Vasopressin During Uncontrolled Hemorrhagic Shock: Less Bleeding Below the Diaphragm, More Perfusion Above

Management of Hemorrhagic Shock: Physiology Approach, Timing and Strategies

Hemorrhagic shock (HS) management is based on a timely, rapid, definitive source control of bleeding/s and on blood loss replacement. Stopping the hemorrhage from progressing from any named and visible vessel is the main stem fundamental praxis of efficacy and effectiveness and an essential, obligatory, life-saving step. Blood loss replacement serves the purpose of preventing ischemia/reperfusion toxemia and optimizing tissue oxygenation and microcirculation dynamics. The "physiological classification of HS" dictates the timely management and suits the 'titrated hypotensive resuscitation' tactics and the 'damage control surgery' strategy. In any hypotensive but not yet critical shock, the body's response to a fluid load test determines the cut-off point between compensation and progression between the time for adopting conservative treatment and preparing for surgery or rushing to the theater for rapid bleeding source control. Up to 20% of the total blood volume is given to refill the unstressed venous return volume. In any critical level of shock where, ab initio, the patient manifests signs indicating critical physiology and impending cardiac arrest or cardiovascular accident, the balance between the life-saving reflexes stretched to the maximum and the insufficient distal perfusion (blood, oxygen, and substrates) remains in a liable and delicate equilibrium, susceptible to any minimal change or interfering variable. In a cardiac arrest by exsanguination, the core of the physiological issue remains the rapid restoration of a sufficient venous return, allowing the heart to pump it back into systemic circulation either by open massage via sternotomy or anterolateral thoracotomy or spontaneously after aorta clamping in the chest or in the abdomen at the epigastrium under extracorporeal resuscitation and induced hypothermia. This is the only way to prevent ischemic damage to the brain and the heart. This is accomplishable rapidly and efficiently only by a direct approach, which is a crush laparotomy if the bleeding is coming from an abdominal +/- lower limb site or rapid sternotomy/anterolateral thoracotomy if the bleeding is coming from a chest +/- upper limbs site. Without first stopping the bleeding and refilling the heart, any further exercise is doomed to failure. Direct source control via laparotomy/thoracotomy, with the concomitant or soon following venous refilling, are the two essential, initial life-saving steps.

Pharmacologic Agents for the Treatment of Vasodilatory Shock

Vasodilatory shock is a life-threatening syndrome in critically ill patients and is characterized by severe hypotension and resultant tissue hypoperfusion. This shock state requires the use of vasopressor agents to restore adequate vascular tone. Norepinephrine is still recommended as first-line vasopressor in the management of critically ill patients suffering from severe vasodilation. In the recent time, catecholaminergic vasopressor drugs have been associated with possible side effects at higher dosages. This so-called catecholamine toxicity has focused on alternative noncatecholaminergic vasopressors or the use of moderate doses of multiple vasopressors with complementary mechanisms of action. Besides vasopressin and terlipressin, angiotensin II may be a promising drug for the management of vasodilatory shock. In addition, adjunctive drugs, such as hydrocortisone, methylene blue or ascorbic acid can be added to conventional vasopressor therapy. The objective of this review is to give an overview of the current available vasopressor agents used in vasodilatory shock. A thorough search of PubMed was conducted in order to identify the majority of studies related to the subject. Data on the outcome of several drugs and future perspective of possible management strategies for the therapy of vasodilatory shock are discussed.

Arginine vasopressin improves cerebral perfusion following controlled haemorrhage in adult ewes

KEY POINTS

Traumatic haemorrhagic shock carries significant morbidity and mortality related to the severity and duration of tissue hypoperfusion, much of which occurs in the pre-hospital environment where therapy must be easy to use and would augment, not replace, local haemorrhage control measures. Vasopressor therapy use in haemorrhagic shock remains controversial. Potential benefits from improved blood pressure and tissue perfusion need to be weighed against possible harm from increased blood loss if haemorrhage is uncontrolled. We demonstrate that 20 IU I.M. vasopressin produces a progressive, sustained and clinically significant increase in blood pressure and carotid blood flow compared to 1 mg I.M. adrenaline or placebo in an animal model of controlled haemorrhagic shock. I.M. vasopressin may play a role in the early management of haemorrhagic shock by improving cerebral perfusion and haemodynamic stability; however, further studies are required to establish the potential benefit against the risk of exacerbating haemorrhage, if it is uncontrolled.

