PATHOPHYSIOLOGIC CONSIDERATIONS

Cardiovascular Considerations

It is one of the wonders of evolution and physiologic regulation that dinosaurs, giraffes, and humans have been able to assume postures in which the head is higher than the heart while perfusion to the brain is maintained. Physiologic mechanisms of changing vascular resistance and cardiac output allow the cardiovascular system to maintain adequate flow to the central nervous system despite position changes.

A complex system of local mechanisms (i.e., autoregulation) and reflexes in the venous and the arterial systems maintain blood pressure and blood flow during changes in position. Although these local mechanisms and reflexes work in concert, anesthesia can blunt the response of each element of the system, thereby altering the final response in either direction. The venous and atrial reflexes are served by mechanosensitive afferent nerves that respond to stretching of the great veins and heart chambers. These nerves tend to inhibit sympathetic outflow

TABLE 28-2 -- Distribution of claims for nerve injury

Nerve	Number of Claims	Percent of Total (n = 670)
Ulnar	190	28
Brachial plexus	137	20
Lumbosacral nerve root	105	16
Spinal cord	84	13
Sciatic	34	5
Median	28	4
Radial	18	3
Femoral	15	2
Other single nerves	43	6
Multiple nerves	16	2
Total	670	100
From Cheney FW, Domino KB, Caplan RA, et al: Nerve injury associated with anesthesia. Anesthesiology 90:1064, 1999.

when central blood volume is elevated. Arterial baroreflexes are also served by mechanosensitive afferent nerves located in the aortic arch and carotid arteries. They respond to stretch and tend to inhibit sympathetic outflow and activate the parasympathetic system when activated by a rise in arterial pressure. Conversely, a fall in pressure usually evokes sympathetic vasoconstriction and vagal withdrawal.

In the upright position, there is a considerable increase in transmural vascular pressure in the lower extremities because of the hydrostatic effects of the columns of blood. This increase is limited by increased pressure in the tissue surrounding the vessels, which is caused by muscle tone and contraction required to maintain the erect position and by venous valves. Even with this compensation, 0.5 to 1.0 L of blood can pool in the lower extremities, central venous pressure can fall to very low values, and cardiac output is reduced by about 20% in the upright position. Cardiac output tends to increase immediately on assumption of the supine position. Venous blood from the lower body returns to the central circulation and stroke volume increases. If contractility and arterial tone remained constant, arterial pressure would rise. Baroreceptor afferent impulses from the great veins, heart, and aortic receptors travel through the vagus nerve and from the carotid sinus through the glossopharyngeal nerve to the medulla. Increased efferent parasympathetic and decreased efferent sympathetic activity change the parasympathetic-sympathetic balance, decreasing heart rate, stroke volume, and contractility and reducing sympathetic vasoconstrictor activity. The result is that blood pressure remains relatively constant.