The effects of sodium nitroprusside on myocardial contractility and haemodynamics

Effects of sodium nitroprusside (MR7S1) and nitroglycerin on the systemic, renal, cerebral, and coronary circulation of dogs anesthetized with enflurane

In beagle dogs anesthetized with enflurane-nitrous oxide, effects of sodium nitroprusside (SNP; MR7S1) and nitroglycerin (NTG) on hemodynamics and main organ circulation were studied to evaluate their effectiveness and safety as hypotensive agents during anesthesia. SNP (MR7S1) infusion (1-10 micrograms/kg/min) decreased arterial blood pressure in a dose-dependent manner. The hypotension was stable during the infusion. After discontinuation of infusion, the blood pressure rapidly returned to the initial level. The hypotension was associated with decreases in cardiac output and total peripheral resistance. NTG infusion (3-10 micrograms/kg/min) decreased arterial blood pressure, too, but the hypotension was less marked and not dose dependent, and the recovery was slower. Neither drug changed the heart rate. Infusion of SNP (MR7S1) and NTG did not change the hypotension induced by the injection of adenosine, SNP, and NTG. Furthermore, cerebral blood flow, cerebral oxygen consumption, and renal blood flow were unchanged during the hypotension produced by either drug. Coronary blood flow was decreased, but this was due to decreases in cardiac oxygen consumption. In conclusion, SNP (MR7S1) is superior to NTG as a hypotensive agent during anesthesia in efficacy, clear dose dependency, and rapid recovery. The hypotension induced by NTG as well as SNP (MR7S1) seems to have no undesirable effects on the circulation of important organs.

Power-afterload relation in the failing human ventricle

Animal studies have shown that the power output of the left ventricle is related to afterload by a bell-shaped curve. Furthermore, the normal ventricle operates at the maximum power point, whereas the diseased ventricle operates off the maximum power point because of increased afterload. We studied this relation in eight patients with dilated cardiomyopathy. A high-fidelity catheter with micromanometer pressure and electromagnetic velocity transducers was used to measure blood pressure and flow velocity in the ascending aorta. The latter was converted into volumetric flow by calibrating with the simultaneously determined thermodilution cardiac output. Ventricular power was calculated by dividing the integral of the aortic blood pressure-flow product by the duration of the cardiac cycle. Intravenous nitroprusside was used to alter afterload and the power-afterload relation was obtained by plotting power against mean aortic blood pressure. In all patients, as blood pressure was lowered initially, the power output of the left ventricle increased. As the dose of nitroprusside was increased further, the total power either plateaued or actually decreased. The averaged power and the mean blood pressure at control were 1.00 +/- 0.62 W and 92 +/- 9.3 mm Hg, respectively. The averaged maximum increase in power with nitroprusside was 22%, to 1.22 +/- 0.73 W, and this occurred at a mean pressure of 80 +/- 8.8 mm Hg. This study showed that the power-afterload relation in the human ventricle has a maximum power point at some intermediate level of afterload, similar to that found in animal studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Self-tuning adaptive control of induced hypotension in humans: a comparison of isoflurane and sodium nitroprusside

Induced hypotension is commonly used during surgery to decrease arterial pressure. Sodium nitroprusside and isoflurane are well-known hypotensive agents. The use of self-tuning adaptive control of induced hypotension was assessed with the use of sodium nitroprusside and isoflurane as hypotensive agents. Nineteen surgical patients were studied during closed-loop control of hypotension induced with sodium nitroprusside. This group of patients was compared with 10 similar patients in whom infusions of sodium nitroprusside were controlled manually by an anesthesiologist. Although the results of the two studies varied, no conclusion could be drawn regarding the superiority of either manual or closed-loop control. When manual versus automatic control of isoflurane-induced hypotension was assessed in a similar fashion, the two methods of induction were found to be comparable.

Pressure-flow relationships of the canine iliac periphery and systemic hemodynamics: effects of sodiumnitroprusside and adenosine

It has been reported that sodiumnitroprusside (SNP) decreases mean systemic pressure and simultaneously increases pressure pulse amplification towards the iliac periphery (Kenner and van Zwieten 1982). This unexpected finding was suggested to be due to a decrease in iliac peripheral resistance but an increase in iliac differential resistance. In order to investigate this apparent contradiction, the iliac periphery was hemodynamically isolated from the rest of the circulation and perfused with the dog's own blood by means of a pump. Perfusion pressure (P) and flow (F), femoral venous pressure (Pv), systemic pressure (Ps) and cardiac output (CO) were measured. Steady state pressure-flow relations of the isolated bed were obtained during control and during various i.v. infusion rates of SNP and adenosine (ADS) and were found to be straight (mean r = 0.99). Their slope (delta P/delta F) was defined as differential resistance (Rd). Peripheral resistance (Rp) of the iliac bed was defined as Rp = (P-Pv)/F, calculated at the flow value where perfusion pressure equalled the prevailing systemic pressure. Total peripheral resistance (TPR) was defined as TPR = Ps/CO. The changes of Rd, Rp, Ps, CO and TPR with respect to control show that during low SNP infusion rates Rd and Rp were both increased while TPR was decreased. During all infusion rates of SNP CO did not change while Ps decreased. During low infusion rates of adenosine CO increased while Ps, Rd and Rp did not change and TPR decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Haemodynamic changes during sodium nitroprusside induced hypotension and halothane/nitrous oxide anaesthesia

The haemodynamic effects of nitroprusside (SNP) were studied in six patients undergoing surgery for intracranial aneurysm under controlled hypotension in endotracheal anaesthesia with halothane-nitrous oxide during hypocapnia. Mean arterial pressure was reduced with SNP from mean 12.25 kPa to mean 8.29 kPa (32%). There were concomitant statistically significant decreases in systemic vascular resistance (-21%), cardiac index (-17%), stroke index (-23%), pulmonary arterial mean pressure (-27%) and pulmonary capillary wedge pressure (-27%). Heart rate, central venous pressure and pulmonary vascular resistance did not change significantly. After the infusion of SNP was discontinued all parameters, except cardiac index and heart rate, returned to values not significantly different from the control values. The hypotension induced by SNP resulted from reductions in cardiac index and systemic vascular resistance. The reduction in cardiac index did not reach a critical level in any of the patients.

