Shock is best defined as a life-threatening, generalized form of acute circulatory failure associated with inadequate oxygen utilization by the cells. It is a state in which the circulation is unable to deliver sufficient oxygen to meet the demands of the tissues, resulting in cellular dysfunction. The result is cellular dysoxia, i.e. the loss of the physiological independence between oxygen delivery and oxygen consumption, associated with increased lactate levels. Some clinical symptoms suggest an impaired microcirculation, including mottled skin, acrocyanosis, slow capillary refill time and an increased central-to-toe temperature gradient.

Shock is a clinical state of acute circulatory failure [3] that can result from one, or a combination, of four mechanisms [4]. The first of these is a decrease in venous return due to a loss of circulating volume (i.e. due to internal or external loss of fluids). The second is a failure of the pump function of the heart that results from a loss of contractility (resulting from ischemia, infarction, myopathy, myocarditis) or a major arrhythmia (such as ventricular tachycardia or a high degree A-V block). The third is an obstruction due to pulmonary embolism, tension pneumothorax or cardiac tamponade. The fourth is loss of vascular tone that results in maldistribution of blood flow (due to sepsis, anaphylaxis or spinal injury). The features of each of these four types of shock often overlap, and patients admitted with one type of shock can develop other types of shock. For example, patients hospitalized with hemorrhagic shock due to trauma or with cardiogenic shock occasionally develop septic shock [5, 6].

Up to one-third of patients admitted to the ICU are in circulatory shock [7], and early recognition of the condition is vital if subsequent tissue injuries are to be avoided. Shock can be categorized according to the underlying cause, including septic shock, cardiogenic shock, anaphylactic shock and shock associated with burns, trauma and hemorrhage. In the 1,679 ICU patients in the European Sepsis Occurrence in Acutely Ill Patients II (SOAP II) trial, septic shock was the most frequent cause of shock, accounting for 62 % of cases, followed by cardiogenic shock (17 %) and hypovolemia (16 %) [8].

Septic shock is the most severe manifestation of sepsis, with reported case-fatality rates in the range of 40–50 %, reaching as high as 80 % [9]. Limited data are available on the epidemiology of septic shock, particularly in low-income countries [9], but the literature suggests that its incidence is increasing [10–20]. The reported incidence of septic shock in patients admitted to the ICU varies between 6.3 and 14.7 % [1, 21].

Cardiogenic shock has most commonly been studied in the setting of acute myocardial infarction; the incidence in this population has remained fairly constant at between 6 and 9 %, over the past several decades [6, 22–25]. In a multinational observational study of 65,119 patients hospitalized for an acute coronary syndrome between 1999 and 2007, 4.6 % developed cardiogenic shock, and the in-hospital case-fatality rate was 59.4 % [26].

We define circulatory shock as a life-threatening, generalized form of acute circulatory failure associated with inadequate oxygen utilization by the cells. Definition.

The diagnosis of acute circulatory failure is based on a combination of clinical, hemodynamic and biochemical signs. The clinical signs of shock typically include arterial hypotension (although this is not always present), associated with signs of altered tissue perfusion, visualized through the three ‘windows’ of the body [27]: the peripheral window (skin that is cold, clammy and blue, pale or discolored); the renal window (decreased urine output: <0.5 mL/kg/h); the neurologic window (altered mental characterized by obtundation, disorientation and confusion). The presence of low blood pressure should not be a prerequisite for defining shock: compensatory mechanisms may preserve blood pressure through vasoconstriction [28], while tissue perfusion and oxygenation are already decreased significantly [29].

Shock is typically associated with evidence of inadequate tissue perfusion on physical examination. The three organs readily accessible to clinical assessment of tissue perfusion are the:

skin (degree of cutaneous perfusion);

In experimental models of acute circulatory failure induced by arterial hypoxemia, low hemoglobin levels, obstructive shock and septic shock, the onset of decreased oxygen utilization relative to oxygen demand is characterized by increasing lactate levels [33, 34] and decreased regional and microcirculatory perfusion [35, 36]. Additionally, the results of limited clinical studies have shown that inadequate oxygen is associated with increased lactate levels [37–39].

