Controlled hypotension effected by sodium nitroprusside

Reductive metabolism of nitroprusside in rat hepatocytes and human erythrocytes

The metabolism of nitroprusside by hepatocytes or subcellular fractions involves a one-electron reduction of nitroprusside to the corresponding metal-nitroxyl radical. Thiol compounds also reduced nitroprusside to the metal-nitroxyl radical apparently via a thiol adduct. The nitroprusside reduction by microsomes was shown to be due to cytochrome P450 reductase as an antibody to cytochrome P450 reductase inhibits the microsomal reduction of nitroprusside, and the inhibitors of cytochrome P450 such as carbon monoxide or metyrapone had no effect. The reduction of nitroprusside by mitochondria in the presence of NADH or NADPH also produced the metal-nitroxyl radical. In hepatocytes, both mitochondria and the cytochrome P450 reductase are involved in the reduction of nitroprusside. The reductive metabolism of nitroprusside was found to produce toxic by-products, namely, free cyanide anion and hydrogen peroxide. We have also detected thiyl radicals formed in the thiol compound reduction of NP. We propose that cyanide and hydrogen peroxide are important toxic species formed in the metabolism of nitroprusside. The rate of reductive metabolism of nitroprusside by rat hepatocytes was much higher than with human erythrocytes. Therefore the major site of nitroprusside metabolism in vivo may be liver and not blood as originally proposed.

Effect of sodium nitroprusside on the biophase concentration of D-tubocurarine at the neuromuscular junction

D-Tubocurarine (DTC) at an intravenous dose of 250 micrograms/kg, or at an intra-arterial dose of 25 micrograms/kg, produced about a 50% block on the twitch tension of the indirectly stimulated tibialis anterior muscle of anaesthetized cats. The DTC-induced neuromuscular block was abolished if it was given intravenously but not intra-arterially, 30 min after the commencement of sodium nitroprusside (SNP) infusion (5 micrograms/kg per min) when there was a maximum decrease in blood pressure. The results indicate that the biophase concentration of DTC at the neuromuscular junction may be decreased as a consequence of the vasodilation induced by SNP leading to a reduced transport of the blocker to the site of action.

Nitroprusside, antihypertensive drug and analytical reagent. Review of (photo)stability, pharmacology and analytical properties

A review of physical, chemical, analytical and pharmacological properties of nitroprusside is presented. In view of the pharmaceutical applications of nitroprusside special attention is given to the discussion of the (photo)degradation, the stability of the pharmaceutical formulations, the application as a reagent in pharmaceutical analysis and the redox behaviour.

Baroreflex function in elderly hypertensives

Baroreflex function was assessed in elderly hypertensive patients and compared with that observed in young hypertensives and young normotensives. Mean arterial pressure was reduced by 20% using intravenous nitroprusside infusion in 10 elderly hypertensive patients (older than 65 years and diastolic pressures over 95 mm Hg), in 10 young hypertensives (under 60 years and diastolic pressures over 95 mm Hg), and in seven young normotensive subjects (under 60 years and diastolic pressures under 95 mm Hg). Elderly subjects demonstrated greater sensitivity (p less than 0.005) and greater variability of response (p less than 0.025) to nitroprusside than either young group. There was no significant difference between the slight heart rate increases observed in the supine position in the three groups. However, in the erect position, heart rate increases were significantly less in the elderly hypertensive group than in the young hypertensive group (p less than 0.01) or the young normotensive group (p less than 0.005). Furthermore, the slope of the regression line relating change in blood pressure with change in R-R interval was less for the elderly patients than for the young hypertensives (p less than 0.05) or the young normotensives (p less than 0.025). We conclude that the heart rate component of the baroreflex is impaired in elderly hypertensives, and anticipate that the clinical response to antihypertensive drugs will be altered.

Effect of sodium nitroprusside during the payback period of cardiopulmonary bypass on the incidence of postoperative arrhythmias

This study was designed to determine whether a sodium nitropruside infusion during the reperfusion (payback) period of cardiopulmonary bypass would minimize arrhythmias during the early postoperative period of coronary artery bypass surgery. A double-blind randomized study was carried out in 38 patients with no previous history of ventricular arrhythmias. Seventeen received 5% dextrose in water (D5W) and 21 received sodium nitroprusside at the rate of 2 microgram per kilogram per minute during the payback period. The pump flow was kept constant at 2.2 liters per square meter per minute, and mean pressure was maintained at greater than 50 mm Hg. There was a statistically significant difference between the two groups in the number of patients who developed ventricular arrhythmias (13 of 17, or 76%, in the D5W group versus 6 of 21, or 29%, in the sodium nitroprusside group; p less than 0.005). Twelve of the 13 patients in the D5W group experienced arrhythmias (6 ventricular tachycardia and 6 ventricular premature depolarization) within the first 24 hours, compared to 5 of 12 patients in the nitroprusside group (3 ventricular tachycardia and 2 ventricular premature depolarization). Only 1 patient in each group developed ventricular arrhythmia after the first postoperative day. One patient in each group experienced atrial arrhythmia during the first 24 hours. After 24 hours, atrial arrhythmias developed in 5 patients in the D5W group (35%) and 3 patients in the sodium nitroprusside group (17%) (p greater than 0.05). The arterial pH ranged from 7.35 to 7.55, with a Po2 greater than 70 torr and a serum potassium of 3.7 +/- 0.36 mEq per liter in the D5W group and 3.4 +/- 0.34 mEq per liter in the nitroprusside group during the period of arrhythmias. Sodium nitroprusside given during the payback period of cardiopulmonary bypass appears to minimize ventricular arrhythmias in the early postoperative period of coronary artery bypass surgery.