ABSTRACT

Haemorrhagic shock causes significant morbidity and mortality. Novel pre-hospital therapy to improve haemodynamic stability and cerebral perfusion may improve outcomes but remains controversial. In an ovine model of controlled haemorrhagic shock, the effects of early intramuscular arginine vasopressin (AVP), adrenaline or placebo on haemodynamic stability and cerebral perfusion were compared. Carotid pressure and flow catheters were placed in healthy, anaesthetized adult ewes. Frontal cortex cerebral oxygenation was measured using near infrared spectroscopy. Controlled, rapid, haemorrhage (∼30% estimated blood volume) was induced. Five minutes post-bleed a 1 ml intramuscular dose of 0.9% saline, adrenaline 1 mg or AVP 20 IU was administered. Carotid blood pressure and flow improved significantly in the AVP group over the first 30 min post-intervention. To emulate standard trauma care, 1 L of 0.9% saline was infused 30 min post-bleed followed by re-transfusion of the sheep's own blood at 60 min post-bleed. Carotid blood pressure and flow in the AVP group remained significantly higher post-crystalloid infusion, but this difference was lost post-blood transfusion. Data were analysed by two-way ANOVA with time, group as the main factors. When compared to saline or adrenaline, a single dose of intramuscular AVP resulted in a progressive and sustained increase in carotid artery blood pressure and flow with commensurate increase in cerebral oxygenation. Intramuscular AVP has potential as an emergency pre-hospital therapy following exsanguinating haemorrhage; however, further studies are required to investigate whether the benefit of improved perfusion pressure outweighs the risks of exacerbating ongoing bleeding.

Evaluation of capillary leakage after vasopressin resuscitation in a hemorrhagic shock model

Background

Hemorrhagic shock (HS) is a major threat to patients with trauma and spontaneous bleeding. The aim of the study was to investigate early effects of vasopressin on metabolic and hemodynamic parameters and endothelium permeability by measuring capillary leakage compared to those of other resuscitation strategies in a HS model.

Methods

Forty-five Sprague-Dawley rats were randomized into five groups: S group (n = 5), sham-operated rats without shock or resuscitation; HS group (n = 10), HS and no resuscitation; RL group (n = 10), HS and resuscitation with Ringer’s lactate (RL); RLB group (n = 10), HS and resuscitation with two-third shed blood plus RL; and vasopressin group (n = 10), HS and resuscitation with RL, followed by continuous infusion of 0.04 U/kg/min vasopressin. The effects of resuscitation on hemodynamic parameters [mean arterial pressure (MAP), superior mesenteric artery blood flow (MBF), and mesenteric vascular resistances (MVR)], arterial blood gases, bicarbonate, base deficit, and lactate levels as well as on capillary leakage in the lung, ileum, and kidney were investigated. Capillary leakage was evaluated with Evans blue dye extravasation.

Results

In the vasopressin group, the MAP was higher than in the RL and RLB groups (p < 0.001), while MBF was decreased (p < 0.001). MVR were increased only in the vasopressin group (p < 0.001). Capillary leakage was increased in the lungs of the animals in the vasopressin group compared to that in the lungs of animals in the RLB group (p < 0.05); this increase was associated with the lowest partial pressure of oxygen (p < 0.05). Conversely, decreased capillary leakage was observed with vasopressin in the ileum (p < 0.05). Increased capillary leakage was observed in the kidney in the RLB and vasopressin groups (p < 0.05). Lastly, vasopressin use was associated with higher base deficit and lactate levels when compared to the RL and RLB groups (p < 0.001).

Conclusion

Although vasopressin was proposed as a vasoactive drug for provisional hemodynamic optimization in the early phase of HS resuscitation, the overall findings of this experimental study focus on the possible critical side effects of vasopressin on metabolic parameters and endothelium permeability.

Thromboembolic Events During Continuous Vasopressin Infusions: A Retrospective Evaluation

Background:

Published guidelines suggest that vasopressin has a role in shock treatment, although its safety has not been adequately evaluated in a clinical setting. Vasopressin causes platelet aggregation and has been associated with the release of factor VIII coagulant and von Willebrand factor.

Objective:

To compare the incidence of venous thromboembolism (VTE) in patients with a diagnosis of shock who received vasopressin with those who did not receive vasopressin for hemodynamic support.

