Reflex syncopes

NCS is a component of what is termed reflex syncopes. These are a group of disorders that occur because of a sudden failure of the ANS to maintain adequate vascular tone during orthostatic stress, resulting in hypotension (fre-qnently associated with bradycardia) and consequently cerebral hypoperfusion and loss of consciousness. 

The two most frequent types of reflex syncopes are NCS and carotid sinus syndrome. While both can represent the consequences of augmented vagal tone with similar resultant clinical manifestations, NCS occurs in a vdifferent patient population and is often associated with sympathetic inhibition as described below. NCS can be quite varied in presentation (1). 

Etiology Although the cause is still controversial (23), NCS is beheved to occur in persons who have a predisposition to the condition as a result of excessive peripheral venous pooling that causes a sudden drop in peripheral venous return (24). There is growing evidence that serotonin plays a key role in CNS regulation of both heart rate and blood pressure (25). In the reflex syncopes, there is thought to be disturbances in the production of serotonin as well as postsynaptic receptor density centrally, which leads to a hypersensitive state, with excessive responses to fluctuations in sensory input on a peripheral level. This results in a cardiac hypercontractile state, which activates mech-anoreceptors that normally only respond to stretch. The increase in afferent neural traffic to the brain mimics the conditions seen in hypertension and provokes an apparent paradoxical reflex bradycardia and a drop in peripheral vascular resistance (26). 

Atropine can be useful in patients with carotid sinus syncope. This condition results from excessive activity of afferent neurons whose stretch receptors are in the carotid sinus. By reflex mechanisms, this excessive afferent input to the medulla oblongata causes pronounced bradycardia, which is reversible by atropine. 

Marked reflex vagal discharge sometimes accompanies the pain of myocardial infarction and may result in sufficient depression of sinoatrial or atrioventricular node function to impair cardiac output. Parenteral atropine or a similar antimuscarinic drug is appropriate therapy in this situation. Rare individuals without other detectable cardiac disease have hyperactive carotid sinus reflexes and may experience faintness or even syncope as a result of vagal discharge in response to pressure on the neck, eg, from a tight collar. Such individuals may benefit from the judicious use of atropine or a related antimuscarinic agent. 

Prazosin, oxazosin and terazosin (see p. 73) produce a competitive block of oci adrenoceptors. They decrease peripheral vascular resistance and lower arterial blood pressure by causing the relaxation of both arterial and venous smooth muscle. These drugs cause only minimal changes in cardiac output, renal blood flow, and glomerular filtration rate. Therefore, long-term tachycardia and increased renin release do not occur. Postural hypotension may occur in some individuals. Prazosin is used to treat mild to moderate hypertension and is prescribed in combination with propranolol or a diuretic for additive effects. Reflex tachycardia and first dose syncope are almost universal adverse effects. Concomitant use of a p-blocker may be necessary to blunt the short-term effect of reflex tachycardia. 

Q5 The baroreceptor reflex, which follows the fall in BP caused by reduced ECF volume (hypovolaemia), causes vasoconstriction and increases the heart rate in an effort to raise BP back towards normal. So Eddie s racing pulse is the result of a sympathetic reflex. But the reduction in BP is likely to make Eddie feel dizzy, because of reduced perfusion of the brain, and in this situation fainting (syncope) often occurs. Fainting is actually beneficial because, when someone falls to the ground, the effects of gravity on the circulation are minimised and a person s BP improves. 

Hypotension is a frequent adverse effect of nitrates (24), resulting from dilatation of arteries and veins. It can be associated with reflex tachycardia and palpitation, and even syncope on assuming an upright posture. Hypotension occurs particularly in old people, especially those with recurrent falls (SEDA-16, 196), and more commonly with glyceryl trinitrate oral spray than with sublingual tablets, because of its faster onset of action (24). It tends to occur more commonly in those who take nitrates infrequently. 

The other group of cocaine poisonings is characterized by delirium, increased reflexes, more or less violent convulsions, the pulse usually being rapid and fairly strong but later may become weak, and syncope and cyanosis may intervene. The delirium is frequently associated with hallucinations and at times the patient may develop a violent mania of even homicidal character, as in a case reported by Mattison. In fatal cases death is usually due to respiratory failure although the circulation is also depressed. 

Prazosin prototype otj selective blocker (others include doxazosin, terazosin, amsulosin). Less reflex tachycardia (because NE feedback mechanism is intact), but postural hypotension occurs (first-dose syncope). Clinical uses include mild-to-moderate HTN and benign prostatic hyperplasia (BPH). 

Baroreceptor reflex effects, such as tachycardia, palpitation, and arrhythmia effects due to decreased tension, such as edema, and postural hypotension and effects due to decreased oxygen delivery, such as peripheral ischemia, dizziness, syncope, vertigo, and lightheadedness. 

The cardiovascular effects of muscarinic receptor antagonists are of limited clinical application. Atropine may be considered in the initial treatment of patients with acute myocardial infarction in whom excessive vagal tone causes sinus or nodal bradycardia. Dosing must be judicious doses that are too low can cause a paradoxical bradycardia excessive doses will cause tachycardia that may extend the infarct by increasing demand. Atropine occasiormlly is useful in reducing the severe bradycardia and syncope associated with a hyperactive carotid sinus reflex. Atropine will protect the SA and AV nodes from the effects of excessive ACh in instances of poisoning with anticholinesterase pesticides. 

There are three potential drawbacks in the use of vasodilators First, vasodilators can lead to a baroreceptor-mediated reflex stimulation of the heart (increased heart rate and inotropy) from systemic vasodilation and arterial pressure reduction. Second, they can impair the normal baroreceptor-mediated reflex vasoconstriction when a person stands up, which can lead to orthostatic hypotension and syncope on standing. Third, they can lead to renal retention of sodium... 

D. Treatment. Do not treat bradycardia or AV block unless the patient is symptomatic (eg, exhibits signs of syncope or hypotension). Note Bradycardia or even AV block may be a protective reflex to lower the blood pressure in a patient with life-threatening hypertension (see item VII below). 

A. Autonomic dysfunction. Dizziness or syncope or both caused by severe orthostatic hy x>tension occur with an onset from 6 hours to 2 days after ingestion. Orthostatic hypotension is usually accompanied by intact reflex tachycardia. Other manifestations of autonomic neuropathy include dysphagia, recurrent vomiting, and constipation. 

Syncope is the abrupt and transient loss of consciousness due to a temporary reduction in cerebral blood flow. It is associated with an absence of postural tone and followed by a rapid and usually complete recovery. Syncope may be both benign or the only warning before an episode causing sudden death (1). Recurrent episodes of syncope may result from a variety of disorders, all of which cause a temporary reductiou iu cerebral blood flow sufficient to disturb the normal functions of the brain. Neurocardiogenic (vasovagal) syncope is the most common of a group of reflex (neurally mediated) syncopes, characterized by a sudden failure of the autonomic nervous system (ANS) to maintain blood pressure, and occasionally heart rate, at a level sufficient to maintain cerebral perfusion and consciousness (2-4). Syncope accounts for 3.5% of all emergency room visits and 1-6% of all hospital admissions annually in the USA (5). 

McLeod K, Wilson N, Hewitt J et al. Cardiac pacing for severe childhood neurally mediated syncope with reflex anoxic seizures. Heart 82, 721-725 (1999). 

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