Hyperlactatemia is indeed typically present in cases of acute circulatory failure, indicating abnormal metabolism. The usual cut-off value is 2 mEq/L (or mmol/L), but lactate levels of >1.5 mmol/L in patients with septic shock are associated with increased mortality [40]. Although generally increased lactate levels are associated with abnormal oxidative phosphorylation and inadequate oxygen utilization [38, 39], other mechanisms may also increase lactate levels in the presence of adequate tissue oxygenation [41–43].

The prognostic value of lactate levels exceeds that of blood pressure [32, 44, 45]. While hyperlactatemia is associated with worse outcome in any type of shock, a correct interpretation also depends on the type of shock, i.e. septic shock versus hemorrhagic shock [46].

From a metabolic perspective, elevations in blood lactate concentration may be due to increased production, a decreased clearance or a combination of the two. As elevated plasma lactate forms part of the definition of shock, the argument to measure it as a diagnostic marker of shock is circular. Many studies have confirmed the association between initial serum lactate level and mortality independently of clinical signs of organ dysfunction in patients not only with severe sepsis [47], but also those in cardiogenic shock [48].

An early decrease in blood lactate levels may indicate the resolution of global tissue hypoxia and has been associated with a decreased mortality rate [49]. Two studies proposed ‘lactate-based management’ of ICU patients [50, 51]. Jones et al. [50] studied 300 patients, of whom more than 80 % had septic shock, who were treated to normalize central venous pressure (CVP) and MAP; additional management to normalize lactate clearance compared with management to normalize ScvO₂ did not result in different rates of hospital mortality. Jansen et al. [51] showed, in patients with hyperlactatemia (>3.0 mEq/L) on ICU admission, that lactate-guided therapy (with the aim of decreasing lactate by ≥20 % every 2 h for the initial 8 h) in comparison with no lactate-guided therapy (in which the treatment team had no knowledge of lactate levels other than the admission level) reduced hospital mortality when predefined risk factors were adjusted (hazard ratio 0.61, 95 % confidence interval 0.43–0.87; P = 0.006). These authors demonstrated a reduced rate of organ failure—but no reduced mortality—in the lactate group compared with the control group in the early observation period (between 9 and 72 h). Lactate levels between the control and lactate groups were similar over a 3-day period. In clinical practice, we suggest serial measurements of lactates and/or base deficit to evaluate not only the outcome and prognosis but also to guide therapy; lactate measurements can be performed every 2 h in the first 8 h and every 8–12 h thereafter.

In the context of hyperlactatemia and mixed venous oxygen saturation (SvO₂), ScvO₂ can provide important information about the balance between oxygen transport and oxygen demand. For example, in the context of septic shock, low ScVO₂ indicates an inadequacy of oxygen transport, especially in the context of hyperlactatemia. In patients with low ScVO₂ values (<70 %), Rivers et al. showed that early goal-directed therapy (EGDT) aimed at increasing the ScVO₂ to >70 % was associated to a better outcome than a different protocolized treatment not using this measurement to guide therapy [31]. In their study, the mean baseline values of ScVO₂ were 49 and 48 % for the control and EGDT groups, respectively. Two recent multicenter RCTs, the ProCESS and the ARISE trials, failed to reproduce similar results. In these studies, patients had a lower mortality and higher mean baseline ScvO₂ values (71 % for the ProCESS trial and 73 % for ARISE trial) [52, 53] compared to the Rivers et al. study [31]. In addition, in both the ProCESS and ARISE trials, clinicians well trained in management of septic shock and well aware of available resuscitation goals and techniques treated the control groups.

In practice, a high ScvO₂ value in the context of hyperlactatemia is of limited use. One of the limitations of ScvO₂ is that normal/high values cannot discriminate if the oxygen transport is adequate, as it may remain ‘blind’ to local hypoperfusion.

The venoarterial carbon dioxide difference (pCO₂ gap), which measures the difference in the partial pressure of carbon dioxide (pCO₂) between mixed or central venous blood and arterial blood, is a marker that can be used to identify patients who are under-resuscitated. Values of >6 mmHg suggest an insufficient blood flow in the tissues even when the ScvO₂ is >70 % [54, 55].

