The toxicity of sodium nitroprusside

Combination of nitrate and sodium nitroprusside dosing for sulfide control with low carbon source loss in sewer biofilm reactors

Nitrate has been widely used in sewer systems for sulfide control. However, significant chemical consumption and the loss of carbon source were observed in previous studies. To find a feasible and cost-effective control strategy of the sulfide control, the effect of nitrate combined with sodium nitroprusside (SNP) dosage strategy was tested in lab-scale sewer biofilm reactors. Results showed that nitrate and SNP were strongly synergistic, with 30 mg N/L nitrate and 20 mg/L SNP being sufficient for sulfide control in this study. While large amount of nitrate alone (100 mg N/L) is required to achieve the same sulfide control effectiveness. Meanwhile, the nitrate combined with SNP could reduce the organic carbon source loss by 80%. Additionally, the high-throughput sequencing results showed that the relative abundance of autotrophic, nitrate reducing-sulfide oxidizing bacteria genera (a-NR-SOB) such as Arcobacter and Sulfurimonas was increased by around 18%, while the heterotrophic, nitrate-reducing bacteria (hNRB) such as Thauera was substantially reduced. It demonstrated that the sulfide control was mainly due to the a-NR-SOB activity under the nitrate and SNP dosing strategy. The microbial functional prediction further revealed that nitrate and SNP promoted the dissimilatory nitrate reduction process which utilizes sulfide as an effective electron donor. Moreover, economic assessment indicated that using the combination of nitrate and SNP for sulfide control in sewers would lower the chemical costs by approximately 35% compared with only nitrate addition.

Nitric oxide modulates mitochondrial activity and apoptosis through protein S-nitrosylation for preimplantation embryo development

PURPOSE

Previous studies reported that patients with endometriosis had excess nitric oxide (NO) in the reproductive tract and poor embryo development in IVF cycles. This study aims to elucidate the effects of NO on early embryo development.

METHODS

Zygotes from superovulated B6CBF1 mice were cultured to blastocysts in a variety of media. Sodium nitroprusside (SNP) and N(G)-nitro-L-arginine (LNA) were added to the culture medium as a NO donor and a NO synthase inhibitor, respectively. The localization and fluorescence intensity of S-nitrosylated (SNO) proteins within 2-cell stage embryos were analyzed with confocal microscopy. Apoptosis and ATP production in the blastocysts were measured.

RESULT(S)

Subsequent to NO exposure, the SNO proteins mainly colocalized with the mitochondria and endoplasmic reticulum and the intensity of SNO proteins increased. The addition of a quanylate cyclase inhibitor and a cyclic GMP mimic agent induced nonsignificant changes in SNO proteins, whereas addition of a superoxide scavenger or a reduced form of glutathione rescued the embryos from the effects of NO. However, superoxide scavenger supplementation resulted in decreased blastocyst ATP production.

CONCLUSION(S)

Elevated NO exerts deleterious effects on embryo development, possibly through protein S-nitrosylation in the mitochondria and endoplasmic reticulum. Including glutathione as a component in the culture medium might counteract this effect.

Sodium nitroprusside is not associated with metabolic acidosis during intraoperative infusion in children

BACKGROUND

Sodium nitroprusside (SNP) is a potent vasodilator that has been used to induce deliberate hypotension in children during surgery involving significant blood loss, including craniofacial and spinal fusion procedures. SNP metabolism liberates cyanide, which may cause interference with cellular energy metabolism, leading to metabolic acidosis and central nervous system injury. We performed a retrospective, case-control study to determine whether the short-term intra-operative use of SNP for deliberate hypotension is associated with metabolic acidosis in children undergoing surgical procedures for craniofacial or spinal anomalies. Cyanide and thiocyanate concentrations were also recorded in patients who received SNP.

METHODS

Data from 166 children undergoing craniofacial and spinal fusion surgery between 2005 and 2010 at Lucile Packard Children's Hospital (LPCH) at Stanford were analyzed. Records from 60 patients who received SNP (SNP group) as part of a multicenter, randomized, double-blind study were compared with records from 106 eligible patients who had blood pressure reduction using anesthetic agents and did not receive SNP (control group). Metabolic acidosis was defined as serum bicarbonate (HCO3) < 18.5 mEq/L. Whole blood CN, plasma thiocyanate and urinary thiocyanate concentrations were measured in patients in the SNP group. Differences in metabolic acidosis rates between the SNP and control groups were assessed through a test of noninferiority in the rate for the SNP group with a noninferiority threshold of 0.2. A z-test was used to test the null hypothesis. The alternative hypothesis was that the difference in these rates was less than 0.2. The same noninferiority threshold of 0.2 was also used to perform separate, secondary tests for noninferiority in the proportion of patients with HCO3 levels below 18.5 mEq/L and the proportion of patients who required HCO3 administration.

