1 Introduction
Early and proper resuscitation is essential to restore tissue perfusion and to preserve cell function in circulatory shock [
1]. Although international guidelines recommend targeting macro-hemodynamic parameters such as mean arterial blood pressure (MAP), central venous pressure (CVP), central venous oxygen saturation (ScvO
2) and blood lactate levels during resuscitation [
2], several clinical studies failed to demonstrate a clear relationship between macro and micro-hemodynamics, which has been termed as “hemodynamic incoherence” [
3,
4].
Microcirculation is a heterogeneous, dynamic and autonomous system with complex regulation and homeostasis [
5]. Several authors have demonstrated that derangements in microcirculation are related to multiple organ failure and death in critically ill patients [
6,
7]. For instance, it has been shown in septic patients that microvascular perfusion improves faster in survivors than in non-survivors [
8]. More interestingly, even after reestablishing systemic hemodynamics, microcirculation parameters may remain impaired while severity of microvascular dysfunction is also related to poor clinical outcomes [
8,
9].
Currently, the diagnosis of shock is based on systemic arterial hypotension, hyperlactatemia and clinical signs of tissue hypoperfusion, which may be apparent at the bedside in three ways: cutaneous (cold and clammy skin), renal (decreased urine output) and neurologic (altered mental state) [
10]. Unlike renal or neurologic dysfunction, skin abnormalities may be subjective. Relevant cutaneous markers of tissue perfusion such as capillary refill time (CRT), peripheral perfusion index (PPI), skin-temperature gradient (Tskin-diff), in addition to tissue oxygen saturation (StO
2) are not cited in the definition of circulatory shock in large international studies or in the consensus, and the assessment of most of these quantitative peripheral perfusion parameters has not been incorporated into routine clinical practice so far [
11,
12].
Moreover, considering the dissociation between macro- and microcirculatory compartments, the assessment of tissue perfusion in intensive care unit (ICU) patients is of paramount importance [
13]. Despite technological advances in this field, the direct identification of severe microcirculatory alterations remains difficult at bedside. Several controversies remain about the behavior of cutaneous peripheral perfusion parameters according to the severity of shock [
14,
15]. For instance, there is considerable overlap between pathological values and the StO
2 values obtained under physiological conditions [
16]. Clinicians should rely on a combination of parameters in detecting “occult” shock and a simultaneous analysis of clinical and laboratory tissue perfusion parameters, in addition to NIRS static and dynamic-derived variables could provide relevant information. Therefore, the objective of this exploratory study was to perform a comprehensive, quantitative and noninvasive evaluation of peripheral perfusion and to investigate the microcirculatory parameters that discriminate patients with and without circulatory shock.
4 Discussion
We found that, differently from clinical and laboratory peripheral perfusion parameters, NIRS-derived static and dynamic parameters discriminated between shock and non-shock patients within the first 24 h of ICU admission. The dynamic parameters derived from NIRS were inversely correlated to the administered dose of norepinephrine. Additionally, although similar values in shock and non-shock patients, CRT exhibited a positive correlation with SOFA score, and was the only peripheral perfusion variable with a significant difference between survivors and non-survivors.
Microcirculatory dysfunction has been associated with increased morbidity and mortality in critically ill patients [
6]. For instance, persistent abnormalities in sublingual microcirculatory, and not global hemodynamic parameters, discriminated between septic shock survivors and septic patients dying of multiple organ failure [
6]. Most importantly, due to the absence of a clear relationship between macro and micro-hemodynamics (hemodynamic incoherence) [
3], the achievement of systemic resuscitation goals may not translate into improved microcirculation and can contribute to fluid overload and additional exposure to catecholamines [
26].
The subjective assessment of peripheral perfusion with physical examination of the skin can be a valuable adjunct in hemodynamic monitoring during circulatory shock [
9]. Lima et al. demonstrated that hemodynamically stable patients have an increased risk of developing organ dysfunction if abnormal clinical signals of peripheral perfusion, such as CRT, Tskin-diff and PPI are detected [
9]. Moreover, a prolonged CRT after 6 h of resuscitation has been shown to be predictive of 14-day mortality in septic shock patients [
27]. Other observational studies have also demonstrated a strong relationship between skin clinical parameters and higher mortality in patients with shock, such as skin temperature gradients and mottling [
28,
29]. In addition, a meta-analysis involving 20 studies and 717 septic patients showed that survivors had higher levels of StO
2 compared with non survivors at different times of measurements [
30]. In our study, we observed that only CRT exhibited a positive correlation with SOFA score and hospital mortality. We studied a mixed population of ICU patients, resuscitated before study enrollment, as demonstrated by the cardiac index (CI) and ScvO
2 values, and without serial StO
2 measurements overtime. Our exploratory study was not powered to investigate associations with mortality. However, the present data supports the hypothesis that NIRS measurements may be more useful when analyzed along with other peripheral perfusion variables, particularly CRT.