Methods:

A retrospective, single-center, cohort study was conducted at an academic, tertiary care center with 350 patients with a diagnosis of shock. Patients from the intensive care unit were randomly selected and separated into 2 groups for comparison of those receiving only catecholamines with those receiving vasopressin with or without catecholamines for hypotension. Patients with diabetes insipidus or variceal hemorrhage and those with any documented history of VTE were excluded. The primary outcome, VTE occurrence, was defined as a positive Doppler ultrasound, spiral computed tomography, or documented diagnosis in the discharge records. Frequency and type of risk factors for VTE were compared between the 2 study arms. A risk factor modeling approach was performed, using logistic regression to identify potential confounders and effect modifiers in the relationship between vasopressin and VTE.

Results:

There were 175 patients in each arm of the study. The crude incidence of VTE was 7.4% and 8% in the vasopressin and catecholamine groups, respectively (p = 0.84). No significant difference in the incidence of deep venous thrombosis (vasopressin 5.1%, control 7.4%; p = 0.51) or pulmonary embolism (vasopressin 2.3%, control 0.6%; p = 0.37) was found between groups. After adjusting for covariates, there was no statistically significant difference in the incidence of VTE between the 2 arms (p = 0.72).

Conclusions:

This investigation provides initial evidence that vasopressin infusions do not increase the risk of VTE in patients with shock.

Hemorrhagic shock: The "physiology approach"

A shift of approach from ‘clinics trying to fit physiology’ to the one of ‘physiology to clinics’, with interpretation of the clinical phenomena from their physiological bases to the tip of the clinical iceberg, and a management exclusively based on modulation of physiology, is finally surging as the safest and most efficacious philosophy in hemorrhagic shock. ATLS^® classification and recommendations on hemorrhagic shock are not helpful because antiphysiological and potentially misleading. Hemorrhagic shock needs to be reclassified in the direction of usefulness and timing of intervention: in particular its assessment and management need to be tailored to physiology.

Therapeutic applications of vasopressin in pediatric patients

CONTEXT

Reports of successful use of vasopressin in various shock states and cardiac arrest has lead to the emergence of vasopressin therapy as a potentially major advancement in the management of critically ill children.

OBJECTIVE

To provide an overview of physiology of vasopressin, rationale of its use and dose schedule in different disease states with special focus on recent advances in the therapeutic applications of vasopressin.

DATA SOURCE

MEDLINE search (1966-September 2011) using terms vasopressin, terlipressin, arginine-vasopressin, shock, septic shock, vasodilatory shock, cardiac arrest, and resuscitation for reports on vasopressin/terlipressin use in children and manual review of article bibliographies. Search was restricted to human studies. Randomized controlled trials, cohort studies, evaluation studies, case series, and case reports on vasopressin/terlipressin use in children (preterm neonates to 21 years of age) were included. Outcome measures were analysed using following clinical questions: indication, dose and duration of vasopressin/terlipressin use, main effects especially on systemic blood pressure, catecholamine requirement, urine output, serum lactate, adverse effects, and mortality.

RESULTS

51 reports on vasopressin (30 reports) and terlipressin (21 reports) use in pediatric population were identified. A total of 602 patients received vasopressin/terlipressin as vasopressors in various catecholamine-resistant states (septic - 176, post-cardiotomy - 136, other vasodilatory/mixed shock - 199, and cardiac arrest - 101). Commonly reported responses include rapid improvement in systemic blood pressure, decline in concurrent catecholamine requirement, and increase in urine output; despite these effects, the mortality rates remained high.

CONCLUSION

In view of the limited clinical experience, and paucity of randomized controlled trials evaluating these drugs in pediatric population, currently no definitive recommendations on vasopressin/terlipressin use can be laid down. Nevertheless, available clinical data supports the use of vasopressin in critically ill children as a rescue therapy in refractory shock and cardiac arrest.

Review article: update in trauma anesthesiology: perioperative resuscitation management

The management of trauma patients has matured significantly since a systematic approach to trauma care was introduced nearly a half century ago. The resuscitation continuum emphasizes the effect that initial therapy has on the outcome of the trauma patient. The initiation of this continuum begins with prompt field medical care and efficient transportation to designated trauma centers, where lifesaving procedures are immediately undertaken. Resuscitation with packed red blood cells and plasma, in parallel with surgical or interventional radiologic source control of bleeding, are the cornerstones of trauma management. Adjunctive pharmacologic therapy can assist with resuscitation. Tranexamic acid is used in Europe with good results, but the drug is slowly being added to the pharmacy formulary of trauma centers in United States. Recombinant factor VIIa can correct abnormal coagulation values, but its outcome benefit is less clear. Vasopressin shows promise in animal studies and case reports, but has not been subjected to a large clinical trial. The concept of "early goal-directed therapy" used in sepsis may be applicable in trauma as well. An early, appropriately aggressive resuscitation with blood products, as well as adjunctive pharmacologic therapy, may attenuate the systemic inflammatory response of trauma. Future investigations will need to determine whether this approach offers a similar survival benefit.