Although all shock states are associated with systemic inflammation either early or later in the course of circulatory failure, and markers of systemic inflammation have been associated with mortality in such cases, the levels of these mediators are generally higher in septic patients [56–59]. However, even in patients with septic shock, lactate levels have a better prognostic value than other markers [60]. There is a lack of evidence that other biomarkers of an early hyper-inflammatory response (e.g. interleukin-1 receptor agonist, intercellular adhesion molecule 1, tumor necrosis factor-α, caspase 3 and interleukin-8) predict early complications, particularly in septic shock [61].

Good animal and human data are available on the role of mediators in the evolution of shock, but current outcome data do not support the routine use of these mediators as biomarkers in the diagnosis and staging of shock.

We recommend measuring blood lactate levels in all cases where shock is suspected. Recommendation. Level 1; QoE low (C).

Identifying the main mechanism responsible for shock—hypovolemic, cardiogenic, obstructive, or distributive [4, 62]––is of paramount importance. Context analysis (trauma, infection, chest pain, etc.) and clinical evaluation which focuses on skin perfusion and jugular vein distension usually orient diagnosis to the type of shock, but complex situations may exist (e.g. cardiac tamponade in a patient with trauma or septic shock in a patient with chronic heart failure) in which a diagnosis is more difficult. Hence, additional hemodynamic measurements are often needed to ascertain the type of shock, especially in complex situations or in patients with comorbidities. Distributive shock is usually characterized by an elevated cardiac output, while the other types of shock are associated with low cardiac output. Hypovolemic shock is associated with low blood pressures and volumes, while these are increased in cardiogenic shock. Obstructive shock is associated with increased pulmonary artery pressure and dilated right-sided cavities. Tamponade, a form of obstructive shock, is associated with compression of all cavities, and thus with elevated intracardiac pressures but small cardiac volumes. In general, hemodynamic variables such as CVP can be useful in identifying the type of shock. However, unless in the extreme ranges of the variables (such as a CVP of 0 mmHg in the case of a history of bleeding), they should always be interpreted together with other variables.

Evaluation of cardiac output, cardiac function and preload is essential when attempting to identify the type of shock and can be obtained using various techniques. Echocardiography allows rapid characterization of the type of shock and is now proposed as the first-line evaluation modality [4]. This information can be obtained rapidly, usually in less than 2 min, even by physicians with minimal training [63]. The situation may evolve over time, however, and repeated echocardiographic evaluations are not always feasible. Hence, a combination of echocardiography with other technologies is often warranted. Advanced hemodynamic monitoring may not be needed in non-severe episodes of shock that rapidly respond to initial therapy based on clinical evaluation and echocardiography. In cases of severe shock and in complex conditions, advanced hemodynamic monitoring is useful for identifying the factor(s) which contribute(s) most to the hemodynamic disturbances and on which therapy should focus. Continuous or semi-continuous measurements of cardiac output and/or SvO₂ are particularly useful as these can be nurse driven.

We recommend efforts to identify the type of shock to better target causal and supportive therapies. Best practice.

Echocardiography cannot provide continuous hemodynamic data. Nevertheless, it is the best bedside method to assess cardiac function repeatedly. Echocardiography can help the ICU physician in three ways: (1) better characterization of the hemodynamic disorders; (2) selection of the best therapeutic options (intravenous fluids, inotropes and ultrafiltration); (3) assessment of the response of the hemodynamic disorders to therapy.

Doppler echocardiography provides an estimation of stroke volume and hence cardiac output using the calculation of the velocity–time integral (VTI) of the subaortic blood flow and the area of the duct crossed by this flow. Since the area of the subaortic tract does not change over time, it is sufficient to follow short-term changes in VTI in order to assess changes in stroke volume.