RESULTS

Fewer patients in the SNP group experienced metabolic acidosis compared to the control group (31.7% vs. 36.8%, respectively; p < .001). No whole blood CN levels above the lower limit of quantification were detected in any of the 51 patients with validated CN data. Plasma and urinary thiocyanate levels were also low.

CONCLUSIONS

Our findings suggest that SNP, when used for short-term deliberate hypotension, does not cause an increased incidence of metabolic acidosis compared with the use of anesthetic agents alone.

TRIAL REGISTRATION

TRIAL REGISTRATION NUMBER

NCT00135668.

Increases in oxygen consumption without cerebral blood volume change during visual stimulation under hypotension condition

The magnitude of the blood oxygenation level-dependent (BOLD) signal depends on cerebral blood flow (CBF), cerebral blood volume (CBV) and cerebral metabolic rate of oxygen (CMRO2). Thus, it is difficult to separate CMRO2 changes from CBF and CBV changes. To detect the BOLD signal changes induced only by CMRO2 responses without significant evoked CBF and CBV changes, BOLD and CBV functional magnetic resonance imaging (fMRI) responses to visual stimulation were measured under normal and hypotension conditions in isoflurane-anesthetized cats at 4.7 T. When the mean arterial blood pressure (MABP) decreased from 89+/-10 to 50+/-1 mm Hg (mean+/-standard deviation, n=5) by infusion of vasodilator sodium nitroprusside, baseline CBV in the visual cortex increased by 28.4%+/-8.3%. The neural activity-evoked CBV increase in the visual cortex was 10.8%+/-3.9% at normal MABP, but was negligible at hypotension. Positive BOLD changes of +1.8%+/-0.5% (gradient echo time=25 ms) at normal MABP condition became prolonged negative changes of -1.2%+/-0.3% at hypotension. The negative BOLD response at hypotension starts approximately 1 sec earlier than positive BOLD response, but similar to CBV change at normal MABP condition. Our finding shows that the negative BOLD signals in an absence of CBV changes are indicative of an increase in CMRO2. The vasodilator-induced hypotension model simplifies the physiological source of the BOLD fMRI signals, providing an insight into spatial and temporal CMRO2 changes.

Spatial specificity of the enhanced dip inherently induced by prolonged oxygen consumption in cat visual cortex: Implication for columnar resolution functional MRI

Since changes in oxygen consumption induced by active neurons are specific to cortical columns, the small and transient "dip" of deoxyhemoglobin signal, which indicates an increase in oxygen consumption, has been of great interest. In this study, we succeeded in enhancing and sustaining the dip in the deoxyhemoglobin-weighted 620-nm intrinsic optical imaging signals from a 10-s orientation-selective stimulation in cat visual cortex by reducing arterial blood pressure with sodium nitroprusside (a vasodilator) to mitigate the contribution of stimulus-induced blood supply. During this condition, intact spiking activity and a significant reduction of stimulus-induced blood volume changes (570-nm intrinsic signals) were confirmed. The deoxyhemoglobin signal from the prolonged dip was highly localized to iso-orientation domains only during the initial approximately 2 s; the signal specificity weakened over time although the domains were still resolvable after 2 s. The most plausible explanation for this time-dependent spatial specificity is that deoxyhemoglobin induced by oxygen consumption drains from active sites, where spiking activity occurs, to spatially non-specific downstream vessels over time. Our results suggest that the draining effect of pial and intracortical veins in dHb-based imaging techniques, such as blood oxygenation-level dependent (BOLD) functional MRI, is intrinsically unavoidable and reduces its spatial specificity of dHb signal regardless of whether the stimulus-induced blood supply is spatially specific.

