Mechanisms of acute cardiovascular response to periodic apneas in sedated pigs.

This study was designed to evaluate the importance of sympathoadrenal activation in the acute cardiovascular response to apneas and the role of hypoxemia in this response. In addition, we evaluated the contribution of the vagus nerve to apnea responses after chemical sympathectomy. In six pigs preinstrumented with an electromagnetic flow probe and five nonpreinstrumented pigs, effects of periodic nonobstructive apneas were tested under the following six conditions: room air breathing, 100% O2 supplementation, both repeated after administration of hexamethonium (Hex), and both repeated again after bilateral vagotomy in addition to Hex. With room air apneas, during the apnea cycle, there were increases in mean arterial pressure (MAP; from baseline of 108 +/- 4 to 124 +/- 6 Torr, P < 0.01), plasma norepinephrine (from 681 +/- 99 to 1,825 +/- 578 pg/ml, P < 0.05), and epinephrine (from 191 +/- 67 to 1,245 +/- 685 pg/ml, P < 0.05) but decreases in cardiac output (CO; from 3.3 +/- 0.6 to 2.4 +/- 0.3 l/min, P < 0.01) and cervical sympathetic nerve activity. With O2 supplementation relative to baseline, apneas were associated with small increases in MAP (from 112 +/- 4 to 118 +/- 3 Torr, P < 0.01) and norepinephrine (from 675 +/- 97 to 861 +/- 170 pg/ml, P < 0.05). After Hex, apneas with room air were associated with small increases in MAP (from 103 +/- 6 to 109 +/- 6 Torr, P < 0.05) and epinephrine (from 136 +/- 45 to 666 +/- 467 pg/ml, P < 0.05) and decreases in CO (from 3.6 +/- 0.4 to 3.2 +/- 0. 5 l/min, P < 0.05). After Hex, apneas with O2 supplementation were associated with decreased MAP (from 107 +/- 5 to 100 +/- 5 Torr, P < 0.05) and no other changes. After vagotomy + Hex, with room air and O2 supplementation, apneas were associated with decreased MAP (from 98 +/- 6 to 76 +/- 7 and from 103 +/- 7 to 95 +/- 6 Torr, respectively, both P < 0.01) but increased CO [from 2.7 +/- 0.3 to 3. 2 +/- 0.4 l/min (P < 0.05) and from 2.4 +/- 0.2 to 2.7 +/- 0.2 l/min (P < 0.01), respectively]. We conclude that sympathoadrenal activation is the major pressor mechanism during apneas. Cervical sympathetic nerve activity does not reflect overall sympathoadrenal activity during apneas. Hypoxemia is an important but not the sole trigger factor for sympathoadrenal activation. There is an important vagally mediated reflex that contributes to the pressor response to apneas.