Doppler echocardiography provides measurements of LVEF, which depends mainly on LV contractility and LV afterload. Thus, LVEF is not a precise marker of LV contractility but rather reflects the way the heart is able to adapt to the actual loading conditions with its intrinsic contractility. This is particularly important in shock patients in whom LV afterload can change markedly over a short period. Therefore, the LVEF must be correctly interpreted to take into account the systolic arterial pressure. Visual estimation of LVEF provides values very close to those measured by the orthogonal plan method [99], and this measurement can be obtained by most ICU physicians, even those who are moderately experienced with echocardiography [100].

LV filling pressures are best evaluated using analysis of the transmitral flow with pulsed Doppler echocardiography and the tissue Doppler imaging of the mitral annulus [101]. Analysis of transmitral flow allows measurement of the peak Doppler velocities of early (E) and late diastolic flow (A). The E/A ratio has been proposed as an estimation of the LV filling pressure [102], but this measurement is affected by diastolic function. Early diastolic mitral annular velocity (E′) measured with tissue Doppler imaging evaluates diastolic function in a load-independent manner [103]. The combination of tissue Doppler imaging and pulsed Doppler echocardiography of transmitral flow allows calculation of the E/E′ ratio, which is considered to be one of the best echocardiographic estimates of LV filling pressure [103, 104]. However, echocardiography provides only a semi-quantitative estimate of LV filling pressures. Although an E/E′ value of <8 is a good predictor of low LV filling pressure and an E/E′ value of >15 is a good predictor of high LV filling pressure, a value between 8 and 15 cannot reliably predict the LV filling pressure [105]. This is not a minor issue as most E/E′ values are between 8 and 15 in critically ill patients [104, 106].

Echocardiography also provides dynamic parameters of preload responsiveness through analysis of the respiratory variability of VTI [88] or the inferior or superior vena cava diameter [88, 107] or through the response of VTI to passive leg-raising [98, 108].

Doppler echocardiography provides an estimation of RV function through comparison of the RV end-diastolic area (RVEDA) with the LV end-diastolic area (LVEDA). A RVEDA/LVEDA ratio of between 0.6 and 1 suggests the presence of moderate RV dilatation, and a ratio >1 suggests the presence of severe RV dilatation. Some authors have defined acute cor pulmonale as the combination of a RVEDA/LVEDA ratio >0.6 and the presence of a paradoxical septal motion [109–111].

The PAC provides the ICU physician with information on important hemodynamic variables [e.g. right atrial pressure, pulmonary artery pressure (PAP), PAOP and cardiac output) and tissue perfusion variables (e.g. SvO₂, oxygen utilization, oxygen delivery, oxygen extraction and PvCO₂]. All of these variables can be useful in the management of patients with shock.

Right atrial pressure and PAP are particularly useful for managing patients with shock associated with RV dysfunction and/or acute respiratory distress syndrome (ARDS). PAOP is assumed to reflect the LV end-diastolic pressure. Correct measurements and appropriate interpretation of PAOP represent a difficult challenge, especially in patients receiving positive end-expiratory pressure (PEEP) [112, 113] or in the presence of intrinsic PEEP [113], where the measured PAOP overestimates the transmural PAOP.

Cardiac output can be measured intermittently according to the thermodilution principle after cold bolus injections. This is classically considered as the reference method, although it suffers from a number of methodological limitations, such as tricuspid regurgitation. The semi-continuous thermodilution method is based on the intermittent and automatic heating of blood by means of a proximal thermal filament and the recording of the temperature changes using a distal thermistor. The results obtained with continuous thermodilution agree with those provided by the intermittent technique, except for high values of cardiac output, which can be underestimated by the continuous method [114]. This technique presents the advantage of a continuous display of cardiac output and avoids repeated manipulations of the catheters and bolus injections. The major limitation is that it does not enable real-time monitoring of cardiac output as it averages several successive cardiac output measurements.

The PAC can also provide intermittent or continuous measurements of SvO₂ and intermittent measurements of PvCO₂; both variables are helpful in assessing the adequacy of the cardiac output for oxygen (O₂) utilization and for the clearance of carbon dioxide (CO₂) produced by cellular metabolism.

