Infusing arginine vasopressin (AVP) in vasodilatory septic shock is usually accompanied by a decrease in cardiac output and systemic oxygen (O₂) transport. It is still a matter of debate whether this vasoconstriction impedes visceral organ blood flow and thereby causes organ dysfunction 12345. In fact, controversial data have been reported in experimental 678910111213141516171819 and clinical studies 202122. The vasopressin-induced vasoconstriction is also associated with reduced coronary flow, but again data are equivocal 2324252627, most likely because of the variable impact of coronary flow and perfusion pressure 27. Consequently, the use of vasopressin is still cautioned in patients with heart and/or peripheral vascular disease 235, and the multicenter Vasopressin and Septic Shock Trial (VASST) explicitly excluded patients with cardiogenic shock, ischemic heart disease, congestive heart failure, and mesenteric ischemia 27.

Given this controversy, we tested the hypothesis whether low-dose AVP infusion (supplemented with noradrenaline) is safe with respect to liver, kidney, and heart function in a clinically relevant porcine model of fecal peritonitis-induced septic shock 28. AVP was compared with noradrenaline, and the two drugs were titrated to maintain comparable blood pressure.

One animal in the control group died following data collection at 18 hours, and thus statistical analysis at 24 hours comprises 23 animals. Colloid resuscitation was identical in the two groups (controls: 15 (14 to 15), AVP: 14 (13 to 14) mL/kg/h). AVP-treated animals did not require any additional noradrenaline during the first 12 hours of the experiment, and, consequently, the median duration and rate of the noradrenaline infusion were significantly lower (duration: 111 (0 to 282) versus 752 (531 to 935) minutes; infusion rate: 0.06 (0.00 to 0.10) versus 0.61 (0.33 to 0.72) μg/kg/min).

Tables 1 and 2 and Figures 1 and 2 summarize the data on systemic hemodynamics and left heart function (Table 1), as well as O₂ exchange, acid-base status, and metabolism (Table 2). AVP-treated animals presented with significantly lower heart rate and cardiac output. In contrast to the AVP group, maintenance of mean blood pressure was only achieved in one-third of the control animals, because the noradrenaline infusion rates were not further increased if tachycardia more than 160 beats/min occurred. Nevertheless, albeit mean blood pressure was significantly lower at 18 and 24 hours of peritonitis, one control animal only developed hypotension with a mean blood pressure less than 60 mmHg (Figure 1). None of the other parameters of systemic and pulmonary hemodynamics showed any significant intergroup difference. Although dp/dt_max was significantly lower in the AVP-treated animals, dp/dt_min and the diastolic relaxation time τ were comparable in the two groups. Troponin I levels progressively increased in the control animals and were significantly higher than in the AVP group at the end of the experiment (Figure 2). Control animals showed a significantly higher systemic O₂ transport as well as O₂ uptake and CO₂ production, whereas arterial blood gas tensions were nearly identical. The progressive fall of arterial pH and base excess was attenuated in the AVP-treated group (P = 0.069 and P = 0.053, respectively, at 24 hours). Although the rate of whole body glucose oxidation increased comparably, the progressive rise of endogenous glucose production rate was less pronounced in the AVP animals (P = 0.053, P = 0.061, and P = 0.053 at 12, 18, and 24 hours of peritonitis). Consequently, the directly oxidized fraction of the glucose released was significantly higher in the AVP group, which coincided with significantly lower arterial lactate levels at 18 and 24 hours.

Table 3 and Figures 3, 4, 5 and 6 summarize the parameters of visceral organ blood flow, O₂ kinetics, acid-base status, and function. Except for a lower portal venous flow (P = 0.053 at 24 hours), liver hemodynamics and O₂ exchange did not significantly differ between the two groups. Nevertheless, AVP attenuated the portal and hepatic venous acidosis (Table 3) and blunted the otherwise significant rise in serum transaminase activities and bilirubin levels (Figures 3, 4 and 5). AVP prevented the time-dependent fall in urine output so that diuresis was significantly higher between 12 and 24 hours (Table 3). Renal dysfunction with reduced creatinine clearance (Table 3) and increased blood creatinine levels (Figure 6) was less severe, while fractional Na⁺ excretion was significantly higher in the AVP-treated animals (Table 3).

Table 4 shows the parameters of the inflammatory response. Although the increase in blood NO₂^-+NO₃^- and TNFα levels was comparable, AVP was associated with significantly lower IL-6 concentrations and expired nitric oxide (NO).

Histomorphologic evaluation showed some non-specific subcapsular inflammatory cell infiltration and a few biliary tract concrements in the liver, and tubular swelling in the kidney; however, this was without any intergroup difference, and no pathologic findings at all in the myocardium. Although TUNEL-positive nuclei were absent or rare (without intergroup difference) in the heart and the liver, respectively, AVP-treated animals showed less TUNEL-positive renal tubular nuclei (3 (3 to 9) versus 11 (5 to 15), respectively, P = 0.061).