Cerebrovascular effects of sodium nitroprusside in the anaesthetized baboon: a positron emission tomographic study

The effects of sodium nitroprusside (SNP), a potent hypotensive agent, on cerebral blood flow (CBF) have been extensively studied in clinical and experimental situations but the results remain controversial. Whereas its properties would predict a dilatation of cerebral blood vessels, most studies report either no change or a decrease in CBF. The aim of this study was to investigate the effects of SNP on CBF, cerebral blood volume (CBV), and cerebral oxygen metabolism (CMRO2), by means of positron emission tomography in the anaesthetized baboon. Measurements were performed during normotension (mean arterial pressure (MABP): 97+/-16 mm Hg) and repeated following SNP-induced hypotension (MABP: 44+/-9 mm Hg). Sodium nitroprusside led to an increase in CBF and CBV (+30% and +37%, respectively, P<0.05), whereas no change in CMRO2 was noted. Linear regression analysis of CBF values as a function of MABP confirmed that CBF increases when MABP is reduced by SNP. The comparison between these cerebrovascular changes and those found during trimetaphan-induced hypotension in our previously published studies further argues for a direct dilatatory effect of SNP on cerebral blood vessels.

Perfusion pressure control by adenosine triphosphate given during cardiopulmonary bypass

Administration of exogenous adenosine triphosphate (ATP) as a vasodilator during cardiopulmonary bypass was assessed in consecutive adult patients (n = 24) who demonstrated a high arterial perfusion pressure (mean, > 90 mm Hg). The action of ATP was characterized by rapid induction and stabilization of the blood pressure level. The dose of ATP ranged from 0.68 to 2.68 mg/min. Within 1 minute after the administration, there was a significant reduction in the perfusion pressure from 102 +/- 18 mm Hg (mean +/- standard deviation) to 72 +/- 19 mm Hg. The ATP was then able to maintain the desired pressure of 69 +/- 12 mm Hg at 5 minutes, 67 +/- 12 mm Hg at 10 minutes, and consistent values thereafter. After the ATP administration was discontinued, there was a prompt recovery of pressure without bradyarrhythmia. The frequency and amount of inotropes used were consistent with the control group (n = 26). Although the administration of ATP reduced the increase in serum catecholamine concentration, there were no significant changes in other vasoactive mediators (eicosanoid, angiotensin II, endothelin) between the two groups during cardiopulmonary bypass. There was neither an accumulation of metabolic products (uric acid, phosphate) nor a decrease in the level of divalent cation (Ca2+), which is observed when the cations combine with phosphates or adenosine nucleotides. This study confirmed the efficacy and safety of ATP infusion during cardiopulmonary bypass.

Nitroprusside intoxication: protection of alpha-ketoglutarate and thiosulphate

Protection against the lethal effects of sodium nitroprusside (SNP) was observed in mice after treatment with alpha-ketoglutarate (AKG), either alone or in combination with sodium thiosulphate (STS). The LD50 of SNP was 12.0 (11.0-13.0) mg/kg in mice. Ip injection of AFG (500 mg/kg twice in 20 min) increased the LD50 1.7-fold in mice. STS (1 g/kg, ip) alone increased the LD50 5.5-fold. Furthermore, combined administration of AKG and STS increased the LD50 6.9-fold. SNP elicited increased cyanide levels in blood of mice in a dose-dependent manner. SNP (10 mg/kg, sc) administration gave rise to blood cyanide levels of 73.2 +/- 3.0 microM, 30 min after treatment. Ip injection of AKG significantly decreased blood cyanide levels by 30% in mice 30 min after treatment with 10 mg SNP/kg. A single injection of STS (1 g/kg) or a combination of AKG and STS reduced in blood cyanide levels by 88 or 98%, respectively, in mice after treatment with 10 mg SNP/kg. In addition, the increase in blood cyanide levels induced by injection of 50 mg SNP/kg was markedly inhibited by a combination of AKG and STS or (to a lesser extent) by STS alone. These results suggest that the combined administration of AKG and STS, by preventing the increase in blood cyanide levels induced by SNP, may afford protection against the toxic effects of SNP.

Esmolol infusion during nitroprusside-induced hypotension: impact on hemodynamics, ventricular performance, and venous admixture