The main limitation of the PAC is its invasiveness, which explains in part the decline in its use during the past decade when less invasive hemodynamic techniques have been developed. Nevertheless, the PAC can still be helpful for the management of shock states refractory to the initial treatment, especially those with RV dysfunction or with complex circulatory conditions in which the knowledge of PAP, PAOP and oxygenation parameters is believed to be important for identifying the main disorders. Monitoring with a PAC is commonplace in the ICU setting despite the lack of high-quality data to support its benefits [115]. While the availability of cardiac output and other hemodynamic variables obtained using a PAC can improve the diagnosis and management of circulatory instability, the device can also cause complications and provide inaccurate measurements, and the data can be difficult to interpret [116].

Published evidence on the use of PACs in the ICU is conflicting, with some early non-randomized trials suggesting increased rates of mortality and morbidity and longer hospital stays. The results of a prospective study with propensity-matched groups showed that patients receiving a PAC had a higher rate of 30-day death, higher hospital costs and increased duration of ICU stay compared with patients who did not receive a PAC [116]. Analysis of data from an observational study in over 4,000 patients undergoing non-cardiac surgery revealed an increase in the rate of postoperative major cardiac and non-cardiac complications in those who underwent perioperative right heart catheterization [117]. In contrast, two recent studies, both of which used propensity scores to account for illness severity, reported similar outcomes in critically ill patients with or without a PAC [118, 119]. In a retrospective study of 119 consecutive patients with ARDS, monitoring with a PAC was not an independent predictor of death [120].

The impact of PAC on mortality and morbidity in critically ill patients has also been reported in several, more recent RCTs [121–124]. One study involving 201 patients reported no difference in mortality related to PAC use, but more fluids were given in the first 24 h to the PAC group, and the rates of acute renal failure and thrombocytopenia 3 days after randomization were greater in this group [121]. Similarly, in 676 patients with shock and/or ARDS, the use of a PAC did not significantly improve morbidity or reduce mortality [122]. Harvey et al. reported that they found no evidence of benefit or harm with the use of a PAC in a study of 1,041 ICU patients [123]. Wheeler et al. conducted a randomized trial in 1,000 patients with ARDS or acute lung injury with the aim of comparing hemodynamic management guided by PAC and by CVC [124]. These authors reported that there were no differences in 60-day mortality, time on the ventilator or days spent in the ICU, but the PAC group did have a higher rate of catheter-related complications, largely arrhythmias [124]. Shah et al., in a meta-analysis of the efficacy and safety of the PAC in 5,051 critically ill patients (13 RCTs) [125], reported that the use of a PAC was not associated with an increased rate of death or length of hospitalization, but the results also failed to demonstrate improved survival associated with PAC use.

Transpulmonary thermodilution devices are considered to be less invasive than the PAC, but they still require insertion of a CVC and femoral arterial catheter. These devices combine transpulmonary thermodilution and pulse contour analysis.

Transpulmonary thermodilution provides intermittent measurements of cardiac output after a cold bolus injection into a central vein and detection of changes in the blood temperature in the femoral artery. The technique has been compared favorably with pulmonary thermodilution in critically ill patients, including some in shock [126]. Transpulmonary thermodilution also provides intermittent measurements of: (1) global end-diastolic volume, a volumetric marker of cardiac preload; (2) cardiac function index, a marker of systolic function of the heart; (3) extravascular lung water, a quantitative index of pulmonary edema.

Arterial pulse wave analysis measures cardiac output by placing sensors placed directly into the femoral artery catheter which record the arterial pressure waveform through the femoral artery catheter. Determination of pulse contour cardiac output uses proprietary algorithms based on the relationship between stroke volume and arterial pressure waveform, which is influenced by the resistive and elastic characteristics of the arterial system. Intermittent transpulmonary thermodilution cardiac output measurements are used to calibrate the pulse contour cardiac output, and good agreement with thermodilution cardiac output has been reported in hemodynamically unstable patients [127]. However, there is a potential drift with time, making recalibration mandatory. After a 1-h calibration-free period, recalibration may be encouraged in patients with septic shock who are receiving vasopressors [128].