Autoregulation of myocardial blood flow under controlled hypotension in the dog

The autoregulation of myocardial blood flow was studied under: (a) haemorrhagic hypotension (mean pressure decrease 40% from baseline); (b) drug-induced hypotension with sodium nitroprusside or trimetaphan (40% pressure decrease); (c) drug-induced hypotension (40% pressure decrease) after moderate blood loss of 20%. Dogs were used in experiments under general anaesthesia with sodium thiopentone and artificially ventilated with N2O/O2 (70%, 30%) and pancuronium. Under haemorrhagic hypotension, the flow was pressure-dependent from the beginning. During nitroprusside hypotension, there was autoregulation of coronary flow to the level of mean pressure decrease 30% from control. Under trimetaphan, a constant flow persisted to the level of mean pressure decrease 20% from the baseline. Identical changes were seen during drug-induced hypotension after moderate blood loss (20%). Central venous pressure, cardiac output, left ventricular stroke work and minute work changes are demonstrated. Blood gas data pH, PO2, PCO2 in the aortic and coronary sinus under drug-induced hypotension are compared with control groups. There is a level of autoregulation of myocardial blood flow under hypotension. This level can be unfavourably changed in cases with hypertension and coronary obstructive diseases.

Comparison of the bronchodilator and vasodilator activity of sodium azide and sodium nitroprusside in the guinea-pig

1. Sodium azide and sodium nitroprusside are potent dilators of the intact guinea-pig tracheal preparation in vitro. 2. Both substances are bronchodilators in the anaesthetized guinea-pig in vivo when administered intravenously or by aerosol inhalation. 3. Sodium azide and sodium nitroprusside are also potent vasodilators in the guinea pig. 4. At all doses and by route of administration, including aerosol inhalation, the hypotensive effect predominates over bronchodilatation.

Systemic hypotension in neurosurgery

The authors review the intraoperative use of elective hypotension to reduce the probability of hemorrhage, to increase pliability of the aneurysmal sac for ease of clip application, and to control hemorrhage. The optimum agent and techniques for lowering systemic blood pressure remain controversial, but trimethaphan, sodium nitroprusside, and halothane have been found most useful. When cerebral blood flow falls below the brain's capacity to autoregulate, distinct time-related alterations occur biochemically and histologically. The profile of prolonged reduced adenosine triphosphate (ATP), low phosphocreatine, low glucose, and elevated lactate and lactate/pyruvate ratio is associated with swelling of perivascular astrocytes and "blebbing" of vascular endothelial cells with subsequent cerebral damage. To prevent permanent alteration it is desirable to observe time constraints and to employ other means of protection such as hypothermia, although the authors believe the latter unnecessary for short hypotensive periods. It has been proposed, but not substantiated, that anesthetics which depress rate of cerebral oxygen consumption but do not affect cerebral ATP level protect the brain from hypotension. Several investigations suggest that halothane, a vasodiltor, satisfies the safety requirement. The most prominent contraindication to halothane, however, is elevation of intracranial pressure. At present hypotensive surgery for aneurysmorrhapy is usually performed when intracranial pressure has returned to normal. Experimentally the electroencephalogram has been observed to show alterations prior to biochemical parameters for following brain vulnerability, so that it conceivably could be an effective monitoring technique during prolonged profound hypotension.

[Conservation of the blood]

Many factors already contribute to limit the amount of bank blood available for therapeutic use; a possible reduction in the amount of available blood could stem from the relative indifference of young people towards the gift of blood and the increase in the size of the pool of positive carriers of Australian antigen. Moreover, new operations appear (coronary by-pass) which increase the demand for blood. It is imperative that we adopt attitudes and practice techniques that will contribute to the conservation of blood. This paper discusses the advantages of an active blood bank committee which, alone, can reduce the consumption of blood by 20 percent. Moreover, actual techniques for conservation of blood are reviewed: controlled hypotension, haemodilution and intraoperative blood salvage.

Sodium nitroprusside: theory and practice

Sodium nitroprusside, by a peripheral vasodilator action, is a powerful hypotensive drug. In contrast to established agents, its hypotensive action is accompanied by an unchanged or augmented cardiac output. Intravenous administration of sodium nitroprusside rapidly produces a profound and controllable hypotension. Normotension is swiftly restored by ceasing to infuse the agent, and antidotes are unnecessary. Its attributes make it especially suitable for neurosurgical anaesthesia, and although free cyanide is liberated during biotransformation, side effects are virtually absent provided that dosage is reasonable. Special contraindications to its use are hypothyroidism and altered cobalamin metabolism.