Vasoactive hemoglobin solution improves survival in hemodilution followed by hemorrhagic shock

OBJECTIVE

To compare survival after exchange transfusion followed by hemorrhage using: 1) the vasoactive, oxygen-carrying, bovine hemoglobin-based blood substitute Oxyglobin (Biopure, 12.9 g hemoglobin/dL); and 2) the hydroxyethyl starch plasma expander Hextend (high molecular weight and low degree of substitution, 6%).

DESIGN

Comparison between treatments.

SETTING

Laboratory.

SUBJECTS

Awake hamster chamber window model.

INTERVENTIONS

Fifty percent blood volume exchange transfusion followed by a 60% hemorrhage over 1 hr, followed by 1 hr of observation. Measurement of blood gases, mean arterial blood pressure, functional capillary density, arteriolar and venular diameter, and Po2 tension distribution.

MEASUREMENTS AND MAIN RESULTS

Survival with Oxyglobin was 100% and only 50% for the Hextend group. Vasoconstriction was evident in the microcirculation. Mean arterial pressure was higher in the Oxyglobin group. Functional capillary density was significantly reduced, although to a lesser extent by Oxyglobin. There was no difference in microvascular Po2 distribution after 1 hr of shock between groups.

CONCLUSIONS

Higher mean arterial pressure during the initial stages of hemorrhage could be due to vasoconstriction in the Oxyglobin group as compared to the Hextend group. It is concluded that the pressor effect due to a vasoactive oxygen carrier may be beneficial in maintaining perfusion in conditions of severe hemodilution followed by hypovolemia.

A treatment protocol including vasopressin and hydroxyethyl starch solution is associated with increased rate of return of spontaneous circulation in blunt trauma patients with pulseless electrical activity

BACKGROUND

Survival after cardiopulmonary resuscitation (CPR) using standard vasopressor therapy is disappointing. Vasopressin is a potent vasopressor that could become a useful therapeutic alternative in the treatment of cardiac arrest.

AIMS

The aim of this prehospital prospective cohort study was to assess the influence of treatment with vasopressin and hydroxyethyl starch solution (HHS) on outcome in resuscitated blunt trauma patients with pulseless electrical activity (PEA) cardiac arrest.

METHODS

Two treatment groups of resuscitated trauma patients in cardiac arrest were compared: in the epinephrine group patients received epinephrine 1 mg IV every 3 min only; in the vasopressin group patients first received hypertonic HHS and arginine vasopressin 40 units IV only or followed by epinephrine 1 mg every 3 min until cessation of CPR. Medical trauma care was provided according to advanced trauma life support (ATLS) guidelines.

RESULTS

The study included 31 patients and there were no significant demographic or clinical differences between the treatment groups. Significantly more circulatory restorations [11/13 (85%) vs 3/18 (17%); P < 0.01] and better 24-h survival rates [8/13 (62%) vs 2/18 (11%); P = 0.001] were observed in the vasopressin group. Average mean arterial pressure (100.4 +/- 11.4 mmHg vs 80.3 +/- 12.4 mmHg) and final end-tidal partial pressure of carbon dioxide (PETCO(2)) at admission (4.5 +/- 0.9 kPa vs 2.8 +/- 0.4 kPa) were also higher in the vasopressin group.

CONCLUSION

Our results suggest that victims of severe blunt trauma with PEA should be initially treated with vasopressin in combination with HHS volume resuscitation followed by standard resuscitation therapy and other procedures when appropriate. Vasopressin might be potentially lifesaving in blunt trauma cardiac arrest compared to standard treatment with epinephrine.

Developing alternative strategies for the treatment of traumatic haemorrhagic shock

PURPOSE OF REVIEW

The optimal strategy of stabilizing haemodynamic function in uncontrolled traumatic haemorrhagic shock states is unclear. Although fluid replacement is established in controlled haemorrhagic shock, its use in uncontrolled haemorrhagic shock is controversial, because it may worsen bleeding.

