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1.Antiadrenergic agents ( Antisympathetic drugs / Sympatholytics ) Non-Selective Alpha Blockers
Dr.G Bhanu Prakash Animated Medical Videos
Overview
This video explains sympatholytic drugs, specifically focusing on non-selective alpha-adrenergic blockers. It details how these drugs work by blocking both alpha-1 and alpha-2 receptors, leading to vasodilation and potential postural hypotension due to alpha-1 blockade. The video also discusses the reflex increase in sympathetic outflow and tachycardia caused by alpha-2 blockade. A key phenomenon, the vasomotor reversal of Dale, is explained, illustrating how prior administration of non-selective alpha-blockers alters the blood pressure response to adrenaline. Finally, the clinical uses of irreversible (phenoxybenzamine) and reversible (phentolamine, tolazoline) non-selective alpha-blockers are presented, including their roles in managing pheochromocytoma and hypertensive crises.
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Chapters
- Sympatholytic drugs counteract the effects of the sympathetic nervous system.
- These drugs primarily work by blocking alpha and beta adrenergic receptors.
- Alpha blockers are categorized into non-selective (blocking both alpha-1 and alpha-2) and selective (blocking either alpha-1 or alpha-2) types.
Understanding the basic mechanism of sympatholytic drugs is crucial for grasping how they modulate the body's stress response and blood pressure regulation.
- Non-selective alpha blockers bind to both alpha-1 and alpha-2 receptors.
- They are further classified into irreversible (e.g., phenoxybenzamine) and reversible (e.g., phentolamine, tolazoline) antagonists.
- Irreversible blockers form a permanent bond with the receptor, while reversible blockers can be displaced.
Knowing the difference between irreversible and reversible blockade helps predict the duration and intensity of drug effects and informs treatment choices.
- Stimulation of alpha-1 receptors normally causes vasoconstriction.
- Blocking alpha-1 receptors leads to vasodilation (widening of blood vessels).
- This vasodilation can result in a drop in blood pressure, particularly when changing posture (postural hypotension).
The vasodilation caused by alpha-1 blockade directly impacts blood pressure, explaining common side effects like dizziness upon standing.
When an individual stands up from a lying position, blood pressure normally drops slightly, but the body compensates. With alpha-1 blockade, this compensation is impaired, leading to a significant drop in blood pressure (systolic by 20 mmHg, diastolic by 10 mmHg).
- Alpha-2 receptors, located in the central nervous system, normally reduce sympathetic outflow when stimulated.
- Blocking alpha-2 receptors removes this inhibitory signal.
- This leads to a reflex increase in sympathetic discharge and tone, causing increased heart rate (tachycardia).
Understanding the dual action on alpha-1 and alpha-2 receptors explains the complex cardiovascular effects of non-selective alpha blockers, including both vasodilation and increased heart rate.
- Normally, intravenous adrenaline causes an initial rise in blood pressure (alpha-1 effect) followed by a fall (beta-2 effect).
- When non-selective alpha blockers are given *before* adrenaline, the alpha-1 mediated pressor response is blocked.
- Consequently, only the beta-2 mediated vasodilation effect is observed, resulting in a pronounced fall in blood pressure, known as the vasomotor reversal of Dale.
This phenomenon demonstrates the interplay between different receptor types and highlights how blocking one pathway can unmask or exaggerate effects mediated by another.
In a normal person, injecting adrenaline causes blood pressure to go up then down. If the person has taken a non-selective alpha blocker first, injecting adrenaline will only cause blood pressure to go down.
- Phenoxybenzamine (irreversible) is used to prevent hypertensive episodes during surgery for pheochromocytoma.
- Phentolamine and tolazoline (reversible) are used to treat hypertensive crises associated with clonidine withdrawal.
- These reversible agents are also used in managing 'cheese reactions' (hypertensive crisis after consuming tyramine-rich foods while on MAO inhibitors).
These drugs have specific, life-saving applications in managing critical conditions involving excessive sympathetic activity or dangerous blood pressure fluctuations.
Phenoxybenzamine is given before surgery to remove a pheochromocytoma (a tumor that releases adrenaline) to stop dangerous blood pressure spikes caused by manipulating the tumor.
Key takeaways
- Non-selective alpha blockers interfere with the sympathetic nervous system by blocking both alpha-1 and alpha-2 receptors.
- Alpha-1 blockade causes vasodilation and can lead to postural hypotension.
- Alpha-2 blockade results in increased sympathetic outflow and reflex tachycardia.
- The vasomotor reversal of Dale illustrates how alpha-blockade alters the cardiovascular response to adrenaline.
- Irreversible non-selective alpha blockers like phenoxybenzamine are used for specific perioperative management of conditions like pheochromocytoma.
- Reversible non-selective alpha blockers like phentolamine are crucial for managing acute hypertensive crises in certain clinical scenarios.
Key terms
Sympatholytic drugsAlpha-adrenergic receptorsBeta-adrenergic receptorsNon-selective alpha blockersSelective alpha blockersAlpha-1 receptorsAlpha-2 receptorsIrreversible antagonistReversible antagonistVasodilationPostural hypotensionSympathetic outflowTachycardiaVasomotor reversal of DalePheochromocytomaHypertensive crisisClonidine withdrawalCheese reaction
Test your understanding
- How do non-selective alpha blockers differ from selective alpha blockers in their receptor targets?
- Explain the physiological consequences of blocking alpha-1 receptors versus alpha-2 receptors.
- What is postural hypotension, and why is it a common side effect of alpha-1 blockade?
- Describe the phenomenon of vasomotor reversal of Dale and the underlying mechanism.
- In what specific clinical situations are irreversible and reversible non-selective alpha blockers utilized?