The impact of esmolol infusion on hemodynamics, ventricular performance, venous admixture, sympathoadrenal, and renin-angiotensin system responses during sodium nitroprusside (SNP)-induced hypotension was studied in 11 patients undergoing lymph node dissection during general anesthesia with 60% nitrous oxide and fentanyl. Radial arterial and thermistor-tipped pulmonary catheters were employed for hemodynamic monitoring. Arterial and mixed venous blood gas tensions, arterial plasma renin activity (PRA), and plasma catecholamine levels were measured. Derived hemodynamic parameters and venous admixture (Qs/Qt) data were obtained from standard equations. Transesophageal echocardiography (6 patients) was used to assess left ventricular performance using the relationship between end-systolic wall stress (ESWS) and velocity of circumferential shortening (VCFC). After surgical incision, arterial hypotension was induced with SNP alone. Esmolol was infused at each of the following rates in sequence: 200, 300, and 400 micrograms/kg/min. Each esmolol infusion lasted 20 minutes and the SNP dose was adjusted to maintain MAP at 55 to 60 mm Hg. The mean dose of SNP required to induce hypotension was 5.5 micrograms/kg/min +/- 0.5 SE. Compared to prehypotension values, SNP induced significant increases in Qs/Qt and reductions in PaO2, systemic vascular resistance (SVR), and stroke volume index (SVI). Esmolol infusion caused dose-dependent (highest with 400 micrograms/kg/min) reductions in the SNP requirement, heart rate (HR), SVI, Qs/Qt, and PRA, and also led to significant increases in SVR and left ventricular (LV) internal diameter in diastole as well as systole. Furthermore, esmolol infusion was associated with a dose-dependent downward and leftward shift of the ESWS versus VCFC relationship, implying diminished contractility.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitroprusside in children after cardiopulmonary bypass: a study of thiocyanate toxicity

Thiocyanate levels, an indicator of nitroprusside toxicity, were studied in 22 children after repair of structural heart disease during cardiopulmonary bypass. At the total dose (2.6 +/- 2.3 mg/kg) and time (34.4 +/- 19 h) ranges of this study, no evidence of toxicity was detected, despite this total dose exceeding recommended maximum in some patients. Nitroprusside infusion, as described, in children with normal hepatic and renal function is safe and may not warrant routine assessment of thiocyanate levels.

Use of sodium nitroprusside in post-coronary bypass surgery. A plea for conservatism

In 292 patients who underwent coronary artery bypass graft (CABG) surgery, seven patients developed sodium nitroprusside (SNP) toxicity in the postoperative period. Duration of infusion varied between 26 to 160 hrs and total SNP dose ranged from 1.8 to 12 mg/kg body weight. All patients were critically ill and required ventilatory support in the postoperative period. Tachyphylaxis to SNP requiring increase of SNP dose for control of hypertension, and loss of consciousness were the major signs of toxicity. Other commonly described signs of SNP toxicity were absent in those patients. Discontinuation of SNP therapy and treatment with sodium thiosulfate was followed by improvement in four patients. Three patients who failed to regain consciousness later died because of hemodynamic, pulmonary and/or renal complications. Our observation suggests that recommended doses of SNP may be toxic in unstable post-CABG patients. We recommend that the dose and duration of SNP infusion be minimized in critically ill patients.

Use of sodium nitroprusside in neonates: efficacy and safety

Sodium nitroprusside was administered to 58 neonates, including 11 with severe respiratory distress syndrome, 15 with persistent pulmonary hypertension of the newborn, 28 with clinical shock, three with systemic hypertension, and two with pulmonary hypoplasia, all refractory to conventional intensive therapy. Nitroprusside was infused at 0.2 to 6.0 micrograms/kg/min for periods of 10 minutes to 126 hours. Infants with severe respiratory distress syndrome had increased PaO2 and decreased PaCO2 or peak inspiratory pressure, and nearly all (82%) survived. Infants with persistent pulmonary hypertension of the newborn had variable responses; improvement did not correlate with survival, but survival (47%) was identical to that in an earlier series of infants given tolazoline. Infants in shock had improved perfusion, urine output, and serum bicarbonate levels, and these responses were significantly related to survival. Hypertension was controlled in all three hypertensive infants. Adverse effects were very uncommon. Toxic effects were not observed. Sodium nitroprusside is effective and can be used safely in circulatory disorders in the neonate.

Sodium nitroprusside for control of severe hypertensive disease of pregnancy: a case report and discussion of potential toxicity

Sodium nitroprusside has been used to alter blood pressure in severe hypertensive disease of pregnancy; however, concern exists in regard to potential lethal complications from cyanide toxicity in both mother and fetus. We recently evaluated levels of cyanide in the liver of an infant stillborn to a woman in whom sodium nitroprusside was used to control gestational hypertension secondary to mitral valve disease. The fetal liver demonstrated levels of cyanide below toxic ranges. Biologic activity, potential toxicity, and treatment of toxic symptoms of nitroprusside are discussed.

