Intestinal and sublingual microcirculation are more severely compromised in hemodilution than in hemorrhage

Abstract

The alterations in O2 extraction in hemodilution have been linked to fast red blood cell (RBC) velocity, which might affect the complete release of O2 from Hb. Fast RBC velocity might also explain the normal mucosal-arterial Pco2 (ΔPco2). Yet sublingual and intestinal microcirculation have not been completely characterized in extreme hemodilution. Our hypothesis was that the unchanged ΔPco2 in hemodilution depends on the preservation of villi microcirculation. For this purpose, pentobarbital-anesthetized and mechanically ventilated sheep were submitted to stepwise hemodilution (n = 8), hemorrhage (n = 8), or no intervention (sham, n = 8). In both hypoxic groups, equivalent reductions in O2 consumption (V̇o2) were targeted. Microcirculation was assessed by videomicroscopy, intestinal ΔPco2 by air tonometry, and V̇o2 by expired gases analysis. Although cardiac output and superior mesenteric flow increased in hemodilution, from the very first step (Hb = 5.0 g/dl), villi functional vascular density and RBC velocity decreased (21.7 ± 0.9 vs. 15.9 ± 1.0 mm/mm(2) and 1,033 ± 75 vs. 850 ± 79 μm/s, P < 0.01). In the last stage (Hb = 1.2 g/dl), these variables were lower in hemodiution than in hemorrhage (11.1 ± 0.5 vs. 15.4 ± 0.9 mm/mm(2) and 544 ± 26 vs. 686 ± 70 μm/s, P < 0.01), and were associated with lower intestinal fractional O2 extraction (0.61 ± 0.04 vs. 0.79 ± 0.02, P < 0.01) but preserved ΔPco2 (5 ± 2 vs. 25 ± 4 mmHg, P < 0.01). Therefore, alterations in O2 extraction in hemodilution seemed related to microvascular shunting, not to fast RBC velocity. The severe microvascular abnormalities suggest that normal ΔPco2 was not dependent on CO2 washout by the villi microcirculation. Increased perfusion in deeper intestinal layers might be an alternative explanation.

Sublingual microcirculatory alterations in cirrhotic patients

“…Hemodilution is another condition that shows a dissociation between systemic hemodynamics and microcirculation. Even though cardiac output is largely augmented, vascular density and RBCV are decreased in most of the microvascular beds . Until now, high RBCV has only been found in cardiac surgery patients during cardiopulmonary bypass …”

“…Even though cardiac output is largely augmented, vascular density and RBCV are decreased in most of the microvascular beds. 30 Until now, high RBCV has only been found in cardiac surgery patients during cardiopulmonary bypass. 31 The presence of capillaries with high RBCV in cirrhotic patients, of similar mean RBCV in controls and in cirrhotic patients, and of Vascular remodeling is a structural modification of the vessel wall that develops in response to long-term modifications in hemodynamic conditions.…”

Systemic and microcirculatory effects of blood transfusion in experimental hemorrhagic shock

“…Since P v-a CO 2 /C a-v O 2 might be a misleading indicator of RQ and anaerobic metabolism, its values should be carefully interpreted. This is additionally emphasized by the fact that ongoing anaerobic metabolism is identified by acute increases in RQ, not by isolated values [6–8]. Actually, the normal range of RQ is 0.67 to 1.30 [32].…”

“…Even though several systemic variables may track the presence of tissue hypoxia, none of them is specific or sensitive. The acute increase in respiratory quotient (RQ) is an excellent marker of ongoing anaerobic metabolism in both exercise and oxygen supply dependency [6–8]. In both circumstances, there is an excess of CO 2 production (VCO 2 ) compared to oxygen consumption (VO 2 ), which results from anaerobic VCO 2 .…”

Effects of Normovolemic Hemodilution on Survival of Skin Flaps: An Experimental Study

“…In clinical practice, decrease in hematocrit levels below 30% is accepted as an indication for transfusion of red blood cells because the compromise of oxygen carriers from excessive hemodilution may result in hypoxic damage in tissues vulnerable to ischemia after trauma and surgery [ 7 ]. Hemodilution can be compensated for by the increased peripheral blood flow that supports the tolerance of lower hematocrit levels, diminishing the risk of infection with blood-borne diseases [ 11 , 12 ]. Failure of the free flaps has been mostly associated with venous and global ischemia, while in random-pattern flaps, factors influencing distal ischemia are more important [ 13 ].…”