RECENT FINDINGS

In the refractory phase of severe haemorrhagic shock, arginine vasopressin has been shown to be beneficial in selected cases due to an increase in arterial blood pressure, shift of blood away from a subdiaphragmatic bleeding site towards the heart and brain, and decrease in fluid resuscitation requirements. Especially in patients with severe traumatic brain injury, rapid stabilization of cardiocirculatory function is essential to ensure adequate brain perfusion and thus to prevent neurological damage and to improve outcome. In addition, despite wide distribution of highly developed and professional emergency medical systems in western industrialized countries, survival chances of patients with uncontrolled traumatic haemorrhagic shock in the prehospital setting are still poor.

SUMMARY

A multicenter, randomized, controlled, international clinical trial is being initiated to assess the effects of arginine vasopressin (10 IU) vs. saline placebo in prehospital traumatic haemorrhagic shock patients, not responding to standard shock treatment, being managed by helicopter emergency medical services [vasopressin in traumatic haemorrhagic shock (VITRIS.at) study].

Vasopressin improves survival in a porcine model of abdominal vascular injury

Introduction

We sought to determine and compare the effects of vasopressin, fluid resuscitation and saline placebo on haemodynamic variables and short-term survival in an abdominal vascular injury model with uncontrolled haemorrhagic shock in pigs.

Methods

During general anaesthesia, a midline laparotomy was performed on 19 domestic pigs, followed by an incision (width about 5 cm and depth 0.5 cm) across the mesenterial shaft. When mean arterial blood pressure was below 20 mmHg, and heart rate had declined progressively, experimental therapy was initiated. At that point, animals were randomly assigned to receive vasopressin (0.4 U/kg; n = 7), fluid resuscitation (25 ml/kg lactated Ringer's and 25 ml/kg 3% gelatine solution; n = 7), or a single injection of saline placebo (n = 5). Vasopressin-treated animals were then given a continuous infusion of 0.08 U/kg per min vasopressin, whereas the remaining two groups received saline placebo at an equal rate of infusion. After 30 min of experimental therapy bleeding was controlled by surgical intervention, and further fluid resuscitation was performed. Thereafter, the animals were observed for an additional hour.

Results

After 68 ± 19 min (mean ± standard deviation) of uncontrolled bleeding, experimental therapy was initiated; at that time total blood loss and mean arterial blood pressure were similar between groups (not significant). Mean arterial blood pressure increased in both vasopressin-treated and fluid-resuscitated animals from about 15 mmHg to about 55 mmHg within 5 min, but afterward it decreased more rapidly in the fluid resuscitation group; mean arterial blood pressure in the placebo group never increased. Seven out of seven vasopressin-treated animals survived, whereas six out of seven fluid-resuscitated and five out of five placebo pigs died before surgical intervention was initiated (P < 0.0001).

Conclusion

Vasopressin, but not fluid resuscitation or saline placebo, ensured short-term survival in this vascular injury model with uncontrolled haemorrhagic shock in sedated pigs.

Liver response to hemorrhagic shock and subsequent resuscitation: MRI analysis

The liver is a target for injury in low flow states. Markers of liver injury are either invasive or not rapidly responding. Magnetic resonance imaging (MRI) may offer a noninvasive alternative to evaluate liver injury due to reduced perfusion. Recently, we reported an MRI method (functional MRI [fMRI]) that enables us to follow liver perfusion by changing the enrichment of inspired gas (air, air-5% carbon dioxide, 95% oxygen-5% carbon dioxide). Rats were subjected to hemorrhagic shock (HS) (bleeding to a MAP of 25 mmHg) and randomized to no resuscitation or resuscitation with Ringer lactate (RL) or adrenaline infusion targeted to a MAP of 50 mmHg or baseline. Significantly decreased fMRI responses to hyperoxia and hypercapnia were observed immediately after HS. Liver enzymes levels, liver histology, and apoptosis assessments were normal immediately after hemorrhage, however, showed significant changes after 6 h. Functional MRI revealed that adrenaline, but not RL infusion, significantly (P < 0.01) improved liver perfusion. Similarly, liver injury, as assessed by liver enzyme levels, liver histology, and apoptosis, was attenuated to a greater extent with adrenaline resuscitation. No significant differences in liver perfusion and injury were noted between resuscitation to low (50 mmHg) versus high (baseline) MAP. This study shows that fMRI enables early assessment of changes in liver perfusion, resulting in liver injury or recovery, and therefore, it may be considered as a noninvasive, rapidly responding tool for following liver outcome subsequent to hemorrhage and resuscitation. Using fMRI, we showed that adrenaline may be preferable to RL as an initial measure to attenuate liver injury after HS.

