EFFECTS OF PERIODIC OBSTRUCTIVE APNEAS ON VENOUS RETURN IN CLOSED-CHEST DOGS

With sleep apnea syndrome, mean cardiac output (CO) may fall. This imp lies a fall in venous return (VR) that is equal to the product of the pressure gradient for VR (GVR) - mean circulatory pressure (MCP) - rig ht ventricular end-diastolic pressure and the conductance for VR (Gv). During airway obstruction there could also be shifts of blood volume between central and peripheral circulatory compartments. We studied th ese parameters in anesthetized closed-chest, spontaneously breathing d ogs when hypoxia occurred (room air RA! breathing) and when hypoxia w as prevented (100% O2 breathing). Obstructive apneas were induced by c ompletely occluding the endotracheal tube at end expiration for 1 min and allowing 1 min of spontaneous ventilation. During RA apneas, arter ial PO2 decreased to 32.7 +/- 6.1 mm Hg (p < 0.01). Apnea reduced resp iratory effort rate by 22.0 to 37.0% and was associated with inspirato ry airway pressure swings in both RA and 02 of approximately -16.0 mm Hg. During apnea, on RA and O2 heart rate and mean arterial pressure d ecreased by 20 to 30 beats/min and by 8 to 12 mm Hg, respectively. On RA, CO decreased by 7.0% (not significant). In contrast, on O2, CO inc reased by 21.0% (p < 0.01). With RA apneas, GVR and MCP increased by 5 5% (p < 0.05) and Gv decreased by 63% (p < 0.05). On O2, there were no changes in GVR or Gv, and changes in CO were predictable from the bas eline VR curve. With O2 there was an increase in central blood volume by 21.0% (p < 0.01) during the apneic phase. We conclude that (1) chan ges in the VR curve with apneas were related to hypoxemia; and (2) cha nges in VR were not related to shifts in blood volume from peripheral to central circulations.