Recent studies have suggested that StO
2 values can be used as a screening tool in potentially critical patients [
31,
32]. Bazerbashi et al. demonstrated that patients with a static value of StO
2 < 70% at presentation in the emergency department (ED) were associated with a 2.64 times increase in ICU admission compared to those with StO
2 of > 70% [
32]. Another prior study evidenced more severe organ dysfunction in septic patients who consistently presented StO
2 < 70% during the first 8 h of resuscitation [
32]. Furthermore, there was no significant relationship between low StO
2 values and global hemodynamic parameters, such as HR and MAP [
32].
Our findings are consistent with previous studies showing that peripheral blood flow variables may be altered in different experimental and clinical shock conditions [
6,
33,
34]. In this regard, a recent study with adult patients presenting to the ED with suspected sepsis diagnosis, used a similar noninvasive optical device to measure the muscle oxygenation (MOx) and found that MOx could stratify patients in mild and moderate shock, defined by degrees of systemic hemodynamic variables and lactate levels [
35]. Our study expands these previous observations demonstrating that changes in NIRS-derived variables assessed early in a mixed ICU population can detect the presence of shock.
By inducing an ischemic stress, VOT provides important information on tissue O
2 extraction and microvascular reactivity [
30,
36]. Dynamic VOT parameters had a higher accuracy in detecting microvascular dysfunction in critically ill patients than static values [
30,
36]. In a mix critically ill adults’ population, Donati et al. showed that the desaturation rate tended to be slower in the late ischemic phase in patients with sepsis, hypotension, high lactate levels or with norepinephrine administration (conditions of a likely hemodynamic instability) [
37]. Although our study, involving a smaller population, evidenced similar descending slope rates between shock and non-shock patients, we observed lower values of StO
2min in patients with shock compared with patients without shock, probably due to the imbalance between supply and demand of oxygen and lower auto regulatory reserve [
38].
Reactive hyperemia can evaluate the tissue’s ability to adjust oxygen extraction capabilities to oxygen delivery after a hypoxic stimulus induced by VOT [
39]. The difference between the maximum StO
2 during the hyperemic phase and baseline StO
2 (ΔStO
2) can be used to estimate the microcirculatory reactivity [
39]. Unlike our findings, a previous study involving 72 patients with severe sepsis or septic shock showed lower slopes (ΔStO
2) in patients with shock than non-shock patients [
24]. More interestingly, there was no correlation between slope and norepinephrine dose [
24]. Nevertheless, we found a moderate negative correlation between the norepinephrine administered dose and dynamic measurements derived from NIRS (recovery time, descending slope and ascending slope) in our study. Our results are consistent with other previous results suggesting that the local vasoconstriction mediated by a pharmacological intervention might be deleterious, regardless of the optimization of global hemodynamic variables [
19,
40,
41]. In addition, our data may suggest that the potentially harmful effect of vasopressor administration on microcirculation may be dose dependent.
Compared with other techniques, the advantages of NIRS are its noninvasiveness, real-time continuous monitoring, with a relatively inexpensive and small device that is easy to use [
35]. However, the utility of NIRS in the management of critically ill patients is still a matter of debate. A recent randomized trial study of StO
2-guided resuscitation with sepsis or septic shock patients at ICU admission found that the inclusion of StO
2 > 80% as a target in the algorithm for early goal-directed therapy did not improve clinical outcomes [
42]. Moreover, this experimental algorithm of resuscitation was associated with more time on mechanical ventilation, more blood transfusion and more use of inotropes [
42]. However, another randomized controlled pilot study was performed comparing a peripheral perfusion–guided early fluid resuscitation with a classical strategy based on MAP, CVP and CI in septic shock patients admitted to the ICU [
43]. Peripheral perfusion was assessed through CRT, Tskin-diff, PPI and StO
2 [
43]. The strategy based on clinical tissue perfusion assessment demonstrated reduction in fluid therapy volume in the first 72 h, reduction in hospital length of stay and lower organ failure scores [
43].
The role of the clinical assessment of peripheral perfusion as a target during early resuscitation in shock was further evaluated in a recent large-scale multicenter randomized trial comparing peripheral perfusion–targeted resuscitation to blood lactate level–targeted resuscitation during an 8-h intervention period [
44]. Patients were randomized to a stepwise resuscitation protocol aimed at either normalizing CRT or decreasing lactate levels at rates greater than 20% per 2 h [
44]. Peripheral perfusion–targeted resuscitation was associated with less organ dysfunction at 72 h. Despite the absence of significant differences in all-cause 28-day mortality, goal-directed therapy protocols based on serial measurements of CRT is a promising therapeutic approach [
44].
This study has some limitations. First, the number of patients included in this study was limited. Moreover, correlations between perfusion parameters, SOFA score and between doses of norepinephrine were not adjusted for confounders. Therefore, the risk of spurious false-positive and false-negative findings must be considered. Second, administered treatment (e.g., dobutamine, fluids and corticosteroids) was not similar between the groups and our patient population with shock was heterogeneous, which may have affected our results. Third, peripheral tissue perfusion parameters alter in a constant dynamic manner and we included patients at variable time points in admission. Although we performed a comprehensive evaluation of several microcirculatory parameters and the assessment of peripheral perfusion could aid in the diagnosis of shock, it is not clear what the clinical consequences should be when these measurements are taken at varying time points and following variable interventions.
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