Acid aspiration results in ileal injury without altering ileal V(O2)-D(O2) relationships

Intramucosal-arterial PCO2 gap fails to reflect intestinal dysoxia in hypoxic hypoxia

INTRODUCTION

An elevation in intramucosal-arterial PCO2 gradient (DeltaPCO2) could be determined either by tissue hypoxia or by reduced blood flow. Our hypothesis was that in hypoxic hypoxia with preserved blood flow, DeltaPCO2 should not be altered.

METHODS

In 17 anesthetized and mechanically ventilated sheep, oxygen delivery was reduced by decreasing flow (ischemic hypoxia, IH) or arterial oxygen saturation (hypoxic hypoxia, HH), or no intervention was made (sham). In the IH group (n = 6), blood flow was lowered by stepwise hemorrhage; in the HH group (n = 6), hydrochloric acid was instilled intratracheally. We measured cardiac output, superior mesenteric blood flow, gases, hemoglobin, and oxygen saturations in arterial blood, mixed venous blood, and mesenteric venous blood, and ileal intramucosal PCO2 by tonometry. Systemic and intestinal oxygen transport and consumption were calculated, as was DeltaPCO2. After basal measurements, measurements were repeated at 30, 60, and 90 minutes.

RESULTS

Both progressive bleeding and hydrochloric acid aspiration provoked critical reductions in systemic and intestinal oxygen delivery and consumption. No changes occurred in the sham group. DeltaPCO2 increased in the IH group (12 +/- 10 [mean +/- SD] versus 40 +/- 13 mmHg; P < 0.001), but remained unchanged in HH and in the sham group (13 +/- 6 versus 10 +/- 13 mmHg and 8 +/- 5 versus 9 +/- 6 mmHg; not significant).

DISCUSSION

In this experimental model of hypoxic hypoxia with preserved blood flow, DeltaPCO2 was not modified during dependence of oxygen uptake on oxygen transport. These results suggest that DeltaPCO2 might be determined primarily by blood flow.

Ileal VO(2)-O(2) alterations induced by endotoxin correlate with severity of mitochondrial injury

Sepsis is usually associated with altered O(2) metabolism in systemic organs. Until recently, inadequate O(2) delivery was thought to be the putative mechanism underlying these metabolic alterations. However, current investigations suggest that impaired O(2) consumption due to disrupted O(2) use by mitochondria may be the culprit. Therefore, we hypothesized that endotoxin (LPS)-induced V O(2)- O(2) alterations would correlate with the severity of mitochondrial injury in a systemic organ (i.e., the ileum). Using an in situ autoperfused feline ileum preparation, we assessed V O(2)- O(2) relationships and mitochondrial ultrastructure after 2 h in LPS-treated (3 mg/kg, intravenous; n = 11) and time-matched control (n = 5) animals. Mitochondrial injury was graded in a blinded fashion on the basis of characteristics associated with established stages of cell injury. LPS-treated animals developed severe mitochondrial injury in the ileal mucosa despite unchanged regional tissue perfusion and ileal oxyhemoglobin levels compared with controls. Worsening of mitochondrial injury correlated with increases in the critical O(2) delivery (r = 0.85; p < 0.002) and decreases in the maximum O(2) extraction (r = -0.61; p < 0.02) in the ileum. These results suggest that mitochondrial injury, leading to impaired O(2) utilization, may be primarily responsible for altered V O(2)- O(2) relationships in systemic organs during sepsis.

Oxygen supply dependency can characterize septic shock

OBJECTIVE

To demonstrate that oxygen consumption (VO2) can be dependent on oxygen delivery (DO2) during hemodynamic instability and independent of DO2 following stabilization.

DESIGN

We retrospectively reviewed hemodynamic and blood gas data collected from ten patients in whom DO2 was acutely altered during an episode of septic shock (phase A) and after recovery from this episode (phase B).

SETTING

General intensive care unit of a university hospital.

PATIENTS

10 critically ill adult patients (aged 55 +/- 19 years).

INTERVENTIONS

DO2 was altered by fluid challenge, administration of vasoactive agents, or application of positive end-expiratory pressure.