Central and splanchnic haemodynamics in the dog during controlled hypotension with sodium nitroprusside

The effects of controlled hypotension induced by sodium nitroprusside (SNP) on central and splanchnic haemodynamics were studied in ten artificially ventilated dogs under neurolept anaesthesia. SNP was given intravenously as a continuous infusion in order to maintain a mean arterial blood pressure (MABP) of about 50 mmHg. Observations were made before (control) and at 20 and 60 min after the start of the SNP infusion. The mean SNP dosage was 13.7 micrograms X kg-1 X min-1. Systemic vascular resistance (SVR) decreased by 47%. After 20 min there was a 17% decrease in cardiac output, while the hepatic arterial blood flow was diminished by 39%, and portal venous blood flow by 16%. Cardiac output and portal venous blood flow tended to return towards control values at 60 min, while the hepatic arterial blood flow remained depressed. The total oxygen uptake was unaltered after 20 min, but slightly decreased after 60 min. There were no changes in hepatic or preportal tissue oxygen consumption, nor in hepatic lactate uptake. It is concluded that SNP-induced hypotension was achieved primarily by a profound reduction of SVR, and initially also by a slight decrease in cardiac output. Although splanchnic and hepatic blood flows decreased, there were no signs of hypoxia in the preportal tissues or in the liver.

A comparison of the cardiovascular and orofacial blood flow changes resulting from hypotension induced by sodium nitroprusside and adenosine triphosphate in the rat

These experiments compared the cardiovascular effects and the alterations in orofacial blood flow resulting from hypotension induced with sodium nitroprusside and adenosine triphosphate infusions in 30 male rats. The authors found that sodium nitroprusside did not significantly change heart rate or cardiac output, while adenosine triphosphate caused a significant decrease in heart rate associated with a small increase in cardiac output. Both hypotensive agents produced similar profound decreases in total peripheral resistance. Blood flows to orofacial structures were qualitatively similar with both agents. Both adenosine triphosphate and sodium nitroprusside caused decreased flows to the maxilla and mandible. Sodium nitroprusside increased flows to masseter and suprahyoid muscles. Adenosine triphosphate caused increased flows to suprahyoid muscles but masseter muscle flows were not significantly changed. Tongue flows responded similarly to both agents, initially decreasing, then returning to normotensive levels. Finally, sodium nitroprusside-induced hypotension was associated with the development of tachyphylaxis in some animals, whereas no similar problem was observed with use of adenosine triphosphate. The demonstration of reduced blood flows to orofacial bony structures and the absence of resistance or tachyphylaxis during adenosine triphosphate-induced hypotension support its usefulness as a hypotensive agent during orthognathic and other surgery.

In-vivo and in-vitro hemodialysis studies of thiocyanate

Dialysis clearance of thiocyanate was studied using in-vivo and in-vitro systems. The in-vivo studies were performed in a patient with renal failure receiving sodium nitroprusside infusion for accelerated hypertension. In-vitro studies were carried out under experimental conditions similar to those of the in-vivo experiment. Plasma thiocyanate level consistently fell with single passage through the dialyzer. In-vivo dialysance of thiocyanate averaged 82.8 ml/min as compared to urea dialysance of 129.6 ml/min. The in-vitro studies revealed an average thiocyanate dialysance of 102.3 as compared to a urea dialysance of 138.6 ml/min. Removal of thiocyanate by hemodialysis was further verified by recovery of significant amounts of thiocyanate in the outgoing dialysate. The thiocyanate clearance calculated directly from the amount recovered in the dialysate and mean plasma concentration was 82.2 ml/min, a value closely approximating that obtained using the transdialyzer concentration gradient. We conclude that hemodialysis is effective in removing thiocyanate and can be used as adjunct in the treatment of thiocyanate toxicity particularly in the presence of renal failure in which thiocyanate excretion is impaired.

Sodium nitroprusside and intracranial pressure

The effects of sodium nitroprusside on intracranial pressure were studied in 10 patients, candidates for surgical treatment, prior to anaesthesia. Blood pressure was lowered to at least 50% of its initial value. In all cases, at the beginning of nitroprusside infusion, both the mean and the pulse intracranial pressures increased (mean increase: 83.2% of the initial value). At a certain moment, however, while the blood pressure continued to fall, the mean intracranial pressure did not increase any more; on the contrary, it decreased. On the other hand, in many cases, the pulse intracranial pressure continued to increase. No neurological or EEG changes were observed. The possible changes of cerebral circulation and CSF dynamics underlying the phenomena observed are discussed.