[Vasopressin for therapy of persistent traumatic hemorrhagic shock: The VITRIS.at study]

While fluid management is established in controlled hemorrhagic shock, its use in uncontrolled hemorrhagic shock is being controversially discussed, because it may worsen bleeding. In the irreversible phase of hemorrhagic shock that was unresponsive to volume replacement, airway management and catecholamines, vasopressin was beneficial due to an increase in arterial blood pressure, shift of blood away from a subdiaphragmatic bleeding site towards the heart and brain and decrease of fluid resuscitation requirements. The purpose of this multicenter, randomized, controlled, international trial is to assess the effects of vasopressin (10 IU IV) vs. saline placebo IV (up to 3 injections at least 5 min apart) in patients with prehospital traumatic hemorrhagic shock that persists despite standard shock treatment. The study will be carried out by helicopter emergency medical service teams in Austria, Germany, Czech Republic, Portugal, the Netherlands and Switzerland. Inclusion criteria are adult trauma patients with presumed traumatic hemorrhagic shock (systolic arterial blood pressure <90 mmHg) that does not respond to the first 10 min of standard shock treatment (endotracheal intubation, fluid resuscitation and use of vasopressors) after arrival of the first emergency physician at the scene. The time window for randomization will close after 30 min of shock treatment. Exclusion criteria are terminal illness, no intravenous access, age <18 years, injury >60 min before randomization, cardiac arrest before randomization, presence of a do-not-resuscitate order, untreated tension pneumothorax, untreated cardiac tamponade, or known pregnancy. Primary study end-point is the hospital admission rate, secondary end-points are hemodynamic variables, fluid resuscitation requirements and hospital discharge rate.

An observational study of vasopressin infusion during uncontrolled haemorrhagic shock in a porcine trauma model: Effects on bowel function

The effects of vasopressin on the gut in a porcine uncontrolled haemorrhagic shock model are described. In eight anaesthetised pigs, a liver laceration was performed; when haemorrhagic shock was decompensated, all animals received 0.4 IU/kg vasopressin, followed by 0.08 IU/kg min over 30 min, which maintained a mean arterial blood pressure >40 mmHg. Subsequent surgical intervention, infusion of whole blood and fluids resulted in a stable cardiocirculatory status. Three hours after stabilisation, all pigs developed non-bloody diarrhoea which converted into normal bowel movements within 24 h. All histological samples retained 7 days after the experiment revealed no histopathological changes. In conclusion, in this small observational study of uncontrolled porcine haemorrhagic shock, a resuscitation strategy that included high dose vasopressin was associated with transient diarrhoea and good long term survival.

Arteriolar vasoconstrictive response: comparing the effects of arginine vasopressin and norepinephrine

INTRODUCTION

This study was designed to examine differences in the arteriolar vasoconstrictive response between arginine vasopressin (AVP) and norepinephrine (NE) on the microcirculatory level in the hamster window chamber model in unanesthetized, normotonic hamsters using intravital microscopy. It is known from patients with advanced vasodilatory shock that AVP exerts strong additional vasoconstriction when incremental dosage increases of NE have no further effect on mean arterial blood pressure (MAP).

METHODS

In a prospective controlled experimental study, eleven awake, male golden Syrian hamsters were instrumented with a viewing window inserted into the dorsal skinfold. NE (2 microg/kg/minute) and AVP (0.0001 IU/kg/minute, equivalent to 4 IU/h in a 70 kg patient) were continuously infused to achieve a similar increase in MAP. According to their position within the arteriolar network, arterioles were grouped into five types: A0 (branch off small artery) to A4 (branch off A3 arteriole).

RESULTS

Reduction of arteriolar diameter (NE, -31 +/- 12% versus AVP, -49 +/- 7%; p = 0.002), cross sectional area (NE, -49 +/- 17% versus AVP, -73 +/- 7%; p = 0.002), and arteriolar blood flow (NE, -62 +/- 13% versus AVP, -80 +/- 6%; p = 0.004) in A0 arterioles was significantly more pronounced in AVP animals. There was no difference in red blood cell velocities in A0 arterioles between groups. The reduction of diameter, cross sectional area, red blood cell velocity, and arteriolar blood flow in A1 to A4 arterioles was comparable in AVP and NE animals.

CONCLUSION

Within the microvascular network, AVP exerted significantly stronger vasoconstriction on large A0 arterioles than NE under physiological conditions. This observation may partly explain why AVP is such a potent vasopressor hormone and can increase systemic vascular resistance even in advanced vasodilatory shock unresponsive to increases in standard catecholamine therapy.