Is plasticity of synapses the mechanism of long-term memory storage?

It has been 70 years since Donald Hebb published his formalized theory of synaptic adaptation during learning. Hebb's seminal work foreshadowed some of the great neuroscientific discoveries of the following decades, including the discovery of long-term potentiation and other lasting forms of synaptic plasticity, and more recently the residence of memories in synaptically connected neuronal assemblies. Our understanding of the processes underlying learning and memory has been dominated by the view that synapses are the principal site of information storage in the brain. This view has received substantial support from research in several model systems, with the vast majority of studies on the topic corroborating a role for synapses in memory storage. Yet, despite the neuroscience community's best efforts, we are still without conclusive proof that memories reside at synapses. Furthermore, an increasing number of non-synaptic mechanisms have emerged that are also capable of acting as memory substrates. In this review, we address the key findings from the synaptic plasticity literature that make these phenomena such attractive memory mechanisms. We then turn our attention to evidence that questions the reliance of memory exclusively on changes at the synapse and attempt to integrate these opposing views.

Do group I metabotropic glutamate receptors mediate LTD?

Synapses undergo significant structural and functional reorganization in response to varying patterns of stimulation. These forms of plasticity are considered fundamental to cognition and neuronal homeostasis. An increasing number of reports highlight the importance of activity-dependent synaptic strengthening (long term potentiation: LTP) for learning. However, the functional significance of activity-dependent weakening of synapses (long term depression: LTD) remains relatively poorly understood. One form of synaptic weakening, induced by group I metabotropic glutamate receptors (mGluRs), has received significant attention from a mechanistic point of view and because of its augmentation in a murine model of Fragile X Syndrome. Yet, studies of this form of plasticity often yield confusing, contradictory results. These conflicting findings are likely attributable to the bulk stimulation and recording techniques often used to study synaptic plasticity (typically involving evoked extracellular recordings, which represent the summed activity of many synapses). Such studies inherently blur the identity of the synapses undergoing change, thus giving the illusion that synapses per se are being modified when in fact this may only be true of a specific subset of synapses. Indeed, studies employing minimal synaptic activation paint a fundamentally different picture of what is commonly called "mGluR-LTD". Here, I review the evidence in favour of group I mGluRs as mediators of various forms of synaptic downregulation and attempt to explain discrepancies in the literature. I argue that, while multiple forms of synaptic weakening may be triggered by these receptors, the canonical form of group I mGluR-mediated depression, mGluR-LTD, is in fact not a depression of basal synaptic responses. Rather, it is a reversal of established LTP and thus a form of depotentiation. Far from being arbitrary, this distinction has significant implications for the role of group I mGluRs in cognition, both in the healthy brain and in pathological conditions. Further, the differential actions of group I mGluRs at naïve and potentiated synapses suggest these receptors signal in a state-dependent manner to regulate various stages of the learning process.

Corticolimbic gating of emotion-driven punishment

Determining the appropriate punishment for a norm violation requires consideration of both the perpetrator's state of mind (for example, purposeful or blameless) and the strong emotions elicited by the harm caused by their actions. It has been hypothesized that such affective responses serve as a heuristic that determines appropriate punishment. However, an actor's mental state often trumps the effect of emotions, as unintended harms may go unpunished, regardless of their magnitude. Using fMRI, we found that emotionally graphic descriptions of harmful acts amplify punishment severity, boost amygdala activity and strengthen amygdala connectivity with lateral prefrontal regions involved in punishment decision-making. However, this was only observed when the actor's harm was intentional; when harm was unintended, a temporoparietal-medial-prefrontal circuit suppressed amygdala activity and the effect of graphic descriptions on punishment was abolished. These results reveal the brain mechanisms by which evaluation of a transgressor's mental state gates our emotional urges to punish.

