Hemodynamic effects of isovolemic hemodilution during descending thoracic aortic cross clamping and lower torso reperfusion

Background. Isovolemic hemodilution has been suggested for blood conservation and to improve hemodynamic tolerance to abdominal aortic cross clamping. However, the hemodynamic effects of hemodilution during descending thoracic aortic cross clamping (DAC) have not been established. We evaluated them in anesthetized swine. Methods. Hemodilution (n = 7) was produced by the isovolemic exchange of blood for 6% hetastarch to, a target hematocrit of 20%. Hematocrit in control pigs (n = 7) remained at 30%. DAC was performed at the T9 level for 45 minutes. During a 60-minute reperfusion period, control pigs were infused with lactated Ringer's solution; shed blood was returned to hemodilution pigs, followed by lactated Ringer's. If hypotension occurred despite left atrial pressure of 10 mm Hg or greater, boluses of phenylephrine were given to keep mean arterial pressure above 60 mm Hg. Results. Hemodilution caused a marked reduction in hematocrit and in global oxygen delivery (DO₂). DAC produced a significant increase in proximal arterial pressure, cardiac index, and DO₂, and oxygen consumption (VO₂) was markedly reduced in both groups. A significant increase in systemic vascular resistance during DAC occurred only in control pigs. After reperfusion, vascular resistance was significantly lower than baseline in hemodilution pigs, requiring a sixfold greater dose of phenylephrine to avoid hypotension. A lower global DO₂ and supply-limited VO₂ were also observed in hemodilution pigs. Conclusions. Isovolemic hemodilution maintains hemodynamic stability during DAC. During lower torso reperfusion, however, hemodilution caused hemodynamic instability, decreased global DO₂, and limited VO₂, which may offset its potential benefits.