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.

Aortic input impedance in man: acute response to vasodilator drugs

In 18 patients who underwent coronary arteriography, aortic velocity and pressure data were obtained during a control state and during either isoproterenol infusion at 1, 2 and 3 micrograms/min or graded nitroprusside infusion (average peak dose 60 micrograms/min). Impedance moduli and phase angles were derived to 10 Hz for controls states, isoproterenol at 2 micrograms/min, and at peak nitroprusside effect. Averaged control data included a mean resistance of 1460 dyn-sec-cm-5 and a characteristic impedance of 88 dyn-sec-cm-5 The characteristic impedance did not correlate with age (r = 0.21), coronary artery disease score (r = 0.17) or mean aortic pressure (r = -0.01). In 11 patients, isoproterenol induced a 38% reduction in mean resistance and a 10% reduction in mean aortic pressure. There was slight reduction in characteristic impedance and phase angles became less negative, to 2 Hz. In seven patients, nitroprusside induced a 38% reduction in mean resistance and a 22% reduction in mean aortic pressure. Impedance moduli decreased to 1.8 Hz and phase angles became less negative, to 3 Hz. Based on the different cardiovascular actions of these two drugs, the data suggest that vasodilators do not induce significant changes in the aortic impedance spectrum when not associated with a decrease in mean aortic pressure.

Hypotension and myocardial metabolism. Drug-induced hypotension and the heart: a comparison between halothane and nitroprusside

This study investigated the effect of hypotension produced by nitroprusside or halothane on haemodynamics and myocardial metabolism. It is postulated that halothane might influence the balance of supply and demand of oxygen to the heart more favourably than nitroprusside. The results in six open-chested dogs do not support this view. Despite the fact that the two drugs lowered the mean arterial pressure by radically different mechanisms, the reduction in demand for oxygen by the heart seemed to be matched by a similar reduction in supply with both drugs. No evidence of anaerobic metabolism or increased a-v O2 content differences was found. It is concluded that the reduction in mean arterial pressure to these levels by either method is not likely to impair myocardial energetics in healthy hearts with normal coronary anatomy.

Cardiovascular effects of sodium nitroprusside in cerebral aneurysm surgery

Cardiovascular effects of sodium nitroprusside (SNP) were studied in 17 patients undergoing cerebral aneurysm surgery under controlled hypotension. In 10 patients oxygen uptake was also calculated. Mean arterial blood pressure was decreased from 91 to 58 mmHg by SNP due to reductions in both total peripheral resistance (29%) and cardiac output (15%). Mean right atrial and pulmonary capillary wedge pressures were reduced during hypotension. Total oxygen uptake remained constant throughout the procedure. There was no significant change in arterial oxygen tension in the hypotensive period, but after discontinuation of SNP infusion it was significantly increased. The haemodynamic effects of SNP in patients with a recent subarachnoid haemorrhage and its effect on blood oxygenation are discussed. It is concluded that the fall in cardiac output is related to a reduction of central blood volume and might be more pronounced in these patients than in normal subjects.

Vasodilator drugs: systemic and regional considerations

Therapeutic use of vasodilator drugs depends on the differential actions of these drugs on regional vascular beds. These differences include selective action on small and large arteries, differential potency on arterial and venous smooth muscle, and selectivity for autonomic receptors. The directly acting sodium nitroprusside and glyceryl trinitrate exemplify many of these differences as well as shared effects in the systemic, coronary and cerebral circulations.

Intracranial pressure changes induced by sodium nitroprusside in patients with intracranial mass lesions

Because of the ability of sodium nitroprusside (SNP) to dilate cerebral blood vessels, intracranial pressure (ICP) should increase with its use. In patients with vascular intracranial tumors following SNP (0.01%) infusion, ICP increased from 14.58 +/- 1.85 to 27.61 +/- 3.33 torr (p greater than 0.0005) and cerebral perfusion pressure decreased from 89.32 +/- 3.5 to 43.23 +/- 4.60 torr (p less than 0.0005) when the mean arterial pressure had reduced by 33%. These results suggest that SNP not be used in patients with raised ICP unless previous measures have been taken to improve intracranial compliance.

Circulatory effects of labetalol during halothane anaesthesia

Labetalol is a drug possessing both alpha and beta adrenergic receptor blocking properties. Its possible use in induced hypotension during halothane anaesthesia has been investigated. It causes a satisfactory decrease in arterial pressure unaccompanied by tachycardia. The circulatory effects of the drug during halothane anaesthesia, both with spontaneous and controlled respiration, have been measured and compared with those of halothane alone. In patients anaesthetised with 1% halothane, labetalol, with both spontaneous and controlled ventilation, was associated with a reduction in MAP from 71.5 mmHg to 54.0 mmHg (P less than 0.001) and 66.8 mmHg to 50.4 mmHg (P less than 0.001) respectively. This reduction was associated with decreases in Qt of 18% and 12% respectively. In the presence of labetalol, with 3% halothane and spontaneous respiration, the depressant effects of the anaesthetic on the heart became rapidly apparent: Qt was reduced by a further 28%. In patients not receiving labetalol, the depressant effects of 3% halothane were frequently countered by the positive inotropic effects of hypercarbia.

[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.