Neuraxial blocks typically produce variable decrease in blood pressure that might be accompanied by a decrease in heart rate and cardiac contractility. These effects are generally proportional to the degree (level) of the sympathectomy. Vasomotor tone is primarily determined by sympathetic fibres arising from T5 to L1, innervating arterial and venous smooth muscle. Blocking these nerves causes vasodilation of the venous capacitance vessels, pooling of blood, and decreased vvenous terurn to the heart; in some instances, arterial vasodilation may also decrease systemic vascular resistance. The effects of arterial vasodilation may be minimized by compensatory vasoconstriction above the level of the block. A high sympathetic block not only prevents compensatory vasoconstriction but also blocks the sympathetic cardiac accelerator fibres that arise at T1-T4.
Profound hypotension may result from vasodilation combined with bradycardia and decreased contractility. These effects are further exaggerated if venous return is further compromised by a head-up position or from the weight of a gravid uterus. Unopposed vagal tone in some persons may explain cardiac arrest with spinal anesthesia.
p.261
The sympathetic system normally maintains some tonic vasoconstriction on the vascular tree. Loss off this tone following induction of anesthesia or sympathectomy frequently contributes to perioperative hypotension.
p.375
AV conduction abnormalities are usually manifested by abnormal ventricular depolarization (bundle-branch block) prolongation of the P-R interval (first degree AV block) failure of some atrial impulses to depolarize the ventricles (second degree AV block) or AV dissociation (third degree AV block or complete heart block).
p.428
Clinical anesthesiology
By G. Edward Morgan, Maged S. Mikhail, Michael J. Murray
McGraw-Hill, Edition: 3 - 2002
McGraw-Hill, Edition: 3 - 2002