Fetal toxicity of nitroprusside in the pregnant ewe

Sodium nitroprusside, a direct-acting vasodilator, has been used to treat the hypertension associated with preeclampsia and eclampsia. Fatal cyanide intoxication has been reported during infusion of nitroprusside. We have investigated placental transfer and fetal toxicity of nitroprusside in the acute pregnant ewe model. A maternal intravenous infusion of nitroprusside solution was maintained at a rate sufficient to decrease mean maternal arterial pressure by 20% for 1 hour. Maternal and fetal levels of nitroprusside were in equilibrium at the 20 minute sample. Five of the eight animals exhibited tachyphylaxis to nitroprusside, and their fetuses died in utero with lethal levels of cyanide. No significant changes were observed in uterine blood flow.

Vasodilator therapy after cardiac surgery: a review of the efficacy and toxicity of nitroglycerin and nitroprusside

Eight-five patients who required vasodilator therapy in the postoperative period after cardiac surgery were studied to compare the haemodynamic effects of nitroglycerin and nitroprusside, to evaluate local and systemic toxicity, and to develop long-range dosage recommendations. Ninety-one per cent of the patients received the vasodilators for postoperative hypertension, while nine per cent had low output syndromes. Both drugs significantly decreased blood pressure and central venous pressure, and increased heart rate. Nitroglycerin decreased both right and left ventricular filling pressures more than nitroprusside. No local toxicity or methaemoglobinaemia was found with either drug. Elevated thiocyanate levels were detected in 44 per cent of the nitroprusside group; however, none of the patients developed progressive metabolic acidosis. For prolonged infusions we found that nitroprusside at 1 microgram.kg-1.min-1 and nitroglycerin at 0.5 microgram.kg-1.min-1 were without significant toxicity.

Sodium nitroprusside and RDS--primum non nocere

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Controlled hypotension in hip joint surgery. An assessment of surgical haemorrhage during sodium nitroprusside infusion

Controlled hypotension, combined with light balanced anaesthesia, was employed during total hip replacement operations on 25 patients. Sodium nitroprusside (Nipride, Roche), in the form of a 0.01 per cent (100 micrograms/ml) infusion, was used as a hypotensive agent. The mean arterial blood pressure (MABP) was lowered from 108 to 64 mmHg (range 60--70) (P less than 0.001). The average blood loss during the operations was 212 ml and none of the patients required homologous blood transfusion. In comparison with 25 normotensive patients undergoing similar surgery, the difference in the mean volume of haemorrhage between the two groups was 826 ml (P less than 0.001). The difference in the total haemorrhage, however, between the same two groups was 518 ml (mean) (P less than 0.01). The results were compared with those of another investigation in which epidural anaesthesia was used to diminish bleeding during surgery on the hip.

The safe use of sodium nitroprusside

The factors necessary for the safe clinical use of sodium nitroprusside are considered in the light of the author's clinical experience. The reasons for abnormal responses to the exhibition of this hypotensive agent are considered and the precautions necessary and the need to avoid overdosage are emphasised.

Prevention of nitroprusside-induced cyanide toxicity with hydroxocobalamin

To investigate hydroxocobalamin's role in preventing cyanide intoxication from sodium nitroprusside, we studied two groups of patients. One group received nitroprusside alone, and the other received nitroprusside and hydroxocobalamin. Red-cell and plasma cyanide levels were 83.44 +/- 23.12 and 3.51 +/- 1.01 microgram per 100 ml after nitroprusside alone and were 33.18 +/- 17.29 and 2.18 +/- 0.65 microgram per 100 ml after nitroprusside plus hydroxocobalamin. Acidosis developed in patients with red-cell cyanide levels higher than 75 microgram per 100 ml. When hydroxocobalamin infusion was stopped before sodium nitroprusside infusion was discontinued, blood cyanide levels and base deficit increased in a manner similar to that in the untreated group. The dose of nitroprusside used in each group did not differ statistically. These data show that hydroxocobalamin prevents cyanide transfer from red cells and plasma to tissue after nitroprusside metabolism, and thereby prevents cyanide toxicity from large intravenous doses of the drug.