Mechanisms of heterosynaptic metaplasticity

Synaptic plasticity is fundamental to the neural processes underlying learning and memory. Interestingly, synaptic plasticity itself can be dynamically regulated by prior activity, in a process termed 'metaplasticity', which can be expressed both homosynaptically and heterosynaptically. Here, we focus on heterosynaptic metaplasticity, particularly long-range interactions between synapses spread across dendritic compartments, and review evidence for intracellular versus intercellular signalling pathways leading to this effect. Of particular interest is our previously reported finding that priming stimulation in stratum oriens of area CA1 in the hippocampal slice heterosynaptically inhibits subsequent long-term potentiation and facilitates long-term depression in stratum radiatum. As we have excluded the most likely intracellular signalling pathways that might mediate this long-range heterosynaptic effect, we consider the hypothesis that intercellular communication may be critically involved. This hypothesis is supported by the finding that extracellular ATP hydrolysis, and activation of adenosine A2 receptors are required to induce the metaplastic state. Moreover, delivery of the priming stimulation in stratum oriens elicited astrocytic calcium responses in stratum radiatum. Both the astrocytic responses and the metaplasticity were blocked by gap junction inhibitors. Taken together, these findings support a novel intercellular communication system, possibly involving astrocytes, being required for this type of heterosynaptic metaplasticity.

Purinergic receptor- and gap junction-mediated intercellular signalling as a mechanism of heterosynaptic metaplasticity

Synaptic plasticity is subject to activity-dependent long-term modification (metaplasticity). We have recently described a novel form of heterosynaptic metaplasticity in hippocampal CA1, whereby 'priming' activity at one set of synapses confers a metaplastic state that inhibits subsequent LTP both within and between dendritic compartments. Here, we investigated the roles of purinergic signalling and gap junctions in mediating this long-distance communication between synapses. We found that the heterosynaptic metaplasticity requires the hydrolysis of extracellular ATP to adenosine, and activation of adenosine A2, but not A1 receptors. The metaplasticity was also blocked by the non-selective gap junction blockers carbenoxolone and meclofenamic acid, and by a connexin43-specific mimetic peptide. These results indicate that an intercellular signalling cascade underlies the long-distance communication required for this form of metaplasticity.

Emerging roles of metaplasticity in behaviour and disease

Since its initial conceptualisation, metaplasticity has come to encompass a wide variety of phenomena and mechanisms, creating the important challenge of understanding how they contribute to network function and behaviour. Here, we present a framework for considering potential roles of metaplasticity across three domains of function. First, metaplasticity appears ideally placed to prepare for subsequent learning by either enhancing learning ability generally or by preparing neuronal networks to encode specific content. Second, metaplasticity can homeostatically regulate synaptic plasticity, and this likely has important behavioural consequences by stabilising synaptic weights while ensuring the ongoing availability of synaptic plasticity. Finally, we discuss emerging evidence that metaplasticity mechanisms may play a role in disease causally and may serve as a potential therapeutic target.

Evolution and the expression of biases: situational value changes the endowment effect in chimpanzees

Cognitive and behavioral biases, which are widespread among humans, have recently been demonstrated in other primates, suggesting a common origin. Here we examine whether the expression of one shared bias, the endowment effect, varies as a function of context. We tested whether objects lacking inherent value elicited a stronger endowment effect (or preference for keeping the object) in a context in which the objects had immediate instrumental value for obtaining valuable resources (food). Chimpanzee subjects had opportunities to trade tools when food was not present, visible but unobtainable, and obtainable using the tools. We found that the endowment effect for these tools existed only when they were immediately useful, showing that the effect varies as a function of context-specific utility. Such context-specific variation suggests that the variation seen in some human biases may trace predictably to behaviors that evolved to maximize gains in specific circumstances.

The neural correlates of third-party punishment

Legal decision-making in criminal contexts includes two essential functions performed by impartial "third parties:" assessing responsibility and determining an appropriate punishment. To explore the neural underpinnings of these processes, we scanned subjects with fMRI while they determined the appropriate punishment for crimes that varied in perpetrator responsibility and crime severity. Activity within regions linked to affective processing (amygdala, medial prefrontal and posterior cingulate cortex) predicted punishment magnitude for a range of criminal scenarios. By contrast, activity in right dorsolateral prefrontal cortex distinguished between scenarios on the basis of criminal responsibility, suggesting that it plays a key role in third-party punishment. The same prefrontal region has previously been shown to be involved in punishing unfair economic behavior in two-party interactions, raising the possibility that the cognitive processes supporting third-party legal decision-making and second-party economic norm enforcement may be supported by a common neural mechanism in human prefrontal cortex.

Idiopathic Pulmonary Trunk Aneurysm Causing Airway Obstruction in an Infant

We report a case of idiopathic pulmonary trunk aneurysm in an infant, in which the main clinical picture was airway obstruction. Aneurysmorraphy was the chosen surgical approach and showed to be successful on 13 months follow-up.