The clinical interest for such real-time cardiac output monitoring is to improve diagnostics so that patients in whom cardiac output is dropping can be identified early and to be able to follow the short-term changes in cardiac output during dynamic tests or therapeutic challenges. Because such devices provide numerous important hemodynamic variables, transpulmonary thermodilution can be helpful in patients with shock refractory to initial treatment and especially in cases of associated ARDS because of the assessment of extravascular lung water.

This system uses the lithium dilution method to calibrate an arterial waveform analysis system. The technique has been validated against pulmonary artery thermodilution in humans [129], and agreement remains acceptable for up to 4 h after calibration in critically ill patients [130]. The system needs a lithium bolus injection for its calibration. The best calibrations are achieved, as is true for any intermittent technique, by averaging more measurements (i.e. 2–3 boluses) [131]. The device can be used to measure and track cardiac output continuously in patients with shock. In complex patients it has the limitation of measuring fewer hemodynamic variables than the PAC and the transpulmonary thermodilution systems.

Several devices provide real-time cardiac output measurements by deriving the arterial pressure waveform recorded from an artery catheter (radial or femoral). These devices use proprietary algorithms that analyze the characteristics of the arterial pressure waveform and use this analysis, along with patient-specific demographic information, to determine continuous cardiac output and PPV and/or SVV. A theoretical advantage is that these devices can be used with a radial artery catheter and do not need calibration. However, the reliability of uncalibrated devices is still debated in the case of shock, particularly septic shock [132–134]. More importantly, the fact that these devices do not provide important variables, such as filling pressures or transpulmonary thermodilution variables (global end-diastolic volume, extravascular lung water, etc.), represents a disadvantage compared with PAC or transpulmonary thermodilution devices.

Recently developed systems use the pulse contour analysis and the volume clamp method to monitor cardiac output in real-time using an inflatable cuff wrapped around a finger connected to a monitor. This non-invasive method should be used during the perioperative period. However, the value of this technique in the context of shock patients and/or patients receiving vasopressors is questionable, as confirmed by results from clinical studies which showed no agreement with thermodilution cardiac output [135] or velocity time integral [136] for estimating either absolute values of cardiac output or for tracking changes in cardiac output during therapy. In addition, patients with shock often need an arterial catheter for their routine management.

The use of esophageal Doppler is aimed at monitoring cardiac output by continuously measuring the blood flow in the descending thoracic aorta. This method takes advantage of the anatomical proximity of the thoracic descending aorta and the esophagus. With this technique, a flexible probe of small diameter is introduced into the esophagus. The tip of the probe is equipped with a Doppler transducer that records the velocity of red blood cells passing into the descending thoracic aorta. To obtain flow from velocity measurements, the diameter of the descending aorta must be taken into account.

Most commercialized devices do not measure the aortic diameter, but rather they estimate it from nomograms based on the patient’s height, weight and age. With these devices, the aortic diameter is thus considered to be a constant in a particular patient. In patients with shock receiving resuscitation, this assumption represents an important limitation of using such devices because the aortic diameter depends on the transmural aortic pressure, such that changes in MAP result in changes in aortic diameter [137]. A practical limitation of the technique is that the probe moves easily into the esophagus when the patient is moving. Thus, continuous monitoring of cardiac output requires frequent replacement of the probe. Esophageal Doppler is more suitable for the operating theater than for the ICU. Unfortunately, esophageal Doppler can only provide blood flow measurements, which limits its potential for hemodynamic monitoring in the context of shock in comparison with other monitoring devices, such as the PAC or transpulmonary thermodilution.

Bioreactance is a non-invasive technique used for monitoring cardiac output in real-time which uses skin surface electrodes placed on the patient’s chest and neck that apply a low-amplitude, high-frequency electrical current which traverses the thorax. The signal is recorded by other electrodes on the skin surface, with a time delay called a phase shift. The underlying scientific rationale is that the higher the cardiac stroke volume, the more significant these phase shifts become. In critically ill patients, including those with shock, a poor agreement between thermodilution cardiac output and bioreactance cardiac output has been reported in two studies [138, 139].

Echocardiography can be used for the sequential evaluation of cardiac function in shock Statement of fact.