Nitroprusside, its metabolites and red cell function

The effects on metabolism and red cell function of blood levels of thiocyanate (SCN-) and cyanide (CN-) were studied in 42 patients undergoing surgery under controlled hypotension (CH) induced by sodium nitroprusside (SNP). The mean dosage of SNP administered was 21.38 mg (SD = 12). The durating of perfusion was 121 minutes (SD = 11). All operations were performed under neuroleptanalgesia without complications. No tachyphylaxis was encountered. Under SNP a slight increase of blood SCN- (from 13.9 mg/l +/- 1.1 to 23 mg/l +/- 2.6) was found. Blood levels of CN- are increased mostly in the red cell, the mean value being 0.300 mg/l +/- 0.10 for whole blood after two hours of perfusion. This value decreased when perfusion was stopped. All blood samples were negative for methaemoglobin and cyanmethaemoglobin. Carbonic anhydrase activity was not modified, CN- toxicity levels for this enzyme being 50 times higher than those found during our study. 2,3-DPG levels did not vary. Blood gases, acid-base balance and Davo2 did not change significantly, although a slight increase in blood lactate was measured. As shown by this study, appreciable amounts of CN- are detected in blood during SNP perfusion while SCN- stays at relatively low levels. Fortunately most of the CN- released from SNP moves into the red cell and does not alter its functions at clinical concentrations. The low plasma concentration of CN- is not sufficient to cause important metabolic disturbances. However, dosages of SNP higher than those administered during this study could increase the blood and tissue CN- to toxic levels. A toxicity study shows that, during a relatively short period of time, SNP dosage should not exceed 1.16 mg/kg or a maximum of 10 microgram/kg/min for a period of two hours.

Sodium nitroprusside: factors which attenuate its action. Studies with the isolated gracilis muscle of the dog

In a laboratory preparation of the isolated, acutely denervated, and separately perfused canine gracilis muscle we have made the following observations: 1. At physiological pH, sodium nitroprusside significantly decreases the vascular resistance; 2. At physiological pH, cyanide significantly attenuates the effect of sodium nitroprusside; 3. In an acidaemic milieu, our data suggest that the effect of sodium nitroprusside may be attenuated. We speculate that patients who manifest resistance to the hypotensive effect of sodium nitroprusside may not normally eliminate the cyanide that is released from the biodegradation of sodium nitroprusside. They accumulate free cyanide which interferes with the action of sodium nitroprusside at the receptor level, leading to administration of more nitroprusside and setting in motion a positive feedback vicious cycle. When one is faced with the problem of an abnormal response to sodium nitroprusside in a fit patient, although many factors may be involved, we suggest that the possibility of rising blood cyanide levels and acidosis be given high priority.

Cyanide toxicity following nitroprusside induced hypotension

Several recently reported deaths following the use of sodium nitroprusside have been attributed to the accumulation of the nitroprusside metabolite, cyanide. In this study, brief nitroprusside infusions (mean = 36 minutes) were administered in currently recommended doses during intracranial surgery. The peak blood cyanide following the infusions was 65.2 +/- 17.5 microgram per cent (mean +/- SE) (n = 13). It occurred within 45 minutes after infusion. The highest cyanide level detected was 205 microgram per cent, which is within the range of reported lethal blood cyanide levels. Metabolic acidosis developed in the four patients with the highest blood cyanide levels (range 90-205 microgram per cent). This occurred between 45 and 180 minutes following the cyanide peak. Blood ATP levels were depressed in the same patients. These findings are indicative of disturbed aerobic metabolism. We conclude that there is evidence of cyanide toxicity when nitroprusside is infused into patients using currently recommended doses. We recommend that for short infusions the dose of sodium nitroprusside should not exceed 0.5 mg/kg.

Evoked potential changes during brain retraction in dogs

Brain retraction and induced hypotension are surgical adjuncts capable of compromising cerebral blood flow. To evaluate their effects upon brain function, cortical evoked potentials, neurological status and cortical histological changes were determined as a function of graded levels of brain retractor and systemic perfusion pressure in the dog. Somatosensory evoked potentials recorded from the site of application of brain retraction showed a decrement as a function of both the amount of retraction pressure and the systemic perfusion pressure. An electrode distant from the retractor site showed similar, though reduced and more variable changes in amplitude. For higher levels of brain retractor pressure, induced hypotension to 50 mm Hg systemic perfusion pressure produced greater reductions in evoked potentials than in normotensive subjects. It was demonstrated that a reduction of 50% of the evoked potential amplitude after sixty minutes brain retraction signaled, with high probability, the occurrence of postoperative sensory and/or motor deficits and cortical histopathology. It was concluded that cortical evoked potentials represent a reliable indicator of the functional effects produced by applied cortical retraction pressure at several levels of systemic perfusion pressure. It was suggested that the recording of evoked potentials would prove most useful during neurosurgical procedures employing induced hypotension and brain retraction.