RESULTS

In phase A, changes in VO2 (121 +/- 32 vs 165 +/- 36 ml/min.m2; p < 0.001) paralleled changes in DO2 (415 +/- 153 vs 607 +/- 217 ml/min.m2; p < 0.001), but oxygen extraction (O2ER) remained stable (31.9 +/- 11.2 vs. 30.2 +/- 8.9%; NS). In phase B, changes in DO2 (412 +/- 118 vs 526 +/- 152 ml/min.m2; p < 0.001) were associated with opposite changes in O2ER (36.1 +/- 4.2 vs 28.9 +/- 4.9%; p < 0.001), and VO2 was unchanged (147 +/- 35 vs 149 +/- 33 ml/min.m2; NS). The mean VO2/DO2 slope was greater in phase A than in phase B (0.26 +/- 0.09 vs. 0.08 +/- 0.08; p < 0.004). Blood lactate levels were higher in phase A than in phase B (3.3 +/- 1.8 vs 1.6 +/- 0.6 mEq/l; p < 0.05).

CONCLUSIONS

Oxygen supply independency and dependency can be found at different times in the same critically ill patient. Our findings are consistent with the concept that VO2/DO2 dependency is a marker of septic shock. Interventions to increase DO2 are probably justified when this phenomenon is present.

Ischemia/reperfusion injury to the ileum does not account for the ileal VO2-DO2 alterations induced by endotoxin

PURPOSE

Endotoxin (lipopolysaccharide [LPS])-induced systemic organ injury leads to disruption of normal systemic organ metabolic processes, which are manifest clinically by signs of accelerated anaerobic metabolism (e.g., tissue acidosis and hyperlactatemia) and altered VO2-DO2 relationships. The association of increased anaerobic metabolism with VO2-DO2 alterations has led to the notion that ischemia/ reperfusion (I/R) injury may be a prerequisite for the development of VO2-DO2 alterations during endotoxemia. However, in contrast to sepsis, in which oxygen consumption is often increased, oxygen consumption is severely decreased after I/R injury. Based on these observations, we hypothesized that I/R injury would result in systemic organ VO2-DO2 alterations, which are distinct from those that occur in sepsis.

MATERIALS AND METHODS

We used the in situ autoperfused feline ileal preparation to simultaneously examine microvascular permeability, reflected as the ileal lymph to plasma protein concentration ratio (CL/CP), and ileal VO2-DO2 relationships after either intravenous LPS (2.0 mg/kg; n = 5) or I/R injury (n = 5), and in matching controls (n = 5).

RESULTS

As expected, all LPS-treated and I/R-injured animals were found to have extensive ileal histological damage and marked increases in the CL/CP compared with controls (0.315 +/- 0.009 and 0.329 +/- 0.034, respectively, v 0.097 +/- 0.009; P < .001, both comparisons). In addition, the critical DO2 (DO2c) was elevated, and the critical oxygen extraction was decreased in both the I/R and LPS groups relative to controls. However, as initially hypothesized, the VO2 at the critical DO2 was markedly decreased in the I/R group compared with that of the LPS group.

CONCLUSIONS

These data indicate that I/R injury is insufficient to account for the systemic organ VO2-DO2 alterations that occur with LPS injury.

Endotoxin-induced ileal Vo2-Do2 alterations do not correlate with the severity of ileal injury

PURPOSE

Altered Vo2-Do2 relationships are most often noted to occur in the setting of sepsis or endotoxin (LPS)-induced systemic organ microvascular injury and are generally thought to be causally linked to that injury. However, we have recently shown that ileal microvascular injury is not associated with altered ileal Vo2-Do2, relationships. Thus, we hypothesized that the severity of LPS-induced systemic organ microvascular injury would not correlate with the development of systemic organ Vo2-Do2 alterations.

MATERIALS AND METHODS

To test this hypothesis, we used the in situ autoperfused feline ileal preparation to simultaneously examine microvascular permeability, reflected as the ileal lymph to plasma protein concentration ratio (CL/CP), and ileal Vo2-Do2 relationships 2 hours after intravenous LPS (0.75-2.0 mg/kg; n = 9) and in matching controls (n = 5).

RESULTS

As expected, all LPS-treated animals were found to have extensive ileal histological damage and marked increases in the CL/CP compared with controls (0.308 +/- 0.019 v 0.097 +/- 0.009; P < .001). In addition, although the critical Do2 (Do2c) was elevated in the LPS-treated animals relative to controls (34.2 +/- 5.0 v 16.7 +/- 1.4 mL/min/kg; P < .03), there was no correlation between the Do2c and the CL/CP in the LPS-treated animals. Finally, ileal wet to dry weight ratios after LPS did not differ from controls.

CONCLUSION

Taken together, these data suggest that factors other than organ injury, as assessed by morphological and permeability alterations, are important in the pathogenesis of altered systemic organ Vo2-Do2 relationships after LPS.