Endowment effects in chimpanzees

Human behavior is not always consistent with standard rational choice predictions. Apparent deviations from rational choice predictions provide a promising arena for the merger of economics and biology [1-6]. Although little is known about the extent to which other species exhibit these seemingly irrational patterns [7-9], similarities across species would suggest a common evolutionary root to the phenomena. The present study investigated whether chimpanzees exhibit an endowment effect, a seemingly paradoxical behavior in which humans tend to value a good they have just come to possess more than they would have only a moment before [10-13]. We show the first evidence that chimpanzees do exhibit an endowment effect, by favoring items they just received more than their preferred items that could be acquired through exchange. Moreover, the effect is stronger for food than for less evolutionarily salient objects, perhaps because of historically greater risks associated with keeping a valuable item versus attempting to exchange it for another [14, 15]. These findings suggest that many seeming deviations from rational choice predictions may be common to humans and chimpanzees and that the evaluation of these through a lens of evolutionary relevance may yield further insights in humans and other species.

Law, responsibility, and the brain

Brain-imaging studies have reinvigorated the neurophilosophical and legal debate of whether we are free agents in control of our own actions or mere prisoners of a biologically determined brain.

Synovial Sarcoma of The Heart

We report a case of synovial sarcoma of the heart in a 14-year-old boy, who presented with signs of mild generalized illness and rapidly progressed to haemodynamic collapse. On operation the tumour was attached to the tricuspid valve, away from the atrioventricular node. He was considered to have incomplete resection due to extensive adhesions to the atrioventricular junction, and was offered chemotherapy. On 18 months follow-up, he was asymptomatic.

Law, evolution and the brain: applications and open questions

This paper discusses several issues at the intersection of law and brain science. It focuses principally on ways in which an improved understanding of how evolutionary processes affect brain function and human behaviour may improve law's ability to regulate behaviour. It explores sample uses of such 'evolutionary analysis in law' and also raises questions about how that analysis might be improved in the future. Among the discussed uses are: (i) clarifying cost-benefit analyses; (ii) providing theoretical foundation and potential predictive power; (iii) assessing comparative effectiveness of legal strategies; and (iv) revealing deep patterns in legal architecture. Throughout, the paper emphasizes the extent to which effective law requires: (i) building effective behavioural models; (ii) integrating life-science perspectives with social-science perspectives; (iii) considering the effects of brain biology on behaviours that law seeks to regulate; and (iv) examining the effects of evolutionary processes on brain design.

Comparative analysis of pattern, management and outcome of pre- versus postnatally diagnosed major congenital heart disease: a population-based study

OBJECTIVES

Most pregnant women in New South Wales undergo obstetric ultrasound examination, including some assessment of fetal cardiac anatomy. We aimed to review the spectrum of cardiac defects, management and outcome data of all fetuses with diagnosis of major congenital heart disease between 1994 and 1996 and compare them to major congenital heart disease in infants born during the same 3-year study period.

METHODS

Descriptive comprehensive study of the New South Wales population. Study centers included the single fetal echocardiographic referral service and the two pediatric cardiac centers of New South Wales.

RESULTS

Ninety-seven fetuses and 562 infants with major congenital heart disease were identified (240,000 livebirths), resulting in a prenatal detection rate of 15%. Anomalies detectable by cardiac four-chamber views were diagnosed at an average rate of 30% (68/229) in utero. By contrast, lesions associated with abnormal ventricular outflow and great artery views were detected in only 6.7% (29/430; P < 0.0001) of cases prior to birth. Of the 97 fetuses, 29 were aborted, 16 died in utero, and 9 died early postnatally without treatment. Within 2 weeks of age, 23% with fetal and 40% (P < 0.05) with infant major congenital heart disease diagnosis required an intervention, mainly for patent ductus arteriosus dependent lesions. Postnatal survival was similar for the fetal and infant series up to 2 years of age: 77% (95% confidence interval 64-90%) vs. 85% (95% confidence interval 82-88%).

CONCLUSIONS

Prenatal diagnosis has important implications for pregnancy outcome, in particular for univentricular lesions. However, the present mode of obstetric routine ultrasound scanning fails to identify most ductus arteriosus dependent cardiac lesions with a predictable need for early postnatal intervention.

