Therefore, it seems that a partial sympathetic blockage exists on substantial area over and under of the level of somatic block. In fact, preganglionic sympathetic fibers, once they quit the dura, enter the paravertebral sympathetic chain. From there, these fibers can ascend or descend, synapsing with up to 18 postganglionic fibers, which may project to dermatomes well above and below the spinal segment from which they originated [9].
Bradycardia associated with spinal block is usually light, and contributes modestly to the drop of blood pressure. Rarely, bradycardia is associated with cardiac collapse. Traditional explanation of this bradycardia originating from a spinal anesthesia is the blockage of cardiac accelerator sympathetic nerves (T1-4). Many studies showed than the incidence and the severity of bradycardia is not related to the height of the sensory block.
Onset time of the bradycardia has poor relation with the timing of the spinal block [10]. Carpenter, in a prospective study on 1000 patients under spinal block., showed that bradycardia occured in 13% (heart rate < 50/min) with an onset time of 47 min (range from1 to 204).
There is a pulse rate paradoxical response to movement of the operation table. Under a spinal or epidural anesthesia, when one lift patient head, blood pressure decreases caused by pooling of the venous blood. But in place of a reflex tachycardia mediated by baroreceptors, there is a paradoxical bradycardia. Interestingly, in situation associated with severe reduction of venous return, paradoxical bradycardia can be seen even in the absence of sympathetic block.
There is similarities between hypotension related to bradycardia of the spinal anesthesia and vasovagal reaction. Vasovagal shock is characterized by hypotension and bradycardia, and can progress to syncope. It has a central or a peripheral etiology.
Because of their rare occurrence, almost all studies on cardiac arrest during spinal anesthesia are retrospective, therefore limited in their ability to identify variables and incidents of such events.
Caplan [14] in 1988 has identified 14 cases of sudden cardiac arrest on patients in good health and undergoing minor surgical procedures. None of these patients had unusually high block, nor received badly inadequate resuscitative care. Despite all this, only 8 of 14 patients survived, and only one survivors had acceptable neurological functions Retrospectively, respiratory insufficiency was suspected, secondary to a strong sedation, as the main etiology of the cardiac arrest. Even a complete sympathectomy leaves a good arterial vascular tone, but in presence of hypoxia and acidosis can lead to a fall in arterial tone, to an exaggerated decrease in blood pressure and cardiac collapse. Early sympathetic responses to hypoxia, which are tachycardia and vasoconstriction, are almost severely blunt by spinal anesthesia [15].
Mackey reported 3 cases of severe bradycardia during spinal anesthesia in the absence of hypoxia and strong sedation [16]. He concludes that severe bradycardya was caused by a drop in venous return triggering Bezold-Jarisch reflex which in presence of sympathetic block led to exaggerated bradycardia, hypotension and arrest.
http://www.esra-learning.com/site/generalites/pathology/b_haemodynamic.htm
