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24. Induced Hypotension
- Hong Kong University Press, HKU
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24. Induced Hypotension Most authoritie s agre e that profoun d hypotensio n fo r surgica l intervention s shoul d be employed mainl y to 'mak e the impossible becom e possible' rather tha n t o mak e the 'difficul t becom e easy' . Deliberat e reductio n o f th e bloo d pressur e ma y b e achieved by interfering eithe r with the peripheral resistance or at other sites. The conventional formula is : B.P. = CO , x P.R . The cardiac output (CO. ) is the sum of (a) the state of the cardiac muscle, (b) the rate and (c ) the venous return. All these contribute t o th e stroke volume. The peripheral resistanc e (P.R. ) depend s o n th e ton e o f th e arteria l tree . Th e blockin g o f vasoconstrictor fibres to resistance vessels will cause a fall in P.R. an d probably als o in B.P. , while blockin g th e vasoconstricto r fibres t o capacitanc e vessel s wil l caus e vasodilatation, decreased venou s return, decreased cardia c output an d furthe r dro p in blood pressure. Central depressants, suc h a s anaestheti c agents , depres s th e vasomoto r centre . Reserpine is believed to have both a central and peripheral depressant effect . At th e leve l o f thoracic nerves, subarachnoi d an d extradura l bloc k wil l depres s conduction along the white rami communicantes (From Tl t o L2) and so cause progressive hypotension the higher the block extends. At th e level of autonomic ganglia, competitio n wit h acetylcholin e i s the mod e o f action here. Because both th e sympathetic and parasympatheti c ar e involved, whe n used i n general medical practic e (outsid e anaesthesia ) side-effect s suc h a s constipa tion , dryness of mouth, derangement of the micturition process and blurring of vision may occur. Pentamethonium, hexamethonium , pentolinium and trimetaphan ar e all in this category. At postganglionic adrenergic receptor sites, adrenergic blockers act specifically o n sympathetic transmissio n (n o involvemen t o f th e parasympathetic). Postganglioni c transmission i s dependent o n th e release o f noradrenaline fro m storag e granules , a process thought t o be initiated by the release of acetylcholine. Adrenergic transmis sion ma y b e blocke d a t th e post-ganglioni c (o r pre-receptor ) site , o r a t th e final receptor site . Reserpine , bretylium , guanethidin e an d methyldop a ar e amongs t th e pre-receptor adrenergi c blockers. O n th e other hand , som e phenothiazines , a s well as dibenamine , phentolamine , chlorpromazin e an d phenoxybenzamin e ar e alph a adrenergic receptor blockers acting at the final receptor sites. Some interesting phar macological effect s ar e produced: (a ) Reserpine, by inhibiting th e transport mecha nism , cause s th e depletio n o f noradrenalin e fro m th e storag e granule s an d th e available pool , (b ) Bretyliu m prevent s noradrenalin e releas e b y th e nerv e impulse . 348 Anaesthesia and Other Specialties (c) Guanethidine , perhap s becaus e o f th e releas e o f activ e noradrenalin e fro m th e pool, also depletes the stores of noradrenaline. In anaesthetic practice the following substance s may be used: Trimetaphan (Arfonad) . Thi s used t o b e given in a 0.1% solution (50 0 mg in 50 0 ml) intravenously a t the rate of 40-60 drops/min until the systolic B.P. fell to 70-8 0 mm Hg. Maintenance of hypotension was achieved by adjusting th e rate of infusio n thereafter a s necessary. Som e authorities no w prefer usin g trimetaphan b y intermit tent intravenou s injections , utilizin g th e 'bolu s effect' . Th e disadvantage s includ e histamine release, tachyphylaxis and, on occasions, tachycardia. It is contraindicated in asthma. Some patients exhibit resistance to hypotensive effects of this substance, in which case additional drugs may have to be given. Phenactropinium (trophenium) . Thi s has a shor t duratio n o f actio n wit h a specifi c ganglion blockin g effec t an d i s responsive t o alteratio n o f the rate o f infusion...