Reproductive autonomy and evolutionary biology: a regulatory framework for trait-selection technologies

The Constitution protects, in some measure, each person's autonomy in making basic decisions about family, parenthood, and procreation. This Article examines the extent to which courts should protect from government intrusions a parent's access to technologies that influence specific characteristics of offspring. Beginning with Supreme Court opinions that articulate constitutional and social values regarding reproductive autonomy, the Article explores how important new insights from evolutionary biology may supplement an understanding of human procreation. Specifically, the Article explains how trait selection can constitute an important part of larger "reproductive strategies" that powerfully affect an individual's "inclusive fitness" (itself a measure of reproductive success). It concludes that access to trait-selection technologies should receive the same federal protection from government intrusions as that afforded access to abortion. It proposes the first limit to that protection, however, when a parent seeks to select for a trait, or to use a technique, that would be clearly and significantly damaging to the future child. The Article subsequently divides the use of trait-selection technologies (TSTs) into eight contexts and proposes a preliminary framework by which a regulatory system could legitimately distinguish among them.

Experience with flecainide for the treatment of cardiac arrhythmias in children

We treated 22 children, aged 3 days to 16 years 6 months (median 11 years 1 month), with flecainide for a variety of arrhythmias where a Class I agent was indicated. In 16, conventional antiarrhythmic treatment had failed. Structural heart disease was present in nine. The arrhythmia was paroxysmal re-entry atrioventricular tachycardia in nine; paroxysmal atrial tachycardia, flutter or fibrillation in five; paroxysmal ventricular tachycardia in five and frequent ventricular extrasystoles (with couplets) in three. Sinus rhythm was achieved in all four children who received flecainide during tachycardia (three received intravenous flecainide, one oral). During follow-up of 3-24 months (median 12 months), arrhythmia control was obtained in 13 children (59%). Combination therapy was used in seven of these; with digoxin in four and a beta blocker in three. Flecainide doses used in this study ranged from 1-11 mg kg-1 day-1 (median 4 mg kg-1 day-1), 25-297 mg m-2 day-1 (median 113 mg m-2 day-1). The median, pre-dose flecainide concentration in those responding to therapy was 225 micrograms l-1 and in those failing to respond was 417 micrograms l-1. An arrhythmogenic effect occurred in one child.

The pathology of balloon pulmonary valvoplasty

The pulmonary valve was examined following balloon dilation of pulmonary valve stenosis in six children. Stenosis was due to commissural fusion in two. In these patients, cineangiography showed that the valve was slightly thickened and "domed" during systole. Balloon valvoplasty produced complete separation of fused commissures in one and partial separation in the other. Commissural splitting is likely to be the most common mechanism of relief of pulmonary stenosis by balloon dilation. Incomplete separation could account for the partial reduction of the gradient in most children treated with this technique. Four children had pulmonary valve dysplasia. Balloon valvoplasty did not produce haemodynamic improvement or morphological changes that were identifiable at the time of surgical correction. Tearing of pulmonary valve tissue and avulsion of pulmonary cusps were not seen in this series.

The value of counter-current aortography in infants

Counter-current aortograms were performed on five infants weighing from 1.6 to 2.8 kg. All had symptoms suggestive of congenital heart disease. The images obtained were all of diagnostic quality. In only one, who had anomalous origin of the subclavian artery on the side chosen for the injection, was the diagnosis not made by this technique. No complications directly attributable to the procedure occurred. Counter-current aortography is a safe and effective minimally invasive method of elucidating the anatomy of the aortic arch in young infants.

Pulmonary regurgitation in transposition of the great arteries

A female infant who presented with cyanosis and tachypnoea was found to have transposition of the great arteries, a ductus arteriosus, and a ventricular septal defect. An aortogram showed evidence of mild pulmonary regurgitation, which persisted after a Senning's operation; this is a hitherto unrecognised clinical finding.

Balloon aortoplasty for recoarctation following the subclavian flap operation

Transluminal balloon aortoplasty was successfully performed 7 times in 5 children between 3 and 14 months of age who had had the subclavian flap operation for coarctation of the aorta in the neonatal period. In two the balloon aortoplasty was performed twice. All recoarctations presented with upper limp hypertension and marked upper to lower limb pressure gradients. The pressure gradient decreased immediately after the procedure from 57.1 +/- 13.8 mm Hg to 17.9 +/- 15.5 mm Hg, P less than 0.001. The diameter of the recoarcted region increased from 2.67 +/- 1.0 mm to 3.85 +/- 1.23 mm, P less than 0.05. There were no complications attributable to the dilatation technique. Intermediate term success was unpredictable from the initial results or the angiographic appearance of the recoarctation. Follow-up has been for an average of 12.7 months (range 2-30 months). Four patients have pressure gradients from upper to lower limbs of 20 mm Hg or less. In one this has been achieved by repeat balloon aortoplasty. Severe restenosis has occurred in one other patient despite repeating the angioplasty. The procedure is safe and although intermediate term success cannot be predicted in all cases, we propose that balloon aortoplasty be the initial treatment of choice for recoarctation of the aorta. The place of repeating the procedure when early restenosis occurs has yet to be defined.