The aim of the present study was to test the hypothesis whether low-dose AVP infusion is safe for heart and visceral organ function in a clinically relevant, resuscitated, and hyperdynamic porcine model of fecal peritonitis-induced septic shock. AVP supplemented with noradrenaline was compared with noradrenaline alone, which were titrated to maintain comparable blood pressure. The key findings were that: AVP decreased heart rate and cardiac output without affecting myocardial relaxation, and significantly decreased troponin I blood levels; increased the rate of direct, aerobic glucose oxidation, and reduced hyperlactatemia; attenuated kidney dysfunction as well as liver injury, which coincided with less severe systemic inflammatory response.

In our experiment, left ventricular dp/dt_max was significantly lower in the AVP group, whereas dp/dt_min remained unchanged. Thus our experiment seems to confirm negative inotrope properties of AVP in isolated hearts 2324 and endotoxin-challenged rabbits 25. As first derivatives of pressure, dp/dt_max and dp/dt_min crucially depend on heart rate. In the mentioned studies, however, heart rate was not affected at all 2324 or decreased by less than 10% only 25. Furthermore, an unresuscitated model with endotoxin-induced cardiac dysfunction 25 or AVP decreased coronary blood flow below baseline levels 2324. Clearly, as we did not measure coronary blood flow, we cannot exclude a vasoconstriction-related reduction in coronary perfusion. Nevertheless, it is unlikely that AVP caused myocardial ischemia: troponin I levels progressively increased in the control animals only and were significantly higher than in the AVP group at the end of the experiment. Our findings are in sharp contrast to data by Müller and colleagues, who recently reported unchanged systolic and compromised diastolic heart function during incremental AVP infusion in swine with transient myocardial ischemia 18. These authors also studied a hypodynamic model characterized by a reduced cardiac output resulting from myocardial dysfunction, while we investigated fluid-resuscitated animals with a sustained increase in cardiac output. In addition, Müller and colleagues infused AVP alone, while we combined AVP with noradrenaline. In fact, the current rationale of AVP use comprises a supplemental infusion, targeted to restore vasopressin levels, simultaneously with catecholamines rather than AVP alone 29. It remains open whether the results reported by Müller and colleagues were due to the AVP-related vasoconstriction, that is, afterload-dependent and/or related to coronary hypoperfusion, or to a genuine myocardial effect. This issue, however, is critical in the discussion on cardiac effects of AVP: 'cardiac efficiency', that is, the product of left ventricular pressure times heart rate normalized for myocardial O₂ consumption, was well maintained under constant flow conditions 26. Finally, the significantly reduced noradrenaline requirements may have contributed to the less severe myocardial injury 30. In the control group, maintaining blood pressure at pre-peritonitis levels necessitated high noradrenaline infusion rates, which were reported to cause myocardial injury due to increased workload 31 and reduced metabolic efficiency resulting from enhanced fatty acid oxidation 32.

Despite the lower portal venous flow infusing AVP did not have any detrimental effect on liver O₂ exchange and, moreover, was associated with less severe hepatic venous metabolic acidosis and attenuated liver injury. Furthermore, AVP infusion resulted in significantly less severe kidney dysfunction. Controversial effects were reported on the effects of AVP infusion on visceral organ blood flow and function during large animal sepsis and septic shock: although AVP decreased mesenteric arterial and portal venous flow during porcine and ovine bacterial sepsis 131516 or endotoxemia 6710, other studies found unchanged hepato-splanchnic perfusion when vasopressin or terlipressin were infused during hyperdynamic porcine endotoxemia and ovine fecal peritonitis 81019. The effect of AVP on the kidney macrocirculation was even more heterogenous, in as much decreased 10, unchanged 1316, and even increased 7 renal blood flow were reported. It should be emphasized that a fall in regional blood flow below baseline levels associated with signs of organ ischemia, for example, regional venous acidosis and/or increased lactate concentrations, only occurred in hypodynamic models with a sustained decrease in cardiac output 710 and/or with AVP doses higher than currently recommended 1516. In fact, Sun and colleagues demonstrated during ovine fecal peritonitis that both low-dose vasopressin alone and in combination with noradrenaline were associated with less severe hyperlactatemia and tissue acidosis than with noradrenaline alone, which ultimately resulted in improved survival 8. In endotoxic swine infusing low doses of the AVP analogue terlipressin also caused hyperlactatemia, which, however, did not originate from the hepato-splanchnic system and was even associated with attenuated portal and hepatic venous metabolic acidosis 33.

