In septic shock patients, the hemodynamic coherence between systemic, regional and microcirculatory blood flow can be tracked by "capillary refill time (CRT) response to an increase in stroke volume induced by a rapid fluid challenge". A parallel improvement in regional blood flow, microcirculation and hypoperfusion-related parameters should be expected in CRT-responders as reflection of preserved hemodynamic coherence. CRT non-response is associated with a more severe systemic inflammatory state, endothelial and microvascular dysfunction, and a higher adrenergic tone.

The objective of this study is to determine if CRT response after a rapid fluid challenge signals a state of hemodynamic coherence as demonstrated by a parallel improvement in regional and microcirculatory blood flow in CRT-responders, and to explore the pathophysiological mechanisms associated to CRT non-response.

INTRODUCTION Septic shock is associated with a high mortality risk of up to 30-60%. Multiple pathogenic factors can lead to progressive tissue hypoperfusion in the context of severe systemic inflammation. However, despite extensive research on the best monitoring and resuscitation strategy many uncertainties persist. Over-resuscitation, particularly when inducing fluid overload, might contribute to a worse outcome. Fluid overload more likely occurs when fluids are administered to fluid unresponsive patients, but also when inappropriate resuscitation goals are pursued. The systematic use of bedside techniques to determine fluid responsiveness (FR) can help to avoid fluid overload. Moreover, further deleterious fluid administration can be prevented by adding the evaluation of hemodynamic coherence in parallel or sequentially to FR. Further research on this topic is imperative considering not only the extremely high morbidity and mortality of septic shock, but also the increasing economic burden over the health system in both developed and low/medium income countries.

CAPILLARY REFILL TIME (CRT) AS A TARGET FOR FLUID RESUSCITATION IN SEPTIC SHOCK The skin territory lacks auto-regulatory flow control, and therefore, sympathetic activation impairs skin perfusion during circulatory dysfunction, a phenomenon that can be evaluated by peripheral perfusion assessment. Abnormal peripheral perfusion after initial or advanced resuscitation is associated with increased morbidity and mortality. A cold clammy skin, mottling or prolonged CRT have been suggested as triggers for fluid resuscitation in patients with septic shock. Moreover, the excellent prognosis associated with CRT recovery, its rapid-response time to fluid loading, its relative simplicity, its availability in resource-limited settings, and its capacity to change in parallel with perfusion of physiologically relevant territories such as the hepatosplanchnic region, constitute strong reasons to consider CRT as a target for fluid resuscitation in septic shock patients.

THE CONCEPT OF A FLUID CHALLENGE Since absolute or relative hypovolemia is almost universally present in early septic shock, resuscitation starts with fluid loading in pre-ICU settings. Fluid loading is the rapid administration of fluids without necessarily monitoring the response in real-time, when confronting severe life-threatening hypotension and hypoperfusion. In this setting, usually 20-30 ml/kg crystalloids are loaded.

If circulatory dysfunction is not resolved with this initial management, patients are transferred to the ICU, where advanced fluid resuscitation is started with the fundamental objective to increase systemic blood flow. The initial step is assessment of FR. Fluid-responsive patients will increase stroke volume >10 to 15% after receiving a fluid bolus (usually 250 to 500 ml of crystalloids) since they are in the ascending part of the Starling curve. On the contrary, being fluid-unresponsive implies to be in the flat part of the curve where fluids will only lead to congestion without increasing stroke volume.

The standard practice is to perform a fluid challenge in fluid-responsive patient who are still hypoperfused. A fluid challenge consists of a fluid bolus, large and rapid enough, to increase venous return and cardiac output (CO) in fluid responsive patients, and eventually improve tissue perfusion, depending on the status of hemodynamic coherence (see below). Fluid is given as a fluid challenge so that response can be assessed looking at the target, and the need for ongoing fluid therapy ascertained.

