although toxic reactions to sodium thiosulfate have not been reported, the co-infusion regimen has not been extensively studied, and it cannot be recommended without reservation. In one study, sodium thiosulfate appeared to potentiate the hypotensive effects of sodium nitroprusside.

Co-infusions of sodium thiosulfate have been administered at rates of 5 to 10 times that of sodium nitroprusside. Care must be taken to avoid the indiscriminate use of prolonged or high doses of sodium nitroprusside with sodium thiosulfate as this may result in thiocyanate toxicity and hypovolemia. Incautious administration of sodium nitroprusside must still be avoided, and all of the precautions concerning sodium nitroprusside administration must still be observed.

Consideration of Methemoglobinemia and Thiocyanate Toxicity: Rare patients receiving more than 10 mg/kg of sodium nitroprusside will develop methemoglobinemia; other patients, especially those with impaired renal function, will predictably develop thiocyanate toxicity after prolonged, rapid infusions.

In accordance with the descriptions in ADVERSE REACTIONS above, patients with suggestive findings should be tested for these toxicities.

WARNING: Do not use flexible container in series connections.

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