AVP did not affect creatinine clearance, and fractional Na⁺ excretion was significantly increased. Therefore, it could be argued that AVP deteriorated or, at best, did not influence kidney function 34, which would be in contrast with previous reports of improved renal function in experimental models 91335 and clinical investigations 2236. It should be noted, however, that AVP significantly attenuated the otherwise progressive increase in creatinine blood levels. Despite its value as a marker of kidney injury, blood creatinine concentrations may not be closely correlated with creatinine clearance in the pig, because in this species some basal tubular creatinine secretion may be present 37. Moreover, in the context of the significantly higher urine output, the lower blood creatinine levels, and the attenuated tubular TUNEL staining, the significantly higher fractional Na⁺ excretion probably mirrors the physiologic response to AVP 38 rather than deteriorated tubular function: intravenous AVP increased fractional Na⁺ elimination both under healthy 3940 and pathologic conditions 3541. Finally, the reduced noradrenaline requirements may have also contributed to the higher fractional Na⁺ excretion: noradrenaline per se was demonstrated to reduce Na⁺ elimination 4243.

Several mechanisms may explain the AVP-related less severe organ dysfunction and tissue injury. First, AVP was associated with significantly lower IL-6 levels, that is, an attenuated systemic inflammatory response, which is in good agreement with the anti-inflammatory properties of AVP reported in endotoxic mice 44. In addition, infusing AVP reduced the amount of exhaled NO, which confirms our own data during terlipressin infusion in endotoxic swine 33, as well as the inhibition of the inducible isoform of the NO synthase in endotoxic rats with biliary cirrhosis 45. In addition to anti-inflammatory properties of vasopressin per se, the lower noradrenaline doses may have attenuated the inflammatory response: catecholamines may mimick 46 and/or enhance 4748 the inflammatory effects of endotoxin. Second, AVP was affiliated with a smaller rise in the endogenous glucose production rate, while glucose oxidation was identical. Consequently, the percentage of direct, aerobic glucose oxidation as a fraction of endogenous glucose release was significantly increased. Such a switch in fuel utilization to the preferential use of glucose improves the yield of oxidative phosphorylation: the ratio of ATP synthesis to O₂ consumption is higher for glycolysis than for β-oxidation, because reduced nicotineamide adenine dinucleotide (NADH) as an electron donor provides three coupling sites rather than two only provided by reduced flavine adenine dinucleotide (FADH₂) 49. Again, it remains open whether this effect is due to AVP per se and/or the reduced catecholamine requirements: Noradrenaline increases endogenous glucose release 50, and Regueria and colleagues showed improved liver mitochondrial function during noradrenaline administration in endotoxic swine 51, whereas other authors emphasized the catecholamine-induced derangement of metabolic efficiency 52.

In our clinically relevant model of fecal peritonitis-induced septic shock, low-dose AVP infusion supplemented with noradrenaline proved to be safe with respect to myocardial and visceral organ function and tissue integrity. Nevertheless, as we observed a reduced dp/dt_max in young animals without underlying heart disease, the use of AVP should be cautioned in patients with heart failure and/or cardiac ischemia, such as in the recent VASST 27. It remains to be elucidated whether the attenuated inflammatory response and improved energy metabolism during AVP was due to the treatment per se and/or to the reduced noradrenaline requirements needed to achieve the hemodynamic targets.

• Low-dose AVP appears to be safe with respect to myocardial function and heart injury and even attenuates kidney and liver dysfunction and tissue damage during well-resuscitated porcine septic shock.

• An increased aerobic glucose oxidation and reduced hyperlactatemia suggests improved cellular energy metabolism, which coincides with less severe systemic inflammation.

• It remains to be elucidated whether this is due to the treatment per se and/or to the decreased exogenous catecholamine requirements.

ALAT: alanine aminotransferase; ASAT: asparatate aminotransferase; AVP: arginine vasopressin; CO₂: carbon dioxide; dp/dt_max: maximal systolic contraction; dp/dt_min: maximal diastolic relaxation; FADH₂: reduced flavine adenine dinucleotide; FiO₂: fraction of inspired oxygen; H&E: hematoxylin and eosin; I/E: inspiratory-to-expiratory; IL-6: interleukin-6; NADH: reduced nicotineamide adenine dinucleotide; NO₂^-+NO₃^-: nitrate+nitrite; O₂: oxygen; PaO₂: partial pressure of arterial oxygen; PaCO₂: partial pressure of arterial carbon dioxide; PEEP: positive end-expiratory pressure; τ: diastolic relaxation time constant; TNFα: tumor necrosis factor-α; TUNEL: terminal deoxynucleotidyltransferase-mediated nick-end labeling assay; VASST: vasopressin and septic shock trial.

RL is a full-time salaried employee of Ferring Research Institute Inc., San Diego, CA, USA. PA, PR, and EC received a research grant from Ferring Research Institute Inc., San Diego, CA, USA. PR and PA received consultant fees from Ferring Pharmaceutical A/S, København, Denmark, for help with designing preclinical experiments. The other authors declare that they have no competing interests.

PA, RL, PR, and EC played a pivotal role in planning and designing the experimental protocol. FS, MG, and FP carried out the anesthesia, surgical instrumentation as well as the on-line data collection. RG, BH, and MG were responsible for the data analysis. AS and PM provided the histomorphology and immunohistochemistry findings and the analysis of these data. JV and UW were responsible for the isotope data acquisition, analysis, and interpretation. MG, PR, and BH wrote the manuscript.