Very few studies have addressed the best way to perform a fluid challenge. A recent study demonstrated that a minimum of 4 ml/kg fluid bolus maximizes the impact on stroke volume. On the other hand, the rate of administration is also important. The FENICE study found that the most common practice in Europe is to administer 500 ml of crystalloids in 30 minutes as a fluid challenge (standard method). However, a more rapid fluid challenge in 5 to 10 minutes might exert more beneficial effects on tissue perfusion by inducing a vasodilatory reflex in addition to the increase in stroke volume. T

THE CONCEPT AND CLINICAL RELEVANCE OF HEMODYNAMIC COHERENCE IN SEPTIC SHOCK Hemodynamic coherence is the condition in which resuscitation of systemic macrohemodynamic variables results in concurrent improvement in regional and microcirculatory flow, and correction of tissue hypoperfusion. Loss of coherence in septic shock is associated with increasing organ dysfunction and a worse prognosis.

The relationship between macrocirculation and regional/microcirculatory blood flow is conditioned by the predominant pathogenic mechanism at different stages of septic shock. At an early stage, hypovolemia and vascular tone depression predominate, leading to low CO and hypotension. An increase in systemic blood flow induced by fluids and/or vasopressors improves regional and microcirculatory flow at this stage. This suggests that macro- and microcirculation are coupled, and should lead to sustained efforts to increase systemic blood flow until hypoperfusion-related variables are corrected.

At a more advanced stage, excessive adrenergic tone (or high-dose vasopressors), and microvascular/endothelial inflammation predominate, leading to abnormal regional flow distribution, and microcirculatory dysfunction that might not respond to systemic blood flow optimization. Microvascular dysfunction occurs because of endothelial dysfunction, leukocyte-endothelium interactions, coagulation and inflammatory disorders, hemorheologic abnormalities, functional shunting, and as an iatrogenic effect of fluid overload/tissue edema.

Hemodynamic coherence is lost in this advanced stage, and efforts to further increase cardiac CO) with fluids or inodilators might lead to fluid overload and the toxicity of vasoactive agents without improving tissue perfusion.

TRACKING THE STATUS OF HEMODYNAMIC COHERENCE IN SEPTIC SHOCK PATIENTS:

A major risk of ICU-based fluid resuscitation is to induce fluid overload. Administering fluids to patients with septic shock after they lost hemodynamic coherence might deteriorate tissue oxygenation, even if they are still fluid-responsive in cardiac function terms. This is a very important consideration. Assessment of hemodynamic coherence is a step forward over the fluid responsiveness concept. This latter looks at the cardiac function curve, but the former instead at the holistic relationship between different components of the cardiovascular system.

The problem is that no single static parameter can predict the status of hemodynamic coherence, and therefore, fluids are abused and probably contribute to progression to refractory shock and death. This is a fundamental contradiction in septic shock resuscitation and highlights the difference between the concepts of FR and hemodynamic coherence. As an example, patients with capillary leak maintain FR along the process because fluids are rapidly lost to the interstitium, and the severe endothelial/microcirculatory dysfunction precludes reperfusion. So, these patients are both fluid-responsive and uncoupled. Moreover, clinicians in despair keep pushing more fluids to try to correct hypoperfusion, which only worsens microcirculatory abnormalities and further impairs perfusion.

Only a novel dynamic test could reveal if the macrocirculation is still coupled or not to regional/microcirculatory blood flow and prevent mismanagement and fluid overload as stated above. The hypothesis of AUSTRALIS is that CRT response to a single rapid fluid challenge can be used as a novel "hemodynamic coherence test." CRT is a sort of bridge between the two worlds (macro-and microcirculation), since it directly represents systemic blood flow (due to the lack of autoregulation), and microcirculation. Normalization of CRT represents an improvement in regional and microcirculatory skin perfusion secondary to an increase in systemic blood flow and/or a reactive decrease in adrenergic tone, thus reflecting hemodynamic coherence. On the contrary, CRT non-response after a rapid fluid challenge is abnormal and a signal of loss of coherence.