The use of sodium nitroprusside in children

The hypotensive response to sodium nitroprusside was investigated in 30 children, age range 5-15 years, weight range 14-60 kg, who were undergoing spinal surgery for kyphoscoliosis. It was possible to achieve a constant level of satisfactory hypotension within the range 70-80 mmHg systolic arterial pressure using sodium nitroprusside infused at a rate of 10 microng/kg/minute and no serious side effects were experienced at this dose rate.

Autoregulation of cerebral blood flow during controlled hypotension in baboons

The effect of graded, progressive hypotension on the autoregulation of cerebral blood flow was studied in anaesthetised baboons. Progressive hypotension was produced over a period of four to five hours, either by graded haemorrhage or by the administration of increasing concentrations of hypotensive drugs. During haemorrhagic hypotension autoregulation was maintained until the mean arterial pressure had decreased to 65% of its baseline value, below which cerebral blood flow was pressure passive. In those animals subjected to drug-induced hypotension, autoregulation persisted to lower levels of mean arterial pressure (35-40% of baseline). It is postulated that under conditions of haemorrhagic hypotension, constriction of the extraparenchymal cerebral vessels in response to sympathetic stimulation decreases the possible range of autoregulation in the anaesthetised baboon.

[EEG effects of sodium nitroprusside and hemodynamic references]

A study was performed on baboons (Papio papio) of various hemodynamic and EEG effects of sodium nitroprussiate (SNP), substance with powerful vasodilator action. Three different procedures of drug injection were used: (1) in isolation, either in doses close to those used in clinical practice or in high doses; (2) after neuroleptanalgesia (NLA); (3) after causing vasospasm. With isolated SNP, effects on blood pressure were unstable; in certain cases hypotension could be maintained only with toxic doses. Hypotension was facilitated when NLA had been previously induced. At low doses the cerebral blood flow (CBF) was practically unmodified, whereas at toxic doses it first increased then decreased. In animals under NLA, the CBF drop was only moderate, suggesting persistence of adequate spasm, CBF was significantly improved through SNP. The EEG did not undergo sizeable change at low SNP doses given in isolation; toxic doses always induced alterations with peculiar morphology, but developing only tardily after SNP perfusion was begun. This lag may indicate that hypotensive and toxic SNP effects are dissociated. These alterations suggest that screening of the EEG is necessary during prolonged administration of SNP, e.g., in surgical procedures. Combining NLA and SNP (at low doses) causes EEG deceleration, but no great alteration of the rhythms, as observed during other types of controlled experimental hypotension. Hence, cerebral circulation may be relatively well protected during SNP hypotension.

Sodium nitroprusside in children: Observations on metabolism during normal and abnormal responses

Three different responses to sodium nitroprusside as a hypotensive agent during anaesthesia in children are described. A constant high dose (greater than 3 mg/Kg) response to, abnormal resistance to and tachyphylaxis to the drug are recognised and are dangerous and potentially lethal. They are probably related to abnormalities of cyanide-thiocyanate metabolism resulting in cyanide accumulation. The abnormal responses produce metabolic acidosis, an increase in mixed venous oxygen tension and a decrease in arterial mixed venous oxygen content difference. Tachyphylaxis may be due to depletion of endogenous thiosulphate and was reversible by sodium thiosulphate therapy (150 mgm/Kg) in two cases in which it was used.

Deliberate hypotension

Deliberate hypotension can reduce major blood loss and indelicate operations can produce a drier field increasing the ease of surgery and the likelihood of a good result. The techniques used to induce hypotension can also be used to avoid dangerous hypertension during and after surgery. These benefits must be weighed against the risks of inadequate perfusion: especially cerebral, myocardial, or renal. In previously normotensive patients these risks are minimal when the arterial pressure is held above 80 torr systolic, and may be acceptably small even at mean pressure of 50 to 60 torr. Previously hypertensive patients show signs of cerebral ischemia at higher pressures; they should probably not be subjected to deliberate hypotension, but they also can be harmed by severe hypertension which can be avioded by the proper use of hypotensive agents. For most situations a balanced technique is suitable: after a stable anesthetic level has been achieved using halothane or enflurane, hypotension can be induced with sodium nitroprusside or trimethaphan camsylate. Longer-acting agents such as pentolinium are sometimes desirable, but the shorter-acting agents are easier to control. Careful monitoring with observation of intra-arterial pressure, electroencephalogram, electrocardiogram, and determination arterial blood gas tensions is likely to make for safer conduct. Close postoperative observation is essential. With careful preparation and monitoring deliberate hypotension can be a safe technique for reducing blood loss or facilitating delicate procedures.