Embolisation procedures in congenital heart disease

Eight therapeutic embolisation procedures were performed by the transcutaneous catheter technique in seven patients with congenital heart disease. After surgical correction of tetralogy of Fallot (four patients), catheter embolisation was used to occlude two large aortopulmonary collaterals (one patient), three small aortopulmonary collaterals (one patient), and two Blalock-Taussig shunts (two patients). In two patients congenital coronary anomalies were occluded--a coronary arteriovenous malformation and a coronary artery/bronchial artery anastomosis. In one patient a pulmonary arteriovenous malformation was embolised. Detachable balloons were used to occlude six large arteries, the three small arteries were occluded with small gelfoam fragments, and the pulmonary arteriovenous malformation was occluded with multiple steel coils and large gelfoam pieces. Successful occlusion was achieved in all cases. No complications were encountered and the procedure was well tolerated even in the two patients receiving postoperative intensive care. Therapeutic embolisation in suitable cases is a safe and effective alternative to surgery and the detachable balloon technique is effective in occluding high flow vessels.

Percutaneous balloon pulmonary valvuloplasty

Percutaneous pulmonary valvuloplasty was performed in 27 patients with congenital pulmonary valve stenosis. A fall in the transvalve gradient of at least 15 mm Hg occurred in 22 patients. In five there was little change in the severity of the stenosis; in three of these the pulmonary valve was dysplastic. None of the successfully treated patients had a dysplastic valve. The two other failures, early in the series, were probably due to inadequate balloon size. In one patient the procedure was performed twice, with a successful result from the second dilatation with a larger balloon. Follow up studies in a further six patients showed no evidence of restenosis in those who had been successfully treated and no late improvement in the remainder. There were no important complications. Percutaneous pulmonary valvuloplasty should be the initial treatment for congenital pulmonary valve stenosis, although when the valve is dysplastic the result is less likely to be satisfactory.

Superiority of radionuclide over oximetric measurement of left to right shunts

In 100 children with suspected left to right shunts the ratio of pulmonary to systemic flow was measured both by oximetry and first pass radionuclide angiography. The pulmonary time activity curve from the radionuclide study was analysed by the method of gamma variate fits. There was strong correlation between the two techniques; weaker correlation was found when the shunt was at atrial rather than ventricular level. This difference can be explained only by problems with the oximetric rather than the radionuclide technique. Although there are important limitations to the radionuclide method, it is the more precise and less invasive of the two and is to be preferred when the accurate measurement of left to right shunts is required.

Acute renal failure following cardiopulmonary bypass in children: results of treatment

Following open heart surgery using cardiopulmonary bypass, 18 (4%) of 441 operated children required treatment with peritoneal dialysis for acute renal failure or refractory oliguria. Nine recovered renal function (50%) and 5 (28%) survived. Only 2 died from renal causes. Despite both adequate symptomatic treatment of renal failure and few complications of dialysis itself, the prognosis remains poor, even with early treatment, but an aggressive approach is justified because some survivors can be expected.

Radionuclide measurement of left ventricular ejection fraction in tricuspid atresia

In 15 patients with tricuspid atresia and one with tricuspid stenosis the left ventricular ejection fraction was measured by equilibrium gated radionuclide angiography and the results compared with those from a control group of 16 patients. The patients with tricuspid atresia had a significantly depressed ejection fraction. Those who had a surgical shunt or who had had pulmonary artery banding had significantly lower ejection fractions than the remainder. There was no significant correlation between the ejection fraction and age, the arterial oxygen saturation, or the haemoglobin concentration. Five patients were also studied during isometric exercise; three had an abnormal response. Volume overload of the ventricle is identified as one cause of the dysfunction, but other factors may be important. Radionuclide angiography offers a non-invasive method of studying ventricular function in this condition.

Use of radionuclide labelled microspheres to show the distribution of the pulmonary perfusion with multifocal pulmonary blood supply

In pulmonary atresia with a ventricular septal defect and similar congenital heart disorders the pulmonary blood supply is often multifocal. The relative distribution of pulmonary perfusion from individual sources is usually demonstrated by selective angiography. A new technique using a selective injection of radionuclide labelled human albumin microspheres was performed in four patients with a complex pulmonary blood supply. In these cases the physiological distribution of each of the multiple sources of pulmonary perfusion was demonstrated. This is a useful additional technique in the assessment of such patients.

Balloon atrial septostomy in the neonatal intensive care unit

In eight infants with complete transposition of the great arteries balloon atrial septostomy was performed in the neonatal intensive care unit solely under cross sectional echocardiographic control. One infant developed necrotising enterocolitis; otherwise there were no complications. This technique eliminates the need for radiographic screening during the procedure, is quick, and causes little disturbance to the infant who does not have to be removed from the incubator.

Internal jugular catheterisation in small children. The use of a posterior approach

Internal jugular catheterisation, using a posterior approach, was attempted in 50 infants and children aged from 1 day to 12 years, weighing 1.25 to 30 kg. Thirty-five of the patients weighed less than 10 kg. Catheterisation was successful in 49 cases. Initial catheter position was satisfactory in 48 of the 49 cases and satisfactory pressure waveform and flow characteristics were achieved in all 49 cases. There were no complications attributable to the technique. The technique proved reliable and easily learnt and is recommended as a particularly suitable approach to central venous catheterisation in small children.

Two dimensional echocardiographic categorisation of the univentricular heart. Ventricular morphology, type, and mode of atrioventricular connection

Most univentricular hearts have two chambers in their ventricular mass, only one of which possesses an atrioventricular connection. Categorisation into univentricular heart of right, left, or indeterminate type, using two dimensional echocardiography has been successfully achieved in 122 out of 132 patients to whom this technique has been applied. Thus, right ventricular rudimentary chambers in 84 univentricular hearts of left ventricular type were shown to be anterosuperior and either to the right or left. In contrast, left ventricular rudimentary chambers in 25 univentricular hearts of right ventricular type were posteroinferior and to the right, left, or directly posterior. Thirteen univentricular hearts of indeterminate morphology were characterised by absence of rudimentary chamber on angiography and echocardiography. The trabecular pattern of both main and rudimentary chambers were separately identified in some of the patients with univentricular hearts of right and left ventricular type and two dimensional echocardiography also illustrated the mode of atrioventricular connection, either via two atrioventricular valves, a common valve, or a valve straddling or overriding the trabecular septum. Finally it was also possible to distinguish absent atrioventricular connection from an imperforate valve in 10 patients.

The use of tolazoline hydrochloride as a pulmonary vasodilator in potentially fatal episodes of pulmonary vasoconstriction after cardiac surgery in children

Infants and children with highly reactive pulmonary vasculature may die of an acutely increased pulmonary vascular resistance (PVR) during or after correction of congenital heart defects. Fifteen patients, 11 infants ages 2--12 months (mean 5.3 months) and 4 children ages 2--10 years (mean 5.8 years), were treated with tolazoline to reduce an acutely elevated PVR during or after operation. Five patients had total anomalous pulmonary venous drainage, six had ventricular septal defect and four had truncus arteriosus. Preoperatively, the ratio of systolic pulmonary artery pressure to systemic artery pressure (PAP/SAP) was 0.57--1.54 (mean 0.93), and PVR was 1.8--20 units/m2 (mean 6.2 units/m2). The pulmonary artery pressure was monitored postoperatively by pulmonary artery lines placed at surgery (12 patients) or by balloon flotation catheters (two patients). Acute elevations in PAP/SAP accompanied by clinical deterioration occurred during attempted withdrawal from cardiopulmonary bypass in three patients, during apparently optimal ventilation postoperatively in eight patients, and after withdrawal of ventilatory support in four patients. Administration of tolazoline as a bolus (1--2 mg/kg) followed by infusion of 1--2 mg/kg/hour resulted in a rapid and sustained decrease in PAP/SAP, from 1.00 +/- 0.18 (mean +/- SD) to 0.40 +/- 0.09 (p less than 0.001), and was effective management for all these clinical crises. The use of tolazoline may prevent a fatal outcome from acute elevation of PVR in the perioperative period.