PATHOPHYSIOLOGICAL DETERMINANTS OF CRT NON-RESPONSE There are many possible explanations on why CRT might not respond to a stroke volume increase induced by a fluid challenge. Some of these possible mechanisms will be addressed in the proposed study. Adrenergic tone and systemic inflammation, and endothelial/coagulation dysfunction will be addressed by a series of biomarkers selected to provide a broad overview of systemic inflammatory/anti-inflammatory response, and of the transition between endothelial/coagulation activation to established dysfunction, plus direct visualization of microcirculatory status under the tongue, and assessment of microvascular reactivity.

CLINICAL RELEVANCE OF THE PRESENT STUDY If the hypothesis is confirmed, CRT-response to a rapid fluid challenge could be used as a hemodynamic coherence test, and help to avoid futile and dangerous further fluid administration in uncoupled patients, and eventually reduce additional iatrogenic-related excess mortality. Fluid resuscitation could then be focused in fluid responsive patients in whom hemodynamic coherence is still preserved while other perfusion parameters are still not normalized.

Furthermore, establishing the status of hemodynamic coherence with this simple test in pre-ICU or resource-limited settings, could eventually aid in taking triage decisions. CRT non-responders who concentrate septic shock mortality might be rapidly transferred to hospitals with ICU facilities for advanced monitoring and treatment, including reinforcement of source control and eventually rescue therapies.

At the end, this study will help to position CRT, a costless, universally available, and simple test, not only as key target for septic shock resuscitation, but also as a dynamic test of the circulatory function that might help clinicians to interpret the stage of evolution, and help to take timely and critical decisions on fluid resuscitation beyond the concept of fluid responsiveness.

For research purposes, CRT response is defined by "CRT-normalization", and not by "CRT improvement but without normalization" which will be categorized as CRT non-response. This is because hemodynamic tests require to be dichotomous to be applied on a decision branch. In addition, normalization is the only alternative to get certainty that reperfusion has been completed. In any case, partial response will be also included in post-hoc analyses, and the results of the test are not of a binding nature for attending intensivists.

OBJECTIVES AND HYPOTHESIS OR RESEARCH QUESTIONS HYPOTHESIS: In septic shock patients, the hemodynamic coherence between systemic, regional and microcirculatory blood flow can be tracked by "CRT response to an increase in stroke volume induced by a rapid fluid challenge". A parallel improvement in regional blood flow, microcirculation and hypoperfusion-related parameters should be expected in CRT-responders as reflection of preserved hemodynamic coherence. CRT non-response is associated with a more severe systemic inflammatory state, endothelial and microvascular dysfunction, and a higher adrenergic tone.

GENERAL OBJECTIVE: To determine if CRT response after a rapid fluid challenge signals a state of hemodynamic coherence as demonstrated by a parallel improvement in regional and microcirculatory blood flow in CRT-responders, and to explore the pathophysiological mechanisms associated to CRT non-response.

SPECIFIC OBJECTIVES

1. To determine if CRT normalization after an increase in stroke volume (>10%) induced by a rapid fluid challenge is associated with a parallel improvement in regional, microcirculatory blood flow and perfusion variables.
2. To determine if the rate of fluid challenge (rapid vs. standard) influences CRT response rate.
3. To determine if CRT non-response is associated with a more severe systemic inflammatory state, endothelial and microvascular dysfunction, and a higher adrenergic tone.

Inclusion Criteria:

1. Septic shock according to the Sepsis-3 Consensus Conference [1], basically septic patients with hypotension requiring norepinephrine (NE) to maintain a MAP of 65 mmHg, and serum lactate levels > 2 mmol/l after initial fluid resuscitation.
2. Less than 24h after fulfilling criteria for septic shock
3. Abnormal CRT (>3 secs)
4. Mechanical ventilation
5. Sinus rhythm with positive predictors of fluid responsiveness [4]
6. Continuous CO monitor, arterial line and central venous catheters in place
7. Required fluid challenge as decided by the attending physician.

Exclusion Criteria:

1. Pregnancy
2. Emergency surgery or dialytic procedure scheduled within the next two hours
3. Do-not-resuscitate status
4. Active bleeding
5. Severe acute respiratory distress syndrome
6. Right ventricular failure